Statistical Mechanics
Slow dynamics from a nested hierarchy of frozen states
Vanja Marić, Luka Paljk and Lenart Zadnik
We identify the mechanism of slow heterogeneous relaxation in
quantum kinetically constrained models (KCMs) in which the potential energy strength is controlled by a coupling parameter. The regime of slow relaxation includes the large-coupling limit. By expanding around that limit, we reveal a nested hierarchy of states that remain frozen on time scales determined by powers of the coupling. The classification of such states, together with the
evolution of their Krylov complexity, reveals that these time scales are related to the distance between the sites where facilitated dynamics is allowed by the kinetic constraint. While correlations within frozen states relax slowly and exhibit metastable plateaus that persist on time scales set by powers of the coupling parameter, the correlations in the rest of the states decay rapidly. We compute the plateau heights of correlations across all frozen states up to second-order corrections in the inverse coupling. Our results explain slow relaxation in
quantum KCMs and elucidate dynamical heterogeneity by relating the relaxation times to the spatial separations between the active regions.
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Soft Condensed Matter
Heterogeneous dynamics in a polymer solution revealed through measurement of ultraslow convection
Thomas P. Chaney, Samuel D. Marks, Dylan M. Ladd et al. (+9 more)
Understanding solution-phase aggregation and dynamics in complex fluids is critical for material processing, yet widely used dynamic light scattering (DLS) fails for strongly attenuating systems such as conjugated
polymers. We use X-ray
photon correlation spectroscopy (XPCS) to probe the dynamics of a
polymer, PM7, in toluene, revealing unexpected oscillations in the autocorrelation function that show vertical flow during measurement. Despite the relatively low X-ray absorption, measured flow velocities scale with X-ray beam power and suggest convective transport. Our analyses reveal both mobile and static scatterers that together produce oscillatory, heterodyne features in the measured correlation functions. Finite element simulations predict flow velocities much larger than observed, suggesting that entanglements of the aggregates slow their motion. These results provide a direct measurement of ultra-slow convection and highlight the need to explicitly account for even modest beam heating in interpreting XPCS results. Moreover, the observation of distinct scatterer populations underscores the structural complexity of conjugated
polymer solutions.
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All authors Thomas P. Chaney, Samuel D. Marks, Dylan M. Ladd, Andrei Fluerasu, Federico Zontone, Yuriy Chushkin, Sebastian Frücht, Dina Sheyfer, Kelsey Levine, Amnahir E. Peña-Alcántara, Hans-Georg Steinrück, Michael F. Toney
Statistical Mechanics
Minimal-Dissipation Learning for Energy-Based Models
Jeff Hnybida and Simon Verret
We show that the bias of the approximate maximum-likelihood estimation (MLE) objective of a persistent chain energy-based model (EBM) is precisely equal to the thermodynamic excess work of an overdamped Langevin dynamical system. We then answer the question of whether such a model can be trained with minimal excess work, that is, energy dissipation, in a finite amount of time. We find that a Gaussian energy function with constant variance can be trained with minimal excess work by controlling only the learning rate. This proves that it is possible to train a persistent chain EBM in a finite amount of time with minimal dissipation and also provides a lower bound on the energy required for the computation. We refer to such a learning process that minimizes the excess work as minimal-dissipation learning. We then provide a generalization of the optimal learning rate schedule to general potentials and find that it induces a natural gradient flow on the MLE objective, a well-known second-order optimization method.
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Mesoscale and Nanoscale Physics
Spatial uniformity of g-tensor and spin-orbit interaction in germanium hole spin qubits
Inga Seidler, Bence Hetényi, Lisa Sommer et al. (+12 more)
Holes in Ge/SiGe
heterostructures are now a leading platform for
semiconductor spin qubits, thanks to the high confinement quality, two-dimensional arrays, high tunability, and larger gate structure dimensions. One limiting factor for the operation of large arrays of
qubits is the considerable variation in
qubit frequencies or properties resulting from the strongly anisotropic $g$-tensor. We study the $g$-tensors of six and seven
qubits in an array with a Y geometry across two devices. We report a mean distribution of the tilts of the $g$-tensor's out-of-plane principal axis of around $1.1 \deg$, where nearby
quantum dots are more likely to have a similar tilt. Independently of this tilt, and unlike simple theoretical predictions, we find a strong in-plane $g$-tensor anisotropy with strong correlations between neighboring
quantum dots. Additionally, in one device where the principal axes of all g-tensors are aligned along the [100]
crystal direction, we extract the
spin-flip tunneling vector from adjacent dot pairs and find a pattern that is consistent with a uniform Dresselhaus-like
spin-orbit field. The Y arrangement of the gate layout and
quantum dots allows us to rule out local factors like electrostatic confinement shape or local strain as the origin of the preferential direction. Our results reveal long-range correlations in the
spin-orbit interaction and $g$-tensors that were not previously predicted or observed, and could prove critical to reliably understand $g$-tensors in germanium
quantum dots.
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All authors Inga Seidler, Bence Hetényi, Lisa Sommer, Leonardo Massai, Konstantinos Tsoukalas, Eoin G. Kelly, Alexei Orekhov, Michele Aldeghi, Stephen W. Bedell, Stephan Paredes, Felix J. Schupp, Matthias Mergenthaler, Gian Salis, Andreas Fuhrer, Patrick Harvey-Collard
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Constraints on Atomistic Disorder for Scalable Electron Spin Shuttling
Raphaël J. Prentki, Pericles Philippopoulos, Mohammad Reza Mostaan et al. (+1 more)
Electron
spin shuttling, the gate-controlled, coherent transport of electrons between
qubit registers, increases
qubit connectivity and enables efficient
quantum-error-correction schemes. It is emerging as a key enabler of scalable silicon
spin-
qubit quantum computing. As an electron travels over micrometers, it encounters angstrom-scale disorder, causing fluctuations in its confinement potential, valley splitting, and valley phase. These lead to leakage into the valley-excited state, limiting high-fidelity shuttling speeds. Accurate predictions of shuttling fidelities thus require modeling tools that link atomistic and mesoscopic physics. We develop a multiscale simulation workflow to quantify these effects in the experimentally-realized Si/SiGe "QuBus" conveyor-belt architecture. First, we resolve the time-dependent, gate-controlled device electrostatics by solving the Poisson equation using the finite-element method. Second, we construct conveyor belt atomic structures with realistic atomistic disorder (random alloying and interface roughness); we resolve strain atomistically using the Keating valence force-field model. Third, we perform position-tracked atomistic tight-binding simulations of the shuttled electrons to obtain their time-dependent valley splittings and phases. Finally, these time traces parametrize a time-dependent Schrödinger equation, which we solve to predict valley dynamics. We find that interface roughness strongly suppresses shuttling fidelities, with a sharp anomaly near the atomic-layer scale. Overall, our predictions set practical, quantitative guidelines to realize scalable, high-fidelity shuttling in silicon
spin-
qubit architectures.
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All authors Raphaël J. Prentki, Pericles Philippopoulos, Mohammad Reza Mostaan, Félix Beaudoin
Statistical Mechanics
Analytical solution of a free-fermion chain with time-dependent ramps
Viktor Eisler, Riccarda Bonsignori and Stefano Scopa
We provide an exact analytical solution of the single-particle Schrödinger equation for a chain of non-interacting
fermions subject to a time-dependent linear potential, with its slope varied as an arbitrary function of time. The resulting dynamics exhibit self-similar behavior, with a structure reminiscent of the domain wall melting problem, albeit characterized by a nontrivial time-dependent length scale and phase. Building on this solution, we derive hydrodynamic predictions for the
evolution of particle density, current, and
entanglement entropy along the chain. In the special case of a sudden quench, the system develops a breathing interface region, which may be interpreted as a realization of Wannier-Stark localization, as previously suggested on the basis of hydrodynamic arguments.
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Mesoscale and Nanoscale Physics
Polarization dependence of spin-electric transitions in molecular exchange qubits
Filippo Troiani and Athanassios K. Boudalis
Quasi-optical experiments are emerging as a powerful technique to probe
magnetic transitions in molecular
spin systems. However, the simultaneous presence of the electric- and
magnetic-dipole induced transitions poses the challenge of discriminating between these two contributions. Besides, the identification of the
spin-electric transitions can hardly rely on the peak intensity, because of the current uncertainties on the value of the
spin-electric coupling in most molecular compounds. Here, we compute the polarizations required for electric- and
magnetic-dipole induced transitions through
spin-
Hamiltonian models of molecular
spin triangles. We show that the polarization allows a clear discrimination between the two kinds of transitions. In addition, it allows one to identify the physical origin of the zero-field splitting in the ground multiplet, a debated issue with significant implications on the coherence properties of the
spin qubit implemented in molecular
spin triangles.
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Materials Science
Thermal assisted transport of biexcitons in monolayer WSe2
Dorian Béret, Louka Hemmen, Vishwas Jindal et al. (+9 more)
Studies of excitonic transport in transition metal dichalcogenide monolayers have attracted increasing interest in recent years in order to develop nano-optoelectronic devices made with 2D materials. These studies began with low to moderate optical excitation regimes, and more recently have focused on high injection regimes where nonlinear effects appear. This article is focused on the transport of biexcitons by spatially and temporally resolved photoluminescence spectroscopy at high excitation flux. The study is carried out on a high-quality WSe$_2$ monolayer encapsulated in hexagonal boron nitride. The results show that a Seebeck current affects transport in connection with the presence of hot biexcitons. In particular, we observe the formation of spatial rings, also called
halos, which have been observed in other excitonic gases. These results tend to generalize the importance of high-energy populations in excitonic transport in TMD, even for complex and heavy excitonic particles.
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All authors Dorian Béret, Louka Hemmen, Vishwas Jindal, Sreyan Raha, Thierry Amand, Delphine Lagarde, Andrea Balocchi, Cédric Robert, Helene Carrere, Xavier Marie, Pierre Renucci, Laurent Lombez
Statistical Mechanics
The systemic impact of edges in financial networks
Michel Alexandre, Thiago Christiano Silva and Francisco A. Rodrigues
In this paper, we assess how the stability of financial networks is affected by interconnectedness considering its tiniest variation: the edge. We compute the impact of edges as the percentage difference in the systemic risk (SR) of the whole network caused by the inclusion of that edge. We apply this framework to a thorough Brazilian
dataset to compute the impact of bank-firm edges. After observing that (i) edges are heterogeneous regarding their impact on the SR, and (ii) the fraction of edges whose impact on the SR is non-positive increases with the level of the initial shock, we use machine learning techniques to try to predict two variables: the criticality of the edges (defining as critical an edge whose impact on SR is significantly greater than that of the others) and the sign of the edge impact. The level of accuracy obtained in these prediction exercises was very high. These results have important implications for the development macroprudential policies aimed at financial stability. Our framework allows to identify, based on features related to the origin and destination nodes of the edge (i.e., the lending bank and the borrowing firm), whether an additional loan will have a significant and positive impact on the SR.
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Other Condensed Matter · Cross-listed from hep-th
General Junction Condition and Casimir Effect for (1+1)-Dimensional Scalar Network CFT
Tian-Ming Zhao, Sinan Pang, Ling Li et al. (+2 more)
Recently, BCFT and ICFT have been generalized to the CFT on networks (NCFT). A key aspect of NCFT is how we connect the CFTs across different edges at the network nodes. Previous research has primarily concentrated on a specific junction condition (JC) that requires the field to be continuous at the nodes. In this paper, we investigate the most general junction conditions for $(1+1)$-dimensional free scalars that are consistent with the variational principle and energy conservation. These general junction conditions are characterized by an $O(p)$ group, where $p$ represents the number of edges connected at a node. We provide exact realizations of two typical JCs in real physical systems. Additionally, we derive both the lower and upper bounds on the network Casimir energy for $(1+1)$-dimensional free scalar fields and extend the lower bound to encompass general NCFTs. Finally, we analyze the Casimir effect in networks composed of regular polyhedra and examine the binding energy required to construct such networks from individual components.
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All authors Tian-Ming Zhao, Sinan Pang, Ling Li, Yu Guo, Rong-Xin Miao
Quantum Gases
Rogue waves in extended Gross-Pitaevskii Models with a Lee-Huang-Yang correction
Sathyanarayanan Chandramouli, Simeon I. Mistakidis, Garyfallia C. Katsimiga et al. (+3 more)
We explore the existence and dynamical generation of rogue waves (RWs) within a one dimensional
quantum droplet bearing environment. RWs are computed by deploying a spacetime fixed point scheme to the relevant extended Gross Pitaevskii equation (eGPE). Parametric regions where the ensuing RWs are different from their counterparts in the nonlinear Schroedinger equation are identified. To corroborate the controllable generation, relevant to ultracold atom experiments, of these rogue patterns we exploit two different protocols. The first is based on interfering dam break flows emanating from Riemann initial conditions and the second refers to the gradient catastrophe of a spatially localized waveform. A multitude of possible RWs are found in this system, spanning waveforms reminiscent of the Peregrine
soliton, its spatially periodic variants, namely, the Akhmediev breathers, and other higher order RW solutions of the nonlinear Schroedinger equation. Key elements of the shape of the corresponding eGPE RWs traced back to nonintegrability and the presence of competing interactions are discussed. Our results set the stage for probing a multitude of unexplored rogue like waveforms in such mixtures with competing interactions and should be accessible to current ultracold atom experiments.
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All authors Sathyanarayanan Chandramouli, Simeon I. Mistakidis, Garyfallia C. Katsimiga, Daniel J. Ratliff, Dimitrios J. Frantzeskakis, Panayotis G. Kevrekidis
Materials Science
Electrically modulated light-emitting diodes driven by resonant and antiresonant tunneling between Cr$_2$Ge$_2$Te$_6$ electrodes
Natalia Zawadzka, Kristina Vaklinova, Tomasz Woźniak et al. (+7 more)
Exploring the electron tunneling mechanisms in diverse materials systems constitutes a versatile strategy for tailoring the properties of optoelectronic devices. In this domain, bipolar vertical tunneling junctions composed of van der Waals materials with vastly different electronic band structures enable simultaneous injection of electrons and holes into an optically active material, providing a universal blueprint for light-emitting diodes (LEDs). Efficient modulation of the injection efficiency has previously been demonstrated by creating resonant states within the energy barrier formed by the luminescent material. Here, we present an alternative approach towards resonant tunneling conditions by fabricating tunneling junctions composed entirely from gapped materials: Cr$_2$Ge$_2$Te$_6$ as electrodes, hBN as a tunneling barrier, and monolayer WSe$_2$ as a luminescent medium. The characterization of such LEDs revealed a nonmonotonous
evolution of the electroluminescence intensity with the tunneling bias. The dominant role driving the characteristics of the electron tunneling was associated with the relative
alignment of the density of states in Cr$_2$Ge$_2$Te$_6$ electrodes. The unique device architecture introduced here presents a universal pathway towards LEDs operating at room temperature with electrically modulated emission intensity.
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All authors Natalia Zawadzka, Kristina Vaklinova, Tomasz Woźniak, Mihai I. Sturza, Holger Kohlmann, Kenji Watanabe, Takashi Taniguchi, Adam Babiński, Maciej Koperski, Maciej R. Molas
Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Ultrafast dynamics of coherent exciton-polaritons in van der Waals semiconductor metasurfaces
Luca Sortino, Armando Genco, Cristina Cruciano et al. (+14 more)
Enabling coherent light-matter interactions is a critical step toward next-generation
quantum technologies. However, achieving this under ambient temperature conditions remains challenging due to rapid dephasing in optically excited systems. Optical metasurfaces based on quasi-bound states in the continuum have recently emerged as a powerful platform for reaching the strong light-matter coupling regime in flat, subwavelength thickness devices. Here, we investigate ultrafast
exciton-polariton dynamics in self-hybridized WS$_2$ thin-film metasurfaces. Using hyperspectral momentum-resolved imaging, we reconstruct the highly anisotropic
exciton-polariton dispersion, with a transition from positive to negative effective mass along orthogonal
symmetry axes. Femtosecond pump-probe and multidimensional spectroscopy reveal detuning-dependent polariton dynamics with a coherence time up to ~110 fs, and allow direct observation of the coherent dynamics through ultrafast Rabi oscillations with ~45 fs period. We describe this behaviour with a three-eigenstate model that couples the photonic resonance with both bright and dark
excitons, extending the conventional two-state picture of strong coupling. Our results establish van der Waals metasurfaces as a promising platform for next-generation polaritonic devices, enabling coherent
quantum transfer of matter excitations at room temperature.
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All authors Luca Sortino, Armando Genco, Cristina Cruciano, Michele Guizzardi, Daniel Timmer, Thomas Weber, Jonathan O. Tollerud, Francesco Gucci, Matteo Corti, Gianluca Valentini, Cristian Manzoni, Stefano Dal Conte, Christoph Lienau, Jeffrey A. Davis, Stefan A. Maier, Andreas Tittl, Giulio Cerullo
Materials Science
Structural Chirality and Natural Optical Activity across the $\alpha$-to-$\beta$ Phase Transition in SiO$_2$ and AlPO$_4$ from first-principles
F. Gómez-Ortiz, A. Zabalo, A. M. Glazer et al. (+3 more)
Natural optical activity (NOA), the ability of a material to rotate the plane of polarized light, has traditionally been associated with structural chirality. However, this relationship has often been oversimplified, leading to conceptual misunderstandings, particularly when attempts are made to directly correlate structural handedness with optical rotatory power. In reality, the relationship between chirality and NOA is more nuanced: optical activity can arise in both
chiral and achiral
crystal structures, and the sign of the rotation cannot necessarily be inferred from the handedness of the space group. % In this work, we conduct a first-principles investigation of natural optical activity in SiO$_2$ and AlPO$_4$
crystals, focusing on their enantiomorphic structural phase transition from high-
symmetry hexagonal ($P6_422$ or $P6_222$) to low-
symmetry trigonal ($P3_121$ or $P3_221$) space groups. This transition, driven by the condensation of a zone-center $\Gamma_3$
phonon mode, reverses the screw axis type given by the space group symbol while leaving the sign of the optical activity unchanged. By following the
evolution of the structure and the optical response along the transition pathway, we clarify the microscopic origin of this behavior. We demonstrate that the sense of optical rotation is determined not by the nominal helicity of the screw axis given in the space group symbol, but by the atomic-scale helicity of the most polarizable atoms of the structure.
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All authors F. Gómez-Ortiz, A. Zabalo, A. M. Glazer, E. E. McCabe, A. H. Romero, E. Bousquet
Disordered Systems and Neural Networks · Cross-listed from physics.soc-ph
Modeling the spatial growth of cities
Ulysse Marquis and Marc Barthelemy
The growth of cities has traditionally been studied from a population perspective, while urban expansion-its spatial growth-has often been approached qualitatively. However, characterizing and modeling this spatial expansion is crucial, particularly given its parallels with surface growth extensively studied in physics. Despite these similarities, approaches to urban expansion modeling are fragmented and scattered across various disciplines and contexts. In this review, we provide a comprehensive overview of the mathematical modeling of this complex phenomenon. We discuss the key challenges hindering progress and examine models inspired by statistical physics, economics and geography, and theoretical ecology. Finally, we highlight critical directions for future research in this interdisciplinary field.
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Strongly Correlated Electrons
Modeling Quantum Geometry for Fractional Chern Insulators with unsupervised learning
Ang-Kun Wu, Louis Primeau, Jingtao Zhang et al. (+3 more)
Fractional Chern
insulators (FCIs) in moire materials present a unique platform for exploring strongly correlated topological phases beyond the paradigm of ideal
quantum geometry. While analytical approaches to FCIs and fractional
quantum Hall states (FQHS) often rely on idealized Bloch wavefunctions, realistic moire models lack direct tunability of
quantum metric and Berry
curvature, limiting theoretical and numerical exploration. Here, we introduce an unsupervised machine learning framework to model interacting
Hamiltonians directly through the distribution of single-particle form factors. Using a variational autoencoder (VAE), we show that unsupervised learning can not only distinguish FCI and non-FCI states, but also generate new form factors with distinct topological character, not present in the training set. This latent space enables the generation and interpolation of form factors for topological flatbands with Chern number $|C|=1$, enabling the discovery of unobserved many-body states such as charge density waves. Principal component analysis (PCA) further reveals that the dominant patterns in the form factors-reflecting correlations across the Brillouin zone-can be decomposed into components with approximately quantized Chern numbers, providing new insights into the global and topological structure of
quantum geometry. Our results highlight the ability of machine learning to generalize and model topological
quantum systems, paving the way for the inverse design of form factors with tailored
quantum geometry and many-body phases in flatband materials.
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All authors Ang-Kun Wu, Louis Primeau, Jingtao Zhang, Kai Sun, Yang Zhang, Shi-Zeng Lin
Mesoscale and Nanoscale Physics
Deconstruction of the anisotropic magnetic interactions from spin-entangled optical excitations in van der Waals antiferromagnets
Dipankar Jana, Swagata Acharya, Milan Orlita et al. (+5 more)
Magneto-optical excitations in
antiferromagnetic d systems can originate from a multiplicity of light-
spin and
spin-
spin interactions, as the light and
spin degrees of freedom can be entangled. This is exemplified in van der Waals systems with attendant strong anisotropy between in-plane and out-of-plane directions, such as MnPS3 and NiPS3 films studied here. The rich interplay between the
magnetic ordering and sub-bandgap optical transitions poses a challenge to resolve the mechanisms driving
spin-entangled optical transitions, as well as the single-particle bandgap itself. Here we employ a high-fidelity ab initio theory to find a realistic estimation of the bandgap by elucidating the atom- and orbital-resolved contributions to the fundamental sub-bands. We further demonstrate that the
spin-entangled excitations, observable as photoluminescence and absorption resonances, originate from an on-site
spin-flip transition confined to a
magnetic atom (Mn or Ni). The
evolution of the
spin-flip transition
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All authors Dipankar Jana, Swagata Acharya, Milan Orlita, Clement Faugeras, Dimitar Pashov, Mark van Schilfgaarde, Marek Potemski, Maciej Koperski
Statistical Mechanics
Percolation and criticality of systems with competing interactions on Bethe lattices: limitations and potential strengths of cluster schemes
Greivin Alfaro Miranda, Mingyuan Zheng, Patrick Charbonneau et al. (+3 more)
The random clusters introduced by Fortuin and Kasteleyn (FK) and analyzed by Coniglio and Klein (CK) for Ising and related models have led first Swendsen and Wang and then Wolff to formulate remarkably efficient Markov chain Monte Carlo sampling schemes that weaken the critical slowing down. In frustrated models, however, no standard way to produce a comparable gain at small frustration – let alone efficiently sample the large frustration regime – has yet been identified. In order to understand why formulating appropriate cluster criteria for frustrated models has thus far been elusive, we here study minimal short-range attractive and long-range repulsive as well as
spin-glass models on Bethe
lattices. Using a generalization of the CK approach and the cavity-field method, the appropriateness and limitations of the FK–CK type clusters are identified. We find that a standard, constructive cluster scheme is then inoperable, and that the frustration range over which generalized FK–CK clusters are even definable is finite. These results demonstrate the futility of seeking constructive cluster schemes for frustrated systems but leaves open the possibility that alternate approaches could be devised.
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All authors Greivin Alfaro Miranda, Mingyuan Zheng, Patrick Charbonneau, Antonio Coniglio, Leticia F. Cugliandolo, Marco Tarzia
Mesoscale and Nanoscale Physics
Dissipation properties of anomalous Hall effect: intrinsic vs. extrinsic magnetic materials
V. Desbuis, D. Lacour, M. Hehn et al. (+5 more)
A comparative study of anomalous-
Hall current injection and anisotropic current injection (through planar
Hall effect) are studied in
Hall devices contacted to a lateral load
circuit.
Hall currents are injected into the load
circuit from three different kinds of
magnetic Hall bars: Mn5Si3 altermagnet, Co75Gd25 ferrimagnet, and Ni80Fe20 ferromagnet. The current, the voltage and the power are measured as a function of the load resistance and the
Hall angle. It is observed that the power dissipated for the three kinds of materials fellow the same law as a function of load resistance and
Hall angle, at the leading order in the
Hall angle. Since the anomalous
Hall effect in the altermagnetic
Hall-bar is due to the intrinsic topological structure (i.e. due to the presence of a Berry phase in the reciprocal space), these observations suggest that the dissipative properties of anomalous
Hall effect are dominated by the injection of electric charges accumulated at the edges (including electric screening), instead of the very mechanism responsible for it.
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All authors V. Desbuis, D. Lacour, M. Hehn, S. Geiskopf, L. Michez, J. Rial, V. Baltz, J.-E. Wegrowe
Superconductivity
Highly-Linear Proximity-Based Bi-SQUID Operating above 4 K
G. Trupiano, E. Riccardi, C. Puglia et al. (+5 more)
We demonstrate a highly linear superconducting
quantum interference device (SQUID) amplifier based on a double-loop (Bi-SQUID) architecture incorporating three
superconductor-normal metal-
superconductor (S-N-S) junctions. Fabricated using niobium-gold technology, the device exhibits robust operation at
liquid helium temperatures, with a superconducting transition temperature of 8.5 K. The flux-to-voltage transfer function demonstrates sharp, symmetric, and highly linear behavior at temperatures up to 5 K. Bi-SQUIDs featuring our single-element S-N-S design represent an interesting and original approach to this field, as they demonstrate a numerically estimated spurious-free dynamic range (SFDR) linearity exceeding 60 dB, achieved in a single element, simplifying the requirements in terms of arrays containing hundreds of junctions. These results highlight the potential of proximity-based Bi-SQUIDs for compact, low-noise, and highly linear cryogenic amplifiers in
quantum sensing, magnetometry, and biomedical diagnostics.
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All authors G. Trupiano, E. Riccardi, C. Puglia, M. Kiczynski, A. Gardin, G. De Simoni, G. C. Tettamanzi, F. Giazotto
Disordered Systems and Neural Networks · Cross-listed from physics.soc-ph
Dynamic roughening of cities driven by multiplicative noise
Martin Hendrick and Gabriele Manoli
The
evolution of urban landscapes is rapidly altering the surface of our
planet. Yet, our understanding of the urbanisation phenomenon remains far from complete. A fundamental challenge is to describe spatiotemporal changes in the built environment. A dynamic theory of urban
evolution should account for both vertical and horizontal city expansion, analogous to the dynamical behaviour of surface growth in physical and biological systems. Here we show that building-height dynamics in cities around the world are well described by a zero-dimensional
geometric Brownian motion (GBM), where multiplicative noise drives
stochastic fluctuations around a deterministic drift associated with economic growth. To account for intra-city correlations, we extend the GBM with spatial coupling, revealing how local interactions effectively mitigate noise-driven fluctuations and shape urban morphology. The continuum limit of this spatial model can be recasted into the Kardar-Parisi-Zhang (KPZ) equation and we find that empirical estimates of the roughness exponent are in the range of the KPZ prediction for most cities. Together, these results show that multiplicative noise, moderated by local interactions, governs the
evolution of urban roughness, anchoring spatiotemporal city dynamics in a well-established statistical physics framework.
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Materials Science
Subthermal Mean Transverse Energies Induced by Electron Refraction on the Jump in Mass at the Surface of Multialkali Photocathodes
S.A. Rozhkov, V.V. Bakin, H.E. Scheibler et al. (+6 more)
The search for photocathode materials with low mean transverse energies (MTEs) and, hence, low intrinsic emittance is of crucial importance for various fields of particle and solid state physics. Here, we demonstrate that polycrystalline multialkali Na$_{2}$KSb(Cs,Sb) photocathodes with negative effective electron affinity (NEA) have MTE values at room temperature by a factor of 2 lower than those of monocrystalline p-GaAs(Cs,O) photocathodes. These low MTE values are due to the electron refraction on the jump in mass, between a small effective mass in Na$_{2}$KSb and free electron mass in
vacuum. It is proved that, at the NEA state, up to half of photoelectrons are emitted in a narrow-angle cone with the fractional MTE of 9\,meV at room temperature. We also showed that the transition from NEA to positive effective affinity results in the subthermal total MTE of the Na$_{2}$KSb(Cs,Sb) photocathode, along with
quantum efficiency of about 10$^{-2}$. The physical reasons for the manifestation of the refraction effect in multialkali photocathodes are discussed, opening up opportunities for the development of high-brightness and ultracold robust electron sources.
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All authors S.A. Rozhkov, V.V. Bakin, H.E. Scheibler, V.S. Rusetsky, D.V. Gorshkov, D.A. Kustov, V.A. Golyashov, V.L. Alperovich, O. E. Tereshchenko
Statistical Mechanics
Numerical methods for quasi-stationary distributions
Sara Oliver-Bonafoux, Javier Aguilar, Tobias Galla et al. (+1 more)
In
stochastic processes with absorbing states, the quasi-stationary distribution provides valuable insights into the long-term behaviour prior to absorption. In this work, we revisit two well-established numerical methods for its computation. The first is an iterative
algorithm for solving the non-linear equation that defines the quasi-stationary distribution. We generalise this technique to accommodate general Markov
stochastic processes, either with discrete or continuous state space, and with multiple absorbing states. The second is a Monte Carlo method with resetting, for which we propose a novel single-trajectory approach that uses the trajectory's own history to perform resets after absorption. In addition to these methodological contributions, we provide a detailed analysis of implementation aspects for both methods. We also compare their accuracy and efficiency across a range of examples. The results indicate that the iterative
algorithm is generally the preferred choice for problems with simple boundaries, while the Monte Carlo approach is more suitable for problems with complex boundaries, where the implementation of the iterative
algorithm is a challenging task.
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All authors Sara Oliver-Bonafoux, Javier Aguilar, Tobias Galla, Raúl Toral
Materials Science
Redox Chemistry of LiCoO$_2$, LiNiO$_2$, and LiNi$_{1/3}$Mn$_{1/3}$Co$_{1/3}$O$_2$ Cathodes: Deduced via XPS, DFT+DMFT, and Charge Transfer Multiplet Simulations
Ruiwen Xie, Maximilian Mellin, Wolfram Jaegermann et al. (+3 more)
Understanding the
evolution of the physicochemical bulk properties during the Li deintercalation (charging) process is critical for optimizing battery cathode materials. In this study, we combine X-ray photoelectron spectroscopy (XPS), density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations, and charge transfer multiplet (CTM) model simulations to investigate how hybridization between transition metal (TM) 3d and oxygen 2p orbitals evolves with Li deintercalation. Based on the presented approach combining theoretical calculations and experimental studies of pristine and deintercalated cathodes, two important problems of ion batteries can be addressed: i) the detailed electronic structure and involved changes with deintercalation providing information of the charge compensation mechanism, and ii) the precise experimental analysis of XPS data which are dominated by charge transfer coupled to final-state effects affecting the satellite structure. As main result for the investigated Li TM
oxides, it can be concluded that the electron transfer coupled to the Li$^{+}$-ion migration does not follow a rigid band model but is modified due to changes in TM 3d and O 2p states hybridization. Furthermore, this integrated approach identifies the 2p XPS satellite peak intensity of TM as an effective indicator of the redox chemistry. With that the redox chemistry of cathodes can be deduced, thus offering a foundation for designing more efficient battery materials.
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All authors Ruiwen Xie, Maximilian Mellin, Wolfram Jaegermann, Jan P. Hofmann, Frank M. F. de Groot, Hongbin Zhang
Materials Science
A physics-informed neural network approach to the point defect model for electrochemical oxide film growth
Mohid Farooqi, Ingmar Bösing and Conrard G. Tetsassi Feugmo
Physics-informed neural networks (PINNs) offer a novel AI-driven framework for integrating physical laws directly into neural network models, facilitating the solution of complex multiphysics problems in materials engineering. This study systematically explores the application of PINNs to simulate
oxide film layer growth in halide-free solutions using the point defect model (PDM). We identify and analyze four key failure modes in this context: imbalanced loss components across different physical processes, numerical instabilities due to variable scale disparities, challenges in enforcing boundary conditions within multiphysics systems, and convergence to mathematically valid but physically meaningless solutions. To overcome these challenges, we implement and validate established techniques including nondimensionalization for training stabilization, Neural Tangent Kernel-based adaptive loss balancing, robust enforcement of boundary conditions and hybrid training with sparse data. Our results demonstrate the effectiveness of these strategies in enhancing the reliability and physical fidelity of PINNs, achieving sub $1\%$ relative error as compared to Finite Element
Benchmarks with the hybrid model. Thereby showing that PINNs can be used for high fidelity electrochemical simulations with minimal data requirements and highlight necesary factors for fully autonomous PINN simulations.
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Other Condensed Matter · Cross-listed from physics.optics
Plasmonic metamaterial time crystal
Tingwen Guo, Jules Sueiro, Gian Marcello Andolina et al. (+18 more)
Periodically driven optical materials and metamaterials have recently emerged as a promising platform for realizing photonic time
crystals (PTCs), which are systems whose optical properties are strongly and periodically modulated on timescales comparable to the optical cycle of light. These time-varying structures are the temporal counterparts of spatial photonic
crystals (SPCs), for which a large and periodic dielectric contrast is achieved spatially on wavelength scales. Just as SPCs have revolutionized control over light-matter interactions by engineering the photonic density of states in space, PTCs promise comparable breakthroughs from a fundamentally new perspective: a temporal one. However, harnessing such phenomena all-optically poses severe experimental challenges, as it requires order-unity modulation depths of a material's optical properties on ultrafast timescales comparable to the light cycle, a regime that has remained elusive to date. Here, we demonstrate the first all-optical realization of a photonic time
crystal, achieved with a surface plasmon cavity metamaterial operating at Terahertz (THz) frequencies. We demonstrate strong (near-unity) and coherent (sub-optical cycle) periodic driving of the plasmonic metamaterial enabled by field-induced dynamical modulation of the carriers' kinetic energy and effective mass - reaching up to 80% of their rest mass. Our spectroscopic measurements reveal a transition into the PTC regime mediated by an exceptional point, at which two Floquet-driven optical eigenmodes coalesce. In the PTC regime, emergent gain is shown to reduce plasmonic losses by more than 50% and we predict plasmonic lasing to be within experimental reach. These results pave the way for temporal engineering of losses and light-matter interactions in plasmonic systems, and establish a robust new platform for time-domain photonics.
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All authors Tingwen Guo, Jules Sueiro, Gian Marcello Andolina, Artem Levchuk, Stefano Ponzoni, Romain Grasset, Donald Monthe, Ian Aupiais, Dmitri Daineka, Javier Briatico, Thales VAG de Oliveira, Alexey Ponomaryov, Atiqa Arshad, Arjun Karimbana-Kandy, Gulloo Lal Prajapati, Igor Ilyakov, Jan-Christoph Deinert, Sebastian F. Maehrlein, Luca Perfetti, Marco Schiro, Yannis Laplace
Statistical Mechanics · Cross-listed from quant-ph
Quantum sensing with discrete time crystals in the Lipkin-Meshkov-Glick Model
Rahul Ghosh, Bandita Das and Victor Mukherjee
Quantum phase transitions have been shown to be highly beneficial for
quantum sensing, owing to diverging
quantum Fisher information close to criticality. In this work we consider a periodically modulated Lipkin-Meshkov-Glick model to show that discrete time
crystal (DTC) phase transition in this setup can enable us to achieve
quantum-enhanced high-precision sensing of field strength. We employ a detailed finite-size scaling analysis, a time-averaged Inverse Participation Ratio analysis, and mean-field analysis in the thermodynamic limit, to determine the critical properties of this second-order phase transition. Our studies provide a comprehensive understanding of how
quantum criticality in DTCs involving long-range interactions can be harnessed for advanced
quantum sensing applications.
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Mesoscale and Nanoscale Physics
Asser Elsayed, Davide Costa, Lucas E. A. Stehouwer et al. (+5 more)
Understanding scattering mechanisms in
semiconductor heterostructures is crucial to reducing sources of disorder and ensuring high yield and uniformity in large
spin qubit arrays. Disorder of the parent two-dimensional electron or hole gas is commonly estimated by the critical, percolation-driven density associated with the metal-
insulator transition. However, a reliable estimation of the critical density within percolation theory is hindered by the need to measure conductivity with high precision at low carrier densities, where experiments are most difficult. Here, we connect experimentally percolation density and
quantum Hall plateau width, in line with an earlier heuristic intuition, and offer an alternative method for characterizing
semiconductor heterostructure disorder.
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All authors Asser Elsayed, Davide Costa, Lucas E. A. Stehouwer, Alberto Tosato, Mario Lodari, Brian Paquelet Wuetz, Davide Degli Esposti, Giordano Scappucci
Materials Science
Photovoltaic Performance of a Rotationally Faulted Multilayer Graphene/n-Si Schottky Junction
Hojun Im and Masahiro Teraoka
We report the fabrication and photovoltaic performance of a rotationally faulted multilayer
graphene (rf-MLG)/n-Si Schottky junction device. A thickness-controlled rf-MLG is synthesized using a 5 {\mu}m Ni foil catalyst via the chemical vapor deposition method and transferred to the n-Si substrate via a
polymer-free process, enabling facile and cost-effective fabrication. The device demonstrates an ideality factor of 1.67, a rectification factor of approximately 4x10^5 at {\pm}1.0 V, and a Schottky barrier height of 0.83 eV. A strong linear relationship between light intensity and photocurrent is also observed. Furthermore, the device exhibits a peak external
quantum efficiency of ~26% at 540 nm and a peak internal
quantum efficiency of ~97% at 410 nm. Transient photocurrent and photovoltaic measurements show approximately one-microsecond extraction and several-millisecond recombination times, respectively, revealing effective charge collection for photovoltaic applications. These results indicate that the rf-MLG/n-Si Schottky junction is well-formed and achieves performance comparable to that of SLG devices, demonstrating its potential for optoelectronic applications.
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Mesoscale and Nanoscale Physics
Magnetocaloric effect for the altermagnetic candidate MnTe
N.N. Orlova, V.D. Esin, A.V. Timonina et al. (+2 more)
We experimentally investigate magnetocaloric effect for single
crystals of MnTe altermagnet at the transition to the state with spontaneous
spin polarization, i.e. well below the Néel temperature of MnTe. The isothermal
magnetic entropy change $\Delta S$ is calculated from the experimental magnetization curves by using Maxwell relation. We observe well-defined magnetocaloric effect as a narrow $\Delta S$ peak around the cricital temperature $T_c\approx 81$~K, which is accompanied by sharp magnetization jump. This behavior is unusual for standard
ferromagnetic transitions, so it confirms the predicted
spin-orbit-induced
spin polarization in the MnTe altermagnetic state.
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All authors N.N. Orlova, V.D. Esin, A.V. Timonina, N.N. Kolesnikov, E.V. Deviatov