Showing 1–50 of 308 results for author: Griffin, M

Collaborative estimation and evaluation of SARS-CoV-2 variant nowcasting in the United States

Authors: Isaac MacArthur, Thomas Robacker, Bren Case, Spencer J. Fox, Dylan H. Morris, Evan L. Ray, Benjamin Rogers, Becky Sweger, Natalie M. Linton, John Huddleston, Andrew Magee, Zachary Susswein, Jover Lee, Trevor Bedford, Marlin D. Figgins, Ehsan Suez, Rajath Prabhakar, Tomas Leon, Brent Siegel, Mugdha Thakur, Christopher M. Hoover, Rahil Ryder, Jesse Elder, Michael Kupperman, Ruian Ke , et al. (5 additional authors not shown)

Abstract: The ability to estimate and predict pathogen variant dynamics can inform public health responses, including planning for increased transmission or severity, shifts in population immunity, or changes to vaccine or therapeutic effectiveness. The COVID-19 pandemic demonstrated the importance of monitoring SARS-CoV-2 variant evolution through viral genome sequencing, enabling predictive models to esti… ▽ More The ability to estimate and predict pathogen variant dynamics can inform public health responses, including planning for increased transmission or severity, shifts in population immunity, or changes to vaccine or therapeutic effectiveness. The COVID-19 pandemic demonstrated the importance of monitoring SARS-CoV-2 variant evolution through viral genome sequencing, enabling predictive models to estimate variant frequencies in the recent past, present, and short-term future. Collaborative forecasting Hubs provided a valuable way to centralize predictive modeling of epidemiological indicators such as cases, hospitalizations, and deaths during the pandemic; however, none existed for variant dynamics. Here, we discuss the creation of the United States SARS-CoV-2 Variant Nowcast Hub, designed to solicit estimates of the relative abundance of a specified set of SARS-CoV-2 variants at the U.S. state level. We discuss the design decisions and challenges in building the Hub and its scoring procedures. Using submissions from the Hub's first respiratory virus season (nowcast dates October 9th, 2024 to June 4th, 2025), we evaluate five individual models and a baseline model. We found that the baseline model, which pools sequences across the U.S., performs well overall, with most individual models performing similarly or slightly worse. Locations with lower sequencing volumes exhibited greater variability in model performance. Models submitted for a single location outperformed those submitted for all locations, potentially due to greater timeliness and magnitude of local data. Much remains to be investigated regarding relative model performance across different phases of variant emergence, and we conclude by proposing future directions within and beyond this Hub. △ Less

Submitted 9 June, 2026; v1 submitted 5 June, 2026; originally announced June 2026.

Comments: 32 pages, 9 figures

Comparison of probabilistic nowcasts and forecasts of SARS-CoV-2 variant proportions made by hierarchical multinomial linear regression models

Authors: Isaac MacArthur, Thomas Robacker, Evan L. Ray, Benjamin W. Rogers, Nicholas G. Reich, Maryclare Griffin

Abstract: Nowcasting and forecasting of infectious diseases have become increasingly important since the SARS-CoV-2 pandemic. In particular, methods for modeling the composition of circulating variants at a given time have seen more use in part due to a large increase in the frequency of genomic sequencing conducted as a part of routine surveillance. However, methods must take into account that locations ha… ▽ More Nowcasting and forecasting of infectious diseases have become increasingly important since the SARS-CoV-2 pandemic. In particular, methods for modeling the composition of circulating variants at a given time have seen more use in part due to a large increase in the frequency of genomic sequencing conducted as a part of routine surveillance. However, methods must take into account that locations have different amounts of data and sometimes have different trends. We discuss hierarchical multinomial logistic regression (HMLR), a commonly used method for forecasting SARS-CoV-2 variants, which allows for data sharing across locations. We show how it has been used in the literature, and define a class of HMLR models for SARS-CoV-2 variant nowcasting and forecasting. We rigorously test a subset of this class of models using the framework of the US SARS-CoV-2 Variant Nowcast Hub, a collaborative modeling project that launched in 2024. We created two years of weekly predictions based on retrospective datasets, with the prediction dates ranging from Wednesday, August 3, 2022, to Wednesday, August 7, 2024. We tested 12 HMLR models against a baseline model on these datasets. We found that the HMLR models outperformed the baseline both in terms of probabilistic accuracy, as measured by the energy score, as well as point accuracy, as measured by the Brier score. Overall, we find that HMLR models perform best with respect to the baseline model in locations with more data, and more complex HMLR models also showed more improvement in those high-data locations; however, there was no one best model across all metrics, and simpler HMLR models perform better in low-data locations. We find that HMLR models perform well in practice for nowcasting and forecasting SARS-CoV-2 variants. △ Less

Submitted 21 May, 2026; originally announced May 2026.

Comments: 24 pages, 8 figures

Colossal Magnetoresistance and Phonon Driven Exchange Dynamics in Eu$_5$Sn$_2$As$_6$

Authors: Luke Pritchard Cairns, Kohtaro Yamakawa, Shengzhi Zhang, Youzhe Chen, Bernard Field, Rainer Reczek, Ryan P. Day, Joel E. Moore, Marcelo Jaime, Sinead M. Griffin, Robert J. Birgeneau, James G. Analytis

Abstract: The emergence of colossal magnetoresistance in a new generation of Eu$^{2+}$-based antiferromagnets is intriguing given stark contrasts to the archetypal perovskite manganites and doped Eu-chalcogenides. In this study the thermal conductivity and magnetostriction of Eu$_5$Sn$_2$As$_6$ -- one such representative -- have been measured to better understand the role of the crystal lattice. Both proper… ▽ More The emergence of colossal magnetoresistance in a new generation of Eu$^{2+}$-based antiferromagnets is intriguing given stark contrasts to the archetypal perovskite manganites and doped Eu-chalcogenides. In this study the thermal conductivity and magnetostriction of Eu$_5$Sn$_2$As$_6$ -- one such representative -- have been measured to better understand the role of the crystal lattice. Both properties are strongly field-dependent and mirror the magnetization, saturating once the Eu$^{2+}$ moments are polarized. The field-enhancement of the phonon-dominated thermal conductivity is interpreted through the lifting of a degeneracy of spin configurations, and the subsequent saturation due to quenched magnetostrain in high field. Comparison with spin-glass insulators suggests that this phenomenon is not a byproduct but rather the driver of electron delocalization due to the suppression of strong phonon scattering arising from exchange frustration. △ Less

Submitted 7 May, 2026; originally announced May 2026.

Comments: 8 pages, 3 appendices and supplement, 13 figures

Polar Topologies in a Ferroelastic Metal Membrane

Authors: Rahil Haria, Noah Schnitzer, T. Ben Britton, Yaqi Li, Tom J. P. Irons, Sophia Linssen Pitsaros, Ella Banyas, Geri Topore, Annabel Hoyes, Mariana Palos, Sinead M. Griffin, Katherine Inzani, Michele Shelly Conroy

Abstract: Polar metals, materials in which electric polarisation and metallicity coexist, are exceptionally rare because itinerant electrons screen long-range dipoles and favour centrosymmetric structures. Engineering polar textures in a conducting magnet holds promise for reconfigurable spin orbit coupling and magnetoelectric functionality. Here we show that releasing epitaxial SrRuO3 films from their subs… ▽ More Polar metals, materials in which electric polarisation and metallicity coexist, are exceptionally rare because itinerant electrons screen long-range dipoles and favour centrosymmetric structures. Engineering polar textures in a conducting magnet holds promise for reconfigurable spin orbit coupling and magnetoelectric functionality. Here we show that releasing epitaxial SrRuO3 films from their substrates drives a hierarchy of ferroelastic domain refinement from micrometre to nanometre length scales, and that this structural reorganisation spontaneously generates two distinct classes of emergent polar texture that are ubiquitous across the freestanding membrane. Using correlative microscopy from mesoscale electron channelling contrast imaging (ECCI) to atomic resolution scanning transmission electron microscopy (STEM), we demonstrate that electric polarisation emerges selectively at translation-inequivalent antiphase boundaries (APBs). At these boundaries multicomponent aac tilt field undergoes Neel-like interpolation that preserves the in-phase tilt component and amplifies roto flexoelectric coupling, while translation-equivalent boundaries remain nonpolar. The Neel like interpolation at hard APBs and Ising like collapse of all tilt components at easy APBs is corroborated with ab initio calculations. While embedded 90 ferroelastic walls provide an additional mechanistically distinct source of electric polarisation resulting in polar nanoclusters (4 nm). These distinct nanotextures at 90 walls from via elastic accommodation of strain mismatch between variants and rotostriction as the tilt field interpolates across the boundaries. These findings show that, in a membrane form, metal oxides provide a robust platform for hosting nanoscale ferroelastic domains that generate polar textures. △ Less

Submitted 30 April, 2026; originally announced April 2026.

Emergency Department Patient Flow Optimization with an Alternative Care Threshold Policy

Authors: Sahba Baniasadi, Paul M. Griffin, Prakash Chakraborty

Abstract: Emergency department (ED) overcrowding and patient boarding represent critical systemic challenges that compromise care quality. We propose a threshold-based admission policy that redirects non-urgent patients to alternative care pathways, such as telemedicine, during peak congestion. The ED is modeled as a two-class $M/M/c$ preemptive-priority queuing system, where high-acuity patients are priori… ▽ More Emergency department (ED) overcrowding and patient boarding represent critical systemic challenges that compromise care quality. We propose a threshold-based admission policy that redirects non-urgent patients to alternative care pathways, such as telemedicine, during peak congestion. The ED is modeled as a two-class $M/M/c$ preemptive-priority queuing system, where high-acuity patients are prioritized and low-acuity patients are subject to state-dependent redirection. Analyzed via a level-dependent Quasi-Birth-Death (QBD) process, the model determines the optimal threshold by maximizing a long-run time-averaged objective function comprising redirection-affected revenue and costs associated with patient balking and system occupancy. Structural analysis establishes monotone comparative statics relating the optimal threshold to all model parameters analytically. Numerical analysis using national healthcare data reveals that optimal policies are highly context-dependent. While rural EDs generally optimize at lower redirection thresholds, urban EDs exhibit performance peaks at moderate thresholds. Results indicate that our optimal policy yields significant performance gains of up to $4.84\%$ in rural settings and $5.90\%$ in urban environments. This research provides a mathematically rigorous framework for balancing clinical priority with operational efficiency across diverse ED settings. △ Less

Submitted 3 July, 2026; v1 submitted 14 January, 2026; originally announced January 2026.

Comments: 44 pages, 14 figures

Discovery of a new weberite-type antiferroelectric: La3NbO7

Authors: Louis Alaerts, Jesse Schimpf, Xinyan Li, Jiongzhi Zheng, Ella Banyas, Jeffrey B. Neaton, Sinéad M. Griffin, Yimo Han, Lane W. Martin, Geoffroy Hautier

Abstract: Antiferroelectrics are antipolar materials which possess an electric field-induced phase transition to a polar, ferroelectric phase and offer significant potential for sensing/actuation and energy-storage applications. Known antiferroelectrics are relatively scarce and mainly based on a limited set of perovskite materials and their alloys (e.g., PbZrO$_3$, AgNbO$_3$, NaNbO$_3$). Here, a new family… ▽ More Antiferroelectrics are antipolar materials which possess an electric field-induced phase transition to a polar, ferroelectric phase and offer significant potential for sensing/actuation and energy-storage applications. Known antiferroelectrics are relatively scarce and mainly based on a limited set of perovskite materials and their alloys (e.g., PbZrO$_3$, AgNbO$_3$, NaNbO$_3$). Here, a new family of lead-free, weberite-type antiferroelectrics, identified through a large-scale, first-principles computational search is introduced. The screening methodology, which connects lattice dynamics to antipolar distortions, predicted that La$_3$NbO$_7$ could exhibit antiferroelectricity. We confirm the prediction through the synthesis and characterization of epitaxial La$_3$NbO$_7$ thin films, which display the signature double hysteresis loops of an antiferroelectric material as well as clear evidence of an antipolar ground state structure from transmission electron microscopy. The antiferroelectricity in La$_3$NbO$_7$ is simpler than most known antiferroelectrics and can be explained by a Kittel-type mechanism involving the movement of niobium atoms in an oxygen octahedron through a single phonon mode which results in a smaller change in the volume during the field-induced phase transition. Additionally, it is found that La$_3$NbO$_7$ combines a high threshold field with a high breakdown field ($\approx$ 6MV/cm) - which opens up opportunities for energy-storage applications. This new weberite-type family of materials offers many opportunities to tune electrical and temperature response especially through substitutions on the rare-earth site. Ultimately, this work demonstrates a successful data-driven theory-to-experiment discovery of an entirely new family of antiferroelectrics and provides a blueprint for the future design of ferroic materials. △ Less

Submitted 12 January, 2026; v1 submitted 8 January, 2026; originally announced January 2026.

Comments: 44 pages, 16 figures

Atomic-scale control of substrate-spin coupling via vertical manipulation of a 2D metal-organic framework

Authors: Benjamin Lowe, Bernard Field, Dhaneesh Kumar, Daniel Moreno Cerrada, Oleksandr Stetsovych, Julian Ceddia, Andrés Pinar Solé, Amelia Domínguez-Celorrio, Jack Hellerstedt, Sinéad M. Griffin, Pavel Jelínek, Agustin Schiffrin

Abstract: Two-dimensional (2D) materials with frustrated crystal geometries can host strongly correlated electrons, potentially leading to a range of exotic many-body quantum phases such as Mott insulators, quantum spin-liquids, and Kondo lattices. The ability to control exchange-coupling within these systems is therefore highly desirable. Here, we use an atomically sharp scanning tunneling microscope probe… ▽ More Two-dimensional (2D) materials with frustrated crystal geometries can host strongly correlated electrons, potentially leading to a range of exotic many-body quantum phases such as Mott insulators, quantum spin-liquids, and Kondo lattices. The ability to control exchange-coupling within these systems is therefore highly desirable. Here, we use an atomically sharp scanning tunneling microscope probe to vertically manipulate a 2D Mott insulating kagome metal-organic framework (MOF) featuring Kondo-screened local magnetic moments on Ag(111). We show that by controlling the adsorption height of the MOF, we can also controllably and reversibly change the strength of Kondo coupling between the MOF's local spins and the substrate's conduction electrons. This mechanical control of Kondo coupling could be extended to other forms of interlayer exchange coupling, potentially allowing for atomic-scale design or control of spintronics technologies. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: 16 pages, 6 figures, and SI 3 pages, 1 figure

Big shells, bigger data: cohort analysis of Chesapeake Bay Crassostrea virginica reefs

Authors: Madison D. Griffin, Grace S. Chiu, Roger L. Mann, Melissa J. Southworth, John K. Thomas

Abstract: Oysters in Virginia Chesapeake Bay oyster reefs are "age-truncated", possibly due to a combination of historical overfishing, disease epizootics, environmental degradation, and climate change. Research has suggested that oysters exhibit resilience to environmental stressors; however, that evidence is based on the current limited understanding of oyster lifespan. Until this paper, the Virginia Oyst… ▽ More Oysters in Virginia Chesapeake Bay oyster reefs are "age-truncated", possibly due to a combination of historical overfishing, disease epizootics, environmental degradation, and climate change. Research has suggested that oysters exhibit resilience to environmental stressors; however, that evidence is based on the current limited understanding of oyster lifespan. Until this paper, the Virginia Oyster Stock Assessment and Replenishment Archive (VOSARA), a spatially and temporally expansive dataset (222 reefs across 2003-2023) of shell lengths (SL, mm), had yet to be examined comprehensively in the context of resilience. We develop a novel method using Gaussian mixture modeling (GMM) to identify the age groups in each reef using yearly SL data and then link those age groups over time to identify cohorts and estimate their lifespan. Sixty-four reefs (29%) are deemed to have sufficient data (at least 300 oysters sampled for a minimum of 8 consecutive years) for this analysis. We fit univariate GMMs for each year ($t$) and reef ($r$) for each of the seven river strata ($R$) to estimate 1) the mean and standard deviation of SL for each $a_{Rrt}$th age group, and 2) the mixture percentage of each $a_{Rrt}$th age group. We link age groups across time to infer age cohorts by developing a mechanistic algorithm that prevents the shrinking of shell length when an $a_{Rrt}$th group becomes an ($a_{R,r,t+1}$)th group. Our method shows promise in identifying oyster cohorts and estimating lifespan solely using SL data. Our results show signals of resiliency in almost all river systems: oyster cohorts live longer and grow larger in the mid-to-late 2010s compared to the early 2000s. △ Less

Submitted 7 December, 2025; originally announced December 2025.

Comments: 29 pages, 7 figures

Twin-boundary-induced nonrelativistic spin splitting

Authors: Kristoffer Eggestad, Marc Vila, Sverre M. Selbach, Sinéad M. Griffin

Abstract: Nonrelativistic spin splitting (NRSS) in compensated magnetic materials is drawing considerable attention due to its potential impact in next-generation spintronic devices. While NRSS is typically restricted to materials with particular symmetry constraints, here we demonstrate, using density functional theory (DFT) and tight-binding transport calculations, that twin boundaries can induce NRSS in… ▽ More Nonrelativistic spin splitting (NRSS) in compensated magnetic materials is drawing considerable attention due to its potential impact in next-generation spintronic devices. While NRSS is typically restricted to materials with particular symmetry constraints, here we demonstrate, using density functional theory (DFT) and tight-binding transport calculations, that twin boundaries can induce NRSS in magnetic systems where it is otherwise forbidden. We focus on two representative material systems: the tetragonal perovskite oxide BiCoO$_3$ with $90^{\circ}$ ferroelastic domain walls, and the rhombohedral layered delafossite-type oxide CoO$_2$, supporting $71^{\circ}$, $109^{\circ}$, and $135^{\circ}$ twin boundaries. Our results reveal that, if these boundaries coexist with ferromagnetic domain walls, they consistently produce NRSS similar to that of d-wave altermagnets, with nodal surfaces dictated by the underlying symmetry of the supercell containing the twin boundary. Tight-binding models further elucidate how the NRSS and derived transport properties scale with domain size and density. Our results put forward twin boundary engineering as a versatile route to realize and control spin splitting in a broader class of materials. △ Less

Submitted 17 November, 2025; originally announced November 2025.

Predicting Metabolic Dysfunction-Associated Steatotic Liver Disease using Machine Learning Methods: A Retrospective Cohort Study

Authors: Mary E. An, Paul M. Griffin, Jonathan G. Stine, Balakrishnan S. Ramakrishna, Soundar R. T. Kumara

Abstract: Background: Metabolic dysfunction-associated steatotic liver disease (MASLD) affects 30-40% of US adults and is the most common chronic liver disease. Although often asymptomatic, progression can lead to cirrhosis. The objective of the study was to develop and evaluate an electronic health record (EHR) based prediction model to support early detection of MASLD in primary care settings. Methods: We… ▽ More Background: Metabolic dysfunction-associated steatotic liver disease (MASLD) affects 30-40% of US adults and is the most common chronic liver disease. Although often asymptomatic, progression can lead to cirrhosis. The objective of the study was to develop and evaluate an electronic health record (EHR) based prediction model to support early detection of MASLD in primary care settings. Methods: We evaluated LASSO logistic regression, random forest, XGBoost, and a neural network model for MASLD prediction using clinical feature subsets from a large EHR database, including the top 10 ranked features. To reduce disparities in true positive rates across racial and ethnic subgroups, we applied an equal opportunity postprocessing method in a prediction model called MASLD EHR Static Risk Prediction (MASER). Results: This retrospective cohort study included 59,492 participants in the training data, 24,198 in the validating data, and 25,188 in the testing data. The LASSO logistic regression model with the top 10 features was selected for its interpretability and comparable performance. Before fairness adjustment, the model achieved AUROC of 0.84, accuracy of 78%, sensitivity of 72%, specificity of 79%, and F1-score of 0.617. After equal opportunity postprocessing, accuracy modestly increased to 81% and specificity to 94%, while sensitivity decreased to 41% and F1-score to 0.515, reflecting the fairness trade-off. Conclusions: MASER achieved competitive performance for MASLD prediction, comparable to previously reported ensemble and tree-based models, while using a limited and routinely collected feature set and a diverse study population. The model is designed to support early detection and potential integration into primary care workflows. MASER demonstrates EHR-ready MASLD prediction with fairness adjustments, supporting future primary care implementation pending prospective validation. △ Less

Submitted 9 April, 2026; v1 submitted 25 October, 2025; originally announced October 2025.

Comments: This manuscript has been submitted for consideration to the Journal of Medical Internet Research. Supplemental material is included in the Appendix. For associated code, see https://github.com/mary-elena-an/MASLD-EHR-Prediction

Polarization Domain Mapping From 4D-STEM Using Deep Learning

Authors: Fintan G. Hardy, Sinead M. Griffin, Mariana Palos, Yaqi Li, Geri Topore, Aron Walsh, Michele Shelly Conroy

Abstract: Polarization in ferroelectric domains arises from atomic-scale structural variations that govern macroscopic functionalities. The interfaces between these domains known as domain walls host distinct physical responses, making their identification and control critical. Four dimensional scanning transmission electron microscopy (4DSTEM) enables simultaneous acquisition of real and reciprocal-space i… ▽ More Polarization in ferroelectric domains arises from atomic-scale structural variations that govern macroscopic functionalities. The interfaces between these domains known as domain walls host distinct physical responses, making their identification and control critical. Four dimensional scanning transmission electron microscopy (4DSTEM) enables simultaneous acquisition of real and reciprocal-space information at the atomic scale, offering a powerful platform for domain mapping. However, conventional analyses rely on computationally intensive processing and manual interpretation, which are time consuming and prone to misalignment and diffraction artefacts. Here, we present a convolutional neural network that, with minimal training, classifies polarization directions from diffraction data and segments domains in real space. We further introduce an adaptive sampling strategy that prioritizes images from domain wall regions, reducing the number of training images required while improving accuracy and interpretability. We demonstrate this approach for domain mapping in ferroelectric boracite, Cu3B7O13Cl. △ Less

Submitted 1 October, 2025; originally announced October 2025.

PRIMA: PRIMAger, a far-infrared hyperspectral and polarimetric instrument

Authors: Laure Ciesla, Charles Darren Dowell, Marc Sauvage, Denis Burgarella, Jochem Baselmans, Matthieu Béthermin, Jeffrey T. Booth, Charles M. Bradford, Florent Canourgues, Ivan Charles, Anne Costille, Thomas Essinger-Hileman, Lorenza Ferrari, Johan Floriot, Marc Foote, Jason Glenn, Renaud Goullioud, Matt Griffin, Oliver Krause, Willem Jellema, Elizabeth Luthman, Laurent Martin, Margaret Meixner, Tony Pamplona, Klaus M. Pontoppidan , et al. (5 additional authors not shown)

Abstract: The PRobe far-Infrared Mission for Astrophysics (PRIMA) is an infrared observatory for the next decade, currently in Phase A, with a 1.8m telescope actively cooled to 4.5K. On board, an infrared camera, PRIMAger, equipped with ultra-sensitive kinetic inductance detector (KID) arrays, will provide observers with coverage of mid-infrared to far-infrared wavelengths from 24 to 264 microns. PRIMAger w… ▽ More The PRobe far-Infrared Mission for Astrophysics (PRIMA) is an infrared observatory for the next decade, currently in Phase A, with a 1.8m telescope actively cooled to 4.5K. On board, an infrared camera, PRIMAger, equipped with ultra-sensitive kinetic inductance detector (KID) arrays, will provide observers with coverage of mid-infrared to far-infrared wavelengths from 24 to 264 microns. PRIMAger will offer two imaging modes: the Hyperspectral mode will cover the 24-84 microns wavelength range with a spectral resolution R=8, while the Polarimetric mode will provide polarimetric imaging in 4 broad bands, from 80 to 264 microns. These observational capabilities have been tailored to answer fundamental astrophysical questions such as black hole and star-formation co-evolution in galaxies, the evolution of small dust grains over a wide range of redshifts, and the effects of interstellar magnetic fields in various environments, as well as to open a vast discovery space with versatile photometric and polarimetric capabilities. PRIMAger is being developed by an international collaboration bringing together French institutes (Laboratoire d'Astrophysique de Marseille and CEA) through the center National d'Etudes Spatiales (CNES, France), the Netherlands Institute for Space Research (SRON, Netherlands), and the Cardiff University (UK) in Europe, as well as the Jet Propulsion Laboratory (JPL) and Goddard Space Flight Center (GSFC) in the USA. △ Less

Submitted 1 September, 2025; originally announced September 2025.

Comments: Published in the PRIMA special issue of JATIS

Journal ref: J. Astron. Telesc. Instrum. Syst. 11(3), 031625 (2025)

First High-Throughput Evaluation of Dark Matter Detector Materials

Authors: Sinéad M. Griffin, Yonit Hochberg, Benjamin V. Lehmann, Rotem Ovadia, Kristin A. Persson, Bethany A. Suter, Ruo Xi Yang, Wayne Zhao

Abstract: We perform the first high-throughput search and evaluation of materials that can serve as excellent low-mass dark matter detectors. Using properties of close to one thousand materials from the Materials Project database, we project the sensitivity in dark matter parameter space for experiments constructed from each material, including both absorption and scattering processes between dark matter an… ▽ More We perform the first high-throughput search and evaluation of materials that can serve as excellent low-mass dark matter detectors. Using properties of close to one thousand materials from the Materials Project database, we project the sensitivity in dark matter parameter space for experiments constructed from each material, including both absorption and scattering processes between dark matter and electrons. Using the anisotropic materials in the dataset, we further compute the level of daily modulation in interaction rates and the resulting directional sensitivities, highlighting materials with prospects to detect the dark matter wind. Our methods provide the basic tools for the data-driven design of dark matter detectors, and our findings lay the groundwork for the next generation of highly optimized direct searches for dark matter as light as the keV scale. △ Less

Submitted 24 June, 2025; originally announced June 2025.

Comments: 6+7 pages, 3+5 figures

Report number: MIT-CTP/5862

First principles computations of the Stark shift of a defect-bound exciton: the case of the T center in silicon

Authors: Louis Alaerts, Yihuang Xiong, Sinéad M. Griffin, Geoffroy Hautier

Abstract: The T center in silicon has recently drawn a lot of attention for its potential in quantum information science. The sensitivity of the zero-phonon line (ZPL) to electrical field was recently investigated by a combination of different experimental methods but there is still no first principles study on the Stark shift of the T center. Dealing with the defect-bound exciton nature of the excited stat… ▽ More The T center in silicon has recently drawn a lot of attention for its potential in quantum information science. The sensitivity of the zero-phonon line (ZPL) to electrical field was recently investigated by a combination of different experimental methods but there is still no first principles study on the Stark shift of the T center. Dealing with the defect-bound exciton nature of the excited state is particularly challenging using density functional theory because of the large spatial delocalization associated with the wavefunction. Here, we tackle this issue by performing a convergence study over the supercell size. We obtain an exciton binding energy of 28.5meV, in good agreement with experimental results. We then calculate the Stark shift through the dipole moment change of the ZPL transition of the T center using the modern theory of polarization formalism and find a modest linear coefficient of $Δμ$=0.79D along X and $Δμ$=0.03D along Y. We discuss our results in light of the recent experimental measurements of the Stark shift. Our analysis suggests that bound-exciton defects could be particularly sensitive to local field effect as a result of their large spatial extent. △ Less

Submitted 30 May, 2025; originally announced May 2025.

Comments: 11 pages, 9 figures

IsaBIL: A Framework for Verifying (In)correctness of Binaries in Isabelle/HOL (Extended Version)

Authors: Matt Griffin, Brijesh Dongol, Azalea Raad

Abstract: This paper presents IsaBIL, a binary analysis framework in Isabelle/HOL that is based on the widely used Binary Analysis Platform (BAP). Specifically, in IsaBIL, we formalise BAP's intermediate language, called BIL and integrate it with Hoare logic (to enable proofs of correctness) as well as incorrectness logic (to enable proofs of incorrectness). IsaBIL inherits the full flexibility of BAP, allo… ▽ More This paper presents IsaBIL, a binary analysis framework in Isabelle/HOL that is based on the widely used Binary Analysis Platform (BAP). Specifically, in IsaBIL, we formalise BAP's intermediate language, called BIL and integrate it with Hoare logic (to enable proofs of correctness) as well as incorrectness logic (to enable proofs of incorrectness). IsaBIL inherits the full flexibility of BAP, allowing us to verify binaries for a wide range of languages (C, C++, Rust), toolchains (LLVM, Ghidra) and target architectures (x86, RISC-V), and can also be used when the source code for a binary is unavailable. To make verification tractable, we develop a number of big-step rules that combine BIL's existing small-step rules at different levels of abstraction to support reuse. We develop high-level reasoning rules for RISC-V instructions (our main target architecture) to further optimise verification. Additionally, we develop Isabelle proof tactics that exploit common patterns in C binaries for RISC-V to discharge large numbers of proof goals (often in the 100s) automatically. IsaBIL includes an Isabelle/ML based parser for BIL programs, allowing one to automatically generate the associated Isabelle/HOL program locale from a BAP output. Taken together, IsaBIL provides a highly flexible proof environment for program binaries. As examples, we prove correctness of key examples from the Joint Strike Fighter coding standards and the MITRE database. △ Less

Submitted 23 April, 2025; originally announced April 2025.

Identifying high performance spectrally-stable quantum defects in diamond

Authors: Yihuang Xiong, Yizhi Zhu, Shay McBride, Sinéad M. Griffin, Geoffroy Hautier

Abstract: Point defects in semiconductors are becoming central to quantum technologies. They can be used as spin qubits interfacing with photons, which are fundamental for building quantum networks. Currently, the most prominent quantum defect in diamond is the nitrogen-vacancy (NV) center. However, it suffers from spectral diffusion that negatively impacts optical coherence and is due to the coupling of th… ▽ More Point defects in semiconductors are becoming central to quantum technologies. They can be used as spin qubits interfacing with photons, which are fundamental for building quantum networks. Currently, the most prominent quantum defect in diamond is the nitrogen-vacancy (NV) center. However, it suffers from spectral diffusion that negatively impacts optical coherence and is due to the coupling of the emission energy with uncontrolled electric fields. The group IV vacancy complexes on the other hand have shown to be significantly more spectrally-stable as they are centrosymmetric and thus immune to the linear Stark shift. They however suffer from several issues ranging from low operation temperature to low optical efficiency due to dark states and difficulty in stabilizing the right defect charge state. Here we search for alternative to the group IV vacancy complex in diamond by systematically evaluating all possible vacancy complex using high-throughput first-principles computational screening. We identify the defects that combine centrosymmetry, emission in the visible range, as well as favorable and achievable electronic structure promoting higher operation temperature and defect levels well within the band gap. We find Zn$V^{-2}$ to be especially appealing. △ Less

Submitted 15 April, 2025; originally announced April 2025.

Some topological genera and Jacobi forms

Authors: Tewodros Amdeberhan, Michael Griffin, Ken Ono

Abstract: We revisit and elucidate the $\widehat{A}$-genus, Hirzebruch's $L$-genus and Witten's $W$-genus, cobordism invariants of special classes of manifolds. After slight modification, involving Hecke's trick, we find that the $\widehat{A}$-genus and $L$-genus arise directly from Jacobi's theta function. For every $k\geq 0,$ we obtain exact formulas for the quasimodular expressions of $\widehat{A}_k$ and… ▽ More We revisit and elucidate the $\widehat{A}$-genus, Hirzebruch's $L$-genus and Witten's $W$-genus, cobordism invariants of special classes of manifolds. After slight modification, involving Hecke's trick, we find that the $\widehat{A}$-genus and $L$-genus arise directly from Jacobi's theta function. For every $k\geq 0,$ we obtain exact formulas for the quasimodular expressions of $\widehat{A}_k$ and $L_k$ as ``traces'' of partition Eisenstein series \[ \widehat{\mathcal{A}}_k(τ)= \operatorname{Tr}_k(φ_{\widehat{A}};τ)\ \ \ \ \ \ {\text {and}}\ \ \ \ \ \ \mathcal{L}_k(τ)= \operatorname{Tr}_k(φ_L;τ), \] which are easily converted to the original topological expressions. Surprisingly, Ramanujan defined twists of the $\widehat{\mathcal{A}}_k(τ)$ in his ``lost notebook'' in his study of derivatives of theta functions, decades before Borel and Hirzebruch rediscovered them in the context of spin manifolds. In addition, we show that the nonholomorphic $G_2^{\star}$-completion of the characteristic series of the Witten genus is the Jacobi theta function avatar of the $\widehat{A}$-genus. △ Less

Submitted 22 July, 2025; v1 submitted 4 February, 2025; originally announced February 2025.

Comments: We have corrected a few minor typos and updated two references. This paper will appear in the Proceedings of the National Academy of Sciences

MSC Class: 11F50; 58J20

A Tale of Three: Magnetic Fields along the Orion Integral-Shaped Filament as Revealed by JCMT BISTRO survey

Authors: Jintai Wu, Keping Qiu, Frederick Poidevin, Pierre Bastien, Junhao Liu, Tao-Chung Ching, Tyler L. Bourke, Derek Ward-Thompson, Kate Pattle, Doug Johnstone, Patrick M. Koch, Doris Arzoumanian, Chang Won Lee, Lapo Fanciullo, Takashi Onaka, Jihye Hwang, Valentin J. M. Le Gouellec, Archana Soam, Motohide Tamura, Mehrnoosh Tahani, Chakali Eswaraiah, Hua-Bai Li, David Berry, Ray S. Furuya, Simon Coude , et al. (130 additional authors not shown)

Abstract: As part of the BISTRO survey, we present JCMT 850 $μ$m polarimetric observations towards the Orion Integral-Shaped Filament (ISF) that covers three portions known as OMC-1, OMC-2, and OMC-3. The magnetic field threading the ISF seen in the JCMT POL-2 map appears as a tale of three: pinched for OMC-1, twisted for OMC-2, and nearly uniform for OMC-3. A multi-scale analysis shows that the magnetic fi… ▽ More As part of the BISTRO survey, we present JCMT 850 $μ$m polarimetric observations towards the Orion Integral-Shaped Filament (ISF) that covers three portions known as OMC-1, OMC-2, and OMC-3. The magnetic field threading the ISF seen in the JCMT POL-2 map appears as a tale of three: pinched for OMC-1, twisted for OMC-2, and nearly uniform for OMC-3. A multi-scale analysis shows that the magnetic field structure in OMC-3 is very consistent at all the scales, whereas the field structure in OMC-2 shows no correlation across different scales. In OMC-1, the field retains its mean orientation from large to small scales, but shows some deviations at small scales. Histograms of relative orientations between the magnetic field and filaments reveal a bimodal distribution for OMC-1, a relatively random distribution for OMC-2, and a distribution with a predominant peak at 90$^\circ$ for OMC-3. Furthermore, the magnetic fields in OMC-1 and OMC-3 both appear to be aligned perpendicular to the fibers, which are denser structures within the filament, but the field in OMC-2 is aligned along with the fibers. All these suggest that gravity, turbulence, and magnetic field are each playing a leading role in OMC-1, 2, and 3, respectively. While OMC-2 and 3 have almost the same gas mass, density, and non-thermal velocity dispersion, there are on average younger and fewer young stellar objects in OMC-3, providing evidence that a stronger magnetic field will induce slower and less efficient star formation in molecular clouds. △ Less

Submitted 23 December, 2024; originally announced December 2024.

Comments: published in the ApJ Letters

Journal ref: ApJL, 977, L31 (2024)

Dark Matter-Electron Detectors for Dark Matter-Nucleon Interactions

Authors: Sinéad M. Griffin, Guy Daniel Hadas, Yonit Hochberg, Katherine Inzani, Benjamin V. Lehmann

Abstract: In a seminal paper now a decade old, it was shown that dark matter detectors geared at probing interactions with nucleons could also be used to probe dark matter interactions with electrons. In this work, we show that new detector concepts designed to probe dark matter-electron interactions at low masses can similarly be used to probe new parameter space for dark matter-nucleon interactions. We de… ▽ More In a seminal paper now a decade old, it was shown that dark matter detectors geared at probing interactions with nucleons could also be used to probe dark matter interactions with electrons. In this work, we show that new detector concepts designed to probe dark matter-electron interactions at low masses can similarly be used to probe new parameter space for dark matter-nucleon interactions. We demonstrate the power of this approach by using existing data from superconducting detectors to place new limits on the interactions of nuclei with MeV-scale dark matter. Further, we show that advances in detector technology that have been anticipated for electronic interactions will automatically extend sensitivity deep into uncharted territory for nuclear interactions. This doubles the effective science output of future low-threshold experiments. △ Less

Submitted 20 December, 2024; originally announced December 2024.

Comments: 5 pages + references, 2 figures

Report number: MIT-CTP/5678

Engineering micromotion in Floquet prethermalization via space-time symmetries

Authors: Ilyoun Na, Jack Kemp, Sinéad M. Griffin, Yang Peng

Abstract: We present a systematic framework for Floquet prethermalization under strong resonant driving, emphasizing the pivotal role of dynamical space-time symmetries. Our approach demonstrates how dynamical space-time symmetries map onto the projective static symmetry group of the prethermal Hamiltonian governing the prethermal regime. We introduce techniques for detecting dynamical symmetries through th… ▽ More We present a systematic framework for Floquet prethermalization under strong resonant driving, emphasizing the pivotal role of dynamical space-time symmetries. Our approach demonstrates how dynamical space-time symmetries map onto the projective static symmetry group of the prethermal Hamiltonian governing the prethermal regime. We introduce techniques for detecting dynamical symmetries through the time evolution of local observables, facilitating a detailed analysis of micromotion within each period and surpassing the limitations of conventional stroboscopic Floquet prethermal dynamics. To implement this framework, we present a prethermal protocol that preserves order-two dynamical symmetry in a spin-ladder model, confirming the predicted relationships between the expectation values of local observables at distinct temporal points in the Floquet cycle, linked by this symmetry. △ Less

Submitted 26 November, 2025; v1 submitted 12 December, 2024; originally announced December 2024.

Comments: 14 pages, 2+3 figures

Relativistic and nonrelativistic spin splitting above and below the Fermi level in a $g$-wave altermagnet

Authors: Nicholas Dale, Omar A. Ashour, Marc Vila, Resham B. Regmi, Justin Fox, Cameron W. Johnson, Edward S. Barnard, Alexei Fedorov, Alexander Stibor, Nirmal J. Ghimire, Sinéad M. Griffin

Abstract: Nonrelativistic spin splitting (NRSS) challenges conventional wisdom about antiferromagnets by allowing spin-split electronic bands even in collinear orders with zero net magnetization. This sub-class of antiferromagnets, recently dubbed "altermagnets," enforces distinctive spin textures via spin-group symmetries in the crystal. However, direct experimental evidence for such symmetry-driven magnet… ▽ More Nonrelativistic spin splitting (NRSS) challenges conventional wisdom about antiferromagnets by allowing spin-split electronic bands even in collinear orders with zero net magnetization. This sub-class of antiferromagnets, recently dubbed "altermagnets," enforces distinctive spin textures via spin-group symmetries in the crystal. However, direct experimental evidence for such symmetry-driven magnetism remains scarce, and distinguishing it from relativistic spin splitting presents additional challenges. Here, we combine first-principles calculations, symmetry analysis, and two spin-resolved spectroscopies--angle-resolved photoemission (spin-ARPES) and our newly developed spin- and angle-resolved electron reflection spectroscopy (spin-ARRES)--to achieve the first complete momentum-resolved mapping of relativistic (RSS) and nonrelastivistic (NRSS) spin splitting in CoNb$_4$Se$_8$. By probing both the occupied (spin-ARPES) and unoccupied (spin-ARRES) electronic states in a single experiment, we uncover a series of momentum-dependent spin splitting phenomena each of which switch sign under sixfold rotations and persists far above and below the Fermi level. Crucially, distinct nodal planes in momentum space distinguish NRSS from RSS features. Additionally, the observed collapse of NRSS and the persistence of RSS above the Néel temperature, distinguishes a genuine magnetic phase transition from inversion symmetry breaking. Our work demonstrates, for the first time, the combined power of spin-ARPES and spin-ARRES in capturing the full spin texture across an extended energy range, positioning CoNb$_4$Se$_8$ as a prototype for exploring spin-group-based phenomena. These findings open new routes for engineering spin-based functionalities ranging from neuromorphic computing to unconventional superconductivity in layered antiferromagnets. △ Less

Submitted 17 March, 2026; v1 submitted 27 November, 2024; originally announced November 2024.

Comments: 29 pages, 19 figures (5 main, 14 supplementary)

The JCMT BISTRO Survey: The Magnetic Fields of the IC 348 Star-forming Region

Authors: Youngwoo Choi, Woojin Kwon, Kate Pattle, Doris Arzoumanian, Tyler L. Bourke, Thiem Hoang, Jihye Hwang, Patrick M. Koch, Sarah Sadavoy, Pierre Bastien, Ray Furuya, Shih-Ping Lai, Keping Qiu, Derek Ward-Thompson, David Berry, Do-Young Byun, Huei-Ru Vivien Chen, Wen Ping Chen, Mike Chen, Zhiwei Chen, Tao-Chung Ching, Jungyeon Cho, Minho Choi, Yunhee Choi, Simon Coudé , et al. (128 additional authors not shown)

Abstract: We present 850 $μ$m polarization observations of the IC 348 star-forming region in the Perseus molecular cloud as part of the B-fields In STar-forming Region Observation (BISTRO) survey. We study the magnetic properties of two cores (HH 211 MMS and IC 348 MMS) and a filamentary structure of IC 348. We find that the overall field tends to be more perpendicular than parallel to the filamentary struc… ▽ More We present 850 $μ$m polarization observations of the IC 348 star-forming region in the Perseus molecular cloud as part of the B-fields In STar-forming Region Observation (BISTRO) survey. We study the magnetic properties of two cores (HH 211 MMS and IC 348 MMS) and a filamentary structure of IC 348. We find that the overall field tends to be more perpendicular than parallel to the filamentary structure of the region. The polarization fraction decreases with intensity, and we estimate the trend by power-law and the mean of the Rice distribution fittings. The power indices for the cores are much smaller than 1, indicative of possible grain growth to micron size in the cores. We also measure the magnetic field strengths of the two cores and the filamentary area separately by applying the Davis-Chandrasekhar-Fermi method and its alternative version for compressed medium. The estimated mass-to-flux ratios are 0.45-2.20 and 0.63-2.76 for HH 211 MMS and IC 348 MMS, respectively, while the ratios for the filament is 0.33-1.50. This result may suggest that the transition from subcritical to supercritical conditions occurs at the core scale ($\sim$ 0.05 pc) in the region. In addition, we study the energy balance of the cores and find that the relative strength of turbulence to the magnetic field tends to be stronger for IC 348 MMS than HH 211 MMS. The result could potentially explain the different configurations inside the two cores: a single protostellar system in HH 211 MMS and multiple protostars in IC 348 MMS. △ Less

Submitted 4 November, 2024; originally announced November 2024.

Comments: Accepted for publication in ApJ. 21 pages, 12 figures

Modeling the Superlattice Phase Diagram of Transition Metal Intercalation in Bilayer 2H-TaS$_2$

Authors: Isaac M. Craig, B. Junsuh Kim, David T. Limmer, D. Kwabena Bediako, Sinéad M. Griffin

Abstract: Van der Waals hosts intercalated with transition metal (TM) ions exhibit a range of magnetic properties strongly influenced by the structural order of the intercalants. However, predictive computational models for the intercalant ordering phase diagram are lacking, complicating experimental pursuits to target key structural phases. Here we use Density Functional Theory (DFT) to construct a pairwis… ▽ More Van der Waals hosts intercalated with transition metal (TM) ions exhibit a range of magnetic properties strongly influenced by the structural order of the intercalants. However, predictive computational models for the intercalant ordering phase diagram are lacking, complicating experimental pursuits to target key structural phases. Here we use Density Functional Theory (DFT) to construct a pairwise lattice model and Monte Carlo to determine its associated thermodynamic phase diagram. To circumvent the complexities of modeling magnetic effects, we use the diamagnetic ions Zn$^{2+}$ and Sc$^{3+}$ as computationally accessible proxies for divalent and trivalent species of interest (Fe$^{2+}$ and Cr$^{3+}$), which provide insights into the high-temperature thermodynamic phase diagram well above the paramagnetic transition temperature. We find that electrostatic coupling between intercalants is almost entirely screened, so the pairwise lattice model represents a coarse-grained charge density reorganization about the intercalated sites. The resulting phase diagram reveals that the entropically-favored $\sqrt{3} \times \sqrt{3}$ ordering and coexisting locally ordered $\sqrt{3} \times \sqrt{3}$ and $2 \times 2$ domains persist across a range of temperatures and intercalation densities. This occurs even at quarter filling of interstitial sites (corresponding to bulk stoichiometries of M$_{0.25}$TaS$_2$; M = intercalant ion) where a preference for long-range $2 \times 2$ order is typically assumed. △ Less

Submitted 25 October, 2024; originally announced October 2024.

Comments: 46 pages (19 main, 16 supplementary, 11 references) and 15 figures (6 main, 9 supplementary)

Symmetries of all lines in monolayer crystals

Authors: Bernard Field, Sinéad M. Griffin

Abstract: As "2D" materials (i.e. materials just a few atoms thick) continue to gain prominence, understanding their symmetries is critical for unlocking their full potential. In this work, we present comprehensive tables that tabulate the rod group symmetries of all crystallographic lines in all 80 layer groups, which describe the symmetries of 2D materials. These tables are analogous to the scanning table… ▽ More As "2D" materials (i.e. materials just a few atoms thick) continue to gain prominence, understanding their symmetries is critical for unlocking their full potential. In this work, we present comprehensive tables that tabulate the rod group symmetries of all crystallographic lines in all 80 layer groups, which describe the symmetries of 2D materials. These tables are analogous to the scanning tables for space groups found in Volume E of the International Tables for Crystallography, but are specifically tailored for layer groups and their applications to 2D materials. This resource will aid in the analysis of line defects, such as domain walls, which play a crucial role in determining the properties and functionality of 2D materials. △ Less

Submitted 12 May, 2025; v1 submitted 24 October, 2024; originally announced October 2024.

Comments: 11 pages, 7 figures, 4 tables. Supplementary information: 39 pages, 80 tables. Substantial revisions to presentation, including explicit auxiliary tables. Very minor corrections to some reported origins in tables

Journal ref: Acta Cryst. A81 (2025)

Factors influencing quantum evaporation of helium from polar semiconductors from first principles

Authors: Lakshay Dheer, Liang Z. Tan, S. A. Lyon, Thomas Schenkel, Sinéad M. Griffin

Abstract: While there is much indirect evidence for the existence of dark matter (DM), to date it has evaded detection. Current efforts focus on DM masses over $\sim$GeV -- to push the sensitivity of DM searches to lower masses, new DM targets and detection schemes are needed. In this work, we focus on the latter - a novel detection scheme recently proposed to detect ~10-100 meV phonons in polar target mate… ▽ More While there is much indirect evidence for the existence of dark matter (DM), to date it has evaded detection. Current efforts focus on DM masses over $\sim$GeV -- to push the sensitivity of DM searches to lower masses, new DM targets and detection schemes are needed. In this work, we focus on the latter - a novel detection scheme recently proposed to detect ~10-100 meV phonons in polar target materials. Previous work showed that well-motivated models of DM can interact with polar semiconductors to produce an athermal population of phonons. This new sensing scheme proposes that these phonons then facilitate quantum evaporation of $^3$He from a van der Waals film deposited on the target material. However, a fundamental understanding of the underlying process is still unclear, with several uncertainties related to the precise rate of evaporation and how it can be controlled. In this work, we use \textit{ab initio} density functional theory (DFT) calculations to compare the adsorption energies of helium atoms on a polar target material, sodium iodide (NaI), to understand the underlying evaporation physics. We explore the role of surface termination, monolayer coverage and elemental species on the rate of He evaporation from the target material. Using this, we discuss the optimal target features for He-evaporation experiments and their range of tunability through chemical and physical modifications such as applied field and surface termination. △ Less

Submitted 5 September, 2024; originally announced September 2024.

Pressure-Tunable Targets for Light Dark Matter Direct Detection: The Case of Solid Helium

Authors: Omar A. Ashour, Sinéad M. Griffin

Abstract: We propose hydrostatic pressure -- a well-established tool for tuning properties of condensed matter -- as a novel route for optimizing targets for light dark matter direct detection, specifically via phonons. Pressure dramatically affects compressible solids by boosting the speed of sound and phonon frequencies. Focusing on helium -- the most compressible solid -- our ab initio calculations illus… ▽ More We propose hydrostatic pressure -- a well-established tool for tuning properties of condensed matter -- as a novel route for optimizing targets for light dark matter direct detection, specifically via phonons. Pressure dramatically affects compressible solids by boosting the speed of sound and phonon frequencies. Focusing on helium -- the most compressible solid -- our ab initio calculations illustrate how high pressure elevates helium from lacking single-phonon reach to rivaling leading candidates. Our work establishes pressure as an unexplored tuning knob for accessing lower dark matter mass regimes. △ Less

Submitted 4 September, 2024; originally announced September 2024.

Comments: 8 pages, 3 figures, plus supplemental. Supplemental materials PDF available with source files

The UK Submillimetre and Millimetre Astronomy Roadmap 2024

Authors: K. Pattle, P. S. Barry, A. W. Blain, M. Booth, R. A. Booth, D. L. Clements, M. J. Currie, S. Doyle, D. Eden, G. A. Fuller, M. Griffin, P. G. Huggard, J. D. Ilee, J. Karoly, Z. A. Khan, N. Klimovich, E. Kontar, P. Klaassen, A. J. Rigby, P. Scicluna, S. Serjeant, B. -K. Tan, D. Ward-Thompson, T. G. Williams, T. A. Davis , et al. (9 additional authors not shown)

Abstract: In this Roadmap, we present a vision for the future of submillimetre and millimetre astronomy in the United Kingdom over the next decade and beyond. This Roadmap has been developed in response to the recommendation of the Astronomy Advisory Panel (AAP) of the STFC in the AAP Astronomy Roadmap 2022. In order to develop our stragetic priorities and recommendations, we surveyed the UK submillimetre a… ▽ More In this Roadmap, we present a vision for the future of submillimetre and millimetre astronomy in the United Kingdom over the next decade and beyond. This Roadmap has been developed in response to the recommendation of the Astronomy Advisory Panel (AAP) of the STFC in the AAP Astronomy Roadmap 2022. In order to develop our stragetic priorities and recommendations, we surveyed the UK submillimetre and millimetre community to determine their key priorities for both the near-term and long-term future of the field. We further performed detailed reviews of UK leadership in submillimetre/millimetre science and instrumentation. Our key strategic priorities are as follows: 1. The UK must be a key partner in the forthcoming AtLAST telescope, for which it is essential that the UK remains a key partner in the JCMT in the intermediate term. 2. The UK must maintain, and if possible enhance, access to ALMA and aim to lead parts of instrument development for ALMA2040. Our strategic priorities complement one another: AtLAST (a 50m single-dish telescope) and an upgraded ALMA (a large configurable interferometric array) would be in synergy, not competition, with one another. Both have identified and are working towards the same overarching science goals, and both are required in order to fully address these goals. △ Less

Submitted 3 September, 2024; v1 submitted 23 August, 2024; originally announced August 2024.

Comments: 91 pages plus cover, 38 figures. Submitted to the Science and Technology Facilities Council, August 2024. One figure corrected (v2); new appendix with STFC Q&A; corrected SMA access statement; updated references, acronyms & author list (v3)

Traces of partition Eisenstein series

Authors: Tewodros Amdeberhan, Michael Griffin, Ken Ono, Ajit Singh

Abstract: We study "partition Eisenstein series", extensions of the Eisenstein series $G_{2k}(τ),$ defined by $$λ=(1^{m_1}, 2^{m_2},\dots, k^{m_k}) \vdash k \ \ \ \ \ \longmapsto \ \ \ \ \ G_λ(τ):= G_2(τ)^{m_1} G_4(τ)^{m_2}\cdots G_{2k}(τ)^{m_k}. $$ For functions $φ: \mathcal{P}\rightarrow \mathbb{C}$ on partitions, the weight $2k$ "partition Eisenstein trace" is the quasimodular form… ▽ More We study "partition Eisenstein series", extensions of the Eisenstein series $G_{2k}(τ),$ defined by $$λ=(1^{m_1}, 2^{m_2},\dots, k^{m_k}) \vdash k \ \ \ \ \ \longmapsto \ \ \ \ \ G_λ(τ):= G_2(τ)^{m_1} G_4(τ)^{m_2}\cdots G_{2k}(τ)^{m_k}. $$ For functions $φ: \mathcal{P}\rightarrow \mathbb{C}$ on partitions, the weight $2k$ "partition Eisenstein trace" is the quasimodular form $$ {\mathrm{Tr}}_k(φ;τ):=\sum_{λ\vdash k} φ(λ)G_λ(τ). $$ These traces give explicit formulas for some well-known generating functions, such as the $k$th elementary symmetric functions of the inverse points of 2-dimensional complex lattices $\mathbb{Z}\oplus \mathbb{Z}τ,$ as well as the $2k$th power moments of the Andrews-Garvan crank function. To underscore the ubiquity of such traces, we show that their generalizations give the Taylor coefficients of generic Jacobi forms with torsional divisor. △ Less

Submitted 3 February, 2025; v1 submitted 16 August, 2024; originally announced August 2024.

Comments: A few typos fixed

Review and Demonstration of a Mixture Representation for Simulation from Densities Involving Sums of Powers

Authors: Maryclare Griffin

Abstract: Penalized and robust regression, especially when approached from a Bayesian perspective, can involve the problem of simulating a random variable $\boldsymbol z$ from a posterior distribution that includes a term proportional to a sum of powers, $\|\boldsymbol z \|^q_q$, on the log scale. However, many popular gradient-based methods for Markov Chain Monte Carlo simulation from such posterior distri… ▽ More Penalized and robust regression, especially when approached from a Bayesian perspective, can involve the problem of simulating a random variable $\boldsymbol z$ from a posterior distribution that includes a term proportional to a sum of powers, $\|\boldsymbol z \|^q_q$, on the log scale. However, many popular gradient-based methods for Markov Chain Monte Carlo simulation from such posterior distributions use Hamiltonian Monte Carlo and accordingly require conditions on the differentiability of the unnormalized posterior distribution that do not hold when $q \leq 1$ (Plummer, 2023). This is limiting; the setting where $q \leq 1$ includes widely used sparsity inducing penalized regression models and heavy tailed robust regression models. In the special case where $q = 1$, a latent variable representation that facilitates simulation from such a posterior distribution is well known. However, the setting where $q < 1$ has not been treated as thoroughly. In this note, we review the availability of a latent variable representation described in Devroye (2009), show how it can be used to simulate from such posterior distributions when $0 < q < 2$, and demonstrate its utility in the context of estimating the parameters of a Bayesian penalized regression model. △ Less

Submitted 2 August, 2024; originally announced August 2024.

Herschel Gould Belt Survey in Taurus. II: A census of dense cores and filaments in the TMC1 region

Authors: Jason Kirk, Derek Ward-Thompson, James Di Francesco, Philippe André, David Bresnahan, Vera Könyves, Kenneth Marsh, Matt Griffin, Nicola Schneider, A. Men'shchikov, Pedro Palmeirim, Sylvain Bontemps, Doris Arzoumanian, Milena Benedettini, Stefania Pezzuto

Abstract: We present a catalogue of dense cores and filaments in a $3.8^{\circ}\times2.4^{\circ}$ field around the TMC1 region of the Taurus Molecular Cloud. The catalogue was created using photometric data from the Herschel SPIRE and PACS instruments in the 70 $μ$m, 160 $μ$m, 250 $μ$m, 350 $μ$m, and 500 $μ$m continuum bands. Extended structure in the region was reconstructed from a Herschel column density… ▽ More We present a catalogue of dense cores and filaments in a $3.8^{\circ}\times2.4^{\circ}$ field around the TMC1 region of the Taurus Molecular Cloud. The catalogue was created using photometric data from the Herschel SPIRE and PACS instruments in the 70 $μ$m, 160 $μ$m, 250 $μ$m, 350 $μ$m, and 500 $μ$m continuum bands. Extended structure in the region was reconstructed from a Herschel column density map. Power spectra and PDFs of this structure are presented. The PDF splits into log-normal and power-law forms, with the high-density power-law component associated primarily with the central part of TMC1. The total mass in the mapped region is 2000 M$_\odot$, of which 34% is above an extinction of AV $\sim$ 3 mag -- a level that appears as a break in the PDF and as the minimum column density at which dense cores are found. A total of 35 dense filaments were extracted from the column density map. These have a characteristic FWHM width of 0.07 pc, but the TMC1 filament itself has a mean FWHM of $\sim$ 0.13 pc. The thermally supercritical filaments in the region are aligned orthogonal to the prevailing magnetic field direction. Derived properties for the supercritical TMC1 filament support the scenario of it being relatively young. A catalogue of 44 robust and candidate prestellar cores is created and is assessed to be complete down to 0.1 M$_\odot$. The combined prestellar CMF for the TMC1 and L1495 regions is well fit by a single log-normal distribution and is comparable to the standard IMF. △ Less

Submitted 1 July, 2024; originally announced July 2024.

Comments: 21 pages, 23 figures, accepted for publication in MNRAS. For data products see http://gouldbelt-herschel.cea.fr/archives

Discovery of T center-like quantum defects in silicon

Authors: Yihuang Xiong, Jiongzhi Zheng, Shay McBride, Xueyue Zhang, Sinéad M. Griffin, Geoffroy Hautier

Abstract: Quantum technologies would benefit from the development of high performance quantum defects acting as single-photon emitters or spin-photon interface. Finding such a quantum defect in silicon is especially appealing in view of its favorable spin bath and high processability. While some color centers in silicon have been emerging in quantum applications, there is still a need to search and develop… ▽ More Quantum technologies would benefit from the development of high performance quantum defects acting as single-photon emitters or spin-photon interface. Finding such a quantum defect in silicon is especially appealing in view of its favorable spin bath and high processability. While some color centers in silicon have been emerging in quantum applications, there is still a need to search and develop new high performance quantum emitters. Searching a high-throughput computational database of more than 22,000 charged complex defects in silicon, we identify a series of defects formed by a group III element combined with carbon ((A-C)$\rm _{Si}$ with A=B,Al,Ga,In,Tl) and substituting on a silicon site. These defects are analogous structurally, electronically and chemically to the well-known T center in silicon ((C-C-H)$\rm_{Si}$) and their optical properties are mainly driven by an unpaired electron in a carbon $p$ orbital. They all emit in the telecom and some of these color centers show improved properties compared to the T center in terms of computed radiative lifetime or emission efficiency. We also show that the synthesis of hydrogenated T center-like defects followed by a dehydrogenation annealing step could be an efficient way of synthesis. All the T center-like defects show a higher symmetry than the T center making them easier to align with magnetic fields. Our work motivates further studies on the synthesis and control of this new family of quantum defects, and also demonstrates the use of high-throughput computational screening to detect new complex quantum defects. △ Less

Submitted 8 May, 2024; originally announced May 2024.

Nematicity of a Magnetic Helix

Authors: Z. Tumbleson, S. A. Morley, E. Hollingworth, A. Singh, T. Bayaraa, N. G. Burdet, A. Us Saleheen, M. R. McCarter, D. Raftrey, R. J. Pandolfi, V. Esposito, G. L. Dakovski, F. -J. Decker, A. H. Reid, T. A. Assefa, P. Fischer, S. M. Griffin, S. D. Kevan, F. Hellman, J. J. Turner, S. Roy

Abstract: A system that possesses translational symmetry but breaks orientational symmetry is known as a nematic phase. While there are many examples of nematic phases in a wide range of contexts, such as in liquid crystals, complex oxides, and superconductors, of particular interest is the magnetic analogue, where the spin, charge, and orbital degrees of freedom of the electron are intertwined. The difficu… ▽ More A system that possesses translational symmetry but breaks orientational symmetry is known as a nematic phase. While there are many examples of nematic phases in a wide range of contexts, such as in liquid crystals, complex oxides, and superconductors, of particular interest is the magnetic analogue, where the spin, charge, and orbital degrees of freedom of the electron are intertwined. The difficulty of spin nematics is the unambiguous realization and characterization of the phase. Here we present an entirely new type of magnetic nematic phase, which replaces the basis of individual spins with magnetic helices. The helical basis allows for the direct measurement of the order parameters with soft X-ray scattering and a thorough characterization of the nematic phase and its thermodynamic transitions. We discover two distinct nematic phases with unique spatio-temporal correlation signatures. Using coherent X-ray methods, we find that near the phase boundary between the two nematic phases, fluctuations coexist on the timescale of both seconds and sub-nanoseconds. Additionally, we have determined that the fluctuations occur simultaneously with a reorientation of the magnetic helices, indicating that there is spontaneous symmetry breaking and new degrees of freedom become available. Our results provide a novel framework for characterizing exotic phases and the phenomena presented can be mapped onto a broad class of physical systems. △ Less

Submitted 30 April, 2025; v1 submitted 19 April, 2024; originally announced April 2024.

Simulations of Classical Three-Body Thermalization in One Dimension

Authors: M. Eltohfa, Xinghan Wang, Colton M. Griffin, F. Robicheaux

Abstract: One-dimensional systems, such as nanowires or electrons moving along strong magnetic field lines, have peculiar thermalization physics. The binary collision of point-like particles, typically the dominant process for reaching thermal equilibrium in higher dimensional systems, cannot thermalize a 1D system. We study how dilute classical 1D gases thermalize through three-body collisions. We consider… ▽ More One-dimensional systems, such as nanowires or electrons moving along strong magnetic field lines, have peculiar thermalization physics. The binary collision of point-like particles, typically the dominant process for reaching thermal equilibrium in higher dimensional systems, cannot thermalize a 1D system. We study how dilute classical 1D gases thermalize through three-body collisions. We consider a system of identical classical point particles with pairwise repulsive inverse power-law potential $V_{ij} \propto 1/|x_i-x_j|^n$ or the pairwise Lennard-Jones potential. Using Monte Carlo methods, we compute a collision kernel and use it in the Boltzmann equation to evolve a perturbed thermal state with temperature $T$ toward equilibrium. We explain the shape of the kernel and its dependence on the system parameters. Additionally, we implement molecular dynamics simulations of a many-body gas and show agreement with the Boltzmann evolution in the low density limit. For the inverse power-law potential, the rate of thermalization is proportional to $ρ^2 T^{\frac{1}{2}-\frac{1}{n}}$ where $ρ$ is the number density. The corresponding proportionality constant decreases with increasing $n$. △ Less

Submitted 9 July, 2024; v1 submitted 29 February, 2024; originally announced March 2024.

Comments: 13 pages, 12 figures

Compressive-Sensing-Enhanced First-Principles Calculation of Photoluminescence Spectra in Color Centers: A Comparison between Theory and Experiment for the G Center in Silicon

Authors: Jiongzhi Zheng, Lukasz Komza, Yihuang Xiong, Natalya Sheremetyeva, Changpeng Lin, Sinéad M. Griffin, Alp Sipahigil, Geoffroy Hautier

Abstract: Photoluminescence (PL) spectra are a versatile tool for exploring the electronic and optical properties of quantum defect systems. In this work, we investigate the PL spectra of the G center in silicon by combining first-principles computations with a machine-learned compressive-sensing technique and experiment. We show that the compressive-sensing technique provides a speed up of approximately 20… ▽ More Photoluminescence (PL) spectra are a versatile tool for exploring the electronic and optical properties of quantum defect systems. In this work, we investigate the PL spectra of the G center in silicon by combining first-principles computations with a machine-learned compressive-sensing technique and experiment. We show that the compressive-sensing technique provides a speed up of approximately 20 times compared with the finite-displacement method with similar numerical accuracy. We compare theory and experiment and show good agreement for the historically proposed configuration B of the G center. In particular, we attribute the experimentally observed E-line of the G center to a local vibration mode mainly involving two substitutional C atoms and one interstitial Si atom. Our theoretical results also well reproduce and explain the experimental E-line energy shifts originating from the carbon isotopic effect. In addition, our results demonstrate that some highly anharmonic modes that are apparent in computed spectra could be absent experimentally because of their short lifetime. Our work not only provides a deeper understanding of the G-center defect but also paves the way to accelerate the calculation of PL spectra for color centers. △ Less

Submitted 16 February, 2024; v1 submitted 12 February, 2024; originally announced February 2024.

Cool Gaseous Exoplanets: surveying the new frontier with Twinkle

Authors: Luke Booth, Subhajit Sarkar, Matt Griffin, Billy Edwards

Abstract: Cool gaseous exoplanets ($1.75\ R_\oplus < R_\text{p} < 3\ R_\text{J}$, $200$ K $<T_\text{eq} < 1000$~K) are an as-yet understudied population, with great potential to expand our understanding of planetary atmospheres and formation mechanisms. In this paper, we outline the basis for a homogeneous survey of cool gaseous planets with Twinkle, a 0.45-m diameter space telescope with simultaneous spect… ▽ More Cool gaseous exoplanets ($1.75\ R_\oplus < R_\text{p} < 3\ R_\text{J}$, $200$ K $<T_\text{eq} < 1000$~K) are an as-yet understudied population, with great potential to expand our understanding of planetary atmospheres and formation mechanisms. In this paper, we outline the basis for a homogeneous survey of cool gaseous planets with Twinkle, a 0.45-m diameter space telescope with simultaneous spectral coverage from 0.5-4.5~$μ$m, set to launch in 2025. We find that Twinkle has the potential to characterise the atmospheres of 36 known cool gaseous exoplanets (11~sub-Neptunian, 11~Neptunian, 14~Jovian) at an SNR $\geq$ 5 during its 3-year primary mission, with the capability of detecting most major molecules predicted by equilibrium chemistry to > $5σ$ significance. We find that an injected mass-metallicity trend is well-recovered, demonstrating Twinkle's ability to elucidate this fundamental relationship into cool regime. We also find Twinkle will be able to detect cloud layers at 3$σ$ or greater in all cool gaseous planets for clouds at $\leq$ 10 Pa pressure level, but will be insensitive to clouds deeper than $10^4$ Pa in all cases. With these results we demonstrate the capability of the Twinkle mission to greatly expand the current knowledge of cool gaseous planets, enabling key insights and constraints to be obtained for this poorly-charted region of exoplanet parameter space. △ Less

Submitted 9 February, 2024; originally announced February 2024.

Comments: 14 pages, 6 figures, 6 tables

Filamentary Network and Magnetic Field Structures Revealed with BISTRO in the High-Mass Star-Forming Region NGC2264 : Global Properties and Local Magnetogravitational Configurations

Authors: Jia-Wei Wang, Patrick M. Koch, Seamus D. Clarke, Gary Fuller, Nicolas Peretto, Ya-Wen Tang, Hsi-Wei Yen, Shih-Ping Lai, Nagayoshi Ohashi, Doris Arzoumanian, Doug Johnstone, Ray Furuya, Shu-ichiro Inutsuka, Chang Won Lee, Derek Ward-Thompson, Valentin J. M. Le Gouellec, Hong-Li Liu, Lapo Fanciullo, Jihye Hwang, Kate Pattle, Frédérick Poidevin, Mehrnoosh Tahani, Takashi Onaka, Mark G. Rawlings, Eun Jung Chung , et al. (132 additional authors not shown)

Abstract: We report 850 $μ$m continuum polarization observations toward the filamentary high-mass star-forming region NGC 2264, taken as part of the B-fields In STar forming Regions Observations (BISTRO) large program on the James Clerk Maxwell Telescope (JCMT). These data reveal a well-structured non-uniform magnetic field in the NGC 2264C and 2264D regions with a prevailing orientation around 30 deg from… ▽ More We report 850 $μ$m continuum polarization observations toward the filamentary high-mass star-forming region NGC 2264, taken as part of the B-fields In STar forming Regions Observations (BISTRO) large program on the James Clerk Maxwell Telescope (JCMT). These data reveal a well-structured non-uniform magnetic field in the NGC 2264C and 2264D regions with a prevailing orientation around 30 deg from north to east. Field strengths estimates and a virial analysis for the major clumps indicate that NGC 2264C is globally dominated by gravity while in 2264D magnetic, gravitational, and kinetic energies are roughly balanced. We present an analysis scheme that utilizes the locally resolved magnetic field structures, together with the locally measured gravitational vector field and the extracted filamentary network. From this, we infer statistical trends showing that this network consists of two main groups of filaments oriented approximately perpendicular to one another. Additionally, gravity shows one dominating converging direction that is roughly perpendicular to one of the filament orientations, which is suggestive of mass accretion along this direction. Beyond these statistical trends, we identify two types of filaments. The type-I filament is perpendicular to the magnetic field with local gravity transitioning from parallel to perpendicular to the magnetic field from the outside to the filament ridge. The type-II filament is parallel to the magnetic field and local gravity. We interpret these two types of filaments as originating from the competition between radial collapsing, driven by filament self-gravity, and the longitudinal collapsing, driven by the region's global gravity. △ Less

Submitted 23 January, 2024; originally announced January 2024.

Comments: Accepted for publication in the Astrophysical Journal. 43 pages, 32 figures, and 4 tables (including Appendix)

A note on odd partition numbers

Authors: Michael Griffin, Ken Ono

Abstract: Ramanujan's celebrated partition congruences modulo $\ell\in \{5, 7, 11\}$ assert that $$ p(\ell n+δ_{\ell})\equiv 0\pmod{\ell}, $$ where $0<δ_{\ell}<\ell$ satisfies $24δ_{\ell}\equiv 1\pmod{\ell}.$ By proving Subbarao's Conjecture, Radu showed that there are no such congruences when it comes to parity. There are infinitely many odd (resp. even) partition numbers in every arithmetic progression. F… ▽ More Ramanujan's celebrated partition congruences modulo $\ell\in \{5, 7, 11\}$ assert that $$ p(\ell n+δ_{\ell})\equiv 0\pmod{\ell}, $$ where $0<δ_{\ell}<\ell$ satisfies $24δ_{\ell}\equiv 1\pmod{\ell}.$ By proving Subbarao's Conjecture, Radu showed that there are no such congruences when it comes to parity. There are infinitely many odd (resp. even) partition numbers in every arithmetic progression. For primes $\ell \geq 5,$ we give a new proof of the conclusion that there are infinitely many $m$ for which $p(\ell m+δ_{\ell})$ is odd. This proof uses a generalization, due to the second author and Ramsey, of a result of Mazur in his classic paper on the Eisenstein ideal. We also refine a classical criterion of Sturm for modular form congruences, which allows us to show that the smallest such $m$ satisfies $m<(\ell^2-1)/24,$ representing a significant improvement to the previous bound. △ Less

Submitted 16 March, 2024; v1 submitted 1 January, 2024; originally announced January 2024.

Comments: Corrects minor typos found by the referees

MSC Class: 11P81; 11P83; 05A17

Edge-sharing quasi-one-dimensional cuprate fragments in optimally substituted Cu/Pb apatite

Authors: Katherine Inzani, John Vinson, Sinéad M. Griffin

Abstract: The flurry of theoretical and experimental studies following the report of room-temperature superconductivity at ambient pressure in Cu-substituted lead apatite Cu$_x$Pb$_{10-x}$(PO$_4$)$_6$O (`LK99') have explored whether and how this system might host strongly correlated physics including superconductivity. While first-principles calculations at low doping ($x\approx1$) have indicated a Cu-… ▽ More The flurry of theoretical and experimental studies following the report of room-temperature superconductivity at ambient pressure in Cu-substituted lead apatite Cu$_x$Pb$_{10-x}$(PO$_4$)$_6$O (`LK99') have explored whether and how this system might host strongly correlated physics including superconductivity. While first-principles calculations at low doping ($x\approx1$) have indicated a Cu-$d^{9}$ configuration coordinated with oxygen giving rise to isolated, correlated bands, its other structural, electronic, and magnetic properties diverge significantly from those of other known cuprate systems. Here we find that higher densities of ordered Cu substitutions can result in the formation of contiguous edge-sharing Cu-O chains, akin to those found in some members of the cuprate superconductor family. Interestingly, while such quasi-one-dimensional edge-sharing chains are typically ferromagnetically coupled along the chain, we find an antiferromagnetic ground-state magnetic order for our cuprate fragments which is in proximity to a ferromagnetic quantum critical point. This is a result of the elongated Cu-Cu distance in Cu-substituted apatite that leads to larger Cu-O-Cu angles supporting antiferromagnetism, which we demonstrate to be controllable by strain. Finally, our electronic structure calculations confirm the low-dimensional nature of the system and show that the bandwidth is driven by the Cu-O plaquette connectivity, resulting in an intermediate correlated regime. △ Less

Submitted 9 January, 2024; v1 submitted 21 December, 2023; originally announced December 2023.

Comments: 6 pages plus Supplemental. Additional citations in revised version

Electron-phonon coupling in copper-substituted lead phosphate apatite

Authors: Alexander C. Tyner, Sinéad M. Griffin, Alexander V. Balatsky

Abstract: Recent reports of room-temperature, ambient pressure superconductivity in copper-substituted lead phosphate apatite, commonly referred to as LK99, have prompted numerous theoretical and experimental studies into its properties. As the electron-phonon interaction is a common mechanism for superconductivity, the electron-phonon coupling strength is an important quantity to compute for LK99. In this… ▽ More Recent reports of room-temperature, ambient pressure superconductivity in copper-substituted lead phosphate apatite, commonly referred to as LK99, have prompted numerous theoretical and experimental studies into its properties. As the electron-phonon interaction is a common mechanism for superconductivity, the electron-phonon coupling strength is an important quantity to compute for LK99. In this work, we compare the electron-phonon coupling strength among the proposed compositions of LK99. The results of our study are in alignment with the conclusion that LK99 is not a likely candidate for room-temperature superconductivity if electron-phonon interaction is to serve as the mechanism. △ Less

Submitted 14 June, 2024; v1 submitted 19 December, 2023; originally announced December 2023.

Comments: 4 pages, 4 figures, Published Version

Journal ref: J Low Temp Phys (2024)

Ab initio amorphous spin Hamiltonian for the description of topological spin textures in FeGe

Authors: Temuujin Bayaraa, Sinéad M. Griffin

Abstract: Topological spin textures in magnetic materials such as skyrmions and hopfions are interesting manifestations of geometric structures in real materials, concurrently having potential applications as information carriers. In the crystalline systems, the formation of these topological spin textures is well understood as a result of the competition between interactions due to symmetry breaking and fr… ▽ More Topological spin textures in magnetic materials such as skyrmions and hopfions are interesting manifestations of geometric structures in real materials, concurrently having potential applications as information carriers. In the crystalline systems, the formation of these topological spin textures is well understood as a result of the competition between interactions due to symmetry breaking and frustration. However, in systems without translation symmetry such as amorphous materials, a fundamental understanding of the driving mechanisms of non-trivial spin structures is lacking owing to the structural and interaction complexity in these systems. In this work, we use a suite of first-principles-based calculations to propose an ab initio spin Hamiltonian that accurately represents the diversity of structural and magnetic properties in the exemplar amorphous FeGe. Monte Carlo simulations of our amorphous Hamiltonian find emergent skyrmions that are driven by frustrated geometric and magnetic exchange, consistent with those observed in experiment. Moreover, we find that the diversity of local structural motifs results in a large range of exchange interactions, far beyond those found in crystalline materials. Finally, we observe the formation of large-scale emergent structures in amorphous materials, far beyond the relevant interaction length-scale in the systems, suggesting a new route to emergent correlated phases beyond the crystalline limit. △ Less

Submitted 13 November, 2023; originally announced November 2023.

Linear magneto-conductivity as a DC probe of time-reversal symmetry breaking

Authors: Veronika Sunko, Chunxiao Liu, Marc Vila, Ilyoun Na, Yuchen Tang, Vladyslav Kozii, Sinéad M. Griffin, Joel E. Moore, Joseph Orenstein

Abstract: Several optical experiments have shown that in magnetic materials the principal axes of response tensors can rotate in a magnetic field. Here we offer a microscopic explanation of this effect, and propose a closely related DC transport phenomenon -- an off-diagonal \emph{symmetric} conductivity linear in a magnetic field, which we refer to as linear magneto-conductivity (LMC). Although LMC has the… ▽ More Several optical experiments have shown that in magnetic materials the principal axes of response tensors can rotate in a magnetic field. Here we offer a microscopic explanation of this effect, and propose a closely related DC transport phenomenon -- an off-diagonal \emph{symmetric} conductivity linear in a magnetic field, which we refer to as linear magneto-conductivity (LMC). Although LMC has the same functional dependence on magnetic field as the Hall effect, its origin is fundamentally different: LMC requires time-reversal symmetry to be broken even before a magnetic field is applied, and is therefore a sensitive probe of magnetism. We demonstrate LMC in three different ways: via a tight-binding toy model, density functional theory calculations on MnPSe$_3$, and a semiclassical calculation. The third approach additionally identifies two distinct mechanisms yielding LMC: momentum-dependent band magnetization and Berry curvature. Finally, we propose an experimental geometry suitable for detecting LMC, and demonstrate its applicability using Landauer-Büttiker simulations. Our results emphasize the importance of measuring the full conductivity tensor in magnetic materials, and introduce LMC as a new transport probe of symmetry. △ Less

Submitted 24 October, 2023; originally announced October 2023.

Comments: 6+8 pages, 4+3 figures

Chiral Majorana hinge modes on a curved surface with magnetic impurities

Authors: Ilyoun Na, James G. McHugh, Sinéad M. Griffin, Luca Chirolli

Abstract: Chiral Majorana one-dimensional modes have been proposed as they key component for topological quantum computing. In this study, we explore their potential realization as hinge modes in higher-order topological superconductors. To create such phases, we engineer a sign-changing, time-reversal symmetry-breaking mass term through an ensemble of magnetic impurities on the surface of a sphere. The mag… ▽ More Chiral Majorana one-dimensional modes have been proposed as they key component for topological quantum computing. In this study, we explore their potential realization as hinge modes in higher-order topological superconductors. To create such phases, we engineer a sign-changing, time-reversal symmetry-breaking mass term through an ensemble of magnetic impurities on the surface of a sphere. The magnetization of this ensemble arises from the competition between the external magnetic field and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction among the impurities, mediated by the surface Majorana modes. We determine the magnetic phase diagram and identify the optimal magnetic field to minimize orbital effects and induce a sign changing mass term. This term opens a gap in the surface spectrum, resulting in a gapless one-dimensional chiral Majorana mode along the nodal line of the mass term, thereby implementing a second-order topological superconductor. △ Less

Submitted 13 October, 2023; originally announced October 2023.

Controlling topology through targeted composite symmetry manipulation in magnetic systems

Authors: Ilyoun Na, Marc Vila, Sinéad M. Griffin

Abstract: The possibility of selecting magnetic space groups by orienting the magnetization direction or tuning magnetic orders offers a vast playground for engineering symmetry protected topological phases in magnetic materials. In this work, we study how selective tuning of symmetry and magnetism can influence and control the resulting topology in a 2D magnetic system, and illustrate such procedure in the… ▽ More The possibility of selecting magnetic space groups by orienting the magnetization direction or tuning magnetic orders offers a vast playground for engineering symmetry protected topological phases in magnetic materials. In this work, we study how selective tuning of symmetry and magnetism can influence and control the resulting topology in a 2D magnetic system, and illustrate such procedure in the ferromagnetic monolayer MnPSe$_3$. Density functional theory calculations reveals a symmetry-protected accidental semimetalic (SM) phase for out-of-plane magnetization which becomes an insulator when the magnetization is tilted in-plane, reaching band gap values close to $100$ meV. We identify an order-two composite antiunitary symmetry and threefold rotational symmetry that induce the band crossing and classify the possible topological phases using symmetry analysis, which we support with tight-binding and $\mathbf{k}\cdot\mathbf{p}$ models. Breaking of inversion symmetry opens a gap in the SM phase, giving rise to a Chern insulator. We demonstrate this explicitly in the isostructural Janus compound Mn$_2$P$_2$S$_3$Se$_3$, which naturally exhibits Rashba spin-orbit coupling that breaks inversion symmetry. Our results map out the phase space of topological properties of ferromagnetic transition metal phosphorus trichalcogenides and demonstrate the potential of the magnetization-dependent metal-to-insulator transition as a spin switch in integrated two-dimensional electronics. △ Less

Submitted 8 November, 2024; v1 submitted 9 October, 2023; originally announced October 2023.

Realistic non-collinear ground states of solids with source-free exchange correlation functional

Authors: Guy C. Moore, Matthew K. Horton, Aaron D. Kaplan, Sinéad M. Griffin, Kristin A. Persson

Abstract: In this work, we extend the source-free (SF) exchange correlation (XC) functional developed by Sangeeta Sharma and co-workers to plane-wave density functional theory (DFT) based on the projector augmented wave (PAW) method. This constraint is implemented by the current authors within the VASP source code, using a fast Poisson solver that capitalizes on the parallel three-dimensional fast Fourier t… ▽ More In this work, we extend the source-free (SF) exchange correlation (XC) functional developed by Sangeeta Sharma and co-workers to plane-wave density functional theory (DFT) based on the projector augmented wave (PAW) method. This constraint is implemented by the current authors within the VASP source code, using a fast Poisson solver that capitalizes on the parallel three-dimensional fast Fourier transforms (FFTs) implemented in VASP. Using this modified XC functional, we explore the improved convergence behavior that results from applying this constraint to the GGA-PBE+$U$+$J$ functional. In the process, we compare the non-collinear magnetic ground state computed by each functional and their SF counterpart for a select number of magnetic materials in order to provide a metric for comparing with experimentally determined magnetic orderings. We observe significantly improved agreement with experimentally measured magnetic ground state structures after applying the source-free constraint. Furthermore, we explore the importance of considering probability current densities in spin polarized systems, even under no applied field. We analyze the XC torque as well, in order to provide theoretical and computational analyses of the net XC magnetic torque induced by the source-free constraint. Along these lines, we highlight the importance of properly considering the real-space integral of the source-free local magnetic XC field. Our analysis on probability currents, net torque, and constant terms draws additional links to the rich body of previous research on spin-current density functional theory (SCDFT), and paves the way for future extensions and corrections to the SF corrected XC functional. △ Less

Submitted 24 May, 2024; v1 submitted 29 September, 2023; originally announced October 2023.

A Simple Electrode Insulation and Channel Fabrication Technique for High-Electric Field Microfluidics

Authors: Gaurav Anand, Samira Safaripour, Jaynie Tercovich, Jenna Capozzi, Mark Griffin, Nathan Schin, Nicholas Mirra, Craig Snoeyink

Abstract: A simple and robust electrode insulation technique that can withstand a voltage as high as $\mathrm{1000~V}$, which is equivalent to an electric field strength of $\sim 1MV/m$ across a $\mathrm{10~μm}$ channel filled with an electrolyte of conductivity $\sim 0.1~S/m$, i.e., higher than sea water's conductivity, is introduced. A multi-dielectric layers approach is adopted to fabricate the blocked e… ▽ More A simple and robust electrode insulation technique that can withstand a voltage as high as $\mathrm{1000~V}$, which is equivalent to an electric field strength of $\sim 1MV/m$ across a $\mathrm{10~μm}$ channel filled with an electrolyte of conductivity $\sim 0.1~S/m$, i.e., higher than sea water's conductivity, is introduced. A multi-dielectric layers approach is adopted to fabricate the blocked electrodes, which helps reduce the number of material defects. Dielectric insulation with an exceptional breakdown electric field strength for an electrolyte confined between electrodes can have a wide range of applications in microfluidics, like high electric field strength-based dielectrophoresis. The voltage-current characteristics are studied for various concentrations of sodium chloride solution to estimate the insulation strength of the proposed materials, and the breakdown strength is calculated at the point where the electrical insulation failed. A detailed adhesion technique is also demonstrated, which will reduce the ambiguity around the fabrication of a sealed channel using SU-8. △ Less

Submitted 7 September, 2023; v1 submitted 1 August, 2023; originally announced August 2023.

Comments: 12 pages

Origin of correlated isolated flat bands in copper-substituted lead phosphate apatite

Authors: Sinéad M. Griffin

Abstract: A recent report of room temperature superconductivity at ambient pressure in Cu-substituted apatite (`LK99') has invigorated interest in the understanding of what materials and mechanisms can allow for high-temperature superconductivity. Here I perform density functional theory calculations on Cu-substituted lead phosphate apatite, identifying correlated isolated flat bands at the Fermi level, a c… ▽ More A recent report of room temperature superconductivity at ambient pressure in Cu-substituted apatite (`LK99') has invigorated interest in the understanding of what materials and mechanisms can allow for high-temperature superconductivity. Here I perform density functional theory calculations on Cu-substituted lead phosphate apatite, identifying correlated isolated flat bands at the Fermi level, a common signature of high transition temperatures in already established families of superconductors. I elucidate the origins of these isolated bands as arising from a structural distortion induced by the Cu ions and a chiral charge density wave from the Pb lone pairs. These results suggest that a minimal two-band model can encompass much of the low-energy physics in this system. Finally, I discuss the implications of my results on possible superconductivity in Cu-doped apatite. △ Less

Submitted 3 August, 2023; v1 submitted 31 July, 2023; originally announced July 2023.

ContriMix: Scalable stain color augmentation for domain generalization without domain labels in digital pathology

Authors: Tan H. Nguyen, Dinkar Juyal, Jin Li, Aaditya Prakash, Shima Nofallah, Chintan Shah, Sai Chowdary Gullapally, Limin Yu, Michael Griffin, Anand Sampat, John Abel, Justin Lee, Amaro Taylor-Weiner

Abstract: Differences in staining and imaging procedures can cause significant color variations in histopathology images, leading to poor generalization when deploying deep-learning models trained from a different data source. Various color augmentation methods have been proposed to generate synthetic images during training to make models more robust, eliminating the need for stain normalization during test… ▽ More Differences in staining and imaging procedures can cause significant color variations in histopathology images, leading to poor generalization when deploying deep-learning models trained from a different data source. Various color augmentation methods have been proposed to generate synthetic images during training to make models more robust, eliminating the need for stain normalization during test time. Many color augmentation methods leverage domain labels to generate synthetic images. This approach causes three significant challenges to scaling such a model. Firstly, incorporating data from a new domain into deep-learning models trained on existing domain labels is not straightforward. Secondly, dependency on domain labels prevents the use of pathology images without domain labels to improve model performance. Finally, implementation of these methods becomes complicated when multiple domain labels (e.g., patient identification, medical center, etc) are associated with a single image. We introduce ContriMix, a novel domain label free stain color augmentation method based on DRIT++, a style-transfer method. Contrimix leverages sample stain color variation within a training minibatch and random mixing to extract content and attribute information from pathology images. This information can be used by a trained ContriMix model to create synthetic images to improve the performance of existing classifiers. ContriMix outperforms competing methods on the Camelyon17-WILDS dataset. Its performance is consistent across different slides in the test set while being robust to the color variation from rare substances in pathology images. We make our code and trained ContriMix models available for research use. The code for ContriMix can be found at https://gitlab.com/huutan86/contrimix △ Less

Submitted 8 March, 2024; v1 submitted 7 June, 2023; originally announced June 2023.

The JCMT BISTRO Survey: Studying the Complex Magnetic Field of L43

Authors: Janik Karoly, Derek Ward-Thompson, Kate Pattle, David Berry, Anthony Whitworth, Jason Kirk, Pierre Bastien, Tao-Chung Ching, Simon Coude, Jihye Hwang, Woojin Kwon, Archana Soam, Jia-Wei Wang, Tetsuo Hasegawa, Shih-Ping Lai, Keping Qiu, Doris Arzoumanian, Tyler L. Bourke, Do-Young Byun, Huei-Ru Vivien Chen, Wen Ping Chen, Mike Chen, Zhiwei Chen, Jungyeon Cho, Minho Choi , et al. (133 additional authors not shown)

Abstract: We present observations of polarized dust emission at 850 $μ$m from the L43 molecular cloud which sits in the Ophiuchus cloud complex. The data were taken using SCUBA-2/POL-2 on the James Clerk Maxwell Telescope as a part of the BISTRO large program. L43 is a dense ($N_{\rm H_2}\sim 10^{22}$-10$^{23}$ cm$^{-2}$) complex molecular cloud with a submillimetre-bright starless core and two protostellar… ▽ More We present observations of polarized dust emission at 850 $μ$m from the L43 molecular cloud which sits in the Ophiuchus cloud complex. The data were taken using SCUBA-2/POL-2 on the James Clerk Maxwell Telescope as a part of the BISTRO large program. L43 is a dense ($N_{\rm H_2}\sim 10^{22}$-10$^{23}$ cm$^{-2}$) complex molecular cloud with a submillimetre-bright starless core and two protostellar sources. There appears to be an evolutionary gradient along the isolated filament that L43 is embedded within, with the most evolved source closest to the Sco OB2 association. One of the protostars drives a CO outflow that has created a cavity to the southeast. We see a magnetic field that appears to be aligned with the cavity walls of the outflow, suggesting interaction with the outflow. We also find a magnetic field strength of up to $\sim$160$\pm$30 $μ$G in the main starless core and up to $\sim$90$\pm$40 $μ$G in the more diffuse, extended region. These field strengths give magnetically super- and sub-critical values respectively and both are found to be roughly trans-Alfvénic. We also present a new method of data reduction for these denser but fainter objects like starless cores. △ Less

Submitted 22 May, 2023; v1 submitted 18 May, 2023; originally announced May 2023.

Comments: Accepted for publication in ApJ. 23 pages, 9 figures (7 main text, 2 appendix)

Intrinsic Origin and Enhancement of Topological Responses in Ferrimagnetic Antiperovskite Mn4N

Authors: Temuujin Bayaraa, Vsevolod Ivanov, Liang Z. Tan, Sinéad M. Griffin

Abstract: Using first-principles calculations we investigate the intrinsic origins of the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) in antiperovskite ferrimagnet Mn4N. We predict that the AHE is significantly enhanced under both compressive and tensile strain, however, the ANE generally decreases under epitaxial strain, except for 1% compressive strain. We connect this behavior to th… ▽ More Using first-principles calculations we investigate the intrinsic origins of the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) in antiperovskite ferrimagnet Mn4N. We predict that the AHE is significantly enhanced under both compressive and tensile strain, however, the ANE generally decreases under epitaxial strain, except for 1% compressive strain. We connect this behavior to the evolution of the Berry curvature with strain, suggesting similar strategies for achieving large AHE and ANE changes with modest amounts of strain. Finally, we find that the non-monotonic characteristics of the AHE and ANE stem from the formation and movement of new Weyl points at the periphery of the Brillouin Zone under compressive and tensile strains. △ Less

Submitted 5 April, 2023; originally announced April 2023.

On the effects of strain, defects, and interactions on the topological properties of HfTe5

Authors: Na Hyun Jo, Omar A. Ashour, Zhixue Shu, Chris Jozwiak, Aaron Bostwick, Sae Hee Ryu, Kai Sun, Tai Kong, Sinead M. Griffin, Eli Rotenberg

Abstract: Topological insulators are characterized by spin-momentum-locked massless surface states which are robust under various perturbations. Manipulating such surface states is a topic of vigorous research, as a possible route for the realization of emergent many-body physics in topological systems. Thus far, time-reversal symmetry breaking via Coulomb and magnetic perturbations has been a dominant appr… ▽ More Topological insulators are characterized by spin-momentum-locked massless surface states which are robust under various perturbations. Manipulating such surface states is a topic of vigorous research, as a possible route for the realization of emergent many-body physics in topological systems. Thus far, time-reversal symmetry breaking via Coulomb and magnetic perturbations has been a dominant approach for the tuning of topological states. However, the effect of the structural degrees of freedom on quasi-particle dynamics in topological materials remains elusive. In this work, we demonstrate a transition in HfTe5 between distinct topological phases as a function of either Te vacancy concentration or applied strain; these phases are characterized theoretically as a transition from strong to weak topological insulator and experimentally by a transition from sharp surface states and Dirac crossing to a Fermi-liquid-like quasiparticle state in which these surface-localized features are heavily suppressed. Although vacancies can result in various consequences such as scattering, doping, and structural distortions, we show that changes in the lattice constants play the foremost role in determining the electronic structure, self-energy, and topological states of HfTe5. Our results demonstrate the possibility of using both defect chemistry and strain as control parameters for topological phase transitions and associated many-body physics. △ Less

Submitted 19 March, 2023; originally announced March 2023.

Journal ref: Phys. Rev. B 109, 235122 (2024)