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Physics Condensed Matter

cond-mat Watch Condensed Matter

Saturday 7 March

Materials Science

High Thermal Conductivity in Back-End-of-Line Compatible AlN Thin Films

Xufei Guo, Zirou Chen, Zifeng Huang et al. (+3 more)

With thermal issues becoming a major challenge to the development of integrated circuits (ICs), high-thermal-conductivity (high-TC) materials are gaining interest from both the industry and academia, especially for high-density back-end-of-line (BEOL) structures. Aluminum nitride (AlN) is an insulating material with high TC, suitable for thermal management in electronic devices. Furthermore, polycrystalline AlN thin films can be deposited at BEOL-compatible low temperatures (lower than 400 C) while retaining relatively high TC, rendering AlN a promising candidate for BEOL heat dissipation. Still, AlN deposition should aim at achieving high TC on a variety of substrates used in IC processes. In this work, we obtained 600- and 1200-nm BEOL-compatible AlN thin film samples on Si, SiO, SiN, and Al2O3 substrates for structural and thermal characterizations. Specifically, time-domain thermoreflectance (TDTR) measurements revealed consistently high TC (higher than 45 W m-1 K-1) on different substrates. Moreover, finite element analysis (FEA) simulations of AlN capped on top of an indium-tin-oxide (ITO) transistor showed a reduction of up to 44% in peak device temperature. Our work provided experimental and calculational evidence for the practicality of leveraging AlN as a high-TC, BEOL-compatible heat spreader material.
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All authors Xufei Guo, Zirou Chen, Zifeng Huang, Yuxiang Wang, Jinwen Liu, Zhe Cheng

Soft Condensed Matter

Realizing microrheological response of configurable viscoelastic media with a dynamic optical trap

Sanatan Halder and Manas Khan

The local viscoelastic (VE) environment governs the motion of an embedded microsphere and consequently, pertinent dynamical phenomena. However, studying such phenomena with varying VE properties remains challenging for various reasons, including the strong coupling among the VE parameters and their dependence on experimental conditions, such as temperature. Here, we demonstrate the experimental realization of configurable VE media with broad variations, wherein the VE properties can be systematically and independently tuned, employing a dynamic optical trap. Specifically, the dynamics of a particle in a slowly diffusing optical trap provides the linear microrheological response of single-relaxation VE fluids, namely Jeffreys or Maxwell-Voigt (MV) fluids, where the trap strength and its diffusion coefficient regulate the elastic response and the low-frequency viscosity, respectively. The characteristic features in the mean square displacement (MSD) of the trapped particle match those of a probe particle in real MV fluids, and the simulation results following harmonically bound Brownian particle with long-time diffusion model describing single-relaxation complex fluids. Our scheme is further validated by demonstrating excellent quantitative agreement between the experimentally observed MSDs of the trapped bead and those from the corresponding analytical predictions. We extend the applicability of this scheme for realizing the microrheological response of double-relaxation VE media by incorporating appropriately correlated noise in the trap trajectory, signifying its validity for any linear VE media with multiple relaxations. Our scheme can be further extended to realize probe particle dynamics in an active VE environment, e.g., an entangled network of active polymers, by translating the trap along an active Brownian trajectory.
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Materials Science

Impacts of Fermi Level Pinning at Hole-Selective Contacts in CdSeTe/CdTe Solar Cells

Ariful Islam, Nathan D. Rock, Kh. Aaditta Arnab et al. (+3 more)

P-type doped CdTe free surfaces Schottky contacts, and even interfaces with isostructural p-ZnTe frequently exhibit downward band bending and moderate to high recombination velocities. Fermi level pinning by donor-like states can explain these band diagram features, as well as device response characteristics such as 1st quadrant rollover in current-voltage (JV) versus temperature (JVT). Parasitic downward band bending also produces voltage-dependent photocurrent collection, producing fill factor (FF) efficiency losses, JV dark/light non-superposition (or JV take-off), and irregularities in Jsc-Voc and Suns-Voc measurements. Herein, we develop a device physics model of state-of-the-art CdSeTe/CdTe solar cells consistent with known characterization of materials and devices, including the optical, thermalization, and trapping effects of band tail states and isolated defects. We use this model to demonstrate that Fermi-level pinning at the p-ZnTe/p-CdSeTe hole contact by donor-like defects reproduces the aforementioned observables, and conclude that (for contemporary few-um absorber thicknesses and low mobilities) it primarily affects FF rather than Voc. We investigate the performance gains possible from hypothetical passivated, hole-selective layers at the ZnTe/CdTe interface, which eliminate the downwards band bending caused by donor-like defects. For thinner devices and larger minority carrier diffusion lengths, these strategies will become more important for continued efficiency improvements.
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All authors Ariful Islam, Nathan D. Rock, Kh. Aaditta Arnab, Nicholas Miller, James Becker, Michael A. Scarpulla

Soft Condensed Matter

Nematic Bubbles and the Breaking of Spherical Symmetry

Gaetano Napoli and Silvia Paparini

The emergence of nematic order on deformable closed surfaces plays a pivotal role in the morphogenesis of active biological matter, such as the regeneration of Hydra. In this work, we present a continuum model that couples the two-dimensional Landau-de Gennes order tensor, describing in-plane nematic ordering, with the mechanics of a mass-conserving, deformable spherical shell. By investigating the isotropic-to-nematic phase transition driven by a reduction in temperature, mimicking the natural induction of nematic order in actomyosin fibres, we perform both linear and weakly non-linear bifurcation analyses. The onset of nematic ordering spontaneously breaks spherical symmetry, yielding distinct equilibrium morphologies governed by the shell's deformability. Axisymmetric configurations, featuring two +1 defects at the poles, emerge via a discontinuous bifurcation, resulting in a globally stable prolate shape, alongside a metastable oblate shape. Non-axisymmetric configurations, featuring four +1/2 defects arranged in a square, arise via a continuous bifurcation. Shell softness drives the first-order character of the transition, while in the limit of infinite stiffness all bifurcations become continuous. Integer defects strongly couple with local mass redistribution, manifesting as shell thinning or thickening, whilst half-integer defects induce no such local deformation. These findings provide a purely mechanical framework for understanding body-axis formation and defect-mediated morphogenesis in biological vesicles.
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Superconductivity

Higgs gap modes in superconducting circuit quantisation

Yun-Chih Liao, Ben J. Powell and Thomas M. Stace

We extend a recently developed projective circuit quantisation approach to incorporate superconducting Higgs modes associated to gap dynamics. This approach starts from a microscopic fermionic Hamiltonian for mesoscopic superconductors, and projects the system onto its low-energy "BCS" Hilbert space. We derive analytical results for the superconducting Higgs mass, "spring" constant, and oscillation frequency of the gap dynamics, which we validate numerically. We compute anharmonic corrections to the Higgs frequency for higher excitations of small superconducting islands, and compare our results to previous long-wavelength calculations.
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Statistical Mechanics · Cross-listed from quant-ph

Quantum criticality in sub-Ohmic systems with three competing terms: beyond conventional spin-boson physics

Nengji Zhou, Yulong Shen and Zhe Sun

Quantum phase transitions (QPTs) in the spin-boson model with/without the rotating-wave approximation (RWA) are systematically investigated through variational calculations using a sub-Ohmic bath with high spectral density. Four cases involving different system-environment interactions are examined, where transition points and critical exponents are accurately determined across varying tunneling strengths. Contrary to prior work, a rich phase diagram is revealed in the tunneling-coupling plane even at the low spectral exponent $s<1/2$, with a novel U(1)-symmetric phase being identified. As coupling increases, a multi-stage QPT sequence arises for the tunneling $0<\Delta < \Delta^*=0.074(1)$, whereas a single transition occurs beyond this range. Furthermore, an odd-parity phase is found to emerge even under the positive tunneling, exhibiting distinct characteristics relative to the prototype model.
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Materials Science

Crystal electric field excitations and spin dynamics in a spin-orbit coupled distorted honeycomb magnet BiErGeO$_5$

S. Mohanty, S. Guchhait, S. S. Islam et al. (+9 more)

The magnetic properties and crystal electric field (CEF) scheme of BiErGeO$_5$ are investigated via magnetization, heat capacity, muon spin relaxation (muSR), and inelastic neutron scattering (INS) experiments on a polycrystalline sample. The Er$^{3+}$ ions form a quasi-two-dimensional distorted honeycomb network with a Kramers doublet ground state. Magnetic susceptibility and heat capacity reveal short-range antiferromagnetic correlations, manifested as a broad maximum around 1.4 K. Heat-capacity data further confirm the onset of a magnetic long-range order at $T_ N = 0.4$ K. The INS spectra exhibit eight CEF excitations and the CEF analysis yields the $g$-factor anisotropy with $g_{xy}/g_{z} = 1.38$ and exchange anisotropy with $J_{xy} = 2.96$ K and $J_{z} = 1.56$ K. The experimental temperature and field dependent magnetization and heat capacity are also reproduced by the simulation using CEF energy scheme. Zero-field muSR measurements down to 30 mK, do not exhibit coherent oscillations or a static 1/3 tail. The spectra are well described by two exponential relaxation components, indicating two magnetically inequivalent muon environments. The relaxation rates display a nearly temperature-independent plateau below $T_{\rm N}$ and follow an Orbach-type activated behavior at higher temperatures involving excited CEF levels, consistent with the INS results. Longitudinal-field $\mu$SR measurements reveal only weak decoupling up to 1.5 T, indicating persistent slow spin fluctuations below $T_{\rm N}$.
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All authors S. Mohanty, S. Guchhait, S. S. Islam, Surya P. Patra, M. P. Saravanan, J. A. Krieger, T. J. Hicken, H. Luetkens, D. T. Adroja, Goran J. Nilsen, M. D. Le, R. Nath

Materials Science

Bulk OsO2 Single Crystals: Superior Catalysts for Water Oxidation

Guojian Zhao, Zhihao Li, Ziang Meng et al. (+13 more)

Although rutile RuO2 has been a well-known and almost the best oxygen evolution reaction (OER) catalyst, the OER properties for the similar rutile oxide OsO2 with the same group element with Ru have been unknown, mainly due to long-standing synthesis difficulties. In this work, we report the successful synthesis of high-quality OsO2 single crystals, and the ground micrometer-size single crystals are chemically stable in alkaline solutions and exhibit robust OER performance. In sharp contrast, OsO2 nanopowder reacts quickly with KOH solutions and cannot work for OER. Compared with commercial RuO2 nanopowder, the OsO2 single crystals show comparable catalytic current densities, remarkably lower overpotentials at high current densities and better stability. These findings question the universal applicability of nanoscaling and highlight crystal integrity as a key descriptor for achieving stable and efficient OER electrocatalysis.
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All authors Guojian Zhao, Zhihao Li, Ziang Meng, Shucheng Wang, Li Liu, Zhiyuan Duan, Xiaoning Wang, Hongyu Chen, Yuzhou He, Jingyu Li, Sixu Jiang, Xiaoyang Tan, Qinghua Zhang, Qianfan Zhang, Peixin Qin, Zhiqi Liu

Mesoscale and Nanoscale Physics

Pressure-Induced Metal-Insulator and Paramagnet-Altermagnet Transitions in Rutile OsO2 Single Crystals

Guojian Zhao, Ziang Meng, Wencheng Huang et al. (+19 more)

Altermagnets with compensated spin structures and nonrelativistic spin splitting have emerged as a new class of magnetic materials. Rutile OsO2 has been theoretically predicted to be altermagnetic, but experimental studies have been limited by synthesis challenges. We have succeeded in synthesizing high-quality single crystals of rutile OsO2. Electrical transport studies reveal that OsO2 is highly conductive and exhibits clear Fermi liquid behavior, indicating strong electron-electron scattering. Magnetic measurements show that the crystals are isotropically paramagnetic. Density-functional theory calculations indicate that bulk OsO2 is semimetallic with coexisting electron and hole pockets, with its magnetic ground state strongly dependent on the on-site Coulomb correlation U. Angle-resolved photoemission spectroscopy studies unveil that the bulk bands do not yet show altermagnetic spin splitting. Interestingly, resistivity is rather pressure sensitive: at 44 GPa, a clear metal-insulator transition occurs. Hybrid functional calculations reveal that applying pressure significantly increases the Hubbard U value, driving a phase transition from a paramagnetic metal to an altermagnetic metal, and eventually to an altermagnetic insulator. These findings suggest that tuning external pressure effectively modulates the magnetic ground state of OsO2, providing a pathway to realize altermagnetism in this material.
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All authors Guojian Zhao, Ziang Meng, Wencheng Huang, Peixin Qin, Shaoheng Ruan, Liang Ma, Lin Zhu, Yuzhou He, Li Liu, Zhiyuan Duan, Xiaoning Wang, Hongyu Chen, Sixu Jiang, Jingyu Li, Xiaoyang Tan, K. Ozawa, Bosen Wang, Jinguang Cheng, Qinghua Zhang, Jianfeng Wang, Chaoyu Chen, Zhiqi Liu

Materials Science

A general statistical framework for vacancy and self-interstitial properties in concentrated multicomponent solids

Jacob Jeffries, Hyunsoo Lee, Anter El-Azab et al. (+1 more)

A rigorous understanding of the thermodynamic properties of point defects, namely vacancies and self-interstitials, is crucial for the discovery and screening of structural materials in clean energy applications. In this work, we extend a previously-developed statistical framework for predicting the thermodynamics of single-site impurities to further predict the thermodynamics of self-interstitial dumbbells in an arbitrarily complex alloy. We then apply this extended framework to compute effective formation energies in fully disordered Fe-Cr and Cu-Ni alloys. Notably, we predict that some self-interstitial dumbbell types that are high-energy in pure Fe become stabilized by Cr. We additionally describe a symmetry-breaking effect, wherein high solute concentrations distort the defect free energy surface, yielding misaligned self-interstitials.
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All authors Jacob Jeffries, Hyunsoo Lee, Anter El-Azab, Enrique Martinez

Materials Science

Cooperative Quantum Optical Effects of Moiré Exciton Superlattices

Haowei Xu, Wang Yao and Ju Li

The unique properties of two-dimensional moiré systems have been widely studied from many perspectives. However, relatively little work has investigated how the real-space structure of moiré systems can directly engender novel properties and functionalities. In this Letter, we explore how intriguing cooperative optical effects can emerge from the real-space superlattice formed by moiré excitons. Particularly, we show that the collective moiré exciton states can have either strongly enhanced (superradiant) or suppressed (subradiant) radiative decay rates, depending on their in-plane wave vector. These super- and subradiant states can be efficiently switched by a gate-induced electric field gradient. Moreover, the cooperative transmittance T of the atomically thin moiré system can be switched from $T\approx 0$ (opaque) to $T \approx 1$ (transparent) with $\lesssim ~1\%$ heterostrain or a $1^{\circ}$ adjustment in the twist angle ${\theta}$. These features are robust against nonradiative losses and disorders, making the moiré system a highly versatile platform for cooperative quantum optics with potential applications in, e.g., single photon storage and switching.
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Strongly Correlated Electrons

Flat Topological Nodal Lines in Heavy-Fermion Compound CeCoGe$_3$

Yuting Wang, Weikang Wu and Jianzhou Zhao

The interplay between strong electronic correlations, unconventional superconductivity, and symmetry-protected topology provides a fertile ground for discovering exotic quantum states. In this work, we investigate the correlated electronic structure and topological properties of the heavy fermion material CeCoGe$_3$ using density functional theory combined with dynamical mean-field theory calculations. Our results reveal a crossover from high temperature incoherent states to low temperature coherent heavy quasiparticles, accompanied by a mass enhancement of $m^*/m_{\text{DFT}}\sim 52.6$ at $T=25$ K. The interplay between electronic correlation, spin-orbit coupling and the noncentrosymmetric $I4mm$ crystal symmetry stabilize flat topological nodal lines within 10 meV of the Fermi level, which could contribute a significant density of states. The proximity of topological nodal lines to the Fermi surface suggests a potential role in mediating pressure induced unconventional superconductivity. Our work establishes CeCoGe$_3$ as a prototype topological nodal line Kondo semimetal. The coexistence of strong correlation, non-trivial band topology and superconductivity indicate CeCoGe$_3$ as a potential candidate for realizing topological superconductivity.
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Materials Science

Electronic manifolds for extrapolative alloy discovery

Pranoy Ray, Sayan Bhowmik, Phanish Suryanarayana et al. (+2 more)

This study presents a computationally efficient framework for accelerated alloy discovery that uses the non-interacting electron density to capture intrinsic structure-property relationships in refractory high-entropy alloys (HEAs). Unlike state-of-the-art approaches relying on expensive, self-consistent density functional theory calculations, our method employs the non-interacting electron density as the primary structural descriptor. By extracting physical features through directionally resolved two-point spatial correlations and compressing them via Principal Component Analysis, we efficiently map the design space. Coupling these descriptors with Bayesian active learning, we achieve a normalized mean absolute error (NMAE) of <2% for the bulk modulus of Al-Nb-Ti-Zr alloys using only 10 training samples (<0.2% of the dataset). Furthermore, we demonstrate that the model learns an electronic packing manifold that is transferable within the refractory BCC alloy family. Validated on a distinct 7-component refractory system (Mo-Nb-Ta-Ti-V-W-Zr) containing four elements entirely absent from the training data, the framework enables zero-shot transfer within the refractory BCC alloy class. Moreover, by augmenting the base model with just 20 samples from the target domain, we achieve high-fidelity predictions (NMAE<3%) for 7-component alloys, reducing data acquisition costs by orders of magnitude compared to standard workflows. A controlled comparison confirms that composition-based descriptors under the identical pipeline do not reach the same accuracy threshold within the same sample budget, establishing that the spatial autocorrelation encoding of the non-interacting electron density provides information beyond elemental composition statistics alone.
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All authors Pranoy Ray, Sayan Bhowmik, Phanish Suryanarayana, Surya R. Kalidindi, Andrew J. Medford

Friday 6 March

Soft Condensed Matter · Cross-listed from cs.LG

Physics-Consistent Neural Networks for Learning Deformation and Director Fields in Microstructured Media with Loss-Based Validation Criteria

Milad Shirani, Pete H. Gueldner, Murat Khidoyatov et al. (+2 more)

In this work, we study the mechanical behavior of solids with microstructure using the framework of Cosserat elasticity with a single unit director. This formulation captures the coupling between deformation and orientational fields that arises in many structured materials. To compute equilibrium configurations of such media, we develop two complementary computational approaches: a finite element formulation based on variational principles and a neural network-based solver that directly minimizes the total potential energy. The neural architecture is constructed to respect the fundamental kinematic structure of the theory. In particular, it enforces frame invariance of the energy, satisfies the unit-length constraint on the director field, and represents deformation and director fields through separate networks to preserve their kinematic independence in the variational setting. Beyond satisfying balance laws, however, physically admissible solutions must also correspond to stable energy minimizers. To assess this requirement, we derive the quasiconvexity condition, rank-one convexity condition, and the Legendre-Hadamard inequalities for the Cosserat model and formulate them in a manner suitable for evaluating neural network predictions. These necessary stability conditions provide a physics-based validation framework: network outputs that violate these necessary conditions cannot correspond to stable energy minimizers and can therefore be rejected. In this way, we integrate classical variational stability theory with modern machine-learning solvers, establishing a computational workflow in which equilibrium solutions are not only learned but also assessed for energetic consistency.
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All authors Milad Shirani, Pete H. Gueldner, Murat Khidoyatov, Jeremy L. Warren, Federica Ninno

Mesoscale and Nanoscale Physics

Non-equilibrium formulation of helicity-dependent thermal field for ultrafast magnetization dynamics

Ezio Iacocca

Far-from-equilibrium magnetization dynamics can be accessed when a magnetic material is subject to a femtosecond excitation, such as an optical laser or an electric current. Numerically, the demagnetization of magnetic materials is typically modeled by atomistic spin dynamics. Micromagnetic models generally fail to reproduce ultrafast demagnetization in a grid independent manner. Here, we propose a non-equilibrium thermal field whose features depend on atomic spin flip probabilities. Under the assumption that each spin flip is equivalent to a quantum of angular momentum, equivalent temperatures on the order of thousands of Kelvin are achieved. Demagnetization is quantitatively reproduced for several cell sizes. The presented approach can be further refined and extended towards a grid-independent and multiscale modeling of ultrafast magnetization dynamics.
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Superconductivity

Capturing nuclear quantum effects in high-pressure superconducting hydrides and ice with nuclear-electronic orbital theory

Logan E. Smith, Paolo Settembri, Alessio Cucciari et al. (+3 more)

Nuclear quantum effects are essential for correctly describing hydrogen-rich materials at high pressures. Superconducting hydrides and ice are prime examples of such systems, requiring the inclusion of lattice anharmonicity and nuclear quantum effects to correctly predict and describe the structures and phase transition pressures observed experimentally. Herein, we show that the nuclear-electronic orbital density functional theory (NEO-DFT) method, which treats specified nuclei quantum mechanically on the same level as the electrons, is capable of accurately describing nuclear quantum effects in superconducting hydrides and ice. NEO-DFT predicts the hydrogen-bond symmetrization pressure in H$_3$S and D$_3$S, benchmarking against the more expensive stochastic self-consistent harmonic approximation (SSCHA) method, and predicts the correct symmetric Fm$\bar{3}$m structure for LaH$_{10}$ at a wide range of pressures. NEO-DFT also predicts the ice VIII to ice X phase transition pressures for H$_2$O and D$_2$O in agreement with experimental measurements. The accuracy, computational efficiency, and broad applicability of the NEO method opens the door for expanded large-scale studies into these types of systems.
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All authors Logan E. Smith, Paolo Settembri, Alessio Cucciari, Lilia Boeri, Gianni Profeta, Sharon Hammes-Schiffer

Materials Science

Correlations Between the Dielectric Properties, Domain Structure Morphology and Phase State of Bi1-xSmxFeO3 Nanoparticles

Oleksandr S. Pylypchuk, Vladyslav O. Kolupaiev, Victor V. Vainberg et al. (+5 more)

Nanoscale multiferroics are basic model objects for studying polar, magnetic and magnetoelectric properties and mutual couplings. Bismuth-samarium ferrite (Bi1-xSmxFeO3) is a model orthoferrite, whose polar, magnetic and magnetoelectric properties have been studied for the bulk and thin film samples. The properties of Bi1-xSmxFeO3 nanoparticles have been much less studied, despite the nanoparticles can be used in a wide range of applications, such as energy storage, magnetic hyperthermia and advanced nanoelectronics. In this work we performed experimental measurements and analysis of the temperature dependence of the Bi1-xSmxFeO3 nanopowders dielectric properties. Calculations of the ferro-ionic coupling influence on the dielectric properties, domain structure morphology and phase states are performed in the framework of the Ginzburg-Landau-Devonshire-Stephenson-Highland approach. Theoretical results explain the main trends of experimentally observed temperature dependences of the effective dielectric permittivity, which allows us to understand the correlations between the temperature behavior of dielectric properties, domain structure morphology and phase state of Bi1-xSmxFeO3 nanoparticles.
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All authors Oleksandr S. Pylypchuk, Vladyslav O. Kolupaiev, Victor V. Vainberg, Vladimir N. Poroshin, Ihor V. Fesych, Lesya D. Demchenko, Eugene A. Eliseev, Anna N. Morozovska

Mesoscale and Nanoscale Physics · Cross-listed from physics.chem-ph

For molecular polaritons, disorder and phonon timescales control the activation of dark states in the thermodynamic limit

Tianchu Li, Pranay Venkatesh, Qiang Shi et al. (+1 more)

Collective light-matter systems host an extensive manifold of dark states whose role in the emergence of thermodynamic behavior remains poorly understood, especially in the presence of disorder and structured environments. Here, we develop a hybrid matrix product state-hierarchical equations of motion (MPS-HEOM) approach that enables numerically exact simulations of polariton dynamics from a few emitters to the thermodynamic limit under both static and dynamic disorder. This allows us, for the first time, to provide a quantitative and operational answer to the long-standing question of what is the minimum system size required to reach the thermodynamic limit in collective polaritonic systems. By introducing a convergence scale, $N_{T}$, i.e., the number of molecules required for the photonic dynamics to reach the thermodynamic limit, we show that dynamic disorder generally poses a greater computational challenge than static disorder. We attribute this behavior to the suppression of collective light-matter dynamics by disorder, which dynamically activates non-collective degrees of freedom. We further find that $N_{T}$ exhibits a turnover behavior as the bath becomes more Markovian, as the bath timescales regulate bright-to-dark energy transfer and the involvement of dark and gray states. Hence, phonon timescales control both the breakdown of collective behavior and the growth of $N_{T}$. Our results establish the suppression of collective behavior as the key mechanism governing thermodynamic convergence in disordered light-matter systems.
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All authors Tianchu Li, Pranay Venkatesh, Qiang Shi, Andrés Montoya-Castillo

Materials Science · Cross-listed from physics.ins-det

Detective Quantum Efficiency of the Timepix4 Hybrid Pixel Detector and its Application to Parallel-Beam Diffraction

Zhiyuan Ding, Nina Dimova, Jonathan S. Barnard et al. (+6 more)

The detective quantum efficiency (DQE) and normalised noise power spectrum (NNPS) of the Timepix4 hybrid pixel detector in event-driven mode in TEM have been measured at 100 kV and 200 kV. In a raw data readout mode, the zero-frequency DQE exceeds 0.9 at both 100 kV and 200 kV. At the Nyquist frequency, the DQE remains above 0.2 at 100 kV but drops close to zero at 200 kV. Initial parallel-beam diffraction data from a polycrystalline gold nanoparticle sample is reported which shows that at 200 kV Timepix4 can detect weak diffracted information beyond a 75 mrad half-angle.
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All authors Zhiyuan Ding, Nina Dimova, Jonathan S. Barnard, Giulio Crevatin, Liam O'Ryan, Richard Plackett, Daniela Bortoletto, Angus I. Kirkland, Marcus Gallagher-Jones

Materials Science

Adsorption-Controlled Epitaxy and Twin Control of $\gamma$-GaSe on GaAs (111)B

Joshua Eickhoff, Wendy L. Sarney, Sina Najmaei et al. (+2 more)

The III-Se layered semiconductors, including InSe and GaSe, are promising optoelectronic materials due to their relatively high electron mobilities at room temperature, nonlinear optical responses, ferroelectricity, self-passivated van der Waals surfaces, and ability to alloy and synthesize heterostructures for bandgap engineering. Adsorption control is a widely utilized strategy for controlling the stoichiometry and phase formation of these materials; however, the bounds of the adsorption-controlled growth window for GaSe have not been systematically established. Additionally, challenges with control over polytype and twinning remain. Here, we use molecular beam epitaxy to experimentally map the adsorption-controlled growth window of GaSe films on vicinal GaAs (111)B substrates. The observed phase boundaries show qualitative agreement with Ellingham diagram predictions. All films crystallize in the $\gamma$ ($R3m$) polytype. Increasing growth and annealing temperature leads to decreased mosaicity measured by x-ray rocking curve and smoother surfaces measured by atomic force microscopy, at the expense of a transition from singly oriented $\gamma$ to twinned $\gamma$ with $60\degree$ rotated domains.
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All authors Joshua Eickhoff, Wendy L. Sarney, Sina Najmaei, Daniel A. Rhodes, Jason Kawasaki

Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Efficient construction of time-invariant process tensors for simulating high-dimensional non-Markovian open quantum systems

Émile Cochin, Jonathan Keeling, Brendon W. Lovett et al. (+1 more)

Numerical methods for obtaining exact dynamics of non-Markovian open quantum systems are mostly limited to either small systems or to short-time evolution only. Here, we propose a new algorithm for computing process tensors–matrix product operator (MPO) representations that capture the environment influence–which achieves greatly enhanced computational scalings with system size, while maintaining linear scaling with simulation length. We build on recent developments in the field which allow for long-time evolutions through process tensors which have a time-translational invariance. These can be built for general Gaussian environments and generic coupling operators with the system using infinite time-evolving block decimation (iTEBD). We introduce a modified iTEBD algorithm using intermediate compression steps which bring down the computation time scaling with system size $d$ from $\mathcal{O}(d^8)$ to $\mathcal{O}(d^4)$, as well as significantly lowering the required memory. To illustrate the power of this method, we apply it to the problem of dispersive qubit readout in circuit QED, which was previously out-of-reach numerically. The full treatment of the measurement resonator, which requires a large system space, combined with the long simulation times precipitated by the separation of timescales between the measurement drive and the environment dissipation, is now possible. The algorithm we introduce not only allows for capturing non-Markovian dynamics in large open quantum systems, but also further extends all the existing capabilities of process tensors, for example in quantum optimal control, or in computation of multi-time correlations or of steady states, to more complex systems with tens of levels.
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All authors Émile Cochin, Jonathan Keeling, Brendon W. Lovett, Alex W. Chin

Materials Science

AIMD-L: An automated laboratory for high-throughput characterization of structural materials for extreme environments

Todd C. Hufnagel, Pranav Addepalli, Anuruddha Bhattacharjee et al. (+14 more)

Rapid developments in artificial intelligence and machine learning as applied to materials science are creating an urgent need for experimental data, which can be provided by high-throughput and autonomous laboratories. To date most demonstrations of such laboratories have focused on functional materials, with less attention paid to structural materials. We present here the Artificial Intelligence in Materials Design Laboratory (AIMD-L), an automated, high-throughput facility for characterizing the microstructure and properties of structural metals and ceramics, with an emphasis on materials in extreme environments. AIMD-L has two custom instruments for characterization of structural materials: HELIX for shock studies of materials, and MAXIMA for X-ray diffraction and X-ray fluorescence spectroscopy. Specifically designed for high-throughput studies, HELIX and MAXIMA are each capable of collecting data at rates two to three orders of magnitude faster than conventional systems. A third experimental station, SPHINX, is a commercial nanoindenter modified for integration into the automated workflow of AIMD-L. A user (which may be human or an AI agent) directs the experiments to be carried out by means of a centralized control program. The experimental stations are linked by a conveyance that moves samples around the lab, with a robot at each station for sample transfer in/out of the instrument. The experimental stations also communicate with a common data layer that streams data autonomously from each instrument to a data portal, where their arrival triggers automated workflows for data reduction and analysis. The processed data are immediately available to the human operator or agentic AI, forming a closed loop for rapid decision-making and experimental control.
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All authors Todd C. Hufnagel, Pranav Addepalli, Anuruddha Bhattacharjee, Rohit Berlia, Jaafar El-Awady, David Elbert, Lori Graham-Brady, Axel Krieger, Harichandana Neralla, T. Joseph Nkansah-Mahaney, Mostafa M. Omar, Hyun Sang Park, K.T. Ramesh, Matthew Shaeffer, Eric Walker, Piyush Wanchoo, Timothy P. Weihs

Mesoscale and Nanoscale Physics

Anharmonicity and Charge-Noise Sensitivity of Fraunhofer Qubit

Longyu Ma, Tony Liu, Javad Shabani et al. (+3 more)

We present a theory of a flux-tunable superconducting qubit, the "Fraunhofer qubit," based on the Fraunhofer interference in a wide ballistic Josephson junction. As magnetic flux threads the junction, the Josephson potential is effectively averaged over a phase window proportional to flux. For perfectly transmitting junctions, as flux approaches one flux quantum h/2e, the flux averaging transforms the potential near its minimum from a quadratic to a triangular shape, resulting in significantly enhanced anharmonicity. This enhancement persists for junctions with lower transparency conducting channels. Microscopic tight-binding simulations that include inhomogeneous electrostatic potential and disorder confirm the enhancement of anharmonicity. These results establish a framework for flux control in hybrid superconducting circuits, providing an operating point where anharmonicity and charge-noise protection can be optimally balanced.
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All authors Longyu Ma, Tony Liu, Javad Shabani, Kasra Sardashti, Vladimir E. Manucharyan, Maxim G. Vavilov

Materials Science

Bridging the lab-to-fab gap in non-fullerene organic solar cells via gravure printing

Svitlana Taranenko, Chen Wang, David Holzner et al. (+10 more)

Organic solar cells have reached record efficiencies with non-fullerene acceptors, yet their translation to industrial printing remains a critical bottleneck. Here we report the highest efficiency achieved for a fully roll-to-roll-compatible gravure-printed non-fullerene organic solar cell. High-performance blends are typically optimised under laboratory coating conditions, while roll-to-roll manufacturing imposes fundamentally different constraints on ink stability, drying dynamics, and multilayer integration. Whether these constraints intrinsically limit device physics has remained unresolved. Here, we demonstrate a gravure-printed PM6:Y12 solar cell architecture using commercially available materials and establish a quantitative framework that disentangles optical, recombination, and transport losses in printed devices. We find that favourable bulk morphology and exciton harvesting can be preserved under gravure printing and non-halogenated solvents. The dominant efficiency penalties arise instead from optical interference within the printed layer stack and slow charge transport. Our results demonstrate that the performance gap between laboratory and printed solar cells is originating from device architecture rather than the intrinsic physics of modern non-fullerene systems, providing a mechanistic roadmap for roll-to-roll manufacturing of non-fullerene solar cells.
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All authors Svitlana Taranenko, Chen Wang, David Holzner, Robert Eland, Christopher Wöpke, Toni Seiler, Alexander Ehm, Fabio Le Piane, Roderick C. I. Mackenzie, Dietrich R. T. Zahn, Carsten Deibel, Arved Carl Hübler, Maria Saladina

Strongly Correlated Electrons

Reply to the comment on "Instability of the ferromagnetic quantum critical point and symmetry of the ferromagnetic ground state in 2D and 3D electron gases with arbitrary spin-orbit splitting”

Dmitry Miserev, Daniel Loss and Jelena Klinovaja

We reply on the comment of D. Belitz and T. R. Kirkpatrick on our article “Instability of the ferromagnetic quantum critical point and symmetry of the ferromagnetic ground state in 2D and 3D electron gases with arbitrary spin-orbit splitting”.

Mesoscale and Nanoscale Physics

A Compact XOR Gate Implemented With a Single Straintronic Magnetic Tunnel Junction

Supriyo Bandyopadhyay

The XOR Boolean logic gate is widely used in many applications such as encryption (XOR ciphers), binary addition (half- and full-adders), error detection (parity bits), etc. but is challenging to construct because of its demanding conditional dynamics. It typically requires multiple logic switches or other types of gates, which results in a large gate footprint and low logic density. Here, we present the design of an XOR gate with a single straintronic magnetic tunnel junction which reduces the footprint dramatically. Such a gate is non-volatile and hence suitable for non-von-Neumann architectures, processor-in-memory, etc. The switching time of the gate is ~200 ps and the energy dissipation per gate operation is ~225 aJ. Cascading of successive stages is accomplished via a CMOS device which plays no role in the gate dynamics but is needed for gain to provide logic level restoration, fan-out and isolation between input and output. This 1 MTJ-1 CMOS design has an energy dissipation that is an order of magnitude smaller than what has been reported for traditional all-transistor XOR designs.
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Superconductivity

Quasiparticle spectroscopy in tantalum films with different Ta/sapphire interfaces

Bicky S. Moirangthem, Kamal R. Joshi, Anthony P. Mcfadden et al. (+8 more)

One of the crucial aspects of current research in quantum information science is the identification and control of loss mechanisms in superconducting circuits. Although microwave measurements directly quantify device performance, additional techniques that probe quasiparticle excitations in superconducting films are needed to understand the microscopic mechanisms underlying dissipation and decoherence. Here, we present results from quasiparticle spectroscopy of Ta/sapphire films by measuring the Meissner-state magnetic susceptibility using a precision frequency-domain resonator specifically designed for thin films. We find direct evidence for additional low-energy excitations in samples with lower internal quality factors. These excitations are consistent with deep subgap states due to two-level systems, Yu-Shiba-Rusinov states near the gap edge, and perhaps other pair-breaking mechanisms. The developed non-destructive frequency-domain quasiparticle spectroscopy is a valuable addition to the quantum materials toolbox.
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All authors Bicky S. Moirangthem, Kamal R. Joshi, Anthony P. Mcfadden, Jin-Su Oh, Amlan Datta, Makariy A. Tanatar, Florent Lecocq, Raymond W. Simmonds, Lin Zhou, Matthew J. Kramer, Ruslan Prozorov

Materials Science · Cross-listed from physics.chem-ph

From Accurate Quantum Chemistry to Converged Thermodynamics for Ion Pairing in Solution

Niamh O'Neill, Benjamin X. Shi, William C. Witt et al. (+6 more)

Quantitative prediction of thermodynamic properties in solution is essential for translating atomistic simulations into reliable chemical insight. As an exemplar system, the behaviour of CaCO$_3$ in water has been widely studied to understand its mineralization in seawater, with potential implications for carbon-capture strategies. However, making accurate computational predictions has been a long-standing challenge, requiring both highly accurate electronic structure methods and extensive statistical sampling. Here, we combine advances in machine learning and electronic structure theory to fully resolve the ion pairing free energy of CaCO$_3$ with explicit solvation. We show that achieving quantitative agreement with experiment requires going beyond the standard density functional theory up to the "gold-standard" coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. We generate a set of systematically improvable models, enabling reliable insights into the initial association mechanism of Ca and CO$_3$ ions prior to nucleation while fully quantifying enthalpic and entropic effects. Our results demonstrate that CCSD(T)-level thermodynamic predictions of complex aqueous systems can now be routinely achieved.
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All authors Niamh O'Neill, Benjamin X. Shi, William C. Witt, Blake I. Armstrong, William J. Baldwin, Paolo Raiteri, Christoph Schran, Angelos Michaelides, Julian D. Gale

Mesoscale and Nanoscale Physics

Enhancing superconductivity using thermal bosons

Ekaterina Vlasiuk, Manfred Salmhofer, Eugene Demler et al. (+1 more)

We investigate how the strong coupling of a superconductor to thermal bosons can enhance its superconducting critical temperature. To tackle this problem, we use a renormalization group approach that allows us to describe the competition between density fluctuations and the build-up of boson-induced attraction between fermions. Capturing the mutual influence of bosonic and fermionic sectors, the self-consistent renormalization group framework predicts a robust increase of the critical temperature across a wide range of interactions. We find a nontrivial dependence of the critical temperature on the boson mass and we establish a phase diagram for enhanced superconductivity driven by bosons being either in the condensed or thermal state. We outline possible experimental realizations in cold atomic systems and discuss implementations using electron-exciton mixtures in van der Waals material heterostructures.
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All authors Ekaterina Vlasiuk, Manfred Salmhofer, Eugene Demler, Richard Schmidt

Materials Science

Patterns of load, elastic energy and damage in network models of architected composite materials

Christian Greff, Leon Pyka, Michael Zaiser et al. (+1 more)

We investigate the role of architected thin films in the interfacial failure properties of bi-layer composites. Our results show that, while graded structures can be used to prescribe failure at the interface, they do not offer significant advantages in terms of fracture toughness. Hierarchically patterned layers can localize failure at the interface and simultaneously enhance interface toughness, by enforcing a buffer region where elastic energy is dissipated in the form of diffuse damage, so that no stress concentration can drive crack growth. To analyze these mechanisms, the associated patterns of local load redistribution and the soft deformation modes, we develop a network formalism that brings together concepts of discrete differential geometry and spectral graph theory.
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All authors Christian Greff, Leon Pyka, Michael Zaiser, Paolo Moretti