Mesoscale and Nanoscale Physics
Evaluation of Wafer-Scale SOT-MRAM for Analog Crossbar Array Applications
Samuel Liu, Chen-Yu Hu, Ming-Yuan Song et al. (+2 more)
Analog crossbar arrays consisting of emerging
memory devices can greatly alleviate the computational strain required by vector
matrix multiplications for neural network applications. The ability to produce
spin orbit torque-
magnetic random-access
memory (SOT-MRAM) at wafer-scale positions SOT-MRAM as a strong
memory candidate. In this work, we fabricate and measure 300 mm-compatible SOT-MRAM with 150% tunnel magnetoresistance ratio, fast (2 ns) and low voltage (<1 V) operation, low energy dissipation (350 fJ), low write noise (0.1%), and low device-to-device variation of 10%. Through 2-bit quantization aware training and noisy training as mitigation techniques, the measured SOT-MRAM devices attain 95% on MNIST. The bi-stable anisotropy and
stochastic switching of SOT-MRAM can additionally be leveraged for
stochastic training of binary neural networks, able to reach ideal accuracy for a single device. Lastly, the devices were evaluated on implementation of probabilistic graph modeling and the interplay of tunnel magnetoresistance ratio, probability curve distribution, and conductance noise was shown to reduce potential errors in implementation. Through these results, SOT-MRAM is shown to be a uniquely effective candidate for implementation of crossbar accelerators in
memory- and energy-limited applications, able to take advantage of
stochastic operation and bi-stability to beneficial results in neural network applications.
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All authors Samuel Liu, Chen-Yu Hu, Ming-Yuan Song, Xinyu Bao, Jean Anne C. Incorvia
Materials Science
Insights into the adhesion and delamination strength of carbon films on metals by high-throughput ab initio calculations
Elisa Damiani, Margherita Marsili and Maria Clelia Righi
Diamond and diamond-like carbon (DLC) coatings are widely employed for their exceptional mechanical, thermal and chemical properties, but their industrial application is often limited by weak adhesion to metallic substrates. In this work, we employ a high-throughput ab initio approach to systematically investigate the adhesion of diamond/metal interfaces, combining a set of technologically relevant metals (Al, Ag, Au, Cr, Cu, Fe, Ir, Mg, Mo, Pt, Rh, Ti, V, W, Zn) with the C(111), C(111)-2x1 (Pandey reconstructed), C(110), C(100) surfaces, that are most common in diamond and are representative of different types of bonds present in DLC. Thanks to our automated and accurate computational protocol for interface construction and characterization, databases are populated and relevant trends are identified on the effect of surface graphitization, ability to form carbides and metal reactivity on carbon film adhesion and delamination strength. Beyond capturing trends, our workflow yields predictive insights. Indeed, we found that adhesion energy scales with the
geometric mean of the constituent surface energies, providing a simple descriptor for rapid screening; while comparing the work of separation with the metal's cohesive energy anticipates the fracture location under tensile loading. A novel method based on the radial distribution function g(r) analysis is introduced to identify when contact with a metal drives rehybridization of surface carbon from sp2 to sp3, the structural signature of improved resistance to delamination. These structural changes are mirrored by an electronic rearrangement at the interface, quantified by a charge-redistribution descriptor that strongly correlates with adhesion.
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Strongly Correlated Electrons
$\mathbb{Z}_2$ Universality of the Mott Transition
Jinchao Zhao, Peizhi Mai, Gaurav Tenkila et al. (+1 more)
We demonstrate that the Mott transition exhibits universal scaling as a consequence of the breaking of a $\mathbb{Z}_2$
symmetry in momentum space. A direct consequence of this discrete
symmetry breaking is the charge or Mott gap itself. From extensive numerics, we proffer that it is the charge compressibility that acts as the underlying order parameter as it is zero in the
insulator and non-zero in the metallic state. Additionally, the Widom line (temperature of the extremum of the compressibility) obeys a universal scaling of $T_m=0.39U$ deep into the insulating state directly from $Z_2$ universality. Furthermore, the temperature at which the second derivative of the compressibility has a minimum is independent of
lattice geometry, exhibiting a universal scaling of $|U-U_c|^\alpha$ where $\alpha\approx 1$. Finally, our computational approach reproduces the key features of the doping dependence of the compressibility demonstrated in recent cold-atom
quantum simulators of the Hubbard model, thereby corroborating our conclusions on $\mathbb{Z}_2$ universality.
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All authors Jinchao Zhao, Peizhi Mai, Gaurav Tenkila, Philip W. Phillips
Strongly Correlated Electrons
Scaling of the disorder operator at (3+1)D O(3) quantum criticality
Xuyang Liang, Xiao-Chuan Wu, Zenan Liu et al. (+3 more)
The disorder operator, as an easily measured nonlocal observable, displays great potential in detecting intrinsic information of field theories. It has been systematically studied in one- and two-dimensional (1D and 2D)
quantum systems, while the knowledge of 3D is still limited. The disorder operator associated with U(1) global
symmetry exhibits rich
geometric dependence on the shape of the spatial region at a
quantum critical point, meanwhile, (3+1)D is the upper critical dimension for O(N) criticality, both of which pose a challenge for exploring the disorder operator in high dimensions. In this Letter, we investigate the scaling behaviors of disorder operators in (3+1)D O(3) models through large-scale
quantum Monte Carlo simulation combined with theoretical analysis. Although the upper critical dimension introduces logarithmic corrections in correlations, we analytically prove and numerically demonstrate that the corrections do not modify the universal trihedral-corner contribution of the disorder operator. The universal contributions, such as the current central charge, have been revealed in our calculation, which establishes a concrete link between
lattice simulations and continuum field theory. This work opens promising directions for the experimental and numerical exploration of universal properties at
quantum critical points in (3+1)D models.
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All authors Xuyang Liang, Xiao-Chuan Wu, Zenan Liu, Zhe Wang, Zheng Yan, Dao-Xin Yao
Superconductivity
Emergence of Fermi-liquid and BCS physics in overdoped cuprates
B.J. Ramshaw and Steven A. Kivelson
Cuprates are the paradigmatic `unconventional'
superconductors: their critical temperature is much higher than can be expected from
phonon-mediated pairing; the superconducting gap has $d$-wave
symmetry; and the normal-metallic state appears to be far from a conventional
Fermi liquid. These and numerous other experimental facts have led to a consensus that the conventional theory — the
Fermi-
liquid-based Bardeen–Cooper–Schrieffer (BCS) theory — is the wrong starting point for understanding superconductivity in the cuprates. In this Perspective, we propose that, although underdoped cuprates do indeed require a different theoretical framework, there is a crossover with increasing doping to an overdoped regime in which a BCS-like approach is warranted (at energy scales of the order of the superconducting gap and below), provided that the various forms of disorder are accounted for. We summarize key experimental studies of the low-energy properties of overdoped cuprates, identify properties that are and are not compatible with this proposal — and argue that features that are inconsistent with this approach can in fact be attributed to the expected effects of material disorder. Finally, we provide falsifiable predictions for the behaviour of an `ideal' (disorder-free) overdoped cuprate through which our approach can be tested.
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Superconductivity
Spatially Inhomogeneous Triplet Pairing Order and Josephson Diode Effect Induced by Frustrated Spin Textures
Grayson R. Frazier and Yi Li
We show that frustrated
spin textures can generate anisotropic Josephson couplings between $d$ vectors that can stabilize spatially varying pairing orders in
spin triplet
superconductors. These couplings depend on the relative orientation of $d$ vectors, analogous to Dzyaloshinskii-Moriya and $\Gamma$-type interactions in magnetism, leading to an effective “pliability” of the pairing order that competes with superfluid stiffness. Such couplings cannot originate from
spin-orbit coupling; rather, they can arise, for example, when itinerant electrons are coupled to a local exchange field composed of frustrated
spin moments. Using a $T$-
matrix expansion, we show that coupling to a local exchange field leads to an effective tunneling of itinerant electrons that is dependent on the underlying
spin configurations at the barrier between superconducting grains. Furthermore, Josephson tunneling through frustrated
spin textures can produce a Josephson diode effect. The diode effect originates either from nonvanishing
spin chirality in the barrier, or from antisymmetric Josephson coupling between noncollinear $d$ vectors, both of which break inversion and time-reversal
symmetries.
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Materials Science
Crystallization Behavior of ZBLAN Glass Under Combined Thermal and Vibrational Effects: Part I – Experimental Investigation
Ayush Subedi, Anthony Torres, Jeff Ganley et al. (+1 more)
ZBLAN glass is a promising material for infrared optical fibers due to its wide transmission window and low theoretical attenuation. However, its strong tendency to crystallize during processing limits optical performance. While microgravity environments have been shown to suppress crystallization, the role of mechanical vibration under normal
gravity conditions remains poorly understood. This study systematically investigates the influence of vibration on the crystallization behavior of ZBLAN using a controlled heating and vibration apparatus. Samples were subjected to varied thermal and vibrational conditions, and their crystallization onset and morphological
evolution were examined through optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM). Results show that vibration reduces the crystallization onset temperature, indicating enhanced atomic mobility and nucleation kinetics. Progressive morphological transitions from needle-like to bow-tie and feather-like
crystals were observed with increasing temperature and vibration intensity. Surface roughness analysis corroborates these findings, revealing a significant increase in nanoscale roughness in crystallized regions. Although brief exposure duration and partial thermal decoupling introduced variability among samples, the overall results confirm that vibration acts as a direct facilitator of nucleation rather than a purely thermal effect. This work provides new insight into vibration-induced crystallization in fluoride glasses and establishes the experimental foundation for follow-up modeling and apparatus optimization studies under terrestrial and microgravity conditions.
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All authors Ayush Subedi, Anthony Torres, Jeff Ganley, Ujjwal Dhakal
Statistical Mechanics
When Heating Isn't Cooling in Reverse: Nosé-Hoover Thermostat Fluctuations from Equilibrium Symmetry to Nonequilibrium Asymmetry
Hesam Arabzadeh and Brad Lee Holian
Recent laboratory experiments suggest an intrinsic asymmetry between heating and cooling, with heating occurring more efficiently. Two decades earlier, molecular dynamics (MD) simulations had examined a related setup - heating one side of a computational
cell while cooling the other via distinct thermostats. We revisit those calculations, recapitulating the underlying theory and showing that earlier MD results already hinted at the observed laboratory asymmetry. Recent realizations of a simple two-dimensional single-particle model, thermostatted in $x$ and $y$ at different temperatures, reproduces key features: at
equilibrium, thermostat variables were identical, but under nonequilibrium conditions, the heating variable is weaker than the cooling one. At the same time, MD simulations from four decades ago by Evans and Holian reported a surprising skew in the Nose–Hoover thermostat variable $\xi$ under
equilibrium - indicating a statistical bias in energy injection versus extraction. We revisit those results with exact reproduction of their setup. We show that when (1) the center-of-mass velocity is set to zero, (2) integration is done carefully with finite differencing, and (3) sampling is sufficiently long, the distribution of $\xi$ is symmetric and Gaussian with zero mean, as predicted by theory and validated by two independent error estimates. However, in the two-temperature
cell, the distribution of thermostat variables become asymmetric, the cold bath requires significantly stronger damping than the hot bath requires anti-damping, with $\langle \xi_x \rangle / \langle \xi_y \rangle = -T_y/T_x$. This exact analytic relation links thermostat effort to thermal bias and the negative rate of change in the
entropy of the system. These results identify the microscopic origin of heating-cooling asymmetry as a genuine nonequilibrium effect, consistent with experimental findings.
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Statistical Mechanics
Critical exponents of fluid-fluid interfacial tensions near a critical endpoint in a nonwetting gap
Joseph O. Indekeu and Kenichiro Koga
Fluid three-phase
equilibria, with phases $\alpha, \beta, \gamma$, are studied close to a tricritical point, analytically and numerically, in a mean-field density-functional theory with two densities. Employing Griffiths' scaling for the densities, the interfacial tensions of the wet and nonwet interfaces are analysed. The mean-field critical exponent is obtained for the vanishing of the critical interfacial tension $\sigma_{\beta\gamma}$ as a function of the deviation of the noncritical interfacial tension $\sigma_{\alpha\gamma}$ from its limiting value at a critical endpoint $\sigma_{\alpha,\beta\gamma}$. In the wet regime, this exponent is $3/2$ as expected. In the nonwetting gap of the model, the exponent is again $3/2$, except for the approach to the critical endpoint on the neutral line where $\sigma_{\alpha\beta} = \sigma_{\alpha\gamma}$. When this point is approached along any path with $\sigma_{\alpha\beta} \neq \sigma_{\alpha\gamma}$, or along the neutral line, $\sigma_{\beta\gamma} \propto | \sigma_{\alpha\gamma} - \sigma_{\alpha,\beta\gamma}|^{3/4}$, featuring an anomalous critical exponent $3/4$, which is an exact result derived by analytic calculation and explained by geometrical arguments.
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Quantum Gases
Universal Random Matrix Behavior of a Fermionic Quantum Gas
Maxime Dixmerias, Giuseppe Del Vecchio Del Vecchio, Cyprien Daix et al. (+6 more)
The pursuit of universal governing principles is a foundational endeavor in physics, driving breakthroughs from thermodynamics to general relativity and
quantum mechanics. In 1951, Wigner introduced the concept of a statistical description of energy levels of heavy atoms, which led to the rise of Random
Matrix Theory (RMT) in physics. The theory successfully captured spectral properties across a wide range of atomic systems, circumventing the complexities of
quantum many-body interactions. Rooted in the fundamental principles of stochasticity and
symmetry, RMT has since found applications and revealed universal laws in diverse physical contexts, from
quantum field theory to disordered systems and wireless communications. A particularly compelling application arises in describing the mathematical structure of the many-body wavefunction of non-interacting
Fermi gases, which underpins a complex spatial organization driven by Pauli's exclusion principle. However, experimental validation of the counting statistics predicted in such systems has remained elusive. Here, we probe at the single-atom level ultracold atomic
Fermi gases made of two interacting
spin states, obtaining direct access to their counting statistics in situ. Our measurements show that, while the system is strongly attractive, each
spin-component is extremely well described by RMT predictions based on Fredholm determinants. Our results constitutes the first experimental validation of the
Fermi-sphere point process through the lens of RMT, and establishes its relevance for strongly-interacting systems.
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All authors Maxime Dixmerias, Giuseppe Del Vecchio Del Vecchio, Cyprien Daix, Joris Verstraten, Tim de Jongh, Bruno Peaudecerf, Pierre Le Doussal, Grégory Schehr, Tarik Yefsah
Materials Science
Intrinsic emittance properties of an Fe-doped Beta-Ga2O3(010) photocathode: Ultracold electron emission at 300K and the polaron self-energy
Louis A. Angeloni, Ir-Jene Shan, J.H. Leach et al. (+1 more)
Measurements of the spectral emission properties of an iron-doped a beta-Ga2O3(010) photocathode at 300 K reveal the presence of an ultracold contribution to the total electron beam emission with a 6 meV mean transverse energy (MTE) in the 3.5-4.4 eV
photon energy range (282-354 nm). This extreme sub-thermal photoemission signal is consistent with direct emission of electrons photoexcited from the Fe dopant states into the low effective mass and positive electron affinity primary conduction band, and it is superimposed on a stronger signal with a larger MTE associated with an (optical)
phonon-mediated momentum resonant Franck-Condon (FC) emission process from a thermally populated and negative electron affinity upper conduction band. For
photon energies above 4.5 eV, a transition from a long to a short transport regime is forced by an absorption depth reduction to below 100 nm and both MTE signals exhibit spectral trends consistent with
phonon-mediated FC emission if the polaron formation self-energy is included in the temperature of the initial thermalized photoexcited electron distribution.
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All authors Louis A. Angeloni, Ir-Jene Shan, J.H. Leach, W. Andreas Schroeder
Statistical Mechanics · Cross-listed from quant-ph
Symmetry and Asymmetry in Bosonic Gaussian Systems: A Resource-Theoretic Framework
Nikolaos Koukoulekidis and Iman Marvian
We study the interplay of
symmetries and Gaussianity in bosonic systems, under closed and open dynamics, and develop a resource theory of Gaussian asymmetry. Specifically, we focus on Gaussian
symmetry-respecting (covariant) operations, which serve as the free operations in this framework. We prove that any such operation can be realized via Gaussian
Hamiltonians that respect the
symmetry under consideration, coupled to an environment prepared in a
symmetry-respecting pure Gaussian state. We further identify a family of tractable monotone functions of states that remain non-increasing under Gaussian
symmetry-respecting dynamics, and are exactly conserved in closed systems. We demonstrate that these monotones are not generally respected under non-Gaussian
symmetry-respecting dynamics. Along the way, we provide several technical results of independent interest to the
quantum information and optics communities, including a new approach to the Stinespring dilation
theorem, and an extension of Williamson's
theorem for the simultaneous normal mode decomposition of Gaussian systems and conserved charges.
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Statistical Mechanics
Monotonicity of eigenstate thermalization hypothesis in two-dimensional systems
Nilakash Sorokhaibam and Anjan Daimari
We study numerically the enveloping $f$-function of the fluctuation term in eigenstate thermalization hypothesis (ETH) statement. We concentrate on the energy (or
entropy) dependence of this function in two-dimensional systems. Our numerical results show that it is, in general, a monotonically increasing function of the
entropy. This is in agreement with the general expectation that fluctuations increase with increasing
entropy. We show that the $f$-function locally flattens with increasing system-size. The flattening rate is directly proportional to the system size. We also show that the flattening rate is directly proportional to the particle number for systems of same spatial size. This variation of the $f$-function is important for physics at subleading order of the system-size. So, it is relevant for intermediate-size systems (upto a few hundred
qubits) which are experimentally accessible. One exception we found is that the $f$-function of the order parameter of a thermal phase transition defy the monotonic behaviour.
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Materials Science
Dual quantum locking: Dynamic coupling of hydrogen and water sublattices in hydrogen filled ice
Loan Renaud, Tomasz Poreba, Simone Di Cataldo et al. (+7 more)
Hydrogen hydrates (HH) are a unique class of materials composed of hydrogen molecules confined within crystalline water frameworks. Among their multiple phases, the filled ice structures, particularly the cubic C2 phase, exhibit exceptionally strong host-guest interactions due to ultra-short H2-H2O distances and a 1:1 stoichiometry leading to two interpenetrated identical diamond-like sublattices, one comprised of water molecules, the other of hydrogen molecules. At high pressures,
nuclear quantum effects involving both hydrogen molecules and the water
lattice become dominant, giving rise to a dual-
lattice quantum system. In this work, we explore the sequence of pressure- and temperature-driven phase transitions in HH, focusing on the interplay between molecular rotation, orientational ordering,
lattice symmetry breaking and hydrogen bond symmetrization. Using a combination of computational modeling based on classical and path-integral molecular dynamics,
quantum embedding, and high pressure experiments, including Raman spectroscopy and synchrotron X-ray diffraction at low temperatures and high pressures, we identify signatures of
quantum-induced ordering and structural transformations in the C2 phase. Our findings reveal that orientational ordering in HH occurs at much lower pressures than in solid hydrogen, by inducing structural changes in the water network and enhancing the coupling of water and hydrogen dynamics. This work provides new insights into the
quantum behavior of hydrogen under extreme mechanochemical confinement and establishes hydrogen-filled ices as a promising platform for the design of hydrogen-rich
quantum materials.
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All authors Loan Renaud, Tomasz Poreba, Simone Di Cataldo, Alasdair Nicholls, Léon Andriambariarijaona, Maria Rescigno, Richard Gaal, Michele Casula, A. Marco Saitta, Livia Eleonora Bove
Statistical Mechanics · Cross-listed from hep-lat
Scaling flow-based approaches for topology sampling in $\mathrm{SU}(3)$ gauge theory
Claudio Bonanno, Andrea Bulgarelli, Elia Cellini et al. (+4 more)
We develop a methodology based on out-of-
equilibrium simulations to mitigate topological freezing when approaching the continuum limit of
lattice gauge theories. We reduce the autocorrelation of the topological charge employing open boundary conditions, while removing exactly their unphysical effects using a non-
equilibrium Monte Carlo approach in which periodic boundary conditions are gradually switched on. We perform a detailed analysis of the computational costs of this strategy in the case of the four-dimensional $\mathrm{SU}(3)$ Yang-Mills theory. After achieving full control of the scaling, we outline a clear strategy to sample
topology efficiently in the continuum limit, which we check at
lattice spacings as small as $0.045$ fm. We also generalize this approach by designing a customized
Stochastic Normalizing Flow for evolutions in the boundary conditions, obtaining superior performances with respect to the purely
stochastic non-
equilibrium approach, and paving the way for more efficient future flow-based solutions.
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All authors Claudio Bonanno, Andrea Bulgarelli, Elia Cellini, Alessandro Nada, Dario Panfalone, Davide Vadacchino, Lorenzo Verzichelli
Mesoscale and Nanoscale Physics
Fast high-fidelity baseband reset of a latched state for quantum dot qubit readout
Piotr Marciniec, M. A. Wolfe, Tyler Kovach et al. (+9 more)
A common method for reading out the state of a
spin qubit is by latching one logical
qubit state, either $|1\rangle$ or $|0\rangle$, onto a different, metastable charge state. Such a latched state can provide a superior charge sensing signal for
qubit readout, and it can have a lifetime chosen to be long enough that the charge sensed readout can be high fidelity. However, the passive reset out of latched states is inherently long, which is not desirable. In this work, we demonstrate an on-demand, high fidelity (> 99%) re-initialization of a
quantum dot
qubit out of a latched readout state. The method is simple to apply as it involves a single baseband voltage pulse to a specific region in the
quantum dot stability diagram where the relaxation time from the latched state to the ground state is over 50 times faster. We describe the mechanism for the reset process as well as the boundaries for the optimal reset region in the
qubit gate voltage space.
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All authors Piotr Marciniec, M. A. Wolfe, Tyler Kovach, J. Reily, Sanghyeok Park, Jared Benson, Mark Friesen, Benjamin D. Woods, Matthew J. Curry, Nathaniel C. Bishop, J. Corrigan, M. A. Eriksson
Materials Science · Cross-listed from eess.SY
Over 3 kV and Ultra-Low leakage Vertical (011) e̱ta}-Ga2O3 Power Diodes with Engineered Schottky Contact and High-permittivity Dielectric Field Plate
Emerson J. Hollar and Esmat Farzana
We report over 3 kV breakdown voltage and ultra-low leakage (011) e̱ta}-Ga2O3 power devices utilizing Schottky barrier engineering and high-permittivity (ąppa}) dielectric (ZrO2) field plate. The (011) orientation of e̱ta}-Ga2O3 enabled low background doping and thick drift layers which are promising to support kV-class vertical e̱ta}-Ga2O3 power switches. The Schottky barrier engineering was performed with a composite Pt cap/PtOx/Pt (1.5 nm) anode contact to take advantage of the enhanced reverse blocking capabilities enabled by PtOx while allowing low turn-on voltage by the interfacing thin Pt layer. We also performed a systematic study using a co-processed Pt/(011) e̱ta}-Ga2O3 Schottky barrier diodes (SBDs) on the same wafer. The bare SBDs revealed a breakdown voltage of ~1.5 kV, while the field-plate Pt/(011) e̱ta}-Ga2O3 SBDs achieved an increased breakdown voltage of 2.75 kV owing to the edge field management. Further enhancement of the breakdown voltage was achieved by tunneling leakage management using composite Pt cap/PtOx/Pt (1.5 nm) Schottky contacts that ultimately enabled breakdown voltage of 3.7 kV for the field-plate diodes. Remarkably, the Pt cap/PtOx/Pt (1.5 nm) Schottky contacts maintained similar turn-on voltage as the Pt/(011) e̱ta}-Ga2O3 SBDs. The combination of efficient tunneling leakage management by composite Pt cap/PtOx/Pt (1.5 nm) contacts with similar turn-on voltage, edge field reduction by high-ąppa} dielectric ZrO2 field plate, as well as the advantageous material properties offered by (011) e̱ta}-Ga2O3 demonstrate a promising strategy for developing ultra-low leakage and multi-kV class vertical (011) e̱ta}-Ga2O3 power devices.
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Mesoscale and Nanoscale Physics
Optical Gain Through Metallic Electro-Optical Effects
N. Roldan-Levchenko, D. J. P. de Sousa, C. O. Ascencio et al. (+3 more)
Optical gain is a critical process in today's
semiconductor technology and it is most often achieved via stimulated emission. In this theoretical study, we find a resonant TE mode in biased low-
symmetry two-dimensional metallic systems which may lead to optical gain in the absence of stimulated emission. We do so by first modeling the optical conductivity using Boltzmann non-
equilibrium transport theory and then simulating the scattering problem using a scattered-wave formalism. Assuming that the system may possess a Berry
curvature dipole (BCD) and a non-zero Magnetoelectric tensor (MET), we find that the optical conductivity has a non-trivial dependence on the direction of the applied bias, which allows for probing the TE mode. After analyzing the system with one of each of the effects, we find that the resonant TE mode is only accessible when both effects are present. Further studies are necessary to find materials with a suitably large BCD and MET, in order to realize the predictions within this study.
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All authors N. Roldan-Levchenko, D. J. P. de Sousa, C. O. Ascencio, J. D. S Forte, L. Martin-Moreno, T. Low
Materials Science
Spin-dependent anisotropic electron-phonon coupling in KTaO$_3$
Giulia Venditti, Francesco Macheda, Paolo Barone et al. (+2 more)
KTaO$_3$ (KTO) is an incipient ferroelectric, characterized by a softening of the lowest transverse optical (TO) mode with decreasing temperature. Cooper pairing in the recently discovered KTO-based
heterostructures has been proposed to be mediated by the soft TO mode. Here we study the electron coupling to the zone-center odd-parity modes of bulk KTO by means of relativistic Density Functional Perturbation Theory (DFPT). The coupling to the soft TO mode is by far the largest, with comparable contributions from both intraband and interband processes. Remarkably, we find that for this mode,
spin-non-conserving
matrix elements are particularly relevant. We develop a three-band microscopic model with
spin-orbit coupled $t_{2g}$ orbitals that reproduces the main features of the ab initio results. For the highest energy band, the coupling can be understood as a "dynamical" isotropic Rashba effect. In contrast, for the two lowest bands, the Rashba-like coupling becomes strongly anisotropic. The DFPT protocol implemented here enables the calculation of the full electron-
phonon coupling
matrix projected onto any mode of interest, and it is easily applicable to other systems.
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All authors Giulia Venditti, Francesco Macheda, Paolo Barone, José Lorenzana, Maria N. Gastiasoro
Mesoscale and Nanoscale Physics
Using Crossed Andreev Reflection to Split Electrons
Austin Marga and Venkat Chandrasekhar
Mesoscopic systems possess shot noise in their currents due to the quantization of the conducting quasiparticles. Measurements of this shot noise are useful to study phenomena that do not manifest themselves in standard conductance or resistance measurements, such as the statistics of the conducting quasiparticles or
quantum entanglement via Bell tests. The corresponding particle statistics can be determined via two particle
quantum interference experiments, such as the Hong-Ou-Mandel effect which demonstrates a bunching effect for
bosons or an anti-bunching effect in
fermions. In superconducting proximity junctions, electrons incident on a
superconductor can induce holes via crossed Andreev reflection (CAR) in spatially separated normal metal leads, where the resulting hole currents have nontrivial partition noise due to the four terminal configuration. These nonlocally generated currents, using a
superconductor as a mesoscopic beam splitter, enable fabrication of mesoscopic analogs to
quantum optics interferometers using metallic and superconducting films with multiport geometries.
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Mesoscale and Nanoscale Physics
Renjie Luo, Tanner J. Legvold, Gage Eichman et al. (+5 more)
The
spin Seebeck effect is useful for probing the
spin correlations and
magnetic order in
magnetic insulators. Here, we report a strong longitudinal
spin Seebeck effect (LSSE) in
antiferromagnetic V2O3 thin films. The LSSE response at cryogenic temperatures increases as a function of the external
magnetic field until it approaches saturation. The response at given power and field exhibits a non-monotonic temperature dependence, with a pronounced peak that shifts toward higher temperatures as the field increases. Furthermore, the magnitude of the LSSE signal decreases consistently with increasing thickness, implying that the bulk SSE dominates any interfacial contribution. This negative correlation between the SSE and the thickness implies that the
magnon energy relaxation length in V2O3 is shorter than the thickness of our thinnest film, 50 nm, consistent with the strong
spin-
lattice coupling in this material.
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All authors Renjie Luo, Tanner J. Legvold, Gage Eichman, Henry Navarro, Ali C. Basaran, Erbin Qiu, Ivan K. Schuller, Douglas Natelson
Materials Science
Combined ab initio and experimental study of phosphorus-based anti-wear additives interacting with iron and iron oxide
Francesca Benini, Paolo Restuccia, Sophie Loehlé et al. (+2 more)
The performance of phosphorus-based lubricant additives is governed by their adsorption, stability, and reactivity at the metal interface. In this study, we investigate the adsorption behavior and tribochemical stability of three additives: Octyl Acid Phosphate (OAP), Dibutyl Hydrogen Phosphite (DBHP), and Amine Neutralized Acid Phosphate (ANAP). These additives are studied on iron and hematite surfaces using both ab initio calculations and experimental analyses on steel. Simulations revealed that ANAP exhibited the strongest adsorption on iron, followed by DBHP, while OAP showed weaker interactions, though its chemisorption was enhanced on hematite via hydrogen loss. Under tribological conditions, the DBHP phosphite dissociated more readily than the other two phosphates molecules due to its lower phosphorus coordination, as confirmed by bond order analysis. Quartz
crystal microbalance (QCM) measurements indicated significant differences in adsorption behavior across temperatures, with DBHP forming stable deposits, while ANAP exhibited poor retention, in agreement with ab initio molecular dynamics simulations. X-ray photoelectron spectroscopy (XPS) confirmed DBHP's strong chemisorption and molecular dissociation, leading to increased phosphorus deposition. OAP, despite forming a phosphorus-based layer, caused a reduction in Fe
oxide, consistent with its hydrogen release mechanism observed in simulations. These findings highlight the critical role of molecular structure and oxidation state in tribofilm formation and stability. Understanding these interactions at the atomic level provides valuable insights for designing high-performance lubricant additives for extreme operating conditions.
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All authors Francesca Benini, Paolo Restuccia, Sophie Loehlé, Quentin Arnoux, Maria Clelia Righi
Mesoscale and Nanoscale Physics
Coupling between vibration and Luttinger liquid in mechanical nanowires
Zeyu Rao, Yue-Xin Huang, Guang-Can Guo et al. (+1 more)
The vibration of the mechanical nanowire coupled to
photons via
photon pressure and coupled to charges via the capacity has been widely explored in experiments in the past decades. This system is electrically neutral, thus its coupling to the other degrees of freedom is always challenging. Here, we show that the vibration can slightly change the nanowire length and the associated
Fermi velocity, which leads to coupling between vibration and Luttinger
liquid. We consider the transverse and longitudinal vibrations of the nanowires, showing that the transverse vibration is much more significant than the longitudinal vibration, which can be measured through the sizable frequency shift. We predict an instability of the vibration induced by this coupling when the frequency becomes negative at a critical temperature for the transverse vibrations in nanowires with low
Fermi energy, which can be reached by tuning the chemical potential and
magnetic field. The time-dependent oscillation of the conductance, which directly measures the Luttinger parameter, can provide evidence for this coupling. Our theory offers a new mechanism for exploring the coupling between the vibration and the electronic excitations, which may lead to intriguing applications in cooling and controlling the mechanical oscillators with currents.
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All authors Zeyu Rao, Yue-Xin Huang, Guang-Can Guo, Ming Gong
Statistical Mechanics
Free-energy REconstruction from Stable Clusters (FRESC): A new method to evaluate nucleation barriers from simulation
Adrian Llamas-Jaramillo, Ivan Latella and David Reguera
We present a simulation technique to evaluate the most important quantity for nucleation processes: the nucleation barrier, i.e. the free energy of formation of the critical cluster. The method is based on stabilizing a small cluster by simulating it in the NVT ensemble and using the thermodynamics of small systems to convert the properties of this stable cluster into the Gibbs free energy of formation of the critical cluster. We demonstrate this approach using condensation in a Lennard-Jones truncated and shifted fluid as an example, showing an excellent agreement with previous Umbrella Sampling simulations. The method is straightforward to implement, computationally inexpensive, requires only a small number of particles comparable to the critical cluster size, does not rely on the use of Classical Nucleation Theory, and does not require any cluster definition or reaction coordinate. All of these advantages hold the promise of opening the door to simulate nucleation processes in complex molecules of atmospheric, chemical or pharmaceutical interest that cannot be easily simulated with current techniques.
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Disordered Systems and Neural Networks
Renormalization group for deep neural networks: Universality of learning and scaling laws
Gorka Peraza Coppola, Moritz Helias and Zohar Ringel
Self-similarity, where observables at different length scales exhibit similar behavior, is ubiquitous in natural systems. Such systems are typically characterized by power-law correlations and universality, and are studied using the powerful framework of the renormalization group (RG). Intriguingly, power laws and weak forms of universality also pervade real-world
datasets and deep learning models, motivating the application of RG ideas to the analysis of deep learning. In this work, we develop an RG framework to analyze self-similarity and its breakdown in learning curves for a class of weakly non-linear (non-lazy) neural networks trained on power-law distributed data. Features often neglected in standard treatments – such as spectrum discreteness and lack of translation invariance – lead to both quantitative and qualitative departures from conventional perturbative RG. In particular, we find that the concept of scaling intervals naturally replaces that of scaling dimensions. Despite these differences, the framework retains key RG features: it enables the classification of perturbations as relevant or irrelevant, and reveals a form of universality at large data limits, governed by a Gaussian Process-like UV fixed point.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Second-order Stark shifts exceeding 10$\,$GHz in electrically contacted SiV$^-$ centers in diamond
Manuel Rieger, Nori N. Chavira Leal, Rubek Poudel et al. (+7 more)
Negatively charged silicon vacancy centers (SiV$^-$) in diamond exhibit excellent
spin coherence and optical properties, making them promising candidates for
quantum technologies. However, the strain-induced inhomogeneous distribution of optical transition frequencies poses a challenge for scalability. We demonstrate electrical tuning of the SiV$^-$ center zero-
phonon lines using in-plane contacts to apply moderate electric fields up to 45$\,$MV/m. The second-order Stark shift exceeds 10$\,$GHz, which is of the same order of magnitude as the 15$\,$GHz inhomogeneous distribution of SiV$^-$ observed in emitters embedded in optical nanostructures such as photonic
crystal nanocavities. Analysis of individual SiV$^-$ centers shows significant variation in polarizabilities between defects indicating that the polarizability strongly depends on local parameters like strain. The observed polarizabilities are 3-25 times larger than those of tin vacancy centers, which we attribute to valence band resonances that delocalize the $e_u$ wavefunctions. Photoluminescence excitation measurements reveal that optical linewidths increase moderately with applied electric field strength. Our results demonstrate that large electrical Stark shifts can overcome the inhomogeneous distribution of transition frequencies, representing a significant step toward scalable SiV$^-$-based
quantum technologies such as
quantum repeaters.
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All authors Manuel Rieger, Nori N. Chavira Leal, Rubek Poudel, Tobias Waldmann, Lina M. Todenhagen, Stefan Kresta, Viviana Villafane, Martin S. Brandt, Kai Müller, Jonathan J. Finley
Strongly Correlated Electrons
Quantum Spin Liquids Stabilized by Disorder in Non-Kramers Pyrochlores
Marcus V. Marinho and Eric C. Andrade
We investigate the emergence of
quantum spin liquid phases in pyrochlore
oxides with non-Kramers ions, in which structural randomness effectively acts as a transverse field, introducing
quantum fluctuations on top of the
spin ice
manifold. This is contrary to the naive expectation that disorder favors phases with short-range
entanglement by adjusting the
spins with their local environment. We study a minimal model for a disordered
quantum spin ice, the transverse-field Ising model, using a real-space formulation of the gauge mean-field theory. This approach allows the inclusion of non-perturbative disorder effects exactly, and thus to assess the stability of the
spin-
liquid phase with respect to the disorder. The analysis shows that the
quantum spin ice remains remarkably stable with respect to disorder up to the transition to the polarized phase at high fields, indicating that it can occur in real materials. A Griffiths region of enhanced disorder-induced fluctuations is restricted to the immediate vicinity of this transition due to the peculiar nature of the low-energy excitations of the problem. For most of the phase diagram, an average description of the disorder captures the physical behavior well, indicating that the inhomogeneous
quantum spin ice behaves closely to its homogeneous counterpart.
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Quantum Gases
Chirality-Induced Spin Currents in a Fermi Gas
Camen A. Royse and J. E. Thomas
We observe and model
spin currents arising from chirality and effective
spin-exchange interactions in a weakly interacting $^6$Li
Fermi gas. Chirality is introduced by a static displacement between the center of the trapped atoms and the center of an applied
magnetic bowl, which produces left- or right-handed spatially varying
spin rotation.
Spin current is directly observed via oscillations in the centers of mass of the
spin-up and
spin-down components, which appear to bounce off of or pass through one another, depending on the degree of handedness and s-wave scattering length. We show that this behavior obeys a driven oscillator equation with an effective
spin-dependent driving force. Our measurements demonstrate chirality-induced
spin selectivity via the direction of the current flow, extending CISS phenomena to
Fermi gases.
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Statistical Mechanics
Fundamental Work Scaling and Non-Extensivity in Critical Engines
Bastian Castorene, Martin HvE Groves, Francisco J. Peña et al. (+2 more)
We present a general analytical framework for critical two-isothermal engines that emerge operationally from quasi-static
quantum Stirling cycles across ground-state level crossings (GLCs) in the low-temperature regime, where reversible heat exchange is governed by the structural
entropy change while the internal energy remains constant. Within this ideal-reservoir, low-temperature
equilibrium model, the Primarch Formula gives an exact quasistatic ensemble-average expression linking extracted work and efficiency directly to macroscopic ground-state degeneracies. In this reversible setting, the engines reach Carnot efficiency without a classical regenerator. For the perturbative population pattern analyzed here, thermal excitations reduce the ideal work and efficiency. Validated against exact numerical simulations of generalized N-th
spin-s Heisenberg models with nontrivial interactions, the framework is applied to the one-dimensional
antiferromagnetic Ising model, revealing a profound connection to number theory. Governed by Fibonacci-Lucas and parity-dependent critical degeneracies, the engine exhibits three distinct work-scaling regimes: persistent logarithmic non-extensivity, recovery of extensivity in the thermodynamic limit, and asymptotic extensivity with a logarithmic finite-size correction. In all three regimes, Carnot efficiency is attained within the reversible
equilibrium model.
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All authors Bastian Castorene, Martin HvE Groves, Francisco J. Peña, Eugenio E. Vogel, Patricio Vargas
Statistical Mechanics
Dynamics of entanglement fluctuations and quantum Mpemba effect in the $\nu=1$ QSSEP model
Angelo Russotto, Filiberto Ares, Pasquale Calabrese et al. (+1 more)
We study the out-of-
equilibrium dynamics of
entanglement fluctuations in the $\nu=1$
Quantum Symmetric Simple Exclusion Process, a free-
fermion chain with hopping amplitudes that are
stochastic in time but homogeneous in space. Previous work showed that the average
entanglement growth after a
quantum quench can be explained in terms of pairs of entangled quasiparticles performing random walks, leading to diffusive
entanglement spreading. By incorporating the noise-induced statistical correlations between the quasiparticles, we extend this description to the full-time probability distribution of the
entanglement entropy. Our generalized quasiparticle picture allows us to compute the average time
evolution of a generic function of the reduced density
matrix of a subsystem. We also apply our result to the
entanglement asymmetry. This allows us to investigate the restoration of particle-number
symmetry in the dynamics from initial states with no well-defined particle number. Regarding the possible existence of the
quantum Mpemba effect, our analysis indicates that its occurrence is an extremely fine-tuned phenomenon, requiring very specific conditions and therefore being rather difficult to observe in practice.
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All authors Angelo Russotto, Filiberto Ares, Pasquale Calabrese, Vincenzo Alba