Showing 1–50 of 353 results for author: Lee, A

Monolithic integration of optically anisotropic GeSe-based films on GaAs by templated solid-phase epitaxy

Authors: Kira J. Martin, Autumn Y. Lee, Pranav Mahaadev, Pooja D. Reddy, Kelly Xiao, Tri Nguyen, Ashlee M. García, Aaron M. Lindenberg, Kunal Mukherjee

Abstract: Layered IV-VI semiconductors such as GeSe exhibit strong in-plane optical anisotropy, making them promising candidates for polarization-sensitive photonic devices. However, realizing these properties in scalable platforms requires heteroepitaxial integration on technologically relevant substrates like GaAs. Direct growth of GeSe is complicated by its glass formation at low temperatures and high va… ▽ More Layered IV-VI semiconductors such as GeSe exhibit strong in-plane optical anisotropy, making them promising candidates for polarization-sensitive photonic devices. However, realizing these properties in scalable platforms requires heteroepitaxial integration on technologically relevant substrates like GaAs. Direct growth of GeSe is complicated by its glass formation at low temperatures and high vapor pressure at elevated temperatures. To overcome this, we develop a method for ex-situ solid-phase epitaxy utilizing a SnSe buffer and offcut GaAs substrate to enable single-orientation crystalline GeSe films. Using polarized reflection measurements, we find that stabilizing a single-in-plane-orientation results in a 2x increase in anisotropic response between the armchair and zigzag directions. This work provides a new integration route to harness the anisotropic optical properties of GeSe and its alloys for polarization-sensitive technologies. △ Less

Submitted 4 August, 2026; originally announced August 2026.

Comments: 11 pages, 5 figures

Effects on a metal that is proximately coupled to hyperbolic photon modes

Authors: Zhiyu Dong, Patrick A. Lee

Abstract: The hyperbolic mode (HM) refers to a polariton mode in a polar insulator where the dielectric function is negative in some direction of propagation. Within a frequency window the light occupies a greatly expanded region in momentum space. The HM in hexagonal Boron Nitride (hBN) has been under intense study and we consider placing a metal directly on top of hBN and ask whether its physical properti… ▽ More The hyperbolic mode (HM) refers to a polariton mode in a polar insulator where the dielectric function is negative in some direction of propagation. Within a frequency window the light occupies a greatly expanded region in momentum space. The HM in hexagonal Boron Nitride (hBN) has been under intense study and we consider placing a metal directly on top of hBN and ask whether its physical properties can be strongly affected. While the problem resembles superficially the electron phonon coupling problem, there are important differences. Due to the longitudinal nature of the HM mode the coupling is driven by time dependent charge fluctuations which results in a coupling that strongly increases with the energy difference of the initial and final states. We find a significant frequency and momentum dependence of the self energy which allows us to identify the dimensionless coupling constant $λ_0$ that controls this effect. There is a suppression of the quasi-particle weight but it turns out that the leading correction to the velocity renormalization is canceled. We compute the single particle spectral function which shows a side band that can be measured experimentally. The virtual exchange of HM leads to a repulsive interaction which is ineffective to leading order because of the energy dependence. We are motivated to seek pairing by going to second order. Unfortunately we find that the leading contributions exactly cancel. This cancellation is not an accident and we give an explanation of why this cancellation would take place. △ Less

Submitted 26 July, 2026; v1 submitted 15 June, 2026; originally announced June 2026.

Comments: The gauge field of the hyperbolic mode is nearly longitudinal. Instead of the Coulomb gauge we had treated it in the Weyl gauge. However, since the electron Green's function is gauge dependent, quantities like Z needs to be treated carefully and the expressions given in the paper are incorrect. This paper is being withdrawn and will be replaced later by one where these issues will be discussed

Effect of the hyperbolic photon mode on a proximate material

Authors: Zhiyu Dong, Patrick A. Lee

Abstract: The hyperbolic mode (HM) refers to a polariton mode where the dielectric function is negative in some direction of propagation. Within a frequency window the light occupies a greatly expanded region in momentum space. We compute the photon density of states and find that the zero point fluctuation in the RMS electric field can reach $MV/cm$, comparable to the largest field used in pump probe exper… ▽ More The hyperbolic mode (HM) refers to a polariton mode where the dielectric function is negative in some direction of propagation. Within a frequency window the light occupies a greatly expanded region in momentum space. We compute the photon density of states and find that the zero point fluctuation in the RMS electric field can reach $MV/cm$, comparable to the largest field used in pump probe experiments in the THz scale. The HM in hexagonal Boron Nitride (hBN) has been under intense study and we consider placing a material directly on top of or sandwiched between hBN crystals. We consider two examples. First the material is a metal and we calculate the modification of the quasi-particle spectral weight, Fermi velocity and pairing interaction. Next we consider a material that is close to the Mott transition. We find that a substantial shift in the metal-insulator transition is possible, but the effect decays rapidly with distance, so that only a few monolayers are affected. We provide estimates and suggestions for a number of materials of interest. △ Less

Submitted 25 August, 2026; v1 submitted 15 June, 2026; originally announced June 2026.

Comments: This version supercedes the earlier withdrawn version and also replaces the content of arXiv:2606.16674 which has been withdrawn

Cryogenically Enhanced Laser-Induced Amorphous Phase Transitions in Crystalline Silicon

Authors: Conrad Kuz, Andy Lee, Shashu Tomar, Ravleen Kaur, Mohamed Yaseen Noor, Justin Twardowski, Liam Clink, Roberto C. Myers, Enam Chowdhury

Abstract: Amorphization of silicon is crucial to applications in photonics, microelectronics and solar cell technologies. Ultrafast lasers have been used to generate amorphous silicon from crystalline silicon using rapid nonthermal melting and solidification in room temperature. As material temperature can affect cooling rates significantly, adding temperature control in ultrafast laser modification of sili… ▽ More Amorphization of silicon is crucial to applications in photonics, microelectronics and solar cell technologies. Ultrafast lasers have been used to generate amorphous silicon from crystalline silicon using rapid nonthermal melting and solidification in room temperature. As material temperature can affect cooling rates significantly, adding temperature control in ultrafast laser modification of silicon may allow a new degree of freedom in ultrafast laser modification. In this work, we investigate the role of cryogenic temperature in governing ultrafast damage pathways via single-shot femtosecond laser irradiation of silicon from room temperature down to 24K at 1030nm. Across this temperature range, we observe a pronounced enhancement of amorphization at lower temperatures, revealed through optical microscopy, Raman spectroscopy, and Kelvin probe force microscopy (KPFM). Raman analysis identifies this ring as an amorphous surface layer, while complementary AFM and SEM imaging show temperature-dependent changes in surface morphology, including localized melt redistribution and refrozen material. To elucidate the physical origins of this behavior, we implement a carrier dependent two-temperature model (nTTM). The simulations reproduce the experimentally observed trends and indicate that reduced phonon population, modified absorption pathways, and altered lattice relaxation dynamics at cryogenic temperatures collectively promote amorphous freezing over recrystallization. This study represents the first detailed examination of silicon under ultrafast irradiation below the liquid-nitrogen regime and reveals temperature-governed mechanisms relevant for advanced silicon microstructuring. △ Less

Submitted 18 May, 2026; originally announced May 2026.

Pseudogap and Condensation in Cuprate Superconductors from NMR Shifts

Authors: Abigail Lee, Juergen Haase

Abstract: The electronic properties of the high-temperature superconducting cuprates are encoded in complex sets of NMR data, but without microscopic theory, reliable NMR phenomenologies are in demand. Early analyses of NMR could only focus on very few materials and discovered spin singlet pairing and the enigmatic pseudogap. However, a coherent phenomenology of shift and relaxation could not be established… ▽ More The electronic properties of the high-temperature superconducting cuprates are encoded in complex sets of NMR data, but without microscopic theory, reliable NMR phenomenologies are in demand. Early analyses of NMR could only focus on very few materials and discovered spin singlet pairing and the enigmatic pseudogap. However, a coherent phenomenology of shift and relaxation could not be established, as incoming data from other cuprates complicated the picture. Today, due to work of many groups worldwide, planar copper and oxygen NMR data are available for most cuprates. Here, based only on symmetry of the two Cu hyperfine couplings, an anisotropic $A_α$ and isotropic $B$, the Cu shifts are disentangled, and two different shift components emerge. Upon doping the cuprates, metallic B-spins are created above the pseudogap temperature which is shared with metallic A-spins. Further doping decreases the pseudogap temperature and increases the B-spin, but less so the A-spin. The apparent linear rate of increase in density of states of the B-spin with doping increases nearly threefold above about $x=0.20$, where the pseudogap has disappeared and A and B turn into superconducting metals, i.e. they disappear rapidly at $T_\mathrm{c}$. The pseudogap temperature is a measure of the coupling between A and B, which suppresses the shifts but not nuclear relaxation. Spin singlet pairing involves A and B according to three simple rules for condensation which will be discussed. The optimal $T_\mathrm{c}$ demands a special match between A and B and involves systems with a pseudogap. However, the highest $T_\mathrm{c}$ of all cuprates is not encoded in the shift, but rather in nuclear relaxation and charge sharing between planar Cu and O. Relations to other probes are discussed. △ Less

Submitted 21 April, 2026; originally announced April 2026.

Comments: 13 pages, 6 figures

Hidden Universal Metal in Cuprate Superconductors

Authors: Abigail Lee, Juergen Haase

Abstract: Nuclear relaxation is a very robust probe of electronic excitations in superconducting materials, above and below the critical temperature of superconductivity, $T_\mathrm{c}$. Here, a phenomenology of it in cuprate superconductors is established based on essentially all cuprate data available in the literature from the CuO$_2$ plane. A universal 'hidden metal' with $1/T_1 T = const$ reigns below… ▽ More Nuclear relaxation is a very robust probe of electronic excitations in superconducting materials, above and below the critical temperature of superconductivity, $T_\mathrm{c}$. Here, a phenomenology of it in cuprate superconductors is established based on essentially all cuprate data available in the literature from the CuO$_2$ plane. A universal 'hidden metal' with $1/T_1 T = const$ reigns below the pseudogap line, and, similar to usual superconducting metals, all cuprates condense at $T_\mathrm{c}$ out of this metal and relaxation ceases rapidly. There is no Hebel-Slichter peak. Above the hidden metal, a renormalized two-component metal is found. Therefore, the hidden metal is identified as the pseudogap matter, which has other important properties. It predominantly lacks a uniform response, unlike the normal metal above $T^*$, and it does not significantly relax planar O. However, for planar Cu it exhibits a special relaxation anisotropy. With the field in the plane, it causes a relaxation rate that is very similar for all cuprates, $1/{^{63}T}_{1\perp}T\approx 25/$Ks, while with the field parallel to the $c$-axis, $1/{^{63}T}_{1\parallel}T$, the proportional rate (in the pseudogap range) changes as a function of doping and material. This relaxation anisotropy is strictly correlated with the maximum critical temperature, $T_\mathrm{c,max}$, of all cuprates. It is stipulated that two partially independent spin components are needed to understand this behavior. The new phenomenology will be discussed and should give a better foundation for the understanding of the cuprates △ Less

Submitted 3 July, 2026; v1 submitted 11 April, 2026; originally announced April 2026.

Comments: 21 pages, 6 figures

Revealing Short- and Long-range Li-ion diffusion in Li$_2$MnO$_3$ from finite-temperature dynamical mean field theory

Authors: Alex Taekyung Lee, Kristin A. Persson, Anh T. Ngo

Abstract: Li$_2$MnO$_3$ is a key component of Li-excess layered cathodes of the form $(1-x),\mathrm{LiMO_2} + x,\mathrm{Li_2MnO_3}$ ($M$ = Mn, Ni, Co, \dots), yet its role in setting Li-ion transport limitations remains under debate. Here we combine DFT+$U$, finite-temperature DFT+DMFT with a continuous-time quantum Monte Carlo impurity solver, and nudged-elastic-band (NEB) calculations to study Li$^{+}$ mi… ▽ More Li$_2$MnO$_3$ is a key component of Li-excess layered cathodes of the form $(1-x),\mathrm{LiMO_2} + x,\mathrm{Li_2MnO_3}$ ($M$ = Mn, Ni, Co, \dots), yet its role in setting Li-ion transport limitations remains under debate. Here we combine DFT+$U$, finite-temperature DFT+DMFT with a continuous-time quantum Monte Carlo impurity solver, and nudged-elastic-band (NEB) calculations to study Li$^{+}$ migration in paramagnetic Li$_2$MnO$_3$ in the presence of a single Li vacancy. Evaluating DMFT total energies along the DFT+$U$ NEB geometries reveals that dynamical correlations strongly renormalize the lowest-barrier processes, reducing the activation energies to $E_a = 0.18$ eV for the shortest-range hop and $E_a = 0.50$ eV for the next-lowest (transport-controlling) step. The 0.18 eV barrier quantitatively reproduces the short-range activation energy from $μ^{+}$SR, while the 0.50 eV barrier is consistent with the long-range transport scale extracted from ac-impedance measurements. This single-vacancy, paramagnetic DMFT description thus provides a unified interpretation of local and macroscopic probes without invoking clustered vacancy configurations or strong extrinsic disorder, consistent with nearly stoichiometric Li$_2$MnO$_3$ powders. More broadly, our results highlight finite-temperature dynamical correlations as an essential ingredient for predicting ionic migration energetics in correlated oxide electrodes. △ Less

Submitted 27 May, 2026; v1 submitted 2 February, 2026; originally announced February 2026.

Comments: 8 pages, 6 figures

Journal ref: Journal of Materials Chemistry A, 14, 20324 (2026)

Excitations and anisotropic sound in planar dipolar supersolids with tilted dipoles

Authors: Reuben Cook, Au-Chen Lee, P. Blair Blakie

Abstract: We investigate the collective excitations of anisotropic dipolar supersolids in planar confinement, focusing on triangular and stripe phases in situations where the dipoles are titled to have a component in the plane. Using Bogoliubov-de Gennes calculations and hydrodynamic theory, we identify the elastic parameters that govern the long-wavelength dynamics, including two orientational coefficients… ▽ More We investigate the collective excitations of anisotropic dipolar supersolids in planar confinement, focusing on triangular and stripe phases in situations where the dipoles are titled to have a component in the plane. Using Bogoliubov-de Gennes calculations and hydrodynamic theory, we identify the elastic parameters that govern the long-wavelength dynamics, including two orientational coefficients that capture the broken rotational symmetry induced by dipole tilt. Analytical expressions for the speeds of sound are obtained along the principal axes for triangular supersolids and along any propagation direction for the stripe supersolid. Our results provide a unified framework for understanding sound propagation in anisotropic dipolar supersolids and establish connections to recent experiments on sound propagation in striped Bose-Einstein condensates. △ Less

Submitted 1 February, 2026; originally announced February 2026.

Comments: 13 pages, 5 figures

Journal ref: Phys. Rev. A 113, 063308 (2026)

Awakening catalytically active surface of BaRuO3 thin film for alkaline hydrogen evolution

Authors: Jegon Lee, Dohyun Kim, Seulgi Ji, Sangmoon Yoon, Seung Hyun Nam, Jucheol Park, Jin Young Oh, Seung Gyo Jeong, Jong-Seong Bae, Sang A Lee, Heechae Choi, Woo Seok Choi

Abstract: The dynamic reconstruction of surfaces during electrochemical reactions plays a crucial role in determining the performance of electrocatalysts. However, because reconstructions occur at the atomic level, direct observation and elucidation of the underlying mechanism are challenging for conventional powder type catalysts with ill defined lattices. In this study, the catalytically active surface of… ▽ More The dynamic reconstruction of surfaces during electrochemical reactions plays a crucial role in determining the performance of electrocatalysts. However, because reconstructions occur at the atomic level, direct observation and elucidation of the underlying mechanism are challenging for conventional powder type catalysts with ill defined lattices. In this study, the catalytically active surface of 3C BaRuO3 (BRO) epitaxial thin films emerges upon the dynamic introduction of surface Ru clusters, for the alkaline hydrogen evolution reaction (HER). Based on the mass activity at overpotential 100 mV, the intrinsic HER performance increases dramatically from 0.11 to 7.72 A/mg immediately after the initial HER cycle and eventually saturates at 1.05 A/mg after continuous operation. The formation of Ru clusters on the catalyst surface, driven by selective Ba leaching under alkaline HER conditions, is observed experimentally. Density functional theory calculations demonstrate that HER activity increased with enhanced H* adsorption owing to the dynamic Ru6 cluster formation. A strategy for stabilizing the 'awakened' active surface of BRO is further proposed by validating that the atomic-scale control of the film thickness can effectively maintain the highly active state. This study offers fundamental insights into the design and stabilization of the highly active Ru-based electrocatalysts for the alkaline HER. △ Less

Submitted 4 December, 2025; originally announced December 2025.

Comments: 24 pages, 5 figures. This work has been accepted for publication in Chinese journal of catalysis in October 2025. Jegon Lee, Dohyun Kim and Seulgi Ji contributed equally to this work. Woo Seok Choi, Heechae Choi and Sang A Lee are corresponding authors

Journal ref: Chinese. J. Catal. 83, 341 (2026)

A path to superconductivity via strong short-range repulsion in a spin-polarized band

Authors: Zhiyu Dong, Patrick A. Lee

Abstract: We predict that the spin-polarized electrons in a two-dimensional triangular lattice with strong electron-electron repulsion gives rise to f-wave pairing. The key point is that the first-order interaction, which is usually pair-breaking, vanishes or nearly vanishes in certain f-wave channels due to symmetry constraints. As a result, these f-wave pairing channels are governed by the subleading-orde… ▽ More We predict that the spin-polarized electrons in a two-dimensional triangular lattice with strong electron-electron repulsion gives rise to f-wave pairing. The key point is that the first-order interaction, which is usually pair-breaking, vanishes or nearly vanishes in certain f-wave channels due to symmetry constraints. As a result, these f-wave pairing channels are governed by the subleading-order processes which enable pairing when the perturbation theory is controlled. We illustrate this using the Hubbard model on the triangular lattice with on-site and nearest-neighbor repulsion, where we find a $T_c\sim 1\% $ of electron's bandwidth. For a general screened interaction, the same idea works asymptotically, but a third-order calculation is needed to fully determine the strength of f-wave pairing. △ Less

Submitted 28 January, 2026; v1 submitted 21 November, 2025; originally announced November 2025.

Comments: This version corrects the results on the nearest-neighbor Hubbard model and the analysis of the generic screened-repulsion problem

Enhanced coherence in the periodically driven two-dimensional XY model

Authors: Duilio De Santis, Marios H. Michael, Sambuddha Chattopadhyay, Andrea Cavalleri, Gil Refael, Patrick A. Lee, Eugene A. Demler

Abstract: Strong optical drives have been shown to induce transient superconducting-like response in materials above their equilibrium $T_c$. Many of these materials already exhibit short-range superconducting correlations in equilibrium. This motivates the question: can external driving enhance coherence in systems with superconducting correlations but no long-range order? We explore this scenario in the t… ▽ More Strong optical drives have been shown to induce transient superconducting-like response in materials above their equilibrium $T_c$. Many of these materials already exhibit short-range superconducting correlations in equilibrium. This motivates the question: can external driving enhance coherence in systems with superconducting correlations but no long-range order? We explore this scenario in the two-dimensional XY model with a periodically modulated stiffness using overdamped Langevin dynamics. We find that, even though the modulation leaves the average coupling unchanged, the drive can markedly increase long-range, time-averaged correlations in systems well above the equilibrium Berezinskii-Kosterlitz-Thouless temperature. The outcome depends on the ratio of the drive frequency to the intrinsic relaxation rate: faster drives primarily heat the system, suppressing correlations and conductivity. For slower drives, the optical conductivity is modified so that the real part exhibits a prolonged effective Drude scattering time, while the imaginary part has a strengthened low-frequency $1/ω$ behavior. We map out these regimes across temperature, frequency, and amplitude, and rationalize them via simple analytics and vortex-thermalization arguments. Overall, we identify a generic nonequilibrium route to enhance coherence in XY-like systems, with potential relevance to experiments reporting light-induced superconductivity. △ Less

Submitted 15 November, 2025; originally announced November 2025.

Comments: Main: 5 pages, 5 figures; Supplement: 5 pages, 6 figures

A GND-based back stress model for reverse loading in metal sheets with consideration of GNB

Authors: Gyu-Jang Sim, Jehyun You, SeongHwan Choi, Youngjae Kim, Chung An Lee, Hyunki Kim, Donghwan Noh, Myoung-Gyu Lee

Abstract: Accurate prediction of springback and formability in sheet metal forming requires understanding reverse loading behavior under complex loading path changes, such as tension followed by compression. However, for ultra-thin sheets experimental characterization of such behavior is difficult due to compressive instability like plastic buckling. This study presents a crystal plasticity finite element m… ▽ More Accurate prediction of springback and formability in sheet metal forming requires understanding reverse loading behavior under complex loading path changes, such as tension followed by compression. However, for ultra-thin sheets experimental characterization of such behavior is difficult due to compressive instability like plastic buckling. This study presents a crystal plasticity finite element method (CPFEM) incorporating a physically motivated back stress model based on geometrically necessary dislocations (GNDs) and boundaries (GNBs). The model captures grain size effects, including the Hall-Petch and Bauschinger effects, through a single grain size-dependent back stress parameter, enabling reverse loading prediction using only tensile data from specimens with different grain sizes. The back stress parameter was calibrated by fitting tensile stress-strain curves from two microstructures - one as-received and one annealed. Without using Tension-Compression (T-C) data for calibration, the model accurately predicted reverse loading behavior in low-carbon steel (0.64 mm thick) and Tension-Bending (T-B) responses in ultra-thin SUS316 (0.083 mm thick) when the developed theory was incorporated to an upscaled anisotropic hardening model. Identifiability analysis confirmed that the model parameters are uniquely determined by the available data. This physically interpretable framework provides an efficient and robust means to predict reverse loading in thin metal sheets, overcoming experimental limitations. △ Less

Submitted 25 September, 2025; originally announced September 2025.

The Future of Artificial Intelligence and the Mathematical and Physical Sciences (AI+MPS)

Authors: Andrew Ferguson, Marisa LaFleur, Lars Ruthotto, Jesse Thaler, Yuan-Sen Ting, Pratyush Tiwary, Soledad Villar, E. Paulo Alves, Jeremy Avigad, Simon Billinge, Camille Bilodeau, Keith Brown, Emmanuel Candes, Arghya Chattopadhyay, Bingqing Cheng, Jonathan Clausen, Connor Coley, Andrew Connolly, Fred Daum, Sijia Dong, Chrisy Xiyu Du, Cora Dvorkin, Cristiano Fanelli, Eric B. Ford, Luis Manuel Frutos , et al. (75 additional authors not shown)

Abstract: This community paper developed out of the NSF Workshop on the Future of Artificial Intelligence (AI) and the Mathematical and Physics Sciences (MPS), which was held in March 2025 with the goal of understanding how the MPS domains (Astronomy, Chemistry, Materials Research, Mathematical Sciences, and Physics) can best capitalize on, and contribute to, the future of AI. We present here a summary and… ▽ More This community paper developed out of the NSF Workshop on the Future of Artificial Intelligence (AI) and the Mathematical and Physics Sciences (MPS), which was held in March 2025 with the goal of understanding how the MPS domains (Astronomy, Chemistry, Materials Research, Mathematical Sciences, and Physics) can best capitalize on, and contribute to, the future of AI. We present here a summary and snapshot of the MPS community's perspective, as of Spring/Summer 2025, in a rapidly developing field. The link between AI and MPS is becoming increasingly inextricable; now is a crucial moment to strengthen the link between AI and Science by pursuing a strategy that proactively and thoughtfully leverages the potential of AI for scientific discovery and optimizes opportunities to impact the development of AI by applying concepts from fundamental science. To achieve this, we propose activities and strategic priorities that: (1) enable AI+MPS research in both directions; (2) build up an interdisciplinary community of AI+MPS researchers; and (3) foster education and workforce development in AI for MPS researchers and students. We conclude with a summary of suggested priorities for funding agencies, educational institutions, and individual researchers to help position the MPS community to be a leader in, and take full advantage of, the transformative potential of AI+MPS. △ Less

Submitted 15 March, 2026; v1 submitted 2 September, 2025; originally announced September 2025.

Comments: Community Paper from the NSF Future of AI+MPS Workshop, Cambridge, Massachusetts, March 24-26, 2025, supported by NSF Award Number 2512945; v2: minor clarifications; v3: approximate version to appear in MLST

Cyclotron reonance in a kagome spin liquid candidate material

Authors: Byungmin Kang, Patrick A. Lee

Abstract: We propose cyclotron resonance as an optical probe for emergent fractionalized excitations in $\mathrm{U}(1)$ quantum spin liquids, focusing on kagome antiferromagnets. In contrast to conventional systems, where cyclotron resonance directly couples to charged carriers, spinons in spin liquids are charge-neutral and interact only through an emergent gauge field. We identify two key mechanisms by wh… ▽ More We propose cyclotron resonance as an optical probe for emergent fractionalized excitations in $\mathrm{U}(1)$ quantum spin liquids, focusing on kagome antiferromagnets. In contrast to conventional systems, where cyclotron resonance directly couples to charged carriers, spinons in spin liquids are charge-neutral and interact only through an emergent gauge field. We identify two key mechanisms by which an external physical electromagnetic field induces emergent electric and magnetic fields, enabling indirect coupling to spinons. Using these mechanisms, we compute the absorption rate of the cyclotron resonance response for Dirac spinons forming Landau levels. Our analysis shows that, although the absorption per layer is small, the absence of a skin-depth limitation in insulating spin liquids allows for cumulative absorption comparable to graphene in realistic sample sizes for the recently discovered spin-liquid candidate material YCu${}_3$(OH)${}_6$Br${}_2$[Br${}_{1-y}$(OH)${}_y$]. Our findings shows that cyclotron resonance is a viable experimental probe of spinon Landau quantization and emergent gauge fields, providing powerful positive experimental signatures of quantum spin liquids. △ Less

Submitted 25 July, 2025; originally announced July 2025.

Comments: 6 pages, 1 figure

Pair density modulation from glide symmetry breaking and nematic superconductivity

Authors: Michał Papaj, Lingyuan Kong, Stevan Nadj-Perge, Patrick A. Lee

Abstract: Pair density modulation is a superconducting state, recently observed in exfoliated iron-based superconductor flakes, in which the superconducting gap oscillates strongly with the same periodicity as the underlying crystalline lattice. We propose a microscopic model that explains this modulation through a combination of glide-mirror symmetry breaking and the emergence of nematic superconductivity.… ▽ More Pair density modulation is a superconducting state, recently observed in exfoliated iron-based superconductor flakes, in which the superconducting gap oscillates strongly with the same periodicity as the underlying crystalline lattice. We propose a microscopic model that explains this modulation through a combination of glide-mirror symmetry breaking and the emergence of nematic superconductivity. The first ingredient results in a sublattice texture on the Fermi surface, which is aligned with the anisotropic superconducting gap of the nematic $s_\pm+d$ state. This gives rise to distinctive gap maxima and minima located on the two inequivalent iron sublattices while still being a zero-momentum pairing state. We discuss how further investigation of such modulations can give insight into the nature of the superconducting pairing, such as the signs of the order parameters and visualization of a phase transition to a mixed two-component state using local probes. △ Less

Submitted 24 June, 2025; originally announced June 2025.

Comments: 7+4 pages, 4+3 figures

Deposition-Dependent Coverage and Performance of Phosphonic Acid Interface Modifiers in Halide Perovskite Optoelectronics

Authors: Hannah Contreras, Aidan O'Brien, Margherita Taddei, Yangwei Shi, Fangyuan Jiang, Robert J. E. Westbrook, Yadong Zhang, Rajiv Giridharagopal, Paul A. Lee, Stephen Barlow, Seth R. Marder, Neal R. Armstrong, David S. Ginger

Abstract: In this work, we study the effect of various deposition methods for phosphonic acid interface modifiers commonly pursued as self-assembled monolayers in high-performance metal halide perovskite photovoltaics and light-emitting diodes. We compare the deposition of (2-(3,6-diiodo-9H-carbazol-9-yl)ethyl)phosphonic acid onto indium tin oxide (ITO) bottom contacts by varying three parameters: the metho… ▽ More In this work, we study the effect of various deposition methods for phosphonic acid interface modifiers commonly pursued as self-assembled monolayers in high-performance metal halide perovskite photovoltaics and light-emitting diodes. We compare the deposition of (2-(3,6-diiodo-9H-carbazol-9-yl)ethyl)phosphonic acid onto indium tin oxide (ITO) bottom contacts by varying three parameters: the method of deposition, specifically spin coating or prolonged dip coating, ITO surface treatment via HCl/FeCl3 etching, and use in combination with a second modifier, 1,6-hexylenediphosphonic acid. We demonstrate that varying these modification protocols can impact time-resolved photoluminescence carrier lifetimes and quasi-Fermi level splitting of perovskite films deposited onto the phosphonic-acid-modified ITO. Ultraviolet photoelectron spectroscopy shows an increase in effective work function after phosphonic acid modification and clear evidence for photoemission from carbazole functional groups at the ITO surface. We use X-ray photoelectron spectroscopy to probe differences in phosphonic acid coverage on the metal oxide contact and show that perovskite samples grown on ITO with the highest phosphonic acid coverage exhibit the longest carrier lifetimes. Finally, we establish that device performance follows these same trends. These results indicate that the reactivity, heterogeneity, and composition of the bottom contact help to control recombination rates and therefore power conversion efficiencies. ITO etching, prolonged deposition times for phosphonic acids via dip coating, and the use of a secondary, more hydrophilic bis-phosphonic acid, all contribute to improvements in surface coverage, carrier lifetime, and device efficiency. These improvements each have a positive impact, and we achieve the best results when all three strategies are implemented. △ Less

Submitted 23 June, 2025; originally announced June 2025.

An Ultra-Low Power and Fast Ising Machine using Voltage-Controlled Magnetoresistive Random Access Memory

Authors: Sai Li, Yihao Zhang, Albert Lee, Zheng Zhu, Lang Zeng, Peng Wang, Lei Gao, Di Wu, Weisheng Zhao

Abstract: Physics-inspired computing paradigms, such as Ising machines, are emerging as promising hardware alternatives to traditional von Neumann architectures for tackling computationally intensive combinatorial optimization problems (COPs). While quantum, optical, and electronic devices have garnered significant attention for their potential in realizing Ising machines, their translation into practical s… ▽ More Physics-inspired computing paradigms, such as Ising machines, are emerging as promising hardware alternatives to traditional von Neumann architectures for tackling computationally intensive combinatorial optimization problems (COPs). While quantum, optical, and electronic devices have garnered significant attention for their potential in realizing Ising machines, their translation into practical systems for industry-relevant applications remains challenging, with each approach facing specific limitations in power consumption and speed. To address this challenge, we report the first chip-level spintronic Ising machine using voltage-controlled magnetoresistive random access memory. The core of our design leverages magnetic tunnel junctions (MTJs) driven by the voltage-controlled magnetic anisotropy effect to realize the probabilistic update of Ising spins through a new mechanism. It enables a latency below 1 ns and an energy consumption under 40 fJ per spin update, achieving a 1000-times improvement over previous current-driven MTJ-based implementations. We map two real-world COPs in electronic design automation-global routing and layer assignment-onto the Ising model and demonstrate high-quality results with an energy efficiency of 25000 solutions per second per watt. This outperforms state-of-the-art quantum and graphics processing units by six and seven orders of magnitude, respectively. These results establish voltage-controlled spintronics as a compelling route towards next-generation physics-inspired machine intelligence, offering a paradigm for ultra-low-power, high-speed, and scalable computation. △ Less

Submitted 14 March, 2026; v1 submitted 25 May, 2025; originally announced May 2025.

Flexible-AR display for near-eye operations

Authors: Alan Lee, Dechuan Sun, Gregory Tanyi, Younger Liang, Christina Lim, Ranjith R Unnithan

Abstract: We propose a new technique to fabricate flexible-near-field Argument-Reality (AR) display using modular-molds. A near-eye flexible-AR-display is fabricated based on parameters extracted from simulations. Our AR-display successfully reconstructed images and videos from a light-engine. It opens a new approach to fabricate flexible-near-field AR display with good physical stress and collision-resilie… ▽ More We propose a new technique to fabricate flexible-near-field Argument-Reality (AR) display using modular-molds. A near-eye flexible-AR-display is fabricated based on parameters extracted from simulations. Our AR-display successfully reconstructed images and videos from a light-engine. It opens a new approach to fabricate flexible-near-field AR display with good physical stress and collision-resilience. △ Less

Submitted 15 May, 2025; originally announced May 2025.

Impact of structural distortions on the correlated electronic structure of orbital-selective Mott insulating Na$_3$Co$_2$SbO$_6$ under strains

Authors: Nam Nguyen, Alex Taekyung Lee, Anh T. Ngo, Hyowon Park

Abstract: Na$_{3}$Co$_{2}$SbO$_6$ is a promising candidate to realize the Kitaev spin liquid phase since the large Kitaev spin exchange interaction is tunable via the change in electronic structure, such as the trigonal crystal field splitting ($Δ_{TCF}$). Here, we show that the uncorrelated electronic structure of Na$_{3}$Co$_{2}$SbO$_6$ is rather insensitive to the strain effect due to the low crystal sym… ▽ More Na$_{3}$Co$_{2}$SbO$_6$ is a promising candidate to realize the Kitaev spin liquid phase since the large Kitaev spin exchange interaction is tunable via the change in electronic structure, such as the trigonal crystal field splitting ($Δ_{TCF}$). Here, we show that the uncorrelated electronic structure of Na$_{3}$Co$_{2}$SbO$_6$ is rather insensitive to the strain effect due to the low crystal symmetry accompanied by oxygen displacements and the presence of Sb $s$ orbitals. This suggests that the Kitaev spin-exchange interaction obtained from perturbation theory also does not depend much on the strain effect. Using density functional theory plus dynamical mean field theory, we find that the correlated electronic structure of Na$_{3}$Co$_{2}$SbO$_6$ is an orbital selective Mott insulating state where the trigonal $a_{1g}$ orbital is insulating due to correlation-assisted hybridization, while other $d$ orbitals behave as typical Mott insulators, resulting in tunability of $Δ_{TCF}$ under the strain effect effectively. Our results show that the local Co-site symmetry and dynamical correlation effects will play an important role in engineering the novel magnetic phase in this and related materials. △ Less

Submitted 9 August, 2025; v1 submitted 15 March, 2025; originally announced March 2025.

Comments: 14 pages, 13 figures

Dirac node pinning from Dzyaloshinskii-Moriya interactions in a Kagome spin liquid

Authors: Ajesh Kumar, Byungmin Kang, Patrick A. Lee

Abstract: Recent experiments on the Kagome spin liquid candidate YCOB suggest the presence of Dirac fermionic spinons near the magnetization plateau at 1/9. Theories suggest that the spinons are charge neutral spin-$1/2$ excitations, in a $2π/3$ flux which triples the unit cell. Generally a gap is expected, and there is no symmetry protection for the Dirac nodes in this system. The question arises as to wha… ▽ More Recent experiments on the Kagome spin liquid candidate YCOB suggest the presence of Dirac fermionic spinons near the magnetization plateau at 1/9. Theories suggest that the spinons are charge neutral spin-$1/2$ excitations, in a $2π/3$ flux which triples the unit cell. Generally a gap is expected, and there is no symmetry protection for the Dirac nodes in this system. The question arises as to what causes the nodes and stabilizes them. In this work, we propose a node-creation and node-pinning mechanism driven by the Dzyaloshinskii-Moriya (DM) interactions. Employing Gutzwiller-projected variational Monte Carlo calculations, we demonstrate that DM interactions induce a band closing phase transition in the spinon spectrum. There is a change in the Chern number when the bands are inverted. Together with the DM-generated internal gauge flux, the coupling to the spinon orbital magnetization counteracts the band reopening. This interplay energetically pins the Dirac nodes over a range of parameters, resulting in a pinning mechanism distinct from the usual one from symmetry protection. △ Less

Submitted 16 April, 2026; v1 submitted 14 March, 2025; originally announced March 2025.

Comments: 5+2 pages, 4+3 figures. v2 adds references, expands discussion of DM-induced gauge flux, and adds a figure on system-size dependence to Appendix A

Journal ref: Phys. Rev. Lett. 136, 146704 (2026)

A controlled expansion for pairing in a polarized band with strong repulsion

Authors: Zhiyu Dong, Patrick A. Lee

Abstract: Can strong repulsive interactions be shown to give rise to pairing in a controlled way? We find that for a single flavor polarized band, there is a small expansion parameter in the low density limit, once the Bloch wavefunction form factor is taken into account. A perturbative expansion is possible, even if the interaction is much stronger than the Fermi energy $ε_F$. As a matter of principle, our… ▽ More Can strong repulsive interactions be shown to give rise to pairing in a controlled way? We find that for a single flavor polarized band, there is a small expansion parameter in the low density limit, once the Bloch wavefunction form factor is taken into account. A perturbative expansion is possible, even if the interaction is much stronger than the Fermi energy $ε_F$. As a matter of principle, our work shows analytically how strong pairing can emerge from strong repulsion. We illustrate our method with two examples: a 2D Dirac model and a 1D tight binding model with two orbitals. In the latter case, using density matrix renormalization group, we show that the analytical theory indeed guided us to discover the parameter regime where p-wave pairing with order-1 strength is dominant. △ Less

Submitted 28 September, 2025; v1 submitted 14 March, 2025; originally announced March 2025.

Comments: This version added some comments on recent papers and fixed typos

Stripe-like correlations in the cuprates from oxygen NMR

Authors: Daniel Bandur, Abigail Lee, Stefan Tsankov, Andreas Erb, Juergen Haase

Abstract: Nuclear magnetic resonance (NMR) of planar oxygen, with its family independent phenomenology, is ideally suited to probe the nature of the quantum matter of superconducting cuprates. Here, with new experiments on La$_{2-x}$Sr$_x$CuO$_4$, in particular also at high doping levels, we report on short-range stripe-like correlations between local charge and spin. Their amplitudes at room temperature ar… ▽ More Nuclear magnetic resonance (NMR) of planar oxygen, with its family independent phenomenology, is ideally suited to probe the nature of the quantum matter of superconducting cuprates. Here, with new experiments on La$_{2-x}$Sr$_x$CuO$_4$, in particular also at high doping levels, we report on short-range stripe-like correlations between local charge and spin. Their amplitudes at room temperature are nearly independent of doping up to at least $x=0.30$, only their relative phase slips near $x=1/4$. Comparisons show the correlations to be generic to the cuprates. Despite the atomic scale length, the variations still resemble the average spin and charge relation, which is not expected from the otherwise simple, apparently metallic behavior, even far into the overdoped regime. Perhaps the phase slip is at the heart of a quantum critical point that demands pseudogap behavior towards lower doping levels in an otherwise strange metal. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Comments: 16 pages, 4 figures

Emergence of Giant Magnetic Chirality during Dimensionality Crossover of Magnetic Materials

Authors: Dae-Yun Kim, Yun-Seok Nam, Younghak Kim, Kyoung-Whan Kim, Gyungchoon Go, Seong-Hyub Lee, Joon Moon, Jun-Young Chang, Ah-Yeon Lee, Seung-Young Park, Byoung-Chul Min, Kyung-Jin Lee, Hyunsoo Yang, Duck-Ho Kim, Sug-Bong Choe

Abstract: Chirality, an intrinsic preference for a specific handedness, is a fundamental characteristic observed in nature. In magnetism, magnetic chirality arises from the anti-symmetric Dzyaloshinskii-Moriya interaction in competition with the symmetric Heisenberg exchange interaction. Traditionally, the anti-symmetric interaction has been considered minor relative to the symmetric interaction. In this st… ▽ More Chirality, an intrinsic preference for a specific handedness, is a fundamental characteristic observed in nature. In magnetism, magnetic chirality arises from the anti-symmetric Dzyaloshinskii-Moriya interaction in competition with the symmetric Heisenberg exchange interaction. Traditionally, the anti-symmetric interaction has been considered minor relative to the symmetric interaction. In this study, we demonstrate an observation of giant magnetic chirality during the dimensionality crossover of magnetic materials from three-dimensional to two-dimensional. The ratio between the anti-symmetric and symmetric interactions exhibits a reversal in their dominance over this crossover, overturning the traditional consideration. This observation is validated theoretically using a non-local interaction model and tight-binding calculation with distinct pairing schemes for each exchange interaction throughout the crossover. Additional experiments investigating the asphericity of orbital moments corroborate the robustness of our findings. Our findings highlight the critical role of dimensionality in shaping magnetic chirality and offer strategies for engineering chiral magnet states with unprecedented strength, desired for the design of spintronic materials. △ Less

Submitted 6 January, 2025; originally announced January 2025.

Comments: 21 pages, 4 figures

Chirality-induced pseudo-magnetic fields, flat bands and enhancement of superconductivity

Authors: Zhiyu Dong, Leonid Levitov, Patrick A. Lee

Abstract: Systems in which exchange interactions couple carrier spins to a spin texture with a net chirality exhibit a spin-dependent Aharonov-Bohm effect, where the geometric gauge field and pseudo-magnetic field have opposite signs for carriers with opposite spins. As a result, Cooper pairs see a net zero vector potential and superconducting pairing is not hindered by pair-breaking effects. This allows su… ▽ More Systems in which exchange interactions couple carrier spins to a spin texture with a net chirality exhibit a spin-dependent Aharonov-Bohm effect, where the geometric gauge field and pseudo-magnetic field have opposite signs for carriers with opposite spins. As a result, Cooper pairs see a net zero vector potential and superconducting pairing is not hindered by pair-breaking effects. This allows superconductivity to occur even when the geometric field induces quantized Landau levels. We identify the dominant pairing order as an s-wave pair density wave of an FFLO type. Flat Landau levels can significantly enhance superconducting $T_c$, favoring superconductivity over competing orders. This exotic paired state features tell-tale signatures such as flat bands of Bogoliubov-deGennes quasiparticles, manifest through Landau level-like resonances in the quasiparticle density of states. △ Less

Submitted 27 December, 2024; originally announced December 2024.

Violation of the Wiedemann-Franz law and ultra-low thermal conductivity of Ti$_3$C$_2$T$_x$ MXene

Authors: Yubin Huang, Jean Spiece, Tetiana Parker, Asaph Lee, Yury Gogotsi, Pascal Gehring

Abstract: The high electrical conductivity and good chemical stability of MXenes offer hopes for their use in many applications, such as wearable electronics, energy storage, or electromagnetic interference shielding. While their optical, electronic and electrochemical properties have been widely studied, the information on thermal properties of MXenes is scarce. In this study, we investigate the heat trans… ▽ More The high electrical conductivity and good chemical stability of MXenes offer hopes for their use in many applications, such as wearable electronics, energy storage, or electromagnetic interference shielding. While their optical, electronic and electrochemical properties have been widely studied, the information on thermal properties of MXenes is scarce. In this study, we investigate the heat transport properties of Ti$_3$C$_2$T$_x$ MXene single flakes using scanning thermal microscopy and find exceptionally low anisotropic thermal conductivities within the Ti$_3$C$_2$T$_x$ flakes, leading to an effective thermal conductivity of 0.78$\pm$0.21 W m$^{-1}$ K$^{-1}$. This observation is in stark contrast to the predictions of the Wiedemann-Franz law, as the estimated Lorenz number is only 0.25 of the classical value. Due to the combination of low thermal conductivity and low emissivity of Ti$_3$C$_2$T$_x$, the heat loss from it is two orders of magnitude smaller than that from common metals. Our study explores the heat transport mechanisms of MXenes and highlights a promising approach for developing thermal insulation, two-dimensional thermoelectric, or infrared stealth materials. △ Less

Submitted 2 December, 2024; originally announced December 2024.

Comments: 22 pages, 4 figures

Journal ref: ACS Nano 2024, 18, 47, 32491-32497

Disorder-induced spin-cluster magnetism in a doped kagome spin liquid candidate

Authors: Arnab Seth, Joseph C. Prestigiacomo, Aini Xu, Zhenyuan Zeng, Trevor D. Ford, B. S. Shivaram, Shiliang Li, Patrick A. Lee, Itamar Kimchi

Abstract: The search for new quantum spin liquid materials relies on systems with strong frustration such as spins on an ideal kagome lattice. However, lattice imperfections can have substantial effects which are as yet not well understood. In recent work, the two-dimensional kagome system YCu$_3$(OH)$_6$[(Cl$_x$Br$_{(1-x)}$)$_{3-y}$(OH)$_y$] has emerged as a leading candidate hosting a Dirac spin liquid wh… ▽ More The search for new quantum spin liquid materials relies on systems with strong frustration such as spins on an ideal kagome lattice. However, lattice imperfections can have substantial effects which are as yet not well understood. In recent work, the two-dimensional kagome system YCu$_3$(OH)$_6$[(Cl$_x$Br$_{(1-x)}$)$_{3-y}$(OH)$_y$] has emerged as a leading candidate hosting a Dirac spin liquid which appears to survive at least for x<0.4, associated with alternating-bond hexagon (ABH) disorder. Here in magnetic samples with x=0.58, y=0.1 we report unusual in-plane ferromagnetic canting (FM) of the in-plane antiferromagnet (AFM), with an unusually wide regime of short-ranged order, and propose theoretical models to explain this behavior. First, we show that Kitaev type exchanges naturally arise on the kagome lattice to second order in the known Dzyaloshinskii-Moriya exchanges, and that these interactions can produce the unusual in-plane FM canting from antichiral AFM. Second, we propose a phenomenological model of weakly-FM-canted spin clusters to describe the short-ranged regime and analyze quantum fluctuations in an ABH toy model to show how ABH disorder can stabilize this regime. The combination of experimental observation and theory suggests that kagome-Kitaev interactions and ABH disorder are necessary for describing the magnetic fluctuations in this family of materials, with potential implications for the proposed proximate spin liquid phase. △ Less

Submitted 24 February, 2026; v1 submitted 18 November, 2024; originally announced November 2024.

Comments: 14 pages, 11 figures. v2: Sections reorganized for clarity. Published version

Journal ref: Phys. Rev. B 113, 064441, 2026

Flux-Floquet instability in fluctuating superconductors

Authors: Marios H. Michael, Duilio De Santis, Eugene A. Demler, Patrick A. Lee

Abstract: In the past decade, photo-induced superconducting-like behaviors have been reported in a number of materials driven by intense pump fields. Of particular interest is the high-Tc cuprate YBCO, where such effect has been reported up to the so-called pseudogap temperature T*~300-400 K. In a recent experiment, a transient magnetic field which is proportional to and in the same direction of an applied… ▽ More In the past decade, photo-induced superconducting-like behaviors have been reported in a number of materials driven by intense pump fields. Of particular interest is the high-Tc cuprate YBCO, where such effect has been reported up to the so-called pseudogap temperature T*~300-400 K. In a recent experiment, a transient magnetic field which is proportional to and in the same direction of an applied field has been observed outside the sample, suggestive of flux exclusion due to the Meissner effect. In this paper, we present an alternative interpretation of these experiments based on a mechanism that we term the flux-Floquet instability of the sine-Gordon (SG) model. We take as our premise the model of preformed Cooper pairs in the pseudogap phase. Starting from the local superconducting order parameter in equilibrium, we introduce an extended SG model to describe the dynamics of the relative phase between the layers of a bilayer. We demonstrate that a combination of external magnetic field and strong terahertz drive used in experiments by Fava et al. results in a novel Floquet instability in the model. This instability leads to currents at the edges of the bilayer formed by defects or grain boundaries, with the current flowing in the opposite direction of the equilibrium screening current, producing a giant paramagnetic magnetization in the same direction as the applied field. We show how this scenario can fit most of the available data. To the extent that this model can account for the data, we conclude that the experiments have the important consequence of revealing the presence of local pairing in the pseudogap phase. More broadly our results reveal a new instability in the SG equation that is of fundamental interest, with potential applications such as providing a mechanism for generating large magnetic fields at ultrafast time scales in Josephson devices. △ Less

Submitted 20 July, 2026; v1 submitted 16 October, 2024; originally announced October 2024.

Comments: 13 pages + 13 pages of appendix, 8 figures + 3 figures in Methods and 5 figures in appendix

Thermodynamic evidence of fermionic behavior in the vicinity of one-ninth plateau in a kagome antiferromagnet

Authors: Guoxin Zheng, Dechen Zhang, Yuan Zhu, Kuan-Wen Chen, Aaron Chan, Kaila Jenkins, Byungmin Kang, Zhenyuan Zeng, Aini Xu, D. Ratkovski, Joanna Blawat, Ali Bangura, John Singleton, Patrick A. Lee, Shiliang Li, Lu Li

Abstract: The spin-1/2 kagome Heisenberg antiferromagnets are believed to host exotic quantum entangled states. Recently, the report of 1/9 magnetization plateau and magnetic oscillations in a kagome antiferromagnet YCu$_3$(OH)$_6$Br$_2$[Br$_x$(OH)$_{1-x}$] (YCOB) have made this material a promising candidate for experimentally realizing quantum spin liquid states. Here we present measurements of the specif… ▽ More The spin-1/2 kagome Heisenberg antiferromagnets are believed to host exotic quantum entangled states. Recently, the report of 1/9 magnetization plateau and magnetic oscillations in a kagome antiferromagnet YCu$_3$(OH)$_6$Br$_2$[Br$_x$(OH)$_{1-x}$] (YCOB) have made this material a promising candidate for experimentally realizing quantum spin liquid states. Here we present measurements of the specific heat $C_p$ in YCOB in high magnetic fields (up to 41.5 Tesla) down to 0.46 Kelvin, and the 1/9 plateau feature has been confirmed. Moreover, the temperature dependence of $C_p/T$ in the vicinity of 1/9 plateau region can be fitted by a linear in $T$ term which indicates the presence of a Dirac spectrum, together with a constant term, which indicates a finite density of states (DOS) contributed by other Fermi surfaces. Surprisingly the constant term is highly anisotropic in the direction of the magnetic field. Additionally, we observe a double-peak feature near $30$~T above the 1/9 plateau which is another hallmark of fermionic excitations in the specific heat. △ Less

Submitted 9 September, 2024; originally announced September 2024.

Comments: 4 figures in the main text, 7 figures in the appendix

Journal ref: Physical Review X 15 (2), 021076 (2025)

Signatures of a Spin-Active Interface and Locally Enhanced Zeeman field in a Superconductor-Chiral Material Heterostructure

Authors: Cliff Chen, Jason Tran, Anthony McFadden, Raymond Simmonds, Keisuke Saito, En-De Chu, Daniel Morales, Varrick Suezaki, Yasen Hou, Joe Aumentado, Patrick A. Lee, Jagadeesh S. Moodera, Peng Wei

Abstract: A localized Zeeman field, intensified at heterostructure interfaces, could play a crucial role in a broad area including spintronics and unconventional superconductors. Conventionally, the generation of a local Zeeman field is achieved through magnetic exchange coupling with a magnetic material. However, magnetic elements often introduce defects, which could weaken or destroy superconductivity. Al… ▽ More A localized Zeeman field, intensified at heterostructure interfaces, could play a crucial role in a broad area including spintronics and unconventional superconductors. Conventionally, the generation of a local Zeeman field is achieved through magnetic exchange coupling with a magnetic material. However, magnetic elements often introduce defects, which could weaken or destroy superconductivity. Alternatively, the coupling between a superconductor with strong spin-orbit coupling and a non-magnetic chiral material could serve as a promising approach to generate a spin active interface. In this study, we leverage an interface superconductor, namely induced superconductivity in noble metal surface states, to probe the spin active interface. Our results unveil an enhanced interface Zeeman field, which selectively closes the surface superconducting gap while preserving the bulk superconducting pairing. The chiral material, i.e. trigonal tellurium, also induces Andreev bound states (ABS) exhibiting spin polarization. The field dependence of ABS manifests a substantially enhanced interface Landé g-factor (g_eff ~ 12), thereby corroborating the enhanced interface Zeeman energy. △ Less

Submitted 28 August, 2024; originally announced August 2024.

Comments: 27 pages, 11 figures

Journal ref: Science Advances 10, eado4875 (2024)

Exploring the Coexistence of Spin States in [Fe-(tpy-ph)$_2$]$^{2+}$ Complexes on Au(111) using ab initio calculations

Authors: Naveen K. Dandu, Alex Taekyung Lee, Sergio Ulloa, Larry Curtiss, Saw Wai Hla, Anh T. Ngo

Abstract: In this work, we systematically study the electronic structure and stability of spin states of the [Fe-(tpy-ph)$_2$]$^{2+}$ molecule in both gas phase and on a Au(111) substrate using density functional theory +U (DFT+U) calculations. We find that the stability of the Fe$^{2+}$ ion's spin states is significantly influenced by the Hubbard U parameter. In the gas phase, the low-spin (LS, S=0) state… ▽ More In this work, we systematically study the electronic structure and stability of spin states of the [Fe-(tpy-ph)$_2$]$^{2+}$ molecule in both gas phase and on a Au(111) substrate using density functional theory +U (DFT+U) calculations. We find that the stability of the Fe$^{2+}$ ion's spin states is significantly influenced by the Hubbard U parameter. In the gas phase, the low-spin (LS, S=0) state is found to be energetically favorable for U(Fe) $\leq$ 3 eV, whereas the high-spin (HS, S=2) state is stabilized for U(Fe) > 3 eV. Interaction with the Au(111) substrate is found to elevate the critical U for the spin-state transition to 3.5 eV. Additionally, we perform L-edge X-ray absorption spectroscopy (XAS) calculations based on time-dependent DFT (TD-DFT) for both HS and LS states. The calculated XAS suggests that the HS state more closely aligns with the experimental observations, indicating the potential coexistence of the HS state as the initial state during the X-ray excitation process. These findings enrich our understanding of spin-state dynamics in [Fe-(tpy-ph)$_2$]$^{2+}$. △ Less

Submitted 20 August, 2024; v1 submitted 13 August, 2024; originally announced August 2024.

Comments: 21 pages, 5 figures, 3 tables

Self-consistent evaluation of proximity and inverse proximity effects with pair-breaking in diffusive SN junctions

Authors: Arpit Raj, Patrick A. Lee, Gregory A. Fiete

Abstract: We consider a planar superconducting-normal-metal (SN) junction with both inelastic and spin-flip scattering processes present. In the diffusive limit, we use a one-dimensional formulation of the Usadel equation to compute the self-consistent energy dependence of the single-particle density of states as a function of distance from the interface on both the superconducting and metallic sides for va… ▽ More We consider a planar superconducting-normal-metal (SN) junction with both inelastic and spin-flip scattering processes present. In the diffusive limit, we use a one-dimensional formulation of the Usadel equation to compute the self-consistent energy dependence of the single-particle density of states as a function of distance from the interface on both the superconducting and metallic sides for various spatial profiles of a pair-breaking spin-flip term. The pair-breaking processes fill in the superconducting gap at zero energy, which is reflected in the zero-bias tunneling conductance in scanning tunneling microscopy/spectroscopy experiments, in the vicinity of the junction. We also investigate the impact of having a partially transparent interface at the junction. We compare our findings with the observed exponential rise in the zero-bias conductance at the 1H step edge in recent experiments on 4Hb-TaS$_2$ [A. K. Nayak et al., Nat. Phys. 17, 1413 (2021)]. △ Less

Submitted 24 May, 2024; originally announced May 2024.

Comments: 12 pages, 11 figures

Journal ref: Phys. Rev. B 110, 184504 (2024)

Charge and spin density wave orders in field-biased Bernal bilayer graphene

Authors: Zhiyu Dong, Patrick A. Lee, Leonid Levitov

Abstract: This paper aims to clarify the nature of a surprising ordered phase recently reported in biased Bernal bilayer graphene that occurs at the phase boundary between the isospin-polarized and unpolarized phases. Strong nonlinearity of transport at abnormally small currents, with $dI/dV$ vs. $I$ sharply rising and then falling back, is typical for a charge/spin-density-wave state (CDW or SDW) sliding t… ▽ More This paper aims to clarify the nature of a surprising ordered phase recently reported in biased Bernal bilayer graphene that occurs at the phase boundary between the isospin-polarized and unpolarized phases. Strong nonlinearity of transport at abnormally small currents, with $dI/dV$ vs. $I$ sharply rising and then falling back, is typical for a charge/spin-density-wave state (CDW or SDW) sliding transport. Here, however, it is observed at an isospin-order phase boundary, prompting a question about the CDW/SDW mechanism and its relation to the quantum critical point. We argue that the observed phase diagram cannot be understood within a standard weak-coupling picture. Rather, it points to a mechanism that relies on an effective interaction enhancement at a quantum critical point. We develop a detailed strong-coupling framework accounting for the soft collective modes that explain these observations. △ Less

Submitted 28 April, 2024; originally announced April 2024.

Comments: 11 pages, 7 figures

Observation of Cooper-pair density modulation state

Authors: Lingyuan Kong, Michał Papaj, Hyunjin Kim, Yiran Zhang, Eli Baum, Hui Li, Kenji Watanabe, Takashi Taniguchi, Genda Gu, Patrick A. Lee, Stevan Nadj-Perge

Abstract: Superconducting states that break space-group symmetries of the underlying crystal can exhibit nontrivial spatial modulation of the order parameter. Previously, such remarkable states were intimately associated with the breaking of translational symmetry, giving rise to the density-wave orders, with wavelengths spanning several unit cells. However, a related basic concept has been long overlooked:… ▽ More Superconducting states that break space-group symmetries of the underlying crystal can exhibit nontrivial spatial modulation of the order parameter. Previously, such remarkable states were intimately associated with the breaking of translational symmetry, giving rise to the density-wave orders, with wavelengths spanning several unit cells. However, a related basic concept has been long overlooked: when only intra-unit-cell symmetries of the space group are broken, the superconducting states can display a distinct type of nontrivial modulation preserving long-range lattice translation. Here, we refer to this new concept as the pair density modulation (PDM), and report the first observation of a PDM state in exfoliated thin flakes of iron-based superconductor FeTe$_{\text{0.55}}$Se$_{\text{0.45}}$. Using scanning tunneling microscopy, we discover robust superconducting gap modulation with the wavelength corresponding to the lattice periodicity and the amplitude exceeding 30% of the gap average. Importantly, we find that the observed modulation originates from the large difference in superconducting gaps on the two nominally equivalent iron sublattices. The experimental findings, backed up by model calculations, suggest that in contrast to the density-wave orders, the PDM state is driven by the interplay of sublattice symmetry breaking and a peculiar nematic distortion specific to the thin flakes. Our results establish new frontiers for exploring the intertwined orders in strong-correlated electronic systems and open a new chapter for iron-based superconductors. △ Less

Submitted 15 April, 2024; originally announced April 2024.

Comments: Full submission including supplementary information, 4 main figures

A model of non-Fermi liquid with power law resistivity: strange metal with a not-so-strange origin

Authors: Patrick A. Lee

Abstract: We construct a model which exhibits resistivity going as a power law in temperature $T$, as $T^α$ down to the lowest temperature. There is no residual resistivity because we assume the absence of disorder and momentum relaxation is due to umklapp scattering. Our model consists of a quantum spin liquid state with spinon Fermi surface and a hole Fermi surface made out of doped holes. The key ingredi… ▽ More We construct a model which exhibits resistivity going as a power law in temperature $T$, as $T^α$ down to the lowest temperature. There is no residual resistivity because we assume the absence of disorder and momentum relaxation is due to umklapp scattering. Our model consists of a quantum spin liquid state with spinon Fermi surface and a hole Fermi surface made out of doped holes. The key ingredient is a set of singular $2k_F$ modes living on a ring in momentum space. Depending on parameters, $α$ may be unity (strange metal) or even smaller. The model may be applicable to a doped organic compound, which has been found to exhibit linear T resistivity. We conclude that it is possible to obtain strange metal behavior starting with a model which is not so strange. △ Less

Submitted 22 March, 2024; v1 submitted 16 February, 2024; originally announced February 2024.

Comments: This version corrects the expression for the polarization function for q>ω^2/3. This does not change the resistivity exponent for σ>1/6. For σ<1/6 we now recovers the Fermi liquid limit, as seen in the updated fig 2

Non-Analytic Magnetic Response and Intrinsic Ferromagnetic Clusters in a Dirac Spin Liquid Candidate

Authors: B. S. Shivaram, J. Prestigiacomo, Aini Xu, Zhenyuan Zeng, Trevor D. Ford, Itamar Kimchi, Shiliang Li, Patrick A. Lee

Abstract: Finding distinct signatures of a quantum spin liquid (QSL) is an ongoing quest in condensed matter physics, invariably complicated by the presence of disorder in real materials. In this regard the 2D Kagome system YCu$_3$(OH)$_6$[(Cl$_x$Br$_{(1-x)}$)$_{3-y}$(OH)$_y$] (YCOB-Cl), where the vast mismatch in size of Y and Cu avoids subsitutional disorder, otherwise present in kagome materials, has eme… ▽ More Finding distinct signatures of a quantum spin liquid (QSL) is an ongoing quest in condensed matter physics, invariably complicated by the presence of disorder in real materials. In this regard the 2D Kagome system YCu$_3$(OH)$_6$[(Cl$_x$Br$_{(1-x)}$)$_{3-y}$(OH)$_y$] (YCOB-Cl), where the vast mismatch in size of Y and Cu avoids subsitutional disorder, otherwise present in kagome materials, has emerged as a favorable candidate. In crystals of this system, with $x<$ 0.4 and no long range order, we report an unusual field dependent magnetization $M(B)$, where $M/B$ changes linearly with $|B|$, the absolute value of the field, in contrast to the expected quadratic behavior. Model calculations with a distribution of ferromagnetic (FM) clusters faithfully capture observed features suggesting such clusters to be intrinsic to real QSL materials. YCOB-Cl has a field enhanced $T^2$ heat capacity as expected for a Dirac QSL but lacks a linear $T$ behavior in the spin susceptibility. By demonstrating that FM clusters dominate the contribution to the susceptibility but not the heat capacity, our work paves the way towards reconciling the apparent inconsistency with a Dirac QSL. △ Less

Submitted 19 January, 2024; originally announced January 2024.

Comments: 6 pages and 4 figures

Pushing the Pareto front of band gap and permittivity: ML-guided search for dielectric materials

Authors: Janosh Riebesell, T. Wesley Surta, Rhys Goodall, Michael Gaultois, Alpha A Lee

Abstract: Materials with high-dielectric constant easily polarize under external electric fields, allowing them to perform essential functions in many modern electronic devices. Their practical utility is determined by two conflicting properties: high dielectric constants tend to occur in materials with narrow band gaps, limiting the operating voltage before dielectric breakdown. We present a high-throughpu… ▽ More Materials with high-dielectric constant easily polarize under external electric fields, allowing them to perform essential functions in many modern electronic devices. Their practical utility is determined by two conflicting properties: high dielectric constants tend to occur in materials with narrow band gaps, limiting the operating voltage before dielectric breakdown. We present a high-throughput workflow that combines element substitution, ML pre-screening, ab initio simulation and human expert intuition to efficiently explore the vast space of unknown materials for potential dielectrics, leading to the synthesis and characterization of two novel dielectric materials, CsTaTeO6 and Bi2Zr2O7. Our key idea is to deploy ML in a multi-objective optimization setting with concave Pareto front. While usually considered more challenging than single-objective optimization, we argue and show preliminary evidence that the $1/x$-correlation between band gap and permittivity in fact makes the task more amenable to ML methods by allowing separate models for band gap and permittivity to each operate in regions of good training support while still predicting materials of exceptional merit. To our knowledge, this is the first instance of successful ML-guided multi-objective materials optimization achieving experimental synthesis and characterization. CsTaTeO6 is a structure generated via element substitution not present in our reference data sources, thus exemplifying successful de-novo materials design. Meanwhile, we report the first high-purity synthesis and dielectric characterization of Bi2Zr2O7 with a band gap of 2.27 eV and a permittivity of 20.5, meeting all target metrics of our multi-objective search. △ Less

Submitted 11 January, 2024; originally announced January 2024.

Comments: 27 pages, 11 figures, 5 authors

Generation of gauge magnetic fields in a kagome spin liqud candidate using the Dzyaloshinskii-Moriya interaction

Authors: Byungmin Kang, Patrick A. Lee

Abstract: The recent discovery of magnetization oscillations in a kagome spin liquid candidate motivates us to examine the origin of the gauge magnetic field term that can give rise to quantum oscillations of fermionic spinons. We find that in the presence of the Dzyaloshinskii-Moriya interaction and an average spin polarization, the spin permutation operator around the unit cell acquires an imaginary part,… ▽ More The recent discovery of magnetization oscillations in a kagome spin liquid candidate motivates us to examine the origin of the gauge magnetic field term that can give rise to quantum oscillations of fermionic spinons. We find that in the presence of the Dzyaloshinskii-Moriya interaction and an average spin polarization, the spin permutation operator around the unit cell acquires an imaginary part, and a net gauge flux is generated through the unit cell of the kagome lattice. This new mechanism of gauge field generation can account for the strength of the gauge magnetic field needed to explain the experiment. △ Less

Submitted 3 January, 2024; originally announced January 2024.

Comments: 7 pages, 3 figures

Excitations and phase ordering of the spin-stripe phase of a binary dipolar condensate

Authors: Au-Chen Lee, D. Baillie, P. B. Blakie

Abstract: We consider the ground states, excitations and dynamics of a quasi-two-dimensional binary dipolar Bose-Einstein condensate. Our focus is on the transition to a spin-stripe ground state in which the translational invariance is spontaneously broken by a striped immiscible pattern of the alternating components. We develop a ground state phase diagram showing the parameter regime where the spin-stripe… ▽ More We consider the ground states, excitations and dynamics of a quasi-two-dimensional binary dipolar Bose-Einstein condensate. Our focus is on the transition to a spin-stripe ground state in which the translational invariance is spontaneously broken by a striped immiscible pattern of the alternating components. We develop a ground state phase diagram showing the parameter regime where the spin-stripe state occurs. Using Bogoliubov theory we calculate the excitation spectrum and structure factors. We identify a balanced regime where the system has a $\mathbb{Z}_2$ symmetry, and in the spin-stripe state this yields a nonsymmorphic symmetry. We consider the evolution of the system following a quench from the uniform to spin-stripe state, revealing novel ordering dynamics involving defects of the stripe order. Using an order parameter to characterize the orientational order of the stripes, we show that the phase ordering exhibits dynamic scaling. △ Less

Submitted 19 December, 2023; originally announced December 2023.

Comments: 11 pages, 7 figures

Journal ref: Phys. Rev. A 109, 023323 (2024)

Multi-year Study of Environmental Stability of Ti$_3$C$_2$T$_x$ MXene Films

Authors: Asaph Lee, Mikhail Shekhirev, Mark Anayee, Yury Gogotsi

Abstract: MXenes are a family of two-dimensional (2D) carbides and nitrides that display extraordinary electrical, optical, chemical, and electrochemical properties. There is a perception that MXenes are unstable and degrade quickly, limiting potential applications and requiring specific storage conditions to last for a long time. This was true for delaminated MXenes flakes in dilute dispersions prepared fr… ▽ More MXenes are a family of two-dimensional (2D) carbides and nitrides that display extraordinary electrical, optical, chemical, and electrochemical properties. There is a perception that MXenes are unstable and degrade quickly, limiting potential applications and requiring specific storage conditions to last for a long time. This was true for delaminated MXenes flakes in dilute dispersions prepared from defective precursors when MXene research was in its infancy. Since then, significant developments in MXene synthesis, processing, and understanding of its chemistry led to dramatic increases in environmental stability. Herein, we analyze Ti$_3$C$_2$T$_x$ free-standing films aged from 4 to 9 years through structural and morphological characterization along with electrical conductivity measurements to reveal the effect, or lack thereof, of prolonged storage under ambient conditions. Further, we show that the decrease in electronic conductivity over time is largely caused by the uptake of water by the hydrophilic surface chemistry of MXenes, which can be easily removed and its effect reversed by vacuum annealing. △ Less

Submitted 18 December, 2023; originally announced December 2023.

High Absorptivity Nanotextured Powders for Additive Manufacturing

Authors: Ottman A. Tertuliano, Philip J. DePond, Andrew C. Lee, Jiho Hong, David Doan, Luc Capaldi, Mark Brongersma, X. Wendy Gu, Manyalibo J. Matthews, Wei Cai, Adrian J. Lew

Abstract: The widespread application of metal additive manufacturing (AM) is limited by the ability to control the complex interactions between the energy source and the feedstock material. Here we develop a generalizable process to introduce nanoscale grooves to the surface of metal powders which increases the powder absorptivity by up to 70% during laser powder bed fusion. Absorptivity enhancements in cop… ▽ More The widespread application of metal additive manufacturing (AM) is limited by the ability to control the complex interactions between the energy source and the feedstock material. Here we develop a generalizable process to introduce nanoscale grooves to the surface of metal powders which increases the powder absorptivity by up to 70% during laser powder bed fusion. Absorptivity enhancements in copper, copper-silver, and tungsten enables energy efficient manufacturing, with printing of pure copper at relative densities up to 92% using laser energy densities as low as 82 J/mm^3. Simulations show the enhanced powder absorptivity results from plasmon-enabled light concentration in nanoscale grooves combined with multiple scattering events. The approach taken here demonstrates a general method to enhance the absorptivity and printability of reflective and refractory metal powders by changing the surface morphology of the feedstock without altering its composition. △ Less

Submitted 8 December, 2023; originally announced December 2023.

Excitations of a binary dipolar supersolid

Authors: W. Kirkby, Au-Chen Lee, D. Baillie, T. Bland, F. Ferlaino, P. B. Blakie, R. N. Bisset

Abstract: We predict a rich excitation spectrum of a binary dipolar supersolid in a linear crystal geometry, where the ground state consists of two partially immiscible components with alternating, interlocking domains. We identify three Goldstone branches, each with first-sound, second-sound or spin-sound character. In analogy with a diatomic crystal, the resulting lattice has a two-domain primitive basis… ▽ More We predict a rich excitation spectrum of a binary dipolar supersolid in a linear crystal geometry, where the ground state consists of two partially immiscible components with alternating, interlocking domains. We identify three Goldstone branches, each with first-sound, second-sound or spin-sound character. In analogy with a diatomic crystal, the resulting lattice has a two-domain primitive basis and we find that the crystal (first-sound-like) branch is split into optical and acoustic phonons. We also find a spin-Higgs branch that is associated with the supersolid modulation amplitude. △ Less

Submitted 5 September, 2024; v1 submitted 6 December, 2023; originally announced December 2023.

Comments: 12 pages, 6 figures

Journal ref: Phys. Rev. Lett. 133, 103401 (2024)

A case study of multi-modal, multi-institutional data management for the combinatorial materials science community

Authors: Sarah I. Allec, Eric S. Muckley, Nathan S. Johnson, Christopher K. H. Borg, Dylan J. Kirsch, Joshua Martin, Rohit Pant, Ichiro Takeuchi, Andrew S. Lee, James E. Saal, Logan Ward, Apurva Mehta

Abstract: Although the convergence of high-performance computing, automation, and machine learning has significantly altered the materials design timeline, transformative advances in functional materials and acceleration of their design will require addressing the deficiencies that currently exist in materials informatics, particularly a lack of standardized experimental data management. The challenges asso… ▽ More Although the convergence of high-performance computing, automation, and machine learning has significantly altered the materials design timeline, transformative advances in functional materials and acceleration of their design will require addressing the deficiencies that currently exist in materials informatics, particularly a lack of standardized experimental data management. The challenges associated with experimental data management are especially true for combinatorial materials science, where advancements in automation of experimental workflows have produced datasets that are often too large and too complex for human reasoning. The data management challenge is further compounded by the multi-modal and multi-institutional nature of these datasets, as they tend to be distributed across multiple institutions and can vary substantially in format, size, and content. To adequately map a materials design space from such datasets, an ideal materials data infrastructure would contain data and metadata describing i) synthesis and processing conditions, ii) characterization results, and iii) property and performance measurements. Here, we present a case study for the low-barrier development of such a dashboard that enables standardized organization, analysis, and visualization of a large data lake consisting of combinatorial datasets of synthesis and processing conditions, X-ray diffraction patterns, and materials property measurements generated at several different institutions. While this dashboard was developed specifically for data-driven thermoelectric materials discovery, we envision the adaptation of this prototype to other materials applications, and, more ambitiously, future integration into an all-encompassing materials data management infrastructure. △ Less

Submitted 6 February, 2024; v1 submitted 16 November, 2023; originally announced November 2023.

Low-energy electronic interactions in ferrimagnetic Sr2CrReO6 thin films

Authors: Guillaume Marcaud, Alex Taekyung Lee, Adam J. Hauser, F. Y. Yang, Sangjae Lee, Diego Casa, Mary Upton, Thomas Gog, Kayahan Saritas, Yilin Wang, Mark P. M. Dean, Hua Zhou, Zhan Zhang, F. J. Walker, Ignace Jarrige, Sohrab Ismail-Beigi, Charles Ahn

Abstract: We reveal in this study the fundamental low-energy landscape in the ferrimagnetic Sr2CrReO6 double perovskite and describe the underlying mechanisms responsible for the three low-energy excitations below 1.4 eV. Based on resonant inelastic x-ray scattering and magnetic dynamics calculations, and experiments collected from both Sr2CrReO6 powders and epitaxially strained thin films, we reveal a stro… ▽ More We reveal in this study the fundamental low-energy landscape in the ferrimagnetic Sr2CrReO6 double perovskite and describe the underlying mechanisms responsible for the three low-energy excitations below 1.4 eV. Based on resonant inelastic x-ray scattering and magnetic dynamics calculations, and experiments collected from both Sr2CrReO6 powders and epitaxially strained thin films, we reveal a strong competition between spin-orbit coupling, Hund's coupling, and the strain-induced tetragonal crystal field. We also demonstrate that a spin-flip process is at the origin of the lowest excitation at 200 meV, and we bring insights into the predicted presence of orbital ordering in this material. We study the nature of the magnons through a combination of ab initio and spin-wave theory calculations, and show that two nondegenerate magnon bands exist and are dominated either by rhenium or chromium spins. The rhenium band is found to be flat at about 200 meV ($\pm$25 meV) through X-L-W-U high-symmetry points and is dispersive toward $Γ$ △ Less

Submitted 17 October, 2023; originally announced October 2023.

Comments: 6 figures

Journal ref: Phys. Rev. B 108, 075132 (2023)

Unconventional Magnetic Oscillations in Kagome Mott Insulators

Authors: Guoxin Zheng, Yuan Zhu, Kuan-Wen Chen, Byungmin Kang, Dechen Zhang, Kaila Jenkins, Aaron Chan, Zhenyuan Zeng, Aini Xu, Oscar A. Valenzuela, Joanna Blawat, John Singleton, Patrick A. Lee, Shiliang Li, Lu Li

Abstract: In metals, electrons in a magnetic field undergo cyclotron motion, leading to oscillations in physical properties called quantum oscillations. This phenomenon has never been seen in a robust insulator because there are no mobile electrons. We report the first exception to this rule. We study a Mott insulator on a kagome lattice which does not order magnetically down to milli-Kelvin temperatures de… ▽ More In metals, electrons in a magnetic field undergo cyclotron motion, leading to oscillations in physical properties called quantum oscillations. This phenomenon has never been seen in a robust insulator because there are no mobile electrons. We report the first exception to this rule. We study a Mott insulator on a kagome lattice which does not order magnetically down to milli-Kelvin temperatures despite antiferromagnetic interactions. We observe a plateau at magnetization equal to 1/9 Bohr magneton per magnetic ion, accompanied by oscillations in the magnetic torque, reminiscent of quantum oscillations in metals. The temperature dependence obeys Fermi distribution. These phenomena are consistent with a quantum spin liquid state whose excitations are fermionic spinons with a Dirac-like spectrum coupled to an emergent gauge field. △ Less

Submitted 7 December, 2024; v1 submitted 11 October, 2023; originally announced October 2023.

Comments: 5 pages and 5 figures in the main text, 11 additional figures and 1 table in the supplement

Journal ref: PNAS, 122, e2421390122 (2025)

Spin-Mediated Direct Photon Scattering by Plasmons in BiTeI

Authors: A. C. Lee, S. Sarkar, K. Du, H. -H. Kung, C. J. Won, K. Wang, S. -W. Cheong, S. Maiti, G. Blumberg

Abstract: We use polarization resolved Raman spectroscopy to demonstrate that for a 3D giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasi-momentum and thus negligibly weak in the Raman spectra, w… ▽ More We use polarization resolved Raman spectroscopy to demonstrate that for a 3D giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasi-momentum and thus negligibly weak in the Raman spectra, we observe a sharp in-gap plasmon mode in the Raman spectrum of BiTeI below the Rashba continuum. This coupling, in a polar system with spin-orbit coupling, occurs without assistance from phonons when the incoming photon excitation is resonant with Rashba-split intermediate states. We discuss the distinctive features of BiTeI's giant Rashba system band structure that enable the direct observation of plasmon in Raman scattering. △ Less

Submitted 18 February, 2024; v1 submitted 6 October, 2023; originally announced October 2023.

Comments: Editors' Suggestion

Journal ref: Phys. Rev. B 109, L041111 (2024)

Two-carrier description of cuprate superconductors from NMR

Authors: Daniel Bandur, Abigail Lee, Jakob Nachtigal, Stefan Tsankov, Juergen Haase

Abstract: Cuprates currently hold the record for the highest temperature superconductivity at ambient pressure, but the microscopic understanding of these materials remains elusive. Here we utilize nuclear magnetic resonance (NMR) data of planar oxygen and copper from essentially all hole-doped cuprates to provide a universal phenomenology relating the NMR spin shifts, which measure the electronic spin pola… ▽ More Cuprates currently hold the record for the highest temperature superconductivity at ambient pressure, but the microscopic understanding of these materials remains elusive. Here we utilize nuclear magnetic resonance (NMR) data of planar oxygen and copper from essentially all hole-doped cuprates to provide a universal phenomenology relating the NMR spin shifts, which measure the electronic spin polarization at a given nucleus, with the superconducting dome and maximum critical temperature. We demonstrate that there are two separate contributions to the spin shift at planar copper, only one of which is seen at oxygen, and associate them with two different carrier types. Upon disentangling these two components, their relative size is shown to determine not only the doping dependence of the superconducting dome, but also the variation in maximum superconducting critical temperature, $T_\mathrm{c}$, between different families. One of these components is independent of family and resides in the hybridized planar orbitals. The second component, in contrast, has a more three-dimensional character and encodes the differences between the families. It is thus related to the charge transfer gap and planar hole sharing. Our findings offer a key, universal insight which should prove useful in the continuing development of a comprehensive theory of the cuprates, as well as an indication of how it may be possible to engineer materials with higher critical temperatures. △ Less

Submitted 3 June, 2025; v1 submitted 21 September, 2023; originally announced September 2023.

Comments: 20 pages, 4 figures

First-principle Study of Multiple Metastable Charge Ordering States in La$_{1/3}$Sr$_{2/3}$FeO$_{3}$

Authors: Nam Nguyen, Alex Taekyung Lee, Vijay Singh, Anh T. Ngo, Hyowon Park

Abstract: La doped SrFeO$_{3}$, La$_{1/3}$Sr$_{2/3}$FeO$_{3}$, exhibits a metal-to-insulator transition accompanied by both antiferromagnetic and charge ordering states along with the Fe-O bond disproportionation below a critical temperature near 200K. Unconventionally slow charge dynamics measured in this material near the critical temperature shows that its excited charge ordering states can exhibit novel… ▽ More La doped SrFeO$_{3}$, La$_{1/3}$Sr$_{2/3}$FeO$_{3}$, exhibits a metal-to-insulator transition accompanied by both antiferromagnetic and charge ordering states along with the Fe-O bond disproportionation below a critical temperature near 200K. Unconventionally slow charge dynamics measured in this material near the critical temperature shows that its excited charge ordering states can exhibit novel electronic structures with nontrivial energy profiles. Here, we reveal possible metastable states of charge ordering structures in La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ using the first-principle and climbing image nudged elastic band methods. In the strong correlation regime, La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ is an antiferromagnetic insulator with a charge ordering state of the big-small-big pattern, consistent with the experimental measurement of this material at the low temperature. As the correlation effect becomes weak, we find at least two possible metastable charge ordering states with the distinct Fe-O bond disproportionation. Remarkably, a ferroelectric metallic state emerges with the small energy barrier of $\sim$7 meV, driven by a metastable CO state of the small-medium-big pattern. The electronic structures of these metastable charge ordering states are noticeably different from those of the ground-state. Our results can provide an insightful explanation to multiple metastable charge ordering states and the slow charge dynamics of this and related oxide materials. △ Less

Submitted 24 June, 2025; v1 submitted 7 September, 2023; originally announced September 2023.

Comments: The paper has 8 pages and 7 figures

Journal ref: Physical Review MATERIALS 8, 014404 (2024)

Probing quantum spin liquids with a quantum twisting microscope

Authors: Valerio Peri, Shahal Ilani, Patrick A. Lee, Gil Refael

Abstract: The experimental characterization of quantum spin liquids poses significant challenges due to the absence of long-range magnetic order, even at absolute zero temperature. The identification of these states of matter often relies on the analysis of their excitations. In this paper, we propose a method for detecting the signatures of the fractionalized excitations in quantum spin liquids using a tun… ▽ More The experimental characterization of quantum spin liquids poses significant challenges due to the absence of long-range magnetic order, even at absolute zero temperature. The identification of these states of matter often relies on the analysis of their excitations. In this paper, we propose a method for detecting the signatures of the fractionalized excitations in quantum spin liquids using a tunneling spectroscopy setup. Inspired by the recent development of the quantum twisting microscope, we consider a planar tunneling junction, in which a candidate quantum spin liquid material is placed between two graphene layers. By tuning the relative twist angle and voltage bias between the leads, we can extract the dynamical spin structure factor of the tunneling barrier with momentum and energy resolution. Our proposal presents a promising tool for experimentally characterizing quantum spin liquids in two-dimensional materials. △ Less

Submitted 21 January, 2024; v1 submitted 29 August, 2023; originally announced August 2023.

Comments: 18 pages, 3 figures

Matbench Discovery -- A framework to evaluate machine learning crystal stability predictions

Authors: Janosh Riebesell, Rhys E. A. Goodall, Philipp Benner, Yuan Chiang, Bowen Deng, Gerbrand Ceder, Mark Asta, Alpha A. Lee, Anubhav Jain, Kristin A. Persson

Abstract: The rapid adoption of machine learning (ML) in domain sciences necessitates best practices and standardized benchmarking for performance evaluation. We present Matbench Discovery, an evaluation framework for ML energy models, applied as pre-filters for high-throughput searches of stable inorganic crystals. This framework addresses the disconnect between thermodynamic stability and formation energy… ▽ More The rapid adoption of machine learning (ML) in domain sciences necessitates best practices and standardized benchmarking for performance evaluation. We present Matbench Discovery, an evaluation framework for ML energy models, applied as pre-filters for high-throughput searches of stable inorganic crystals. This framework addresses the disconnect between thermodynamic stability and formation energy, as well as retrospective vs. prospective benchmarking in materials discovery. We release a Python package to support model submissions and maintain an online leaderboard, offering insights into performance trade-offs. To identify the best-performing ML methodologies for materials discovery, we benchmarked various approaches, including random forests, graph neural networks (GNNs), one-shot predictors, iterative Bayesian optimizers, and universal interatomic potentials (UIP). Our initial results rank models by test set F1 scores for thermodynamic stability prediction: EquiformerV2 + DeNS > Orb > SevenNet > MACE > CHGNet > M3GNet > ALIGNN > MEGNet > CGCNN > CGCNN+P > Wrenformer > BOWSR > Voronoi fingerprint random forest. UIPs emerge as the top performers, achieving F1 scores of 0.57-0.82 and discovery acceleration factors (DAF) of up to 6x on the first 10k stable predictions compared to random selection. We also identify a misalignment between regression metrics and task-relevant classification metrics. Accurate regressors can yield high false-positive rates near the decision boundary at 0 eV/atom above the convex hull. Our results demonstrate UIPs' ability to optimize computational budget allocation for expanding materials databases. However, their limitations remain underexplored in traditional benchmarks. We advocate for task-based evaluation frameworks, as implemented here, to address these limitations and advance ML-guided materials discovery. △ Less

Submitted 10 December, 2024; v1 submitted 28 August, 2023; originally announced August 2023.

Comments: Please see online leaderboard at: https://matbench-discovery.materialsproject.org/

Effective model for Pb$_9$Cu(PO$_4$)$_6$O

Authors: Patrick A. Lee, Zhehao Dai

Abstract: The copper substituted Pb-apatite has attracted a great deal of attention recently, due to the claim of the observation of room temperature superconductivity. Based on LDA calculations in the literature, we propose an effective model that describe the low energy physics. It consists of stacks of buckled honeycomb lattices, with Cu and O occupying the A and B sites respectively. In addition to the… ▽ More The copper substituted Pb-apatite has attracted a great deal of attention recently, due to the claim of the observation of room temperature superconductivity. Based on LDA calculations in the literature, we propose an effective model that describe the low energy physics. It consists of stacks of buckled honeycomb lattices, with Cu and O occupying the A and B sites respectively. In addition to the narrow Cu bands that have been emphasized, we call attention to the relatively small energy separation between the Cu and O orbitals. Thus despite the small hoping energies, the model may be in an interesting regime near the metal insulator transition driven by the charge transfer mechanism. Relationship with cuprates and the organic superconductors are discussed. △ Less

Submitted 13 August, 2023; v1 submitted 8 August, 2023; originally announced August 2023.