Showing 1–50 of 71 results for author: Hahn, H

Epitaxial CeO2 Films as a Host for Quantum Applications

Authors: Pralay Paul, Kusal M. Abeywickrama, Nisha Geng, Mritunjaya Parashar, Levi Brown, Mohin Sharma, Darshpreet Kaur Saini, Melissa Ayala Artola, Todd A. Byers, Bibhudutta Rout, Yiwei Ju, Xiaoqing Pan, Sumit Goswami, Sreehari Puthan Purayil, Casey Kerr, Dhiman Biswas, Ben Summers, Bin Wang, Horst Hahn, Alisa Javadi, T. Venkatesan

Abstract: In highly purified host, the coherence of quantum emitters is ultimately limited by hyperfine interactions between the emitter and lattice nuclei possessing non-zero nuclear magnetic moments. This limitation can only be mitigated through isotopic purification. In this work, we investigate CeO2 as a host composed entirely of nuclei with zero nuclear moment. High-quality CeO2 thin films were grown b… ▽ More In highly purified host, the coherence of quantum emitters is ultimately limited by hyperfine interactions between the emitter and lattice nuclei possessing non-zero nuclear magnetic moments. This limitation can only be mitigated through isotopic purification. In this work, we investigate CeO2 as a host composed entirely of nuclei with zero nuclear moment. High-quality CeO2 thin films were grown by PLD and doped with Tm and Er ions. Structural characterization using X-ray diffraction, atomic force microscopy, and ion channeling confirms single-crystalline, atomically smooth films with dopants substitutionally incorporated at Ce lattice sites. Photoluminescence lifetime measurements show significantly longer lifetimes for Er-doped CeO2 (2.9 - 5.3 ms) compared with Tm-doped films (14 - 68 μs). Moreover, the Er-doped PLD films exhibit longer lifetimes at ~1% dopant concentration than previously reported for MBE-grown films. Density functional theory calculations reveal a substantial overlap between unoccupied O 2p and Tm 4f states near the valence band maximum, whereas Er 4f states remain well isolated. This electronic interaction likely introduces non-radiative recombination pathways in Tm-doped CeO2, explaining the reduced lifetimes. These findings highlight the importance of selecting appropriate dopant-host combinations and optimized growth conditions to minimize non-radiative channels for quantum applications. △ Less

Submitted 26 March, 2026; originally announced March 2026.

Pulsed Laser Template Engineering- PLATEN

Authors: Dhiman Biswas, Junyeob Song, Francisco Guzman, Levi Brown, Yiwei Ju, Nisha Geng, Pralay Paul, Sumit Goswami, Casey Kerr, Sreehari Puthan Purayil, Ben Summers, Preston Larson, Binbin Weng, Bin Wang, Horst Hahn, Xiaoxing Pan, Alisa Javadi, Henri Lezec, Thirumalai Venkatesan

Abstract: Thin films of functional inorganic materials, particularly oxides, play a vital role in optoelectronics, enabling applications that range from active optical components to MEMS-based architectures. Achieving high aspect ratio patterning of these functional materials remains a significant challenge, as many of their constituent elements do not readily form volatile compounds required for convention… ▽ More Thin films of functional inorganic materials, particularly oxides, play a vital role in optoelectronics, enabling applications that range from active optical components to MEMS-based architectures. Achieving high aspect ratio patterning of these functional materials remains a significant challenge, as many of their constituent elements do not readily form volatile compounds required for conventional reactive ion etch processes. We introduce a novel approach, Pulsed Laser Template ENgineering (PLATEN), which offers a more accessible route for patterning materials that are typically difficult to etch. This technique involves depositing functional films using the Pulsed Laser Deposition (PLD) process onto silicon substrates that have been pre-patterned using reactive ion etching to create high aspect ratio features. Due to the highly forward-directed nature of the PLD process, the deposited films replicate closely the topography of the patterned silicon, without coatings the sidewalls. This process remains effective even at feature sizes down to approximately 50 nm. The oxide films replicate the underlying silicon pattern to a thickness of 80 nm. For thickness beyond 80 nm the patterns develop a waist at the midpoint which scales with film thickness and is not dependent on the feature size. In this paper, we present a detailed analysis of the PLATEN process, including deviations from ideal pattern replication in sub-micron features as a function of film thickness, and demonstrate near single crystalline growth of oxides on the patterned silicon substrate, demonstrating the potential of PLATEN technique for active opto-electronic materials. △ Less

Submitted 26 March, 2026; originally announced March 2026.

Fragmenting Diffusion Pathways Confers Extraordinary Radiation Resistance in Refractory Multicomponent Alloys

Authors: Bin Xing, Bijun Xie, Wanjuan Zou, Eric Lang, Evgeniy Boltynjuk, Hangman Chen, Michael P Short, George Tynan, Timothy J Rupert, Jason Trelewicz, Horst Hahn, Blas P Uberuaga, Khalid Hattar, Penghui Cao

Abstract: The accumulation and growth of vacancy clusters under irradiation is a pivotal degradation mode for structural materials in extreme environments. Even tungsten undergoes rapid defect coarsening compromising its integrity. Here we show a tungsten multicomponent alloy that effectively fragments the vacancy diffusion network, kinetically trapping defects within localized domains. This effect originat… ▽ More The accumulation and growth of vacancy clusters under irradiation is a pivotal degradation mode for structural materials in extreme environments. Even tungsten undergoes rapid defect coarsening compromising its integrity. Here we show a tungsten multicomponent alloy that effectively fragments the vacancy diffusion network, kinetically trapping defects within localized domains. This effect originates from a broad spectrum of migration barriers and substantial vacancy-jump heterogeneity, which drives the interconnectivity of diffusion paths below the percolation threshold. Starving clusters of the necessary vacancy supply, irradiation experiments and atomic-scale defect characterizations confirm negligible defect growth as radiation doses increase by four orders of magnitude. These results provide a fundamental paradigm for percolation-engineered kinetics, offering a predictive pathway for tailoring defect diffusion and discovering inherently radiation-tolerant materials. △ Less

Submitted 3 March, 2026; originally announced March 2026.

A voltage-responsive strongly dipolar-coupled macrospin network with emergent dynamics for computing

Authors: Xinglong Ye, Zhibo Zhao, Qian Wang, Jiangnan Li, Fernando Maccari, Ning Lu, Christian Dietz, Esmaeil Adabifiroozjaei, Leopoldo Molina-Luna, Yufeng Tian, Lihui Bai, Guodong Wang, Konstantin Skokov, Yanxue Chen, Shishen Yan, Robert Kruk, Horst Hahn, Oliver Gutfleisch

Abstract: Emergent behavior, which arises from local interactions between simple elements, is pervasive in nature. It underlies the energy-efficient computing in our brains. However, realizing such dynamics in artificial materials, particularly under low-energy stimuli, remains a fundamental challenge. While dipole-dipole interactions are typically suppressed in magnetic storage, here we harness and amplify… ▽ More Emergent behavior, which arises from local interactions between simple elements, is pervasive in nature. It underlies the energy-efficient computing in our brains. However, realizing such dynamics in artificial materials, particularly under low-energy stimuli, remains a fundamental challenge. While dipole-dipole interactions are typically suppressed in magnetic storage, here we harness and amplify them to construct a strongly dipolar-coupled network of SmCo5 macrospins at wafer scale, which can exhibit intrinsic interaction-driven collective dynamics in response to voltage pulses. The network combines three essential ingredients: strong dipolar coupling by large single-domain macrospin, giant voltage control of coercivity over nearly 1000-fold, and disordered network topology with frustrated Ising-like energy landscape. When stimulated by 1 V pulses, the network enters a regime where interaction-driven magnetic behaviors emerge, including spontaneous demagnetization, greatly enhanced magnetization modulation, reversible freeze and resume evolution and stochastic convergence toward low-energy magnetic configurations. All these behaviors are completely absent at the single-nanomagnet level. Furthermore, by constructing micromagnetic models of the strongly dipolar-coupled macrospin networks, we show that the resulting nonlinear, high-dimensional collective dynamics, intrinsic to strongly-interacting systems, can enable accurate chaotic Mackey-Glass prediction and multiclass drone-signal classification. Our work establishes the voltage-responsive strongly-coupled SmCo5 network as a mesoscopic platform for probing emergent magnetic dynamics previously inaccessible under ambient conditions. It also suggests a fundamental distinct route towards scalable, low-voltage computing, one rooted in native physical interaction-driven collective dynamics at the network level. △ Less

Submitted 25 December, 2025; v1 submitted 23 December, 2025; originally announced December 2025.

High-resolution valence band RIXS at the actinide M$_{4,5}$-edges

Authors: Martin Sundermann, Henrik Hahn, Denise S. Christovam, Maurits W. Haverkort, Roberto Caciuffo, Bernhard Keimer, Liu Hao Tjeng, Andrea Severing, Hlynur Gretarsson

Abstract: Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence-band resonant inelastic x-ray spectrosco… ▽ More Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence-band resonant inelastic x-ray spectroscopy (VB-RIXS) experiments at the uranium M$_{4,5}$ edges (3551 and 3725\,eV). We present data of UO$_2$, a well-established model actinide compound. VB-RIXS is particularly well suited for probing the 5f-shell electronic structure, as it probes, in contrast to core-to-core RIXS, excitations without leaving a high-energy core hole in the final state. In VB-RIXS, we achieve energy resolutions of 50\,meV (M$_5$) and 90\,meV (M$_4$), enabling the resolution of multiplet excitations and crystal-field effects, as well as charge-transfer and fluorescence-like features with unprecedented clarity. As such, high resolution VB-RIXS offers direct insights into both low-energy, near ground-state properties and high-energy hybridization and covalency effects. Our results demonstrate the power of VB-RIXS as a versatile and powerful tool for probing the strongly correlated electronic structure of actinide materials, providing essential input for quantitative modeling and the validation of theoretical concepts. △ Less

Submitted 27 October, 2025; v1 submitted 22 September, 2025; originally announced September 2025.

Comments: 10 pages, 6 figures

Journal ref: Physical Review Research 7, 043081 (2025)

Triple product $L$-functions and the Ramanujan conjecture

Authors: Jayce R. Getz, Heekyoung Hahn, HaoYun Yao

Abstract: We prove that the Ramanujan conjecture is true under the assumption that the expected analytic properties of triple product $L$-functions hold. Further, we explain how these analytic properties imply certain reduction steps in the construction of functorial transfers in the sense of Langlands. Roughly, at the level of stably automorphic representations, they allow one to reduce any functorial tran… ▽ More We prove that the Ramanujan conjecture is true under the assumption that the expected analytic properties of triple product $L$-functions hold. Further, we explain how these analytic properties imply certain reduction steps in the construction of functorial transfers in the sense of Langlands. Roughly, at the level of stably automorphic representations, they allow one to reduce any functorial transfer from a given reductive group $G$ to a general linear group to a finite family of transfers depending on $G.$ △ Less

Submitted 17 September, 2025; originally announced September 2025.

Comments: welcome!

MSC Class: Primary 11F70; Secondary 11F66

Predicting New Research Directions in Materials Science using Large Language Models and Concept Graphs

Authors: Thomas Marwitz, Alexander Colsmann, Ben Breitung, Christoph Brabec, Christoph Kirchlechner, Eva Blasco, Gabriel Cadilha Marques, Horst Hahn, Michael Hirtz, Pavel A. Levkin, Yolita M. Eggeler, Tobias Schlöder, Pascal Friederich

Abstract: Due to an exponential increase in published research articles, it is impossible for individual scientists to read all publications, even within their own research field. In this work, we investigate the use of large language models (LLMs) for the purpose of extracting the main concepts and semantic information from scientific abstracts in the domain of materials science to find links that were not… ▽ More Due to an exponential increase in published research articles, it is impossible for individual scientists to read all publications, even within their own research field. In this work, we investigate the use of large language models (LLMs) for the purpose of extracting the main concepts and semantic information from scientific abstracts in the domain of materials science to find links that were not noticed by humans and thus to suggest inspiring near/mid-term future research directions. We show that LLMs can extract concepts more efficiently than automated keyword extraction methods to build a concept graph as an abstraction of the scientific literature. A machine learning model is trained to predict emerging combinations of concepts, i.e. new research ideas, based on historical data. We demonstrate that integrating semantic concept information leads to an increased prediction performance. The applicability of our model is demonstrated in qualitative interviews with domain experts based on individualized model suggestions. We show that the model can inspire materials scientists in their creative thinking process by predicting innovative combinations of topics that have not yet been investigated. △ Less

Submitted 23 February, 2026; v1 submitted 20 June, 2025; originally announced June 2025.

On the Encapsulation of Medical Imaging AI Algorithms

Authors: Hans Meine, Yongli Mou, Guido Prause, Horst Hahn

Abstract: In the context of collaborative AI research and development projects, it would be ideal to have self-contained encapsulated algorithms that can be easily shared between different parties, executed and validated on data at different sites, or trained in a federated manner. In practice, all of this is possible but greatly complicated, because human supervision and expert knowledge is needed to set u… ▽ More In the context of collaborative AI research and development projects, it would be ideal to have self-contained encapsulated algorithms that can be easily shared between different parties, executed and validated on data at different sites, or trained in a federated manner. In practice, all of this is possible but greatly complicated, because human supervision and expert knowledge is needed to set up the execution of algorithms based on their documentation, possibly implicit assumptions, and knowledge about the execution environment and data involved. We derive and formulate a range of detailed requirements from the above goal and from specific use cases, focusing on medical imaging AI algorithms. Furthermore, we refer to a number of existing APIs and implementations and review which aspects each of them addresses, which problems are still open, and which public standards and ontologies may be relevant. Our contribution is a comprehensive collection of aspects that have not yet been addressed in their entirety by any single solution. Working towards the formulated goals should lead to more sustainable algorithm ecosystems and relates to the FAIR principles for research data, where this paper focuses on interoperability and (re)usability of medical imaging AI algorithms. △ Less

Submitted 11 September, 2025; v1 submitted 30 April, 2025; originally announced April 2025.

Comments: v2: mention FAIR4ML, MLentory, some minor elaboration and spelling fixes

Requirements for Quality Assurance of AI Models for Early Detection of Lung Cancer

Authors: Horst K. Hahn, Matthias S. May, Volker Dicken, Michael Walz, Rainer Eßeling, Bianca Lassen-Schmidt, Robert Rischen, Jens Vogel-Claussen, Konstantin Nikolaou, Jörg Barkhausen

Abstract: Lung cancer is the second most common cancer and the leading cause of cancer-related deaths worldwide. Survival largely depends on tumor stage at diagnosis, and early detection with low-dose CT can significantly reduce mortality in high-risk patients. AI can improve the detection, measurement, and characterization of pulmonary nodules while reducing assessment time. However, the training data, fun… ▽ More Lung cancer is the second most common cancer and the leading cause of cancer-related deaths worldwide. Survival largely depends on tumor stage at diagnosis, and early detection with low-dose CT can significantly reduce mortality in high-risk patients. AI can improve the detection, measurement, and characterization of pulmonary nodules while reducing assessment time. However, the training data, functionality, and performance of available AI systems vary considerably, complicating software selection and regulatory evaluation. Manufacturers must specify intended use and provide test statistics, but they can choose their training and test data, limiting standardization and comparability. Under the EU AI Act, consistent quality assurance is required for AI-based nodule detection, measurement, and characterization. This position paper proposes systematic quality assurance grounded in a validated reference dataset, including real screening cases plus phantom data to verify volume and growth rate measurements. Regular updates shall reflect demographic shifts and technological advances, ensuring ongoing relevance. Consequently, ongoing AI quality assurance is vital. Regulatory challenges are also adressed. While the MDR and the EU AI Act set baseline requirements, they do not adequately address self-learning algorithms or their updates. A standardized, transparent quality assessment - based on sensitivity, specificity, and volumetric accuracy - enables an objective evaluation of each AI solution's strengths and weaknesses. Establishing clear testing criteria and systematically using updated reference data lay the groundwork for comparable performance metrics, informing tenders, guidelines, and recommendations. △ Less

Submitted 24 February, 2025; originally announced February 2025.

Comments: 12 pages incl. 2 figures, 2 charts, and references, summary in English (page 2), article in German (original title: Anforderungen an die Qualitätssicherung von KI-Modellen für die Lungenkrebs-Früherkennung)

ACM Class: I.2.1; J.3; K.6.4

High quality superconducting tantalum resonators with beta phase defects

Authors: Ritika Dhundhwal, Haoran Duan, Lucas Brauch, Soroush Arabi, Dirk Fuchs, Amir-Abbas Haghighirad, Alexander Welle, Florentine Scharwaechter, Sudip Pal, Marc Scheffler, José Palomo, Zaki Leghtas, Anil Murani, Horst Hahn, Jasmin Aghassi-Hagmann, Christian Kübel, Wulf Wulfhekel, Ioan M. Pop, Thomas Reisinger

Abstract: For practical superconducting quantum processors, orders of magnitude improvement in coherence is required, motivating efforts to optimize hardware design and explore new materials. Among the latter, the coherence of superconducting transmon qubits has been shown to improve by forming the qubit capacitor pads from $α$-tantalum, avoiding the meta-stable $β$-phase that forms when depositing tantalum… ▽ More For practical superconducting quantum processors, orders of magnitude improvement in coherence is required, motivating efforts to optimize hardware design and explore new materials. Among the latter, the coherence of superconducting transmon qubits has been shown to improve by forming the qubit capacitor pads from $α$-tantalum, avoiding the meta-stable $β$-phase that forms when depositing tantalum at room temperature, and has been previously identified to be a source of microwave losses. In this work, we show lumped element resonators containing $β$-phase tantalum in the form of inclusions near the metal-substrate interface with internal quality factors ($Q_\text{i}$) up to $(5.0 \pm 2.5) \times 10^6$ in the single photon regime. They outperform resonators with no sign of the $β$-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of $β$-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum based superconducting circuits. △ Less

Submitted 24 February, 2025; originally announced February 2025.

AlN/Si interface engineering to mitigate RF losses in MOCVD grown GaN-on-Si substrates

Authors: Pieter Cardinael, Sachin Yadav, Herwig Hahn, Ming Zhao, Sourish Banerjee, Babak Kazemi Esfeh, Christof Mauder, Barry O Sullivan, Uthayasankaran Peralagu, Anurag Vohra, Robert Langer, Nadine Collaert, Bertrand Parvais, Jean-Pierre Raskin

Abstract: Fabrication of low-RF loss GaN-on-Si HEMT stacks is critical to enable competitive front-end-modules for 5G and 6G applications. The main contribution to RF losses is the interface between the III-N layer and the HR Si wafer, more specifically the AlN/Si interface. At this interface, a parasitic surface conduction layer exists in Si, which decreases the substrate effective resistivity sensed by ov… ▽ More Fabrication of low-RF loss GaN-on-Si HEMT stacks is critical to enable competitive front-end-modules for 5G and 6G applications. The main contribution to RF losses is the interface between the III-N layer and the HR Si wafer, more specifically the AlN/Si interface. At this interface, a parasitic surface conduction layer exists in Si, which decreases the substrate effective resistivity sensed by overlying circuitry below the nominal Si resistivity. However, a clear understanding of this interface with control of the parasitic channel is lacking. In this letter, a detailed physical and electrical description of MOCVD-grown AlN/Si structures is presented. The presence of a $\text{SiC}_\text{x}\text{N}_\text{y}$ interfacial layer is revealed and its importance for RF losses is shown. Through C-V and I-V characterisation, an increase in the C concentration of this interfacial layer is linked to the formation of negative charge at the AlN/Si interface, which counteracts the positive charge present in the 0-predose limit. The variation of TMAl predose is shown to allow precise tuning of the C composition and, consequently, the resulting interface charge. Notably, a linear relationship between predose and net interface charge is observed and confirmed by the fabrication of an AlN/Si sample with close to zero net charge. In addition, a higher $D_{it}$ ($\sim 2\times 10^{12}$ cm$^\text{-2}$) for such compensated samples is observed and can contribute to low RF loss. An exceptionally high effective resistivity of above 8 k$Ω\cdot$cm is achieved, corresponding to an RF loss below 0.3 dB/mm at 10 GHz. △ Less

Submitted 13 August, 2024; v1 submitted 3 April, 2024; originally announced April 2024.

Comments: The following article has been accepted for publication in Applied Physics Letters. After it is published, it will be found at https://pubs.aip.org/aip/apl

Voltage-driven 90 switching of bulk perpendicular magnetic anisotropy in ferrimagnets

Authors: Zhengyu Xiao, Ruiwen Xie, Fernando Maccari, Philipp Klaassen, Benedikt Eggert, Di Wang, Yuting Dai, Raquel Lizarraga, Johanna Lill, Tom Helbig, Heiko Wende, Kurt Kummer, Katharina Ollefs, Konstantin Skokov, Hongbin Zhang, Zhiyong Quan, Xiaohong Xu, Robert Kruk, Horst Hahn, Oliver Gutfleisch, Xinglong Ye

Abstract: Unravelling the mechanism behind bulk perpendicular magnetic anisotropy (PMA) in amorphous rare earth-transition metal films has proven challenging. This is largely due to the inherent complexity of amorphous structure and the entangled potential origins arising from microstructure and atomic structure factors. Here, we present an approach wherein the magneto-electric effect is harnessed to induce… ▽ More Unravelling the mechanism behind bulk perpendicular magnetic anisotropy (PMA) in amorphous rare earth-transition metal films has proven challenging. This is largely due to the inherent complexity of amorphous structure and the entangled potential origins arising from microstructure and atomic structure factors. Here, we present an approach wherein the magneto-electric effect is harnessed to induce 90° switching of bulk PMA in Tb-Co films to in-plane directions by applying voltages of only -1.2 V. This manipulation is achieved by voltage-driven insertion of hydrogen atoms into interstitial sites between Tb and Co atoms, which serves as a perturbation to local atomic structure. Using angle-dependent X-ray magnetic circular dichroism, we find that the anisotropy switching originates from the distortion of crystal field around Tb which reorients the alignment of Tb orbital moments. Initially aligned along Tb-Co bonding directions, the easy magnetization axis undergoes reorientation and switches by 90°, as substantiated by ab-initio calculations. Our study not only concludes the atomic origin of Tb-Co atom bonding configuration in shaping bulk PMA, but also establishes the groundwork for electrically programmable ferrimagnetic spintronics, such as controlling domain wall motion and programming artificial spin textures. △ Less

Submitted 7 January, 2025; v1 submitted 1 December, 2023; originally announced December 2023.

Comments: 4 Figures in manuscript (15 pages); 16 Figures, two Tables in Supplementary materials (21 pages)

Comparison of different automatic solutions for resection cavity segmentation in postoperative MRI volumes including longitudinal acquisitions

Authors: Luca Canalini, Jan Klein, Nuno Pedrosa de Barros, Diana Maria Sima, Dorothea Miller, Horst Hahn

Abstract: In this work, we compare five deep learning solutions to automatically segment the resection cavity in postoperative MRI. The proposed methods are based on the same 3D U-Net architecture. We use a dataset of postoperative MRI volumes, each including four MRI sequences and the ground truth of the corresponding resection cavity. Four solutions are trained with a different MRI sequence. Besides, a me… ▽ More In this work, we compare five deep learning solutions to automatically segment the resection cavity in postoperative MRI. The proposed methods are based on the same 3D U-Net architecture. We use a dataset of postoperative MRI volumes, each including four MRI sequences and the ground truth of the corresponding resection cavity. Four solutions are trained with a different MRI sequence. Besides, a method designed with all the available sequences is also presented. Our experiments show that the method trained only with the T1 weighted contrast-enhanced MRI sequence achieves the best results, with a median DICE index of 0.81. △ Less

Submitted 14 October, 2022; originally announced October 2022.

Journal ref: SPIE Proceedings Vol. 11598 - Medical Imaging 2021: Image-Guided Procedures, Robotic Interventions, and Modeling

Topographic De-adhesion in the Viscoelastic Limit

Authors: Nhung Nguyen, Eugenio Hamm Hahn, Sachin Velankar, Enrique Cerda, Luka Pocivavsek

Abstract: The superiority of many natural surfaces at resisting soft, sticky biofoulants has inspired the integration of dynamic topography with mechanical instability to promote self-cleaning artificial surfaces. The physics behind this novel mechanism is currently limited to elastic biofoulants where surface energy, bending stiffness, and topographical wavelength are key factors. However, the viscoelastic… ▽ More The superiority of many natural surfaces at resisting soft, sticky biofoulants has inspired the integration of dynamic topography with mechanical instability to promote self-cleaning artificial surfaces. The physics behind this novel mechanism is currently limited to elastic biofoulants where surface energy, bending stiffness, and topographical wavelength are key factors. However, the viscoelastic nature of many biofoulants causes a complex interplay between these factors with time-dependent characteristics such as material softening and loading rate. Here, we enrich the current elastic theory of topographic de-adhesion using analytical and finite element models to elucidate the non-linear, time-dependent interaction of three physical, dimensionless parameters: biofoulant's stiffness reduction, product of relaxation time and loading rate, and the critical strain for short-term elastic de-adhesion. Theoretical predictions, in good agreement with numerical simulations, provide insight into tuning these control parameters to optimize surface renewal via topographic de-adhesion in the viscoelastic regime. △ Less

Submitted 19 September, 2022; originally announced September 2022.

Chemical order transitions within extended interfacial segregation zones in NbMoTaW

Authors: Doruk Aksoy, Megan J. McCarthy, Ian Geiger, Diran Apelian, Horst Hahn, Enrique J. Lavernia, Jian Luo, Huolin Xin, Timothy J. Rupert

Abstract: Interfacial segregation and chemical short-range ordering influence the behavior of grain boundaries in complex concentrated alloys. In this study, we use atomistic modeling of a NbMoTaW refractory complex concentrated alloy to provide insight into the interplay between these two phenomena. Hybrid Monte Carlo and molecular dynamics simulations are performed on columnar grain models to identify equ… ▽ More Interfacial segregation and chemical short-range ordering influence the behavior of grain boundaries in complex concentrated alloys. In this study, we use atomistic modeling of a NbMoTaW refractory complex concentrated alloy to provide insight into the interplay between these two phenomena. Hybrid Monte Carlo and molecular dynamics simulations are performed on columnar grain models to identify equilibrium grain boundary structures. Our results reveal extended near-boundary segregation zones that are much larger than traditional segregation regions, which also exhibit chemical patterning that bridges the interfacial and grain interior regions. Furthermore, structural transitions pertaining to an A2-to-B2 transformation are observed within these extended segregation zones. Both grain size and temperature are found to significantly alter the widths of these regions. Analysis of chemical short-range order indicates that not all pairwise elemental interactions are affected by the presence of a grain boundary equally, as only a subset of elemental clustering types are more likely to reside near certain boundaries. The results emphasize the increased chemical complexity that is associated with near-boundary segregation zones and demonstrate the unique nature of interfacial segregation in complex concentrated alloys. △ Less

Submitted 19 November, 2022; v1 submitted 24 August, 2022; originally announced August 2022.

Tailoring epitaxial growth and magnetism in La1-xSrxMnO3 / SrTiO3 heterostructures via temperature-driven defect engineering

Authors: Alan Molinari, Saleh Gorji, Jan Michalička, Christian Kübel, Horst Hahn, Robert Kruk

Abstract: Among the class of strongly-correlated oxides, La1-xSrxMnO3 $-$ a half metallic ferromagnet with a Curie temperature above room temperature $-$ has sparked a huge interest as a functional building block for memory storage and spintronic applications. In this respect, defect engineering has been in the focus of a long-standing quest for fabricating LSMO thin films with highest quality in terms of b… ▽ More Among the class of strongly-correlated oxides, La1-xSrxMnO3 $-$ a half metallic ferromagnet with a Curie temperature above room temperature $-$ has sparked a huge interest as a functional building block for memory storage and spintronic applications. In this respect, defect engineering has been in the focus of a long-standing quest for fabricating LSMO thin films with highest quality in terms of both structural and magnetic properties. Here, we discuss the correlation between structural defects, such as oxygen vacancies and impurity islands, and magnetism in La0.74Sr0.26MnO3/SrTiO3 (LSMO/STO) epitaxial heterostructures by systematic control of the growth temperature and post-deposition annealing conditions. Upon increasing the growth temperature within the 500 $-$ 700 $^{\circ}$C range, the epitaxial LSMO films experience a progressive improvement in oxygen stoichiometry, leading to enhanced magnetic characteristics. Concurrently, however, the use of a high growth temperature triggers the diffusion of impurities from the bulk of STO, which cause the creation of off-stoichiometric, dendritic-like SrMoOx islands at the film/substrate interface. As a valuable workaround, post-deposition annealing of the LSMO films grown at a relatively-low temperature of about 500 $^{\circ}$C permits to obtain high-quality epitaxy, atomically-flat surface as well as a sharp magnetic transition above room temperature and robust ferromagnetism. Furthermore, under such optimized fabrication conditions possible scenarios for the formation of the magnetic dead layer as a function of LSMO film thickness are discussed. Our findings offer effective routes to finely tailor the complex interplay between structural and magnetic properties of LSMO thin films via temperature-controlled defect engineering. △ Less

Submitted 15 January, 2022; originally announced January 2022.

Disconnection-Mediated Twin/Twin-Junction Migration in FCC metals

Authors: Mingjie Xu, Kongtao Chen, Fan Cao, Leonardo Velasco Estrada, Thomas M. Kaufman, Fan Ye, Horst Hahn, Jian Han, David J. Srolovitz, Xiaoqing Pan

Abstract: We present the results of novel, time-resolved, in situ HRTEM observations, molecular dynamics (MD) simulations, and disconnection theory that elucidate the mechanism by which the motion of grain boundaries (GBs) in polycrystalline materials are coupled through disconnection motion/reactions at/adjacent to GB triple junctions (TJs). We focus on TJs composed of a pair of coherent twin boundaries (C… ▽ More We present the results of novel, time-resolved, in situ HRTEM observations, molecular dynamics (MD) simulations, and disconnection theory that elucidate the mechanism by which the motion of grain boundaries (GBs) in polycrystalline materials are coupled through disconnection motion/reactions at/adjacent to GB triple junctions (TJs). We focus on TJs composed of a pair of coherent twin boundaries (CTBs) and a Σ9 GB. As for all GBs, disconnection theory implies that multiple modes/local mechanisms for CTB migration are possible and that the mode selection is affected by the nature of the driving force for migration. While we observe (HRTEM and MD) CTB migration through the motion of pure steps driven by chemical potential jump, other experimental observations (and our simulations) show that stress-driven CTB migration occurs through the motion of disconnections with a non-zero Burgers vector; these are pure-step and twinning-partial CTB migration mechanisms. Our experimental observations and simulations demonstrate that the motion of a GB drags its delimiting TJ and may force the motion of the other GBs meeting at the TJ. Our experiments and simulations focus on two types of TJs composed of a pair of CTBs and a Σ9 GB; a 107° TJ readily migrates while a 70° TJ is immobile (experiment, simulation) in agreement with our disconnection theory even though the intrinsic mobilities of the constituent GBs do not depend on TJ-type. We also demonstrate that disconnections may be formed at TJs (chemical potential jump/stress driven) and at GB/free surface junctions (stress-driven). △ Less

Submitted 11 January, 2022; originally announced January 2022.

The Brain Tumor Sequence Registration (BraTS-Reg) Challenge: Establishing Correspondence Between Pre-Operative and Follow-up MRI Scans of Diffuse Glioma Patients

Authors: Bhakti Baheti, Satrajit Chakrabarty, Hamed Akbari, Michel Bilello, Benedikt Wiestler, Julian Schwarting, Evan Calabrese, Jeffrey Rudie, Syed Abidi, Mina Mousa, Javier Villanueva-Meyer, Brandon K. K. Fields, Florian Kofler, Russell Takeshi Shinohara, Juan Eugenio Iglesias, Tony C. W. Mok, Albert C. S. Chung, Marek Wodzinski, Artur Jurgas, Niccolo Marini, Manfredo Atzori, Henning Muller, Christoph Grobroehmer, Hanna Siebert, Lasse Hansen , et al. (48 additional authors not shown)

Abstract: Registration of longitudinal brain MRI scans containing pathologies is challenging due to dramatic changes in tissue appearance. Although there has been progress in developing general-purpose medical image registration techniques, they have not yet attained the requisite precision and reliability for this task, highlighting its inherent complexity. Here we describe the Brain Tumor Sequence Registr… ▽ More Registration of longitudinal brain MRI scans containing pathologies is challenging due to dramatic changes in tissue appearance. Although there has been progress in developing general-purpose medical image registration techniques, they have not yet attained the requisite precision and reliability for this task, highlighting its inherent complexity. Here we describe the Brain Tumor Sequence Registration (BraTS-Reg) challenge, as the first public benchmark environment for deformable registration algorithms focusing on estimating correspondences between pre-operative and follow-up scans of the same patient diagnosed with a diffuse brain glioma. The BraTS-Reg data comprise de-identified multi-institutional multi-parametric MRI (mpMRI) scans, curated for size and resolution according to a canonical anatomical template, and divided into training, validation, and testing sets. Clinical experts annotated ground truth (GT) landmark points of anatomical locations distinct across the temporal domain. Quantitative evaluation and ranking were based on the Median Euclidean Error (MEE), Robustness, and the determinant of the Jacobian of the displacement field. The top-ranked methodologies yielded similar performance across all evaluation metrics and shared several methodological commonalities, including pre-alignment, deep neural networks, inverse consistency analysis, and test-time instance optimization per-case basis as a post-processing step. The top-ranked method attained the MEE at or below that of the inter-rater variability for approximately 60% of the evaluated landmarks, underscoring the scope for further accuracy and robustness improvements, especially relative to human experts. The aim of BraTS-Reg is to continue to serve as an active resource for research, with the data and online evaluation tools accessible at https://bratsreg.github.io/. △ Less

Submitted 17 April, 2024; v1 submitted 13 December, 2021; originally announced December 2021.

Robust Segmentation Models using an Uncertainty Slice Sampling Based Annotation Workflow

Authors: Grzegorz Chlebus, Andrea Schenk, Horst K. Hahn, Bram van Ginneken, Hans Meine

Abstract: Semantic segmentation neural networks require pixel-level annotations in large quantities to achieve a good performance. In the medical domain, such annotations are expensive, because they are time-consuming and require expert knowledge. Active learning optimizes the annotation effort by devising strategies to select cases for labeling that are most informative to the model. In this work, we propo… ▽ More Semantic segmentation neural networks require pixel-level annotations in large quantities to achieve a good performance. In the medical domain, such annotations are expensive, because they are time-consuming and require expert knowledge. Active learning optimizes the annotation effort by devising strategies to select cases for labeling that are most informative to the model. In this work, we propose an uncertainty slice sampling (USS) strategy for semantic segmentation of 3D medical volumes that selects 2D image slices for annotation and compare it with various other strategies. We demonstrate the efficiency of USS on a CT liver segmentation task using multi-site data. After five iterations, the training data resulting from USS consisted of 2410 slices (4% of all slices in the data pool) compared to 8121 (13%), 8641 (14%), and 3730 (6%) for uncertainty volume (UVS), random volume (RVS), and random slice (RSS) sampling, respectively. Despite being trained on the smallest amount of data, the model based on the USS strategy evaluated on 234 test volumes significantly outperformed models trained according to other strategies and achieved a mean Dice index of 0.964, a relative volume error of 4.2%, a mean surface distance of 1.35 mm, and a Hausdorff distance of 23.4 mm. This was only slightly inferior to 0.967, 3.8%, 1.18 mm, and 22.9 mm achieved by a model trained on all available data, but the robustness analysis using the 5th percentile of Dice and the 95th percentile of the remaining metrics demonstrated that USS resulted not only in the most robust model compared to other sampling schemes, but also outperformed the model trained on all data according to Dice (0.946 vs. 0.945) and mean surface distance (1.92 mm vs. 2.03 mm). △ Less

Submitted 30 September, 2021; originally announced September 2021.

Comments: This work has been submitted to the IEEE for possible publication

Cryogenic Penning-Trap Apparatus for Precision Experiments with Sympathetically Cooled (anti)protons

Authors: M. Niemann, T. Meiners, J. Mielke, N. Pulido, J. Schaper, M. J. Borchert, J. M. Cornejo, A. -G. Paschke, G. Zarantonello, H. Hahn, T. Lang, C. Manzoni, M. Marangoni, G. Cerullo, U. Morgner, J. -A. Fenske, A. Bautista-Salvador, R. Lehnert, S. Ulmer, C. Ospelkaus

Abstract: Current precision experiments with single (anti)protons to test CPT symmetry progress at a rapid pace, but are complicated by the need to cool particles to sub-thermal energies. We describe a cryogenic Penning-trap setup for $^9$Be$^+$ ions designed to allow coupling of single (anti)protons to laser-cooled atomic ions for sympathetic cooling and quantum logic spectroscopy. We report on trapping an… ▽ More Current precision experiments with single (anti)protons to test CPT symmetry progress at a rapid pace, but are complicated by the need to cool particles to sub-thermal energies. We describe a cryogenic Penning-trap setup for $^9$Be$^+$ ions designed to allow coupling of single (anti)protons to laser-cooled atomic ions for sympathetic cooling and quantum logic spectroscopy. We report on trapping and laser cooling of clouds and single $^9$Be$^+$ ions. We discuss prospects for a microfabricated trap to allow coupling of single (anti)protons to laser-cooled $^9$Be$^+$ ions for sympathetic laser cooling to sub-mK temperatures on ms time scales. △ Less

Submitted 18 July, 2021; originally announced July 2021.

Comments: Presented at the Eighth Meeting on CPT and Lorentz Symmetry, Bloomington, Indiana, May 12-16, 2019

Journal ref: World Scientific, Singapore, Proceedings of the Eighth Meeting on CPT and Lorentz Symmetry (2020)

Complete crystallographic, spin-electronic and magnetic structure of (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)3O4: Unraveling the suppression of entropy in high entropy oxides

Authors: Abhishek Sarkar, Benedikt Eggert, Ralf Witte, Johanna Lill, Leonardo Velasco, Qingsong Wang, Janhavika Sonar, Katharina Ollefs, Subramshu S. Bhattacharya, Richard A. Brand, Heiko Wende, Frank M. F. de Groot, Oliver Clemens, Horst Hahn, Robert Kruk

Abstract: High entropy oxides (HEOs) are a rapidly emerging class of functional materials consisting of multiple principal cations. The original paradigm of HEOs assumes cationic occupations with the highest possible configurational entropy allowed by the composition and crystallographic structure. However, the fundamental question remains on the actual degree of configurational disorder in HEOs, especially… ▽ More High entropy oxides (HEOs) are a rapidly emerging class of functional materials consisting of multiple principal cations. The original paradigm of HEOs assumes cationic occupations with the highest possible configurational entropy allowed by the composition and crystallographic structure. However, the fundamental question remains on the actual degree of configurational disorder in HEOs, especially, in systems with a low enthalpy barriers for cation anti-site mixing. Considering the experimental limitations due to the presence of multiple principal cations in HEOs, here we utilize a robust and cross-referenced characterization approach using soft X-ray magnetic circular dichroism, hard X-ray absorption spectroscopy, Mössbauer spectroscopy, neutron powder diffraction and SQUID magnetometry to study the competition between crystal field stabilization energy and configurational entropy governing the cation occupation in a spinel HEO (S-HEO), (Co$_{0.2}$Cr$_{0.2}$Fe$_{0.2}$Mn$_{0.2}$Ni$_{0.2}$)$_3$O$_4$. In contrast to the previous studies, the derived complete structural and spin-electronic model, (Co$_{0.6}$Fe$_{0.4}$)(Cr$_{0.3}$Fe$_{0.1}$Mn$_{0.3}$Ni$_{0.3}$)$_2$O$_4$, highlights a significant deviation from the hitherto assumed paradigm of entropy-driven non-preferential distribution of cations in HEOs. An immediate correlation of this result can be drawn with bulk as well as the local element specific magnetic properties, which are intrinsically dictated by cationic occupations in spinels. The real local lattice picture presented here provides an alternate viewpoint on ionic arrangement in HEOs, which is of fundamental interest for predicting and designing their structure-dependent functionalities. △ Less

Submitted 22 December, 2021; v1 submitted 9 July, 2021; originally announced July 2021.

Vertical GaN Devices: Process and Reliability

Authors: Shuzhen You, Karen Geens, Matteo Borga, Hu Liang, Herwig Hahn, Dirk Fahle, Michael Heuken, Kalparupa Mukherjee, Carlo De Santi, Matteo Meneghini, Enrico Zanoni, Martin Berg, Peter Ramvall, Ashutosh Kumar, Mikael T. Björk, B. Jonas Ohlsson, Stefaan Decoutere

Abstract: This paper reviews recent progress and key challenges in process and reliability for high-performance vertical GaN transistors and diodes, focusing on the 200 mm CMOS-compatible technology. We particularly demonstrated the potential of using 200 mm diameter CTE matched substrates for vertical power transistors, and gate module optimizations for device robustness. An alternative technology path bas… ▽ More This paper reviews recent progress and key challenges in process and reliability for high-performance vertical GaN transistors and diodes, focusing on the 200 mm CMOS-compatible technology. We particularly demonstrated the potential of using 200 mm diameter CTE matched substrates for vertical power transistors, and gate module optimizations for device robustness. An alternative technology path based on coalescence epitaxy of GaN-on-Silicon is also introduced, which could enable thick drift layers of very low dislocation density. △ Less

Submitted 7 July, 2021; v1 submitted 6 July, 2021; originally announced July 2021.

Comments: ["European Union (EU)" & "Horizon 2020"]["Euratom" & Euratom research & training programme 2014-2018"][ECSEL Joint Undertaking (JU)][UltimateGaN][grant agreement No 826392]

Emergence of near-boundary segregation zones in face-centered cubic multi-principal element alloys

Authors: Megan J. McCarthy, Hui Zheng, Diran Apelian, William J. Bowman, Horst Hahn, Jian Luo, Shyue Ping Ong, Xiaoqing Pan, Timothy J. Rupert

Abstract: Grain boundaries have been shown to dramatically influence the behavior of relatively simple materials such as monatomic metals and binary alloys. The increased chemical complexity associated with multi-principal element alloys is hypothesized to lead to new grain boundary phenomena. To explore the relationship between grain boundary structure and chemistry in these materials, hybrid molecular dyn… ▽ More Grain boundaries have been shown to dramatically influence the behavior of relatively simple materials such as monatomic metals and binary alloys. The increased chemical complexity associated with multi-principal element alloys is hypothesized to lead to new grain boundary phenomena. To explore the relationship between grain boundary structure and chemistry in these materials, hybrid molecular dynamics/Monte Carlo simulations of a faceted Σ11 <110> tilt boundary, chosen to sample both high- and low-energy boundary configurations, are performed in face-centered cubic CrFeCoNiCu and CrFeCoNi equiatomic alloys. Unexpected enrichment of Fe is discovered in the face-centered cubic regions adjacent to the interface and found to be correlated with a structurally-distinct region of reduced atomic volume. Comparison with the boundary of the same type in monatomic Cu demonstrates that altered near-boundary regions exist in simpler systems as well, with the chemical complexity of the multi-principal element alloys highlighting its existence and importance. △ Less

Submitted 10 October, 2021; v1 submitted 17 June, 2021; originally announced June 2021.

Magneto-electric Tuning of Pinning-Type Permanent Magnets through Atomic-Scale Engineering of Grain Boundaries

Authors: Xinglong Ye, Fengkai Yan, Lukas Schaefer, Di Wang, Holger Geßwein, Wu Wang, Mohammed Reda Chellali, Leigh T. Stephenson, Konstantin Skokov, Oliver Gutfleisch, Dierk Raabe, Horst Hahn, Baptiste Gault, Robert Kruk

Abstract: Pinning-type magnets maintaining high coercivity, i.e. the ability to sustain magnetization, at high temperature are at the core of thriving clean-energy technologies. Among these, Sm2Co17-based magnets are excellent candidates owing to their high-temperature stability. However, despite decades of efforts to optimize the intragranular microstructure, the coercivity currently only reaches 20~30% of… ▽ More Pinning-type magnets maintaining high coercivity, i.e. the ability to sustain magnetization, at high temperature are at the core of thriving clean-energy technologies. Among these, Sm2Co17-based magnets are excellent candidates owing to their high-temperature stability. However, despite decades of efforts to optimize the intragranular microstructure, the coercivity currently only reaches 20~30% of the theoretical limits. Here, the roles of the grain-interior nanostructure and the grain boundaries in controlling coercivity are disentangled by an emerging magneto-electric approach. Through hydrogen charging/discharging by applying voltages of only ~ 1 V, the coercivity is reversibly tuned by an unprecedented value of ~ 1.3 T. In situ magneto-structural measurements and atomic-scale tracking of hydrogen atoms reveal that the segregation of hydrogen atoms at the grain boundaries, rather than the change of the crystal structure, dominates the reversible and substantial change of coercivity. Hydrogen lowers the local magnetocrystalline anisotropy and facilitates the magnetization reversal starting from the grain boundaries. Our study reveals the previously neglected critical role of grain boundaries in the conventional magnetisation-switching paradigm, suggesting a critical reconsideration of strategies to overcome the coercivity limits in permanent magnets, via for instance atomic-scale grain boundary engineering. △ Less

Submitted 10 February, 2021; originally announced February 2021.

Anisotropic 3D Multi-Stream CNN for Accurate Prostate Segmentation from Multi-Planar MRI

Authors: Anneke Meyer, Grzegorz Chlebus, Marko Rak, Daniel Schindele, Martin Schostak, Bram van Ginneken, Andrea Schenk, Hans Meine, Horst K. Hahn, Andreas Schreiber, Christian Hansen

Abstract: Background and Objective: Accurate and reliable segmentation of the prostate gland in MR images can support the clinical assessment of prostate cancer, as well as the planning and monitoring of focal and loco-regional therapeutic interventions. Despite the availability of multi-planar MR scans due to standardized protocols, the majority of segmentation approaches presented in the literature consid… ▽ More Background and Objective: Accurate and reliable segmentation of the prostate gland in MR images can support the clinical assessment of prostate cancer, as well as the planning and monitoring of focal and loco-regional therapeutic interventions. Despite the availability of multi-planar MR scans due to standardized protocols, the majority of segmentation approaches presented in the literature consider the axial scans only. Methods: We propose an anisotropic 3D multi-stream CNN architecture, which processes additional scan directions to produce a higher-resolution isotropic prostate segmentation. We investigate two variants of our architecture, which work on two (dual-plane) and three (triple-plane) image orientations, respectively. We compare them with the standard baseline (single-plane) used in literature, i.e., plain axial segmentation. To realize a fair comparison, we employ a hyperparameter optimization strategy to select optimal configurations for the individual approaches. Results: Training and evaluation on two datasets spanning multiple sites obtain statistical significant improvement over the plain axial segmentation ($p<0.05$ on the Dice similarity coefficient). The improvement can be observed especially at the base ($0.898$ single-plane vs. $0.906$ triple-plane) and apex ($0.888$ single-plane vs. $0.901$ dual-plane). Conclusion: This study indicates that models employing two or three scan directions are superior to plain axial segmentation. The knowledge of precise boundaries of the prostate is crucial for the conservation of risk structures. Thus, the proposed models have the potential to improve the outcome of prostate cancer diagnosis and therapies. △ Less

Submitted 2 December, 2020; v1 submitted 23 September, 2020; originally announced September 2020.

Comments: Accepted manuscript in Elsevier Computer Methods and Programs in Biomedicine. Anneke Meyer and Grzegorz Chlebus contributed equally to this work. Sourcecode available at https://github.com/AnnekeMeyer/AnisotropicMultiStreamCNN. Data available at https://doi.org/10.7937/TCIA.2019.DEG7ZG1U

Ultra-low vibration closed-cycle cryogenic surface-electrode ion trap apparatus

Authors: Timko Dubielzig, Sebastian Halama, Henning Hahn, Giorgio Zarantonello, Malte Niemann, Amado Bautista-Salvador, Christian Ospelkaus

Abstract: We describe the design, commissioning and operation of an ultra-low-vibration closed-cycle cryogenic ion trap apparatus. One hundred lines for low-frequency signals and eight microwave / radio frequency coaxial feed lines offer the possibility of implementing a small-scale ion-trap quantum processor or simulator. With all supply cables attached, more than 1.3 W of cooling power at 5 K is still ava… ▽ More We describe the design, commissioning and operation of an ultra-low-vibration closed-cycle cryogenic ion trap apparatus. One hundred lines for low-frequency signals and eight microwave / radio frequency coaxial feed lines offer the possibility of implementing a small-scale ion-trap quantum processor or simulator. With all supply cables attached, more than 1.3 W of cooling power at 5 K is still available for absorbing energy from electrical pulses introduced to control ions. The trap itself is isolated from vibrations induced by the cold head using a helium exchange gas interface. The performance of the vibration isolation system has been characterized using a Michelson interferometer, finding residual vibration amplitudes on the order of 10 nm rms. Trapping of $^9$Be$^+$ ions has been demonstrated using a combination of laser ablation and photoionization. △ Less

Submitted 9 June, 2021; v1 submitted 8 August, 2020; originally announced August 2020.

Comments: 10 figures

Journal ref: Review of Scientific Instruments 92, 043201 (2021)

Role of intermediate 4$f$ states in tuning the band structure of high entropy oxides

Authors: Abhishek Sarkar, Benedikt Eggert, Leonardo Velasco, Xiaoke Mu, Johanna Lill, Katharina Ollefs, Subramshu S. Bhattacharya, Heiko Wende, Robert Kruk, Richard A. Brand, Horst Hahn

Abstract: High entropy oxides (HEOs) are single phase solid solutions consisting of 5 or more cations in approximately equiatomic proportions. In this study, we show reversible control of optical properties in a rare-earth (RE) based HEO-(Ce$_{0.2}$La$_{0.2}$Pr$_{0.2}$Sm$_{0.2}$Y$_{0.2}$)O$_{2-δ}$ and subsequently utilize a combination of spectroscopic techniques to derive the features of the electronic ban… ▽ More High entropy oxides (HEOs) are single phase solid solutions consisting of 5 or more cations in approximately equiatomic proportions. In this study, we show reversible control of optical properties in a rare-earth (RE) based HEO-(Ce$_{0.2}$La$_{0.2}$Pr$_{0.2}$Sm$_{0.2}$Y$_{0.2}$)O$_{2-δ}$ and subsequently utilize a combination of spectroscopic techniques to derive the features of the electronic band structure underpinning the observed optical phenomena. Heat treatment of the HEO under vacuum atmosphere followed by reheat-treatment in air results in a reversible change of the band gap energy, from 1.9 eV to 2.5 eV. The finding is consistent with the reversible changes in the oxidation state and related $f$-orbital occupancy of Pr. However, no pertinent changes in the phase composition or crystal structure is observed upon the vacuum heat treatment. Further annealing of this HEO under H$_2$ atmosphere, followed by reheat-treatment in air, results in even larger but still reversible change of the band gap energy from 1.9 eV to 3.2 eV. This is accompanied by a disorder-order type crystal structure transition and changes in the O 2$p$-RE 5$d$ hybridization evidenced from X-ray absorption near edge spectra (XANES). The O $K$ and RE ${M_{4,5}}$/$L_{3}$ XANES indicate that the presence of Ce and Pr (in 3+/4+) state leads to the formation of intermediate 4$f$ energy levels between the O 2$p$ and RE 5$d$ gap in HEO. It is concluded that heat treatment under reducing/oxidizing atmospheres affects these intermediate levels, thus, offering the possibility to tune the band gap energy in HEO. △ Less

Submitted 29 February, 2020; originally announced March 2020.

Magnetic properties of rare earth and transition metal based perovskite type high entropy oxides

Authors: Ralf Witte, Abhishek Sarkar, Leonardo Velasco, Robert Kruk, Richard A. Brand, Benedikt Eggert, Katharina Ollefs, Eugen Weschke, Heiko Wende, Horst Hahn

Abstract: High entropy oxides (HEO) are a recently introduced class of oxide materials, which are characterized by a large number of elements (i.e. five or more) sharing one lattice site which crystallize in a single phase structure. One complex example of the rather young HEO family are the rare-earth transition metal perovskite high entropy oxides. In this comprehensive study, we provide an overview over… ▽ More High entropy oxides (HEO) are a recently introduced class of oxide materials, which are characterized by a large number of elements (i.e. five or more) sharing one lattice site which crystallize in a single phase structure. One complex example of the rather young HEO family are the rare-earth transition metal perovskite high entropy oxides. In this comprehensive study, we provide an overview over the magnetic properties of three perovskite type high entropy oxides. The compounds have a rare-earth site which is occupied by five different rare-earth elements, while the transition metal site is occupied by a single transition metal. In this way a comparison to the parent binary oxides, namely the orthocobaltites, -chromites and -ferrites is possible. X-ray absorption near edge spectroscopy (XANES), magnetometry and Mössbauer spectroscopy are employed to characterize these complex materials. In general, we find surprising similarities to the magnetic properties of the binary oxides, despite the chemical disorder on the rare-earth site. However distinct differences and interesting magnetic properties are also observed such as noncollinearity, spin reorientation transitions as well as large coercive fields of up to 2\,T at ambient temperature. Both the chemical disorder on the RE A-site, and the nature of the TM on the B-site play an important role in the physical properties of these high entropy oxides. △ Less

Submitted 31 January, 2020; originally announced February 2020.

Poles of triple product $L$-functions involving monomial representations

Authors: Heekyoung Hahn

Abstract: In this paper, we study the order of the pole of the triple tensor product $L$-functions $L(s,π_1\timesπ_2\timesπ_3,\otimes^3)$ for cuspidal automorphic representations $π_i$ of $\mathrm{GL}_{n_i}(\mathbb{A}_F)$ in the setting where one of the $π_i$ is a monomial representation. In the view of Brauer theory, this is a natural setting to consider. The results provided in this paper give crucial exa… ▽ More In this paper, we study the order of the pole of the triple tensor product $L$-functions $L(s,π_1\timesπ_2\timesπ_3,\otimes^3)$ for cuspidal automorphic representations $π_i$ of $\mathrm{GL}_{n_i}(\mathbb{A}_F)$ in the setting where one of the $π_i$ is a monomial representation. In the view of Brauer theory, this is a natural setting to consider. The results provided in this paper give crucial examples that can be used as a point of reference for Langlands' beyond endoscopy proposal. △ Less

Submitted 9 December, 2019; originally announced December 2019.

Robust and resource-efficient microwave near-field entangling $^9$Be$^+$ gate

Authors: G. Zarantonello, H. Hahn, J. Morgner, M. Schulte, A. Bautista-Salvador, R. F. Werner, K. Hammerer, C. Ospelkaus

Abstract: Microwave trapped-ion quantum logic gates avoid spontaneous emission as a fundamental source of decoherence. However, microwave two-qubit gates are still slower than laser-induced gates and hence more sensitive to fluctuations and noise of the motional mode frequency. We propose and implement amplitude-shaped gate drives to obtain resilience to such frequency changes without increasing the pulse e… ▽ More Microwave trapped-ion quantum logic gates avoid spontaneous emission as a fundamental source of decoherence. However, microwave two-qubit gates are still slower than laser-induced gates and hence more sensitive to fluctuations and noise of the motional mode frequency. We propose and implement amplitude-shaped gate drives to obtain resilience to such frequency changes without increasing the pulse energy per gate operation. We demonstrate the resilience by noise injection during a two-qubit entangling gate with $^9$Be$^+$ ion qubits. In absence of injected noise, amplitude modulation gives an operation infidelity in the $10^{-3}$ range. △ Less

Submitted 10 November, 2019; originally announced November 2019.

Comments: 4 figures

Journal ref: Phys. Rev. Lett. 123, 260503 (2019)

Versatile control of $^9$Be$^+$ ions using a spectrally tailored UV frequency comb

Authors: A. -G. Paschke, G. Zarantonello, H. Hahn, T. Lang, C. Manzoni, M. Marangoni, G. Cerullo, U. Morgner, C. Ospelkaus

Abstract: We demonstrate quantum control of $^9$Be$^+$ ions directly implemented by an optical frequency comb. Based on numerical simulations of the relevant processes in $^9$Be$^+$ for different magnetic field regimes, we demonstrate a wide applicability when controlling the comb's spectral properties. We introduce a novel technique for the selective and efficient generation of a spectrally tailored narrow… ▽ More We demonstrate quantum control of $^9$Be$^+$ ions directly implemented by an optical frequency comb. Based on numerical simulations of the relevant processes in $^9$Be$^+$ for different magnetic field regimes, we demonstrate a wide applicability when controlling the comb's spectral properties. We introduce a novel technique for the selective and efficient generation of a spectrally tailored narrow-bandwidth optical frequency comb near 313 nm. We experimentally demonstrate internal state control and internal-motional state coupling of $^9$Be$^+$ ions implemented by stimulated-Raman manipulation using a spectrally optimized optical frequency comb. Our pulsed laser approach is a key enabling step for the implementation of quantum logic and quantum information experiments in Penning traps. △ Less

Submitted 7 March, 2019; originally announced March 2019.

Comments: Includes supplementary material

Higher order mode damper for low energy RHIC electron cooler SRF booster cavity

Authors: Binping Xiao, A. Fedotov, H. Hahn, D. Holmes, G. McIntyre, C. Pai, S. Seberg, K. Smith, R. Than, P. Thieberger, J. Tuozzolo, Q. Wu, T. Xin, Wencan Xu, A. Zaltsman

Abstract: To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC electron Cooler (LEReC) is currently under commissioning at BNL. The Linac of LEReC is designed to deliver a 1.6 MeV to 2.6 MeV electron beam, with rms dp/p less than 5e-4. A 704 MHz superconducting radio frequency (SRF) booster cavity in this Linac provides up to 2.2 MeV accelerating voltage. With suc… ▽ More To improve RHIC luminosity for heavy ion beam energies below 10 GeV/nucleon, the Low Energy RHIC electron Cooler (LEReC) is currently under commissioning at BNL. The Linac of LEReC is designed to deliver a 1.6 MeV to 2.6 MeV electron beam, with rms dp/p less than 5e-4. A 704 MHz superconducting radio frequency (SRF) booster cavity in this Linac provides up to 2.2 MeV accelerating voltage. With such a low energy and very demanding energy spread requirement, control of Higher Order Modes (HOMs) in the cavities becomes critical and needs to be carefully evaluated to ensure minimum impact on the beam. In this paper, we report the multiphysics design of the HOM damper for this cavity to meet the energy spread requirement, as well as experimental results of the cavity with and without the HOM damper. △ Less

Submitted 1 March, 2019; originally announced March 2019.

Comments: 9 pages, 7 figures

Journal ref: Phys. Rev. Accel. Beams 22, 050101 (2019)

Epitaxial strain adaption in chemically disordered FeRh thin films

Authors: Ralf Witte, Robert Kruk, Di Wang, Sabine Schlabach, Richard A. Brand, Markus E. Gruner, Heiko Wende, Horst Hahn

Abstract: Strain and strain adaption mechanisms in modern functional materials are of crucial importance for their performance. Understanding these mechanisms will advance innovative approaches for material properties engineering. Here we study the strain adaption mechanism in a thin film model system as function of epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V buffer layers,… ▽ More Strain and strain adaption mechanisms in modern functional materials are of crucial importance for their performance. Understanding these mechanisms will advance innovative approaches for material properties engineering. Here we study the strain adaption mechanism in a thin film model system as function of epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V buffer layers, which allow for large variation of the preset lattice constants, e.g. epitaxial boundary condition. It is shown by means of high resolution X-ray reciprocal space maps and transmission electron microscopy that the system reacts with a tilting mechanism of the structural units in order to adapt to the lattice constants of the buffer layer. This response explained by density functional theory calculations, which evidence an energetic minimum for structures with a distortion of c/a =0.87. The experimentally observed tilting mechanism is induced by this energy gain and allows the system to remain in the most favorable structure. In general, it is shown that the use of epitaxial model heterostructures consisting of alloy buffer layers of fully miscible elements and the functional material of interest allows to study strain adaption behaviors in great detail. This approach makes even small secondary effects observable, such as the directional tilting of the structural domains identified in the present case study. △ Less

Submitted 29 April, 2019; v1 submitted 20 February, 2019; originally announced February 2019.

Journal ref: Phys. Rev. B 99, 134109 (2019)

Integrated $^{9}$Be$^{+}$ multi-qubit gate device for the ion-trap quantum computer

Authors: Henning Hahn, Giorgio Zarantonello, Marius Schulte, Amado Bautista-Salvador, Klemens Hammerer, Christian Ospelkaus

Abstract: We demonstrate the experimental realization of a two-qubit Mølmer-Sørensen gate on a magnetic field-insensitive hyperfine transition in $^9$Be$^+$ ions using microwave-near fields emitted by a single microwave conductor embedded in a surface-electrode ion trap. The design of the conductor was optimized to produce a high oscillating magnetic field gradient at the ion position. The measured gate fid… ▽ More We demonstrate the experimental realization of a two-qubit Mølmer-Sørensen gate on a magnetic field-insensitive hyperfine transition in $^9$Be$^+$ ions using microwave-near fields emitted by a single microwave conductor embedded in a surface-electrode ion trap. The design of the conductor was optimized to produce a high oscillating magnetic field gradient at the ion position. The measured gate fidelity is determined to be $98.2\pm1.2\,\%$ and is limited by technical imperfections, as is confirmed by a comprehensive numerical error analysis. The conductor design can potentially simplify the implementation of multi-qubit gates and represents a self-contained, scalable module for entangling gates within the quantum CCD architecture for an ion-trap quantum computer. △ Less

Submitted 4 September, 2019; v1 submitted 19 February, 2019; originally announced February 2019.

Comments: 4 figures, 1 table

Journal ref: npj Quantum Inf 5, 70 (2019)

High entropy oxides: An emerging prospect for magnetic rare earth - transition metal perovskites

Authors: Ralf Witte, Abhishek Sarkar, Robert Kruk, Benedikt Eggert, Richard A. Brand, Heiko Wende, Horst Hahn

Abstract: It has been shown that oxide ceramics containing multiple transition and/or rare-earth elements in equimolar ratios have a strong tendency to crystallize in simple single phase structures, stabilized by the high configurational entropy. In analogy to the metallic alloy systems, these oxides are denoted high entropy oxides (HEOs). The HEO concept allows to access hitherto uncharted areas in the mul… ▽ More It has been shown that oxide ceramics containing multiple transition and/or rare-earth elements in equimolar ratios have a strong tendency to crystallize in simple single phase structures, stabilized by the high configurational entropy. In analogy to the metallic alloy systems, these oxides are denoted high entropy oxides (HEOs). The HEO concept allows to access hitherto uncharted areas in the multi-element phase diagram. Among the already realized structures there is the highly complex class of rare earth - transition element perovskites. This fascinating class of materials generated by applying the innovative concept of high entropy stabilization provides a new and manyfold research space with promise of discoveries of unprecedented properties and phenomena. The present study provides a first investigation of the magnetic properties of selected compounds of this novel class of materials. Comprehensive studies by DC and AC magnetometry are combined with element specific spectroscopy in order to understand the interplay between magnetic exchange and the high degree of chemical disorder in the systems. We observe a predominant antiferromagnetic behavior in the single phase materials, combined with a small ferromagnetic contribution possibly stemming from small ferromagnetic clusters or configurations in the antiferromagnetic matrix. In the long term perspective it is proposed to screen the properties of this family of compounds with high throughput methods, including combined experimental and theoretical approaches. △ Less

Submitted 12 March, 2019; v1 submitted 8 January, 2019; originally announced January 2019.

Journal ref: Phys. Rev. Materials 3, 034406 (2019)

Multilayer ion trap with three-dimensional microwave circuitry for scalable quantum logic applications

Authors: Henning Hahn, Giorgio Zarantonello, Amado Bautista-Salvador, Martina Wahnschaffe, Matthias Kohnen, Joerg Schoebel, Piet O. Schmidt, Christian Ospelkaus

Abstract: We present a multilayer surface-electrode ion trap with embedded 3D microwave circuitry for implementing entangling quantum logic gates. We discuss the electromagnetic full-wave simulation procedure that has led to the trap design and the characterization of the resulting microwave field-pattern using a single ion as a local field probe. The results agree with simulations within the uncertainty; c… ▽ More We present a multilayer surface-electrode ion trap with embedded 3D microwave circuitry for implementing entangling quantum logic gates. We discuss the electromagnetic full-wave simulation procedure that has led to the trap design and the characterization of the resulting microwave field-pattern using a single ion as a local field probe. The results agree with simulations within the uncertainty; compared to previous traps, this design reduces detrimental AC Zeeman shifts by three orders of magnitude. The design presented here can be viewed as an entangling gate component in a library for surface-electrode ion traps intended for quantum logic operations. △ Less

Submitted 6 July, 2021; v1 submitted 6 December, 2018; originally announced December 2018.

Journal ref: Appl. Phys. B 125, 154 (2019)

Multilayer ion trap technology for scalable quantum computing and quantum simulation

Authors: Amado Bautista-Salvador, Giorgio Zarantonello, Henning Hahn, Alan Preciado-Grijalva, Jonathan Morgner, Martina Wahnschaffe, Christian Ospelkaus

Abstract: We present a novel ion trap fabrication method enabling the realization of multilayer ion traps scalable to an in principle arbitrary number of metal-dielectric levels. We benchmark our method by fabricating a multilayer ion trap with integrated three-dimensional microwave circuitry. We demonstrate ion trapping and microwave control of the hyperfine states of a laser cooled $\,^{9}$Be$^{+}$ ion he… ▽ More We present a novel ion trap fabrication method enabling the realization of multilayer ion traps scalable to an in principle arbitrary number of metal-dielectric levels. We benchmark our method by fabricating a multilayer ion trap with integrated three-dimensional microwave circuitry. We demonstrate ion trapping and microwave control of the hyperfine states of a laser cooled $\,^{9}$Be$^{+}$ ion held at a distance of 35$\,μ$m above the trap surface. This method can be used to implement large-scale ion trap arrays for scalable quantum information processing and quantum simulation. △ Less

Submitted 5 December, 2018; originally announced December 2018.

Comments: 11 pages, 4 figures

Optomechanics with one-dimensional gallium phosphide photonic crystal cavities

Authors: Katharina Schneider, Yannick Baumgartner, Simon Hönl, Pol Welter, Herwig Hahn, Dalziel J. Wilson, Lukas Czornomaz, Paul Seidler

Abstract: Gallium phosphide offers an attractive combination of a high refractive index ($n>3$ for vacuum wavelengths up to 4 μm) and a wide electronic bandgap (2.26 eV), enabling optical cavities with small mode volumes and low two-photon absorption at telecommunication wavelengths. Heating due to strongly confined light fields is therefore greatly reduced. Here, we investigate the benefits of these proper… ▽ More Gallium phosphide offers an attractive combination of a high refractive index ($n>3$ for vacuum wavelengths up to 4 μm) and a wide electronic bandgap (2.26 eV), enabling optical cavities with small mode volumes and low two-photon absorption at telecommunication wavelengths. Heating due to strongly confined light fields is therefore greatly reduced. Here, we investigate the benefits of these properties for cavity optomechanics. Utilizing a recently developed fabrication scheme based on direct wafer bonding, we realize integrated one-dimensional photonic crystal cavities made of gallium phosphide with optical quality factors as high as $1.1\times10^5$. We optimize their design to couple the optical eigenmode at $\approx 200$ THz via radiation pressure to a co-localized mechanical mode with a frequency of 3 GHz, yielding sideband-resolved devices. The high vacuum optomechanical coupling rate ($g_0=2π\times 400$ kHz) permits amplification of the mechanical mode into the so-called mechanical lasing regime with input power as low as $\approx 20$ μW. The observation of mechanical lasing implies a multiphoton cooperativity of $C>1$, an important threshold for the realization of quantum state transfer protocols. Because of the reduced thermo-optic resonance shift, optomechanically induced transparency can be detected at room temperature in addition to the normally observed optomechanically induced absorption. △ Less

Submitted 3 December, 2018; originally announced December 2018.

Comments: 11 pages, 13 figures

Bayesian with Gaussian process based missing input imputation scheme for reconstructing magnetic equilibria in real time

Authors: Semin Joung, Jaewook Kim, Sehyun Kwak, Kyeo-reh Park, S. H. Hahn, H. S. Han, H. S. Kim, J. G. Bak, S. G. Lee, Y. -c. Ghim

Abstract: A Bayesian with GP(Gaussian Process)-based numerical method to impute a few missing magnetic signals caused by impaired magnetic probes during tokamak operations is developed such that the real-time reconstruction of magnetic equilibria, whose performance strongly depends on the measured magnetic signals and their intactness, are affected minimally. Likelihood of the Bayesian model constructed wit… ▽ More A Bayesian with GP(Gaussian Process)-based numerical method to impute a few missing magnetic signals caused by impaired magnetic probes during tokamak operations is developed such that the real-time reconstruction of magnetic equilibria, whose performance strongly depends on the measured magnetic signals and their intactness, are affected minimally. Likelihood of the Bayesian model constructed with the Maxwell's equations, specifically Gauss's law of magnetism and Ampère's law, results in infinite number of solutions if two or more magnetic signals are missing. This undesirable characteristic of the Bayesian model is remediated by coupling the model with the Gaussian process. Our proposed numerical method infers the missing magnetic signals correctly in less than $1$\:msec suitable for real-time reconstruction of magnetic equilibria during tokamak operations. The method can also be used for a neural network that reconstructs magnetic equilibria trained with a complete set of magnetic signals. Without our proposed imputation method, such a neural network would become useless if missing signals are not tolerable by the network. △ Less

Submitted 11 June, 2018; originally announced June 2018.

Highly selective dry etching of GaP in the presence of Al$_\textrm{x}$Ga$_{1-\textrm{x}}$P

Authors: Simon Hönl, Herwig Hahn, Yannick Baumgartner, Lukas Czornomaz, Paul Seidler

Abstract: We present an inductively coupled-plasma reactive-ion etching process that simultaneously provides both a high etch rate and unprecedented selectivity for gallium phosphide (GaP) in the presence of aluminum gallium phosphide (Al$_\textrm{x}$Ga$_{1-\textrm{x}}$P). Utilizing mixtures of silicon tetrachloride (SiCl$_4$) and sulfur hexafluoride (SF$_6$), selectivities exceeding 2700:1 are achieved at… ▽ More We present an inductively coupled-plasma reactive-ion etching process that simultaneously provides both a high etch rate and unprecedented selectivity for gallium phosphide (GaP) in the presence of aluminum gallium phosphide (Al$_\textrm{x}$Ga$_{1-\textrm{x}}$P). Utilizing mixtures of silicon tetrachloride (SiCl$_4$) and sulfur hexafluoride (SF$_6$), selectivities exceeding 2700:1 are achieved at GaP etch rates above 3000 nm/min. A design of experiments has been employed to investigate the influence of the inductively coupled-plasma power, the chamber pressure, the DC bias and the ratio of SiCl$_4$ to SF$_6$. The process enables the use of thin Al$_\textrm{x}$Ga$_{1-\textrm{x}}$P stop layers even at aluminum contents of a few percent. △ Less

Submitted 19 January, 2018; originally announced January 2018.

Comments: 21 pages, 9 figures

Journal ref: J. Phys. D: Appl. Phys. 51 (2018) 185203

Gallium phosphide-on-silicon dioxide photonic devices

Authors: Katharina Schneider, Pol Welter, Yannick Baumgartner, Herwig Hahn, Lukas Czornomaz, Paul Seidler

Abstract: The development of integrated photonic circuits utilizing gallium phosphide requires a robust, scalable process for fabrication of GaP-on-insulator devices. Here we present the first GaP photonic devices on SiO$_2$. The process exploits direct wafer bonding of a GaP/Al$_x$Ga$_{1-x}$P/GaP heterostructure onto a SiO$_2$-on-Si wafer followed by removal of the GaP substrate and the Al$_x$Ga$_{1-x}$P s… ▽ More The development of integrated photonic circuits utilizing gallium phosphide requires a robust, scalable process for fabrication of GaP-on-insulator devices. Here we present the first GaP photonic devices on SiO$_2$. The process exploits direct wafer bonding of a GaP/Al$_x$Ga$_{1-x}$P/GaP heterostructure onto a SiO$_2$-on-Si wafer followed by removal of the GaP substrate and the Al$_x$Ga$_{1-x}$P stop layer. Photonic devices such as grating couplers, waveguides, and ring resonators are patterned by inductively coupled-plasma reactive-ion etching in the top GaP device layer. The peak coupling efficiency of the fabricated grating couplers is as high as 4.8 dB. Optical quality factors of 17000 as well as second- and third-harmonic generation are observed with the ring resonators. Because the large bandgap of GaP provides for low two-photon absorption at telecommunication wavelengths, the high-yield fabrication of GaP-on-insulator photonic devices enabled by this work is especially interesting for applications in nanophotonics, where high quality factors or low mode volumes can produce high electric field intensities. The large bandgap also enables integrated photonic devices operating at visible wavelengths. △ Less

Submitted 8 January, 2018; originally announced January 2018.

Journal ref: Journal of Lightwave Technology ( Volume: 36, Issue: 14, July15, 15 2018 )

From partition identities to a combinatorial approach to explicit Satake inversion

Authors: Heekyoung Hahn, JiSun Huh, EunSung Lim, Jaebum Sohn

Abstract: In this paper, we provide combinatorial proofs for certain partition identities which arise naturally in the context of Langlands' beyond endoscopy proposal. These partition identities motivate an explicit plethysm expansion of $\mathrm{Sym}^j(\mathrm{Sym}^kV)$ for $\mathrm{GL}_2$ in the case $k=3$. We compute the plethysm explicitly for the cases $k=3, 4$. Moreover, we use these expansions to exp… ▽ More In this paper, we provide combinatorial proofs for certain partition identities which arise naturally in the context of Langlands' beyond endoscopy proposal. These partition identities motivate an explicit plethysm expansion of $\mathrm{Sym}^j(\mathrm{Sym}^kV)$ for $\mathrm{GL}_2$ in the case $k=3$. We compute the plethysm explicitly for the cases $k=3, 4$. Moreover, we use these expansions to explicitly compute the basic function attached to the symmetric power $L$-function of $\mathrm{GL}_2$ for these two cases. △ Less

Submitted 9 June, 2017; originally announced June 2017.

MSC Class: Primary 11P84; 11S40; Secondary 05E05; 05E10

Impact of Casimir-Polder interaction on Poisson-spot diffraction at a dielectric sphere

Authors: Joshua Leo Hemmerich, Robert Bennett, Thomas Reisinger, Stefan Nimmrichter, Johannes Fiedler, Horst Hahn, Herbert Gleiter, Stefan Yoshi Buhmann

Abstract: Diffraction of matter-waves is an important demonstration of the fact that objects in nature possess a mixture of particle-like and wave-like properties. Unlike in the case of light diffraction, matter-waves are subject to a vacuum-mediated interaction with diffraction obstacles. Here we present a detailed account of this effect through the calculation of the attractive Casimir-Polder potential be… ▽ More Diffraction of matter-waves is an important demonstration of the fact that objects in nature possess a mixture of particle-like and wave-like properties. Unlike in the case of light diffraction, matter-waves are subject to a vacuum-mediated interaction with diffraction obstacles. Here we present a detailed account of this effect through the calculation of the attractive Casimir-Polder potential between a dielectric sphere and an atomic beam. Furthermore, we use our calculated potential to make predictions about the diffraction patterns to be observed in an ongoing experiment where a beam of indium atoms is diffracted around a silicon dioxide sphere. The result is an amplification of the on-axis bright feature which is the matter-wave analogue of the well-known `Poisson spot' from optics. Our treatment confirms that the diffraction patterns resulting from our complete account of the sphere Casimir-Polder potential are indistinguishable from those found via a large-sphere non-retarded approximation in the discussed experiments, establishing the latter as an adequate model. △ Less

Submitted 30 June, 2016; originally announced June 2016.

Comments: 13 pages, 10 figures

Journal ref: Phys. Rev. A 94, 023621 (2016)

Tailoring Magnetic Frustration in Strained Epitaxial FeRh Films

Authors: Ralf Witte, Robert Kruk, Markus E. Gruner, Richard A. Brand, Di Wang, Sabine Schlabach, Andre Beck, Virgil Provenzano, Rossitza Pentcheva, Heiko Wende, Horst Hahn

Abstract: We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electron microscopy and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonal-to-orthorhombi… ▽ More We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electron microscopy and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonal-to-orthorhombic transition, triggered by a band-Jahn-Teller-type lattice instability. The collapse of magnetic order is a direct consequence of this structural change, which upsets the subtle balance between ferromagnetic nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated antiferromagnetic coupling among localized Fe moments at larger distances. △ Less

Submitted 29 February, 2016; originally announced February 2016.

Comments: 9 pages, 9 figures, accepted for publication in Physical Review B

Journal ref: Phys. Rev. B 93, (2016) 104416

Single-ion microwave near-field quantum sensor

Authors: M. Wahnschaffe, H. Hahn, G. Zarantonello, T. Dubielzig, S. Grondkowski, A. Bautista-Salvador, M. Kohnen, C. Ospelkaus

Abstract: We develop an intuitive model of 2D microwave near-fields in the unusual regime of centimeter waves localized to tens of microns. Close to an intensity minimum, a simple effective description emerges with five parameters which characterize the strength and spatial orientation of the zero and first order terms of the near-field, as well as the field polarization. Such a field configuration is reali… ▽ More We develop an intuitive model of 2D microwave near-fields in the unusual regime of centimeter waves localized to tens of microns. Close to an intensity minimum, a simple effective description emerges with five parameters which characterize the strength and spatial orientation of the zero and first order terms of the near-field, as well as the field polarization. Such a field configuration is realized in a microfabricated planar structure with an integrated microwave conductor operating near 1 GHz. We use a single 9Be+ ion as a high-resolution quantum sensor to measure the field distribution through energy shifts in its hyperfine structure. We find agreement with simulations at the sub-micron and few-degree level. Our findings give a clear and general picture of the basic properties of oscillatory 2D near-fields with applications in quantum information processing, neutral atom trapping and manipulation, chip-scale atomic clocks, and integrated microwave circuits. △ Less

Submitted 6 July, 2021; v1 submitted 24 January, 2016; originally announced January 2016.

Journal ref: Applied Physics Letters 110, 034103 (2017)

On Classical groups detected by the tensor third representation

Authors: Heekyoung Hahn

Abstract: Motivated by the Langlands' beyond endoscopy proposal for establishing functoriality, we study the representation $\otimes^3$ in a setting related to the Langlands $L$-functions $L(s,π,\,\otimes^3),$ where $π$ is a cuspidal automorphic representation of $G$ where $G$ is either $\mathrm{SO}(2n+1)$, $\mathrm{Sp}(2n)$ and $\mathrm{SO}(2n)$. In particular, under what conditions on partitions $λ$, we e… ▽ More Motivated by the Langlands' beyond endoscopy proposal for establishing functoriality, we study the representation $\otimes^3$ in a setting related to the Langlands $L$-functions $L(s,π,\,\otimes^3),$ where $π$ is a cuspidal automorphic representation of $G$ where $G$ is either $\mathrm{SO}(2n+1)$, $\mathrm{Sp}(2n)$ and $\mathrm{SO}(2n)$. In particular, under what conditions on partitions $λ$, we examine whether or not $\otimes^3$ detects the subgroups $\mathbb{S}_{[λ]}(G)$ for $G$ with type $B_n$ and $D_{2n}$ or $\mathbb{S}_{\langleλ\rangle}(G)$ for $G$ with type $C_n$. Here $\mathbb{S}_{[λ]}$ and $\mathbb{S}_{\langleλ\rangle}$ are the usual Schur functors associated to the partition $λ$. △ Less

Submitted 23 November, 2015; v1 submitted 10 October, 2015; originally announced October 2015.

MSC Class: Primary 11F70; Secondary 20G05

The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Modeling the clustering and halo occupation distribution of BOSS-CMASS galaxies in the Final Data Release

Authors: Sergio A. Rodríguez-Torres, Chia-Hsun Chuang, Francisco Prada, Hong Guo, Anatoly Klypin, Peter Behroozi, Chang Hoon Hahn, Johan Comparat, Gustavo Yepes, Antonio D. Montero-Dorta, Joel R. Brownstein, Claudia Maraston, Cameron K. McBride, Jeremy Tinker, Stefan Gottlöber, Ginevra Favole, Yiping Shu, Francisco-Shu Kitaura, Adam Bolton, Román Scoccimarro, Lado Samushia, David Schlegel, Donald P. Schneider, Daniel Thomas

Abstract: We present a study of the clustering and halo occupation distribution of BOSS CMASS galaxies in the redshift range 0.43 < z < 0.7 drawn from the Final SDSS-III Data Release. We compare the BOSS results with the predictions of a Halo Abundance Matching (HAM) clustering model that assigns galaxies to dark matter halos selected from the large BigMultiDark $N$-body simulation of a flat $Λ$CDM Planck c… ▽ More We present a study of the clustering and halo occupation distribution of BOSS CMASS galaxies in the redshift range 0.43 < z < 0.7 drawn from the Final SDSS-III Data Release. We compare the BOSS results with the predictions of a Halo Abundance Matching (HAM) clustering model that assigns galaxies to dark matter halos selected from the large BigMultiDark $N$-body simulation of a flat $Λ$CDM Planck cosmology. We compare the observational data with the simulated ones on a light-cone constructed from 20 subsequent outputs of the simulation. Observational effects such as incompleteness, geometry, veto masks and fiber collisions are included in the model, which reproduces within 1-$σ$ errors the observed monopole of the 2-point correlation function at all relevant scales: from the smallest scales, 0.5 $h^{-1}$ Mpc, up to scales beyond the Baryonic Acoustic Oscillation feature. This model also agrees remarkably well with the BOSS galaxy power spectrum (up to $k\sim1$ $h$ Mpc$^{-1}$), and the Three-point correlation function. The quadrupole of the correlation function presents some tensions with observations. We discuss possible causes that can explain this disagreement, including target selection effects. Overall, the standard HAM model describes remarkably well the clustering statistics of the CMASS sample. We compare the stellar to halo mass relation for the CMASS sample measured using weak lensing in the CFHT Stripe 82 Survey with the prediction of our clustering model, and find a good agreement within 1-$σ$. The BigMD-BOSS light-cone including properties of BOSS galaxies and halo properties is made publicly available. △ Less

Submitted 3 May, 2016; v1 submitted 21 September, 2015; originally announced September 2015.

Comments: 16 pages, 16 figures. See companion papers that share the "The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey:" title

On tensor third $L$-functions of automorphic representations of $\mathrm{GL}_n(\mathbb{A}_F)$

Authors: Heekyoung Hahn

Abstract: Langlands' beyond endoscopy proposal for establishing functoriality motivates interesting and concrete problems in the representation theory of algebraic groups. We study these problems in a setting related to the Langlands $L$-functions $L(s,π,\,\otimes^3),$ where $π$ is a cuspidal automorphic representation of $\mathrm{GL}_n(\mathbb{A}_F)$ where $F$ is a global field. Langlands' beyond endoscopy proposal for establishing functoriality motivates interesting and concrete problems in the representation theory of algebraic groups. We study these problems in a setting related to the Langlands $L$-functions $L(s,π,\,\otimes^3),$ where $π$ is a cuspidal automorphic representation of $\mathrm{GL}_n(\mathbb{A}_F)$ where $F$ is a global field. △ Less

Submitted 6 September, 2015; originally announced September 2015.

Longitudinal Antigenic Sequences and Sites from Intra-Host Evolution (LASSIE) Identifies Immune-Selected HIV Variants

Authors: Peter Hraber, Bette Korber, Kshitij Wagh, Elena E. Giorgi, Tanmoy Bhattacharya, S. Gnanakaran, Alan S. Lapedes, Gerald H. Learn, Edward F. Kreider, Yingying Li, George M. Shaw, Beatrice H. Hahn, David C. Montefiori, S. Munir Alam, Mattia Bonsignori, M. Anthony Moody, Hua-Xin Liao, Feng Gao, Barton F. Haynes

Abstract: Within-host genetic sequencing from samples collected over time provides a dynamic view of how viruses evade host immunity. Immune-driven mutations might stimulate neutralization breadth by selecting antibodies adapted to cycles of immune escape that generate within-subject epitope diversity. Comprehensive identification of immune-escape mutations is experimentally and computationally challenging.… ▽ More Within-host genetic sequencing from samples collected over time provides a dynamic view of how viruses evade host immunity. Immune-driven mutations might stimulate neutralization breadth by selecting antibodies adapted to cycles of immune escape that generate within-subject epitope diversity. Comprehensive identification of immune-escape mutations is experimentally and computationally challenging. With current technology, many more viral sequences can readily be obtained than can be tested for binding and neutralization, making down-selection necessary. Typically, this is done manually, by picking variants that represent different time-points and branches on a phylogenetic tree. Such strategies are likely to miss many relevant mutations and combinations of mutations, and to be redundant for other mutations. Longitudinal Antigenic Sequences and Sites from Intrahost Evolution (LASSIE) uses transmitted-founder loss to identify virus "hot-spots" under putative immune selection and chooses sequences that represent recurrent mutations in selected sites. LASSIE favors earliest sequences in which mutations arise. With well-characterized longitudinal Env sequences, we confirmed selected sites were concentrated in antibody contacts and selected sequences represented diverse antigenic phenotypes. Practical applications include rapidly identifying immune targets under selective pressure within a subject, selecting minimal sets of reagents for immunological assays that characterize evolving antibody responses, and for immunogens in polyvalent "cocktail" vaccines. △ Less

Submitted 4 August, 2015; v1 submitted 3 August, 2015; originally announced August 2015.

Comments: 73 pages, 10 figures, 8 supplemental figures; 18 full-resolution figures appear at the end of the manuscript. Presented at 22nd International Workshop on HIV Dynamics and Evolution, Budapest, May 2015

Convolution sums of some functions on divisors

Authors: Heekyoung Hahn

Abstract: One of the main goals in this paper is to establish convolution sums of functions for the divisor sums $\widetildeσ_s(n)=\sum_{d|n}(-1)^{d-1}d^s$ and $\widehatσ_s(n)=\sum_{d|n}(-1)^{\frac{n}{d}-1}d^s$, for certain $s$, which were first defined by Glaisher. We first introduce three functions $\mathcal{P}(q)$, $\mathcal{E}(q)$, and $\mathcal{Q}(q)$ related to $\widetildeσ(n)$, $\widehatσ(n)$, and… ▽ More One of the main goals in this paper is to establish convolution sums of functions for the divisor sums $\widetildeσ_s(n)=\sum_{d|n}(-1)^{d-1}d^s$ and $\widehatσ_s(n)=\sum_{d|n}(-1)^{\frac{n}{d}-1}d^s$, for certain $s$, which were first defined by Glaisher. We first introduce three functions $\mathcal{P}(q)$, $\mathcal{E}(q)$, and $\mathcal{Q}(q)$ related to $\widetildeσ(n)$, $\widehatσ(n)$, and $\widetildeσ_3(n)$, respectively, and then we evaluate them in terms of two parameters $x$ and $z$ in Ramanujan's theory of elliptic functions. Using these formulas, we derive some identities from which we can deduce convolution sum identities. We discuss some formulae for determining $r_s(n)$ and $δ_s(n)$, $s=4,$ $8$, in terms of $\widetildeσ(n)$, $\widehatσ(n)$, and $\widetildeσ_3(n)$, where $r_s(n)$ denotes the number of representations of $n$ as a sum of $s$ squares and $δ_s(n)$ denotes the number of representations of $n$ as a sum of $s$ triangular numbers. Finally, we find some partition congruences by using the notion of colored partitions. △ Less

Submitted 15 July, 2015; originally announced July 2015.

Comments: This is an old paper uploaded for archival purposes

Journal ref: Rocky Mt. J. Math., Vol 37, No 5 (2007), 315-344