Disordered Systems and Neural Networks
Soft vector spins with dimensional annealing for combinatorial optimization
Marvin Syed, Richard Zhipeng Wang and Natalia G. Berloff
Recently, purpose-built analog hardware that can efficiently minimize the Ising energy and thereby solve a variety of combinatorial optimization problems has been receiving widespread
attention. In this work, we show how multidimensional, vectorial degrees of freedom, that are either naturally present or can be artificially created in such hardware, could strengthen the capability to find optimal solutions to optimization problems. In order to achieve this, we introduce a simple model of soft vector
spins that should be implementable on a variety of analog hardware platforms as well as three different dimensional annealing methods which harness the enlarged phase space of the vectorial degrees of freedom to minimize the Ising energy. We perform simulations on different
benchmark problems and show that for all dimensional annealing methods we tested, vectorial degrees of freedom improve upon one-dimensional degrees of freedom when it comes to finding the ground state of the Ising model. In particular, we find that this advantage becomes most pronounced for $d \gtrsim 3$ dimensional degrees of freedom, with diminishing returns as the dimension is increased further. Our results could inspire new analog optimization hardware and
algorithms that explicitly incorporate the advantage of vectorial degrees of freedom.
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Strongly Correlated Electrons
Excitations across the equilibrium and photoinduced `hidden' states of magnetoresistive manganites
Shiyu Fan, Feng Jin, Taehun Kim et al. (+16 more)
"Hidden" phases, generated using ultrafast
laser pulses (few hundred femtoseconds), with properties distinct from thermodynamic
equilibrium, are appealing for technologies because they can be long-lived, with lifetimes of hours or weeks, and reversible with temperature sweeping or extra pulses. In this regard, La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LCMO) stands out due to its tunability through epitaxial strain, which can drive the bulk
ferromagnetic metal (FMM) into an
antiferromagnetic insulator (AFI), and its susceptibility to photo-induced transitions. Indeed, AFI LCMO displays a long-lived photo-induced transition into a putative 'hidden' phase whose exact nature and excitations are still largely unknown. Here, we combine ultrafast photo-excitation in the near infrared with in situ transport, x-ray absorption (XAS), and Resonant Inelastic X-ray Scattering (RIXS) to investigate the excitations (polarons,
phonons, and orbital) of the photo-excited phase of LCMO and contrast them with the thermodynamic phases achieved through strain and temperature. In the thermodynamic regime, we establish the correlation between polarons and transport, placing them in the 'strong coupling' regime of the Holstein model. Upon photo-excitation of LCMO-AFI, we uncover a long-lived phase characterized by the softening of the polaron excitations, the partial suppression of the Jahn-Teller distortion, and nearly unchanged
phonons, showing the emergence of a photo-excited state absent in the
equilibrium phase diagram. Finally, by varying temperature, epitaxial strain, and photo-excitation fluence, we construct a polaron phase diagram and identify the key spectroscopic signatures of each phase. Our
laser-RIXS approach establishes a versatile platform for exploring photo-induced 'hidden' phases in
quantum materials in non-stroboscopic conditions.
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All authors Shiyu Fan, Feng Jin, Taehun Kim, Umesh Kumar, Zixun Zhang, Vivek Bhartiya, Jiemin Li, Brandon Yalin, Yanhong Gu, Mingqiang Gu, Wen Hu, Claudio Mazzoli, G. Lawrence Carr, Osor S. Barišić, Andrey S. Mishchenko, Valentina Bisogni, Sobhit Singh, Wenbin Wu, Jonathan Pelliciari
Statistical Mechanics · Cross-listed from cs.CL
Phase transition on a context-sensitive random language model with short range interactions
Yuma Toji, Jun Takahashi, Vwani Roychowdhury et al. (+1 more)
Since the random
language model was proposed by E. DeGiuli [Phys. Rev. Lett. 122, 128301],
language models have been investigated intensively from the viewpoint of statistical mechanics. Recently, the existence of a Berezinskii–Kosterlitz–Thouless transition was numerically demonstrated in models with long-range interactions between symbols. In statistical mechanics, it has long been known that long-range interactions can induce phase transitions. Therefore, it has remained unclear whether phase transitions observed in
language models originate from genuinely linguistic properties that are absent in conventional
spin models. In this study, we construct a random
language model with short-range interactions and numerically investigate its statistical properties. Our model belongs to the class of context-sensitive grammars in the Chomsky hierarchy and allows explicit reference to contexts. We find that a phase transition occurs even when the model refers only to contexts whose length remains constant with respect to the sentence length. This result indicates that finite-temperature phase transitions in
language models are genuinely induced by the intrinsic nature of
language, rather than by long-range interactions.
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All authors Yuma Toji, Jun Takahashi, Vwani Roychowdhury, Hideyuki Miyahara
Statistical Mechanics
Absence of $\mathrm{O} (2)$ symmetry in the Vicsek model
Yushin Takahashi, Kota Mitsui, Tsuyoshi Mizohata et al. (+1 more)
The phase transition in the Vicsek model is widely believed to be associated with spontaneous
symmetry breaking of the two-dimensional rotational
symmetry $\mathrm{O} (2)$. In this paper, we revisit the original Vicsek model introduced by Vicsek et al. [Phys.~Rev.~Lett.~75,~1226~(1995)] and demonstrate that the original angle-based update rule is not invariant under global phase shifts at the level of angular increments when it is implemented using the principal-value arctangent, as in the original definition. As a consequence, we numerically demonstrate that the phase transition reported in the original paper vanishes when the global phase is adaptively chosen.
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All authors Yushin Takahashi, Kota Mitsui, Tsuyoshi Mizohata, Hideyuki Miyahara
Mesoscale and Nanoscale Physics
Nonreciprocal spin waves of helical magnetization states in CoFeB/NiFe bilayers
Claudia Negrete, Omar J. Suarez, Attila Kákay et al. (+1 more)
We investigated the nonreciprocal
spin-wave properties, including the frequency shift, of a helical
equilibrium state in a versatile CoFeB/NiFe
bilayer. Through an extension of the dynamic
matrix formalism (developed in this work) to an arbitrary non-collinear configuration along a heterostructured multilayered system thickness, we explained the frequency shift via differences in the dynamic dipolar and interlayer exchange interactions arising from the distinct
spin-wave mode profiles across the
bilayer thickness for counterpropagating modes at the same wave vector. In contrast to recent literature wherein the frequency shift is attributed solely to the dipolar interaction, our results and explanations hereby presented involve a starring role of the interlayer exchange interaction not accounted in current literature. Furthermore, we also found a combination of large frequency shift values and sub-100 nm
spin wave wavelengths that can be tuned or even enhanced with the twisting degree of the helical magnetization state by the application of the external field, and with the thickness of the NiFe sublayer, which might be highly relevant for magnonic applications. We validated our model and the physical mechanism that explains the frequency shift using recent simulations and experimental results.
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All authors Claudia Negrete, Omar J. Suarez, Attila Kákay, Jorge A. Otálora
Statistical Mechanics · Cross-listed from quant-ph
Multi-Mode Quantum Annealing for Generative Representation Learning with Boltzmann Priors
Gilhan Kim and Daniel K. Park
Energy-based models provide a natural bridge between statistical physics and machine learning by representing data through structured energy landscapes. Boltzmann machines are a particularly compelling class of such models for capturing complex interactions among latent variables, but their use in modern generative learning has been limited by the classical intractability of sampling from general (non-restricted) Boltzmann distributions. Here we develop a
quantum-annealing-based framework that enables variational autoencoders with general Boltzmann priors. The framework employs three complementary annealing modes tailored to different stages of learning and deployment: diabatic
quantum annealing provides unbiased Boltzmann samples for efficient training, slower annealing concentrates samples near low-energy configurations of the learned prior for unconditional generation, and conditional annealing with external fields steers the learned energy landscape toward attribute-specific regions for conditional generation and semantic editing. Using up to 2000
qubits on a D-Wave Advantage2 processor, we demonstrate stable training and high-quality generation on MNIST, Fashion-MNIST, and CelebA, achieving faster convergence and lower reconstruction loss than a Gaussian-prior VAE with the same encoder-decoder architecture. Beyond generation, the learned energy function provides out-of-distribution detection signals that add discriminative power beyond reconstruction loss. We demonstrate that these scores separate in-distribution samples from held-out digit classes in one-class MNIST experiments and improve the detection of
market regime shifts in financial data. These results establish
quantum annealing as a practical and controllable physical mechanism for energy-based representation learning and generative modeling beyond the reach of tractable classical approaches.
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Strongly Correlated Electrons
The $\mathbb{Z}_N^{\times 3}$ symmetry protected boundary modes in two-dimensional Potts paramagnets
Hrant Topchyan
We construct and analyze a class of one-dimensional boundary
Hamiltonians arising from two-dimensional
symmetry-protected topological phases with $\mathbb{Z}_N^{\times 3}$
symmetry on a triangular
lattice. Using a
cohomology-based transformation, the
lattice models for the edge modes are explicitly obtained, and their structure is shown to be governed by the arithmetic properties of $N$. For
prime $N$, the boundary theory admits a formulation in terms of mutually commuting Temperley-Lieb
algebras. For the composite values of $N$, the models exhibit hierarchical or factorized structures. We demonstrate that the phases can be understood in terms of primary models augmented by local defect degrees of freedom that partition the system into independent segments. Finally, the global
symmetry is realized on the boundary in a non-on-site and anomalous manner, directly realizing the corresponding 't Hooft anomaly.
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Disordered Systems and Neural Networks · Cross-listed from quant-ph
Quantum walk on a random comb
François David and Thordur Jonsson
We study continuous time
quantum walk on a random comb graph with infinite teeth. Due to localization effects along the spine, the walk cannot go to infinity in the spine direction, while it can escape to infinity along the teeth of the comb. Starting from an initial vertex, the walk has a nonzero probability to stay trapped in a finite region. These results are obtained by studying the spectrum and eigenstates of the random
Hamiltonian for the graphand analysing its properties. We use both analytic and numerical methods, many of which come from the theory of Anderson localization in one dimension.
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Materials Science
Cryogenic stabilization of molecular hydrogen in dense cubic ice
Tomasz Poręba, Leon Andriambariarijaona, Richard Gaal et al. (+5 more)
Hydrogen is widely regarded as a cornerstone of future low-carbon energy technologies, yet the lack of safe, efficient, and reversible solid-state storage materials remains a major barrier to its large-scale deployment. Although porous frameworks and metal hydrides have been extensively explored, far less is known about the ability of dense molecular solids to stabilize hydrogen at near-ambient pressure. Here we show that fully crystalline cubic ice, despite its non-porous nature, can retain molecular hydrogen as an interstitial guest following controlled decompression from a high-pressure hydrogen hydrate precursor. Using synchrotron X-ray diffraction,
neutron diffraction, and Raman spectroscopy, we demonstrate that hydrogen is retained within the ice structure up to about 130 K, producing reproducible
lattice expansion and distinct spectroscopic signatures. We further show that pure cubic ice can be partially refilled with hydrogen at 0.18 GPa and 130 K, while fully hydrogen-filled cubic structure can be preserved at the same pressure up to 90 K. The retained hydrogen content reaches several percent of the parent hydrate composition, corresponding to gravimetric and volumetric storage densities comparable to those of interstitial hydrogen in metals. These results reveal an unexpected ability of a dense hydrogen-bonded
crystal structure to host molecular hydrogen without permanent porosity or chemical bonding, establishing cubic ice as a minimal model for hydrogen-
lattice interactions. More broadly, our findings identify dense hydrogen-bonded solids as an unexplored class of materials for hydrogen storage physics, with implications extending from energy materials to planetary and astrophysical ice environments.
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All authors Tomasz Poręba, Leon Andriambariarijaona, Richard Gaal, Kazuki Komatsu, Gaston Garbarino, Thomas Hansen, Stanislav Savvin, Livia E. Bove
Mesoscale and Nanoscale Physics
The origin of KPZ-scaling in arrays of polariton condensates
Denis Novokreschenov and Alexey Kavokin
This work investigates the origin of Kardar-Parisi-Zhang (KPZ) scaling in the phase dynamics of one-dimensional and two-dimensional polariton condensates. We demonstrate that the key mechanism leading to the observed power laws for the first-order correlation function $g^{(1)}$ is the fluctuation of the population of Goldstone modes, which arise due to the spontaneous breaking of $U(1)$
symmetry. Numerical simulations and analytical theory confirm that the critical exponents describing the KPZ universality class directly follow from the dynamics of Goldstone excitations. Our results establish a direct connection between the microscopic parameters of arrays of
exciton-polariton condensates and the coherent properties of the light they emit.
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Mesoscale and Nanoscale Physics
The effect of staggered nonlinearity on the Su-Schrieffer-Heeger model
Ahmed Alharthy and RW Bomantara
We investigate the spectral properties of the Su-Schrieffer-Heeger (SSH) model with sublattice-dependent onsite nonlinearity. Two complementary approaches are employed in our studies. First, Bloch state solutions under periodic boundary conditions are assumed to enable semi-analytical treatment, which allows us to obtain the system's energy band structure and further derive a general expression of the Zak phase that incorporates nonlinearity-induced correction (referred to as nonlinear Zak phase). This analysis reveals that, at sufficiently high nonlinearities, a nonlinearity-induced topological phase transition occurs, marked by a discontinuity in the nonlinear Zak phase. The second approach amounts to numerically obtaining other (non-Bloch) solutions under open boundary conditions, employing the Self-Consistent Field Iterative Method. Its main results include the observation of an edge state's energy that is independent of a nonlinear parameter, a persisting band touching point that only shifts in the presence of perturbations reminiscent of Weyl points in a Weyl semimetal, as well as delocalized solutions that persist even at extreme nonlinearity strengths. These findings illuminate the rich interplay between
topology and nonlinearity in
lattice models with potential realization in optical/acoustic waveguide settings.
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Quantum Gases
Ground-state solution of quantum droplets in Bose-Bose mixtures
Wei Liu and Limin Xu
In this paper, we present a systematic study on the ground state computation of
quantum droplets in homonuclear Bose-Bose mixtures, governed by the extended Gross-Pitaevskii equations (eGPEs) with Lee-Huang-Yang (LHY) corrections. This model captures the formation of self-bound droplets stabilized by the delicate balance between the attractive mean-field interaction and the repulsive
quantum fluctuations. We formulate dimensionless energy functionals for both the general two-component system and the reduced single-component density-locked model. To compute the ground states efficiently, we adapt and
benchmark various gradient flow discretization schemes, identifying a backward-forward sine-pseudospectral scheme based on the gradient flow with Lagrange multiplier method (GFLM-BFSP) as the robust solver for our simulations. Utilizing this method, we report three main numerical observations: (i) the density-locked model is quantitatively validated as a reliable approximation for ground state properties; (ii) the dimension-dependent convergence rates of the Thomas-
Fermi approximation are established in the strong-coupling regime; and (iii) the critical particle number for self-binding in free space is numerically determined, providing a precise correction to the analytical prediction by Petrov [Phys. Rev. Lett. 115, 155302 (2015)].
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Statistical Mechanics
The Klein bottle ratio of two-dimensional ferromagnetic Potts models
Zi-Han Wang and Li-Ping Yang
The weakly first-order nature of the two-dimensional 5-state
ferromagnetic Potts model poses challenges for numerical study. Using density-
matrix and tensor-network renormalization group methods, we investigate these transitions of the Potts-$q$ model via the Klein bottle ratio $g$ on original and dual
lattices. Finite-size scaling of $g$ as a function of transverse system size $L_y$ accurately locates the critical points for $q = 4, 5, 6$. We further examine the transfer-
matrix spectra and
entanglement entropy, extracting central charges through toroidal and Klein bottle boundary conditions. For $q = 5$, the extracted central charge ($c \approx 1.14811$) is close to the real part of the theoretical value $c_{5\text{-Potts}} = 1.1375 \pm 0.0211 i$ predicted by complex conformal field theories. The observed drift in the scaling exponent $b$ effectively distinguishes the continuous transition from the weakly first-order regime. Furthermore, the extrapolated divergence of $g$ confirms the first-order nature of the $q=5$ Potts model.
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Statistical Mechanics
Statistical Physics of Coding for the Integers
Neri Merhav
We study a paradigm of coding for compression of the natural numbers via the
zeta distribution and develop a statistical-mechanical interpretation, both in terms of Hagedorn systems and a Bose gas with energy levels given by logarithms of
prime numbers. We also propose a simple coding scheme for the
zeta distribution that nearly achieves the ideal code length. For block coding of vectors of natural numbers, we derive the micro-canonical
entropy function and demonstrate its asymptotic linearity implying that its behavior is analogous to that of a Hagedorn system. We also derive the large deviations rate function, and provide a formula for the best coding parameter in the large deviations sense. We show that due the Hagedorn-type phase transition there is only partial equivalence of ensembles, due to the degeneration of the domain of the partition function.
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Mesoscale and Nanoscale Physics
Impact of gate voltage on switching field of perpendicular magnetic tunnel junctions with a synthetic antiferromagnetic free layer
K. Fan, S. V. Beek, G. Talmelli et al. (+11 more)
We present micromagnetic simulations and experiments on voltage-assisted field switching in perpendicular
magnetic tunnel junctions (MTJs) with a synthetic
antiferromagnetic (SAF) free layer, where the
magnetic state of one sublayer is detected via tunneling magnetoresistance (TMR). Simulations reveal that local modulation of perpendicular
magnetic anisotropy (PMA) in one SAF sublayer leads to distinct switching characteristics. The switching field varies linearly with the anisotropy field, indicating voltage-controlled
magnetic anisotropy (VCMA)-dominated dynamics similar to single free-layer devices. We then experimentally study the
magnetic switching field of MTJ devices with SAF free layers under applied gate voltage. By varying the MgO tunnel barrier thickness to systematically modulate the resistance-area (RA) product, we enable quantitative separation of
spin-transfer torque (STT), VCMA, and Joule heating contributions. Our findings indicate that VCMA dominates in devices with a high RA product, while low-RA devices exhibit nonlinear switching behavior due to enhanced contributions from STT and Joule heating. Furthermore, the effective fields derived from STT, VCMA, and Joule heating contributions under various gate voltages show minimal dependence on device critical dimensions, indicating favorable scaling behavior. This work presents a unified framework analyzing the roles of STT, VCMA, and Joule heating in SAF-based voltage-gated
spin-orbit torque (SOT) MRAM, offering key insights for the optimization of performance, energy efficiency, and scalability in SOT-MRAM technologies.
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All authors K. Fan, S. V. Beek, G. Talmelli, V. Kateel, D. Giuliano, B. Vermeulen, K. Cai, B. Sorée, J. D. Boeck, R. Carpenter, S. Rao, S. Couet, V. D. Nguyen, G. S. Kar
Superconductivity
Emergent superconductivity at 16.3 K in an altermagnetic candidate Na$_{2-x}$V$_2$Se$_2$O with broken inversion symmetry
Y. Sun, Z. Yin, T. Zhang et al. (+14 more)
Altermagnets (AMs), characterized by zero net magnetization and momentum-dependent
spin splitting, are anticipated to hold significant potential for generating multiple exotic and uncommon superconducting states. However, superconductivity has not yet been realized in AMs to date. Recently, two-dimensional (2D) V$_2$Ch$_2$O (Ch = Se, Te) monolayers, as well as AV$_2$Ch$_2$O (A = K, Rb, Cs)
crystals containing [V$_2$Ch$_2$O]$^{\delta -}$ building layers, have been predicted and/or demonstrated to be promising altermagnetic materials. Our preliminary attempts to explore superconductivity in these materials by applying pressure or chemical doping were unsuccessful. Here we report the discovery of superconductivity at a relatively high transition temperature of ~ 16.3 K in a newly synthesized layered compound, Na$_{2-x}$V$_2$Se$_2$O, a variant of AV$_2$Ch$_2$O. In this structure, the [V$_2$Ch$_2$O]$^{\delta -}$ layers are interspersed with double layers of Na$^+$ instead of a single layer of A$^+$, with sodium sites being only half-filled. This new family of layered vanadium oxychalcogenides, lacking inversion
symmetry, represents an intriguing platform for exploring altermagnetic
superconductors, and holds the potential to reveal novel phenomena, such as topological states, van Hove singularities, and finite-momentum superconductivity. Furthermore, this material acts as a "bridge" between the cuprate/nickelate and iron-pnictide high temperature
superconductors, providing new hope and opportunity to expand the category of layered
superconductors with higher critical temperatures (T$_c$) and enhancing our understanding of the underlying mechanisms in these systems.
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All authors Y. Sun, Z. Yin, T. Zhang, L. Wang, B. Ruan, Y. Huang, J. He, W. Zhu, M. Ma, J. Bai, J. Cheng, Q. Dong, C. Li, P. Liu, Q. Liu, C. Zhang, G. Chen
Strongly Correlated Electrons · Cross-listed from physics.comp-ph
Stable Determinant Monte Carlo Simulations at Large Inverse Temperature $\beta$
Thomas Luu, Johann Ostmeyer, Petar Sinilkov et al. (+1 more)
At low temperatures $T$ where $1/T=\beta\gg1$ the naïve implementation of determinant
quantum Monte Carlo (DQMC) methods suffers from loss of precision and numerical instabilities when evaluating the
fermion determinant. This instability propagates into the calculation of observables that rely on the evaluation of the inverse of the
fermion matrix, or the Greens function. For DQMC methods that rely on the
Hamiltonian Monte Carlo (HMC)
algorithm, an additional complication comes from evaluating the force terms required for integrating Hamilton's equations of motion, since here loss of precision and numerical instabilities are also prevalent. We show how to address all these issues using various choices of
matrix decompositions, allowing us to simulate at $\beta\gtrsim 90$, which corresponds to room temperature for
graphene structures. Furthermore, our implementation has numerical costs that scale similarly to the naïve implementation, namely as $\mathcal{O}(N_x^3N_t)$, where $N_x$ ($N_t$) is the number of spatial (temporal) sites.
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All authors Thomas Luu, Johann Ostmeyer, Petar Sinilkov, Finn L. Temmen
Materials Science · Cross-listed from physics.optics
Fractal hierarchy enables exponential scaling of topological boundary states
Limin Song, Zhichan Hu, Ziteng Wang et al. (+7 more)
Exponential growth describes an extremely rapid process ubiquitous across mathematics and diverse physical, biological, and technological systems. Here, we introduce a class of fractal-inspired
lattices that combine long-range periodic order with self-similar hierarchy, establishing a structural motif that enables exponential scaling of topological boundary states. We demonstrate this phenomenon in (i) a quasi-one-dimensional
lattice chain constructed from Koch-curve unit
cells and (ii) a two-dimensional periodic tiling
lattice composed of Sierpinski-gasket unit
cells. We show that, for suitable coupling parameters, both the number of topological boundary states $N_{\ell}$ and the number of topological minigaps $M_{\ell}$ grow exponentially with the fractal generation index $\ell$. We find that $N_{\ell}$ is an integer multiple of $M_{\ell}$, with the integer determined by the underlying
symmetry. This hierarchical scaling law is captured by multi-topological-phase theory and confirmed experimentally in
laser-written photonic
lattices. Our results identify fractal hierarchy as a materials architecture principle for controlling boundary-state multiplicity, revealing an interplay between
topology, self-similar geometry, and periodic order. More broadly, this work suggests a route to synthetic materials and integrated photonic platforms in which large numbers of robust boundary modes can be engineered within compact architectures.
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All authors Limin Song, Zhichan Hu, Ziteng Wang, Domenico Bongiovanni, Liqin Tang, Daohong Song, Roberto Morandotti, Jingjun Xu, Hrvoje Buljan, Zhigang Chen
Quantum Gases
Emergent Weyl Nodes and Berry Curvature in Bose Polarons via $p$-Wave Feshbach Coupling
Hiroyuki Tajima, Eiji Nakano and Kei Iida
We show that an
impurity quasiparticle immersed in a Bose-Einstein condensate, known as a Bose polaron, exhibits topological properties characterized by a nonzero Berry
curvature, which is induced by Weyl nodes that emerge via interspecies $p$-wave Feshbach resonance. Such nodes occur even in the absence of
spin degrees of freedom and
spin-orbit coupling. For charged
impurities, the corresponding $p$-wave polarons are shown to be accompanied by
chiral anomaly. The above predictions can be tested in a cold atomic environment by observing the
Hall transport of the atomic or ionic
impurity cloud.
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Soft Condensed Matter
Glassy Signatures in Water's Second Liquid
Florian Pabst and Ali Hassanali
The origin of water's anomalous behavior remains a central open problem in the physical sciences and is often attributed to a
liquid-
liquid transition (LLT) between high- and low-density
liquid states deep in the supercooled regime. Experimental access to this region has been challenging due to rapid crystallization, leaving atomistic simulations as a major source of supporting evidence. Using extensive machine-learning-accelerated first-principles simulations in direct comparison with spectroscopic, structural, and dynamical experimental measurements, we show that features commonly interpreted as signatures of two-
liquid behavior coincide with the onset of dramatic dynamical slowing down characteristic of an emerging non-ergodic glassy state. Specifically, we find that two-state fluctuations associated with an LLT, are also consistent with a transformation from a high-density
liquid to a kinetically constrained low-density glassy-like state. By mapping
equilibrium dynamics across pressure and temperature, our results call for a closer examination of water's metastable landscape, in which two-state behavior may reflect a relatively high glass-transition temperature of low-density water, 189~$\pm$~8 K—curiously close to the temperature commonly associated with the proposed LLT.
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Materials Science
Laser-generated CuPdAgPtAu High-Entropy Alloy Nanoparticles – Thermal Segregation Threshold and Elemental Segregation
Felix Pohl, Robert Stuckert, Florent Calvo et al. (+5 more)
High-
entropy alloy nanoparticles synthesized via
laser ablation in
liquid are promising for catalysis due to their ability to form simple solid solutions despite chemical complexity. In this study, noble metal HEA NPs (CuPdAgPtAu) are produced from equimolar and Cu- or Ag-enriched bulk targets. Advanced electron microscopy, XRD, and atomistic simulations are used for structural and compositional analysis. Equimolar targets and NPs exhibit a single fcc phase. In contrast, Cu- or Ag-enriched targets show phase segregation into two fcc phases, which is not observed in the synthesized NPs. Simulations predict segregation tendencies, including Ag surface enrichment and Pt core enrichment due to surface energy differences. However, experimentally, individual NPs remain compositionally homogeneous. Thermal stability studies reveal that phase segregation can be induced post-synthesis. Upon heating, Cu-Ag segregation occurs, forming a second fcc phase similar to bulk targets. These findings demonstrate that rapid quenching during
laser ablation suppresses thermodynamically driven segregation and stabilizes metastable solid solutions under kinetic control. Subsequent slow heating overcomes kinetic barriers, enabling
equilibrium phase formation at higher temperatures. The thermal stability of these NPs and their tunable composition, including Cu enrichment beyond
equilibrium limits, make them promising for high-temperature catalytic applications while reducing noble metal usage.
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All authors Felix Pohl, Robert Stuckert, Florent Calvo, Oleg Prymak, Christoph Rehbock, Ulrich Schürmann, Stephan Barcikowski, Lorenz Kienle
Statistical Mechanics
Negative Differential Heat Conductivity in a Harmonic Chain Coupled to a Particle Reservoir
Simon Krekels, Christian Maes, Ion Santra et al. (+1 more)
When coupling thermal baths at different temperatures, negative differential thermal conductivity is typically attributed to nonlinear interactions in the connecting medium. In this work, we demonstrate that such an effect can arise purely from the nature of the thermal baths and their coupling with the medium. Specifically, we construct a bath composed of overdamped thermal particles, which is coupled to one end of a harmonic chain, while the other end is connected to a standard Langevin heat bath. By analyzing the steady-state heat current, we observe significant negative differential thermal conductivity. In particular, as the temperature difference between the two baths diverges, the steady-state heat current through the chain vanishes. The effect is thermokinetic: we compute the effective dissipative coefficient and we find that it scales inversely with the square of the temperature of the particle bath in the high-temperature limit, resulting in an asymptotic decoupling between the bath and the chain. Our results highlight that nonequilibrium transport properties can be strongly influenced by the structure of the environment and its coupling to the system, even in otherwise linear systems.
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All authors Simon Krekels, Christian Maes, Ion Santra, Ruoxun Zhai
Materials Science
First principles study of thermoelectric properties of $\text{Nb}_2\text{Co}_2\text{InSb}$ and $\text{Nb}_2\text{Co}_2\text{GaSb}$ double half-Heuslers
Rajeev Ranjan
Valence electron count (VEC) 18 half-Heusler (hH)
alloys are considered promising for high-temperature thermoelectric applications due to their high Seebeck coefficient, mechanical stability, and robustness. However, their relatively large
lattice thermal conductivity ($k_{L}$) significantly limits their thermoelectric performance. Introducing mass disorder at
lattice sites is an effective approach to suppress $k_{L}$ through enhanced
phonon scattering. For instance, NbCoSn exhibits a low figure of merit ($zT \sim 0.05$) despite having a reasonably high power factor of 2.1~$\text{mW}/\text{mK}^{2}$ at room temperature, mainly due to its large
lattice thermal conductivity, reported to be 13.25~$\text{W}/\text{mK}$ experimentally and 18~$\text{W}/\text{mK}$ theoretically. In this work, we explore the thermoelectric properties of $\text{Nb}_2\text{Co}_2\text{InSb}$ and $\text{Nb}_2\text{Co}_2\text{GaSb}$, which can be regarded as derivatives of NbCoSn with substitution at the Sn site. Both ordered and Special Quasirandom Structures (SQSs) are considered to understand the role of configurational disorder. Energetic analysis indicates that the ordered phase is most stable for $\text{Nb}_2\text{Co}_2\text{InSb}$, whereas the SQS phase is energetically favored for $\text{Nb}_2\text{Co}_2\text{GaSb}$. The
lattice thermal conductivity is evaluated using the Debye-Callaway model, yielding values in the range of 5.5-6.9~$\text{W}/\text{mK}$ for $\text{Nb}_2\text{Co}_2\text{InSb}$ and 4.7-5.8~$\text{W}/\text{mK}$ for $\text{Nb}_2\text{Co}_2\text{GaSb}$ at room temperature. These values are significantly lower than those of the parent NbCoSn system, highlighting the effectiveness of mass disorder in reducing thermal conductivity. The results suggest that these double half-Heusler compounds are promising candidates for improved thermoelectric performance.
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Superconductivity
Improving YBa$_2$Cu$_3$O$_{7-\delta}$ annealing times through a combining-temperatures route
R. F. Luccas and L. Gallo
The oxygenation process at constant temperature of YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) was systematically investigated in the temperature range from 300 $^o$C to 800 $^o$C. With this purpose, fully deoxygenated powder samples was exposed to an oxygen saturated atmosphere, and the
evolution of their mass was recorded as a function of time using a thermogravimetric balance. Results reveal a strong dependence of both: the oxygenation kinetics and the final oxygen saturation level, on the temperature used for oxygenation. Moreover, results show that higher oxygen temperatures promote faster oxygen absorption but lead to lower saturation levels (higher final $\delta$ values), whereas lower oxygen temperatures result in slower kinetics but enable the system to approach better oxygenation conditions in order to improve the final superconducting properties of the material. In addition to our measurements, a comparative analysis between oxygenation levels at the oxygen temperatures under study was performed in the range around oxygen saturation ($\delta$ $<$ 0.3). Consequently, an oxygenation protocol based on a combination of several oxygenation temperatures is proposed. As a first approach, results from a protocol with just two different oxygenation temperatures is compared with results coming from using just one oxygenation temperature. Outstandingly, a protocol with a first oxygenation step at high temperatures and a second one at low temperatures demonstrates to improve oxygenation times in near a 30 % for reaching $\delta$ values below 0.1 and in near a 60 % for reaching $\delta$ values around 0.12. Finally, we trust that our results are of direct application on industry since size of grain used herein are in scales of typical thickness of
superconductor tapes.
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Mesoscale and Nanoscale Physics
Andreev-enhanced conductance quantization and gate-tunable induced superconducting gap in germanium
Elyjah Kiyooka, Chotivut Tangchingchai, Gonzalo Troncoso Fernandez-Bada et al. (+8 more)
Ge/SiGe
quantum well
heterostructures confining a high-mobility two-dimensional hole gas (2DHG) have emerged as a compelling platform for hybrid
superconductor(S)-
semiconductor(Sm)
quantum devices. Here, we investigate the low-temperature transport properties of split-gate
quantum point contacts (QPC) defined in one such
heterostructure and positioned at different distances from an aluminum superconducting contact. We observe ballistic one-dimensional transport evidenced by conductance quantization with at least four clearly visible plateaus. Andreev reflection at the S/Sm interface induces a 40% enhancement of the conductance steps relative to the normal-state conductance staircase measured under a 100-mT out-of-plane
magnetic field. This result is in excellent agreement with the theoretical expectation for an interface transparency of 0.88. By operating the QPCs in the tunneling regime, we probe the local density of states of the proximitized 2DHG. We report direct experimental evidence of an induced superconducting gap, demonstrating that its magnitude can be tuned by a gate voltage acting on the carrier density in the 2DHG.
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All authors Elyjah Kiyooka, Chotivut Tangchingchai, Gonzalo Troncoso Fernandez-Bada, Boris Brun-Barriere, Simon Zihlmann, Romain Maurand, Francois Lefloch, Vivien Schmitt, Jean-Michel Hartmann, Manuel Houzet, Silvano De Franceschi
Soft Condensed Matter
Slip-link simulations of long-fiber networks under uniaxial compression
Yuichi Masubuchi
A coarse-grained molecular simulation approach originally developed for entangled polymeric
liquids is extended to model the mechanical behavior of long-fiber networks. The model, based on the slip-link picture of chain entanglements, resolves the force balance at contact points and accounts for fiber slippage under these topological constraints. Two key governing equations describe the time
evolution of contact-point positions and the local fiber fraction between adjacent contact points. A yield-force criterion determines whether contact points are displaced or remain pinned, as well as whether fiber slippage occurs at contact points. Uniaxial compression simulations corresponding to press molding of fiber-reinforced thermoplastics were performed for networks with varying fiber lengths and compression rates. The results were qualitatively consistent with experimental observations of long-fiber thermoplastics. The model captures physics inaccessible to the classical van Wyk theory of fiber network compression, which is quasi-static and insensitive to fiber length. This work demonstrates that the slip-link framework, already validated for
polymer melts, provides a promising mesoscale simulation tool for understanding and predicting the processing behavior of non-thermal fiber networks.
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Quantum Gases · Cross-listed from physics.atom-ph
Probing topological edge states in a molecular synthetic dimension
Adarsh P. Raghuram, Francesca M. Blondell, Jonathan M. Mortlock et al. (+7 more)
Engineering synthetic dimensions, where the physics of additional spatial dimensions is simulated within the internal states of a
quantum system, allows the realisation of phenomena not otherwise accessible in experiments. Ultracold ground-state polar molecules are an ideal platform to encode synthetic dimensions, offering access to large
Hilbert spaces of long-lived internal states associated with the rotational and hyperfine degrees of freedom, that can be coupled together with microwave fields to simulate tunnelling. Here, to
benchmark the advantages of ultracold molecules, we encode a 1D synthetic
lattice in the rotational states of ultracold RbCs molecules and use it to investigate the well-known Su-Schrieffer-Heeger (SSH) model, a minimal model displaying topological properties. To probe the system, we perform spectroscopy using an auxiliary rotational state and study the time dynamics after deterministic state preparation. We demonstrate long coherence times, typically ~500 times the
lattice tunnelling period, even for a synthetic
lattice using 8 rotational states. Observations of dynamics at long times with full site-resolved readout of the synthetic dimension allow us to test the effects of
chiral and non-
chiral perturbations on the topologically protected edge states. Our work lays the foundation for further
quantum simulations using the rich internal structure of molecules, including dipolar string phases in interacting samples of molecules, and adiabatic state preparation of many-body
Hamiltonians.
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All authors Adarsh P. Raghuram, Francesca M. Blondell, Jonathan M. Mortlock, Benjamin P. Maddox, Sohail Dasgupta, Holly A. J. Middleton-Spencer, Kaden R. A. Hazzard, Hannah M. Price, Philip D. Gregory, Simon L. Cornish
Statistical Mechanics
Fate of emergent Hilbert space fragmentation and information insulation upon softening of hard dual constraints at PXP-chain junctions
Akshay Panda and Anwesha Chattopadhyay
We demonstrate emergent
Hilbert-space fragmentation in a composite chain of PXP and dual-PXP subchains separated by junctions with simultaneous blockade and anti-blockade projection constraints. The
Hilbert space shatters into disjoint Krylov fragments growing exponentially with junction number. Freezing does not require a unique allowed junction state: multiple configurations remain locally allowed, but the dual constraints remove dynamical links between them. Each fragment is labeled by frozen junction configurations and contains compatible basis states. This fragmentation produces dynamical caging of
quantum information between neighboring junctions and novel states with partially thermal and athermal regions. Softening the junction by admitting other local states with variable nonzero diagonal amplitudes destroys the direct-sum block structure and fragmentation. Nevertheless, information remains dynamically insulated and inhomogeneous thermal properties persist up to an onset time scaling with the diagonal energy penalty of the previously restricted junction states, followed by longer prethermal behavior. Energy-level statistics in each
symmetry-resolved sector evolve from Poissonian in the hard-junction limit toward Wigner-Dyson behavior upon softening, with finite-size non-monotonicity. This signals the breakdown of the tensor-sum structure yielding pairwise sums of chaotic subchain spectra, restoring chaotic thermalization. In the hard-junction case, we find a chirally protected zero-energy mode with density peaks at physical edges and near the junction, pinned to zero under chirality-preserving disorder. The tensor-product eigenfunction structure also enhances the number of nonzero-energy scar states. Finally, we discuss routes to these junction architectures in programmable Rydberg atom platforms using optical tweezers, local addressing, and facilitation techniques.
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Other Condensed Matter
The motion of tracer particles in turbulent superfluid $^4$He down to the zero-temperature limit
C. O. Goodwin, M. J. Doyle, J. A. Hay et al. (+4 more)
An injection system for
polymer particles, with diameters ranging from 1 to 6 $\mu$m, has been developed for visualizing flows in superfluid $^4$He at temperatures down to 0.14 K. Using an ultrasound transducer, bursts of particles were launched into a sample of superfluid and allowed to descend under
gravity. The particles were imaged using their fluorescence in the presence of a sheet of
laser light. We report on the statistical behavior of particles during their descent, including descriptions of a mixture of smooth and erratic trajectories, indicative of the interactions with thermal excitations and quantized
vortex lines. Temperature-dependent velocity distributions were measured and analyzed, yielding Gaussian distributions with power law tails persisting into the zero temperature limit. When sampled over increasing length scales, these distributions bifurcated into exponential for the smallest particles and bimodal Gaussian for the largest. We also report observations of long-lived suspensions of small particles at temperatures near 1 K, which appear to be associated with the trapping of large numbers of particles in a turbulent
vortex tangle. A method was developed for identifying and quantifying the numbers of particles bound to
vortex lines, allowing for a description of the temporal dynamics of their population by an analytical model.
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All authors C. O. Goodwin, M. J. Doyle, J. A. Hay, I. Skachko, W. Guo, P. M. Walmsley, A. I. Golov
Mesoscale and Nanoscale Physics
Topological magnetotransport in modified-Haldane systems
A. Uzair, Muzamil Shah, Imtiaz Khan et al. (+1 more)
We present a theoretical study of
quantum magneto-transport and magneto-optical (M-O) properties in modified-Haldane model; which is applicable to diverse classes of two-dimensional (2D)
quantum materials such as buckled Xene monolayers and transition metal dichalcogenide (TMDC) monolayers. By varying the staggered sublattice potential and intrinsic
spin-orbit coupling, we identify distinct topological regimes and analyze their manifestations in the emergence of Landau levels, the
evolution of the density of states, and the characteristics of M-O absorption spectra. Using the Kubo formalism, we compute the longitudinal and
Hall M-O conductivities and show that inter-Landau-level (inter-LL) transitions produce characteristic resonance features that provide optical signatures of the underlying topological phases. Within this framework, we demonstrate electrically tunable topological phase transitions in buckled silicene. Extending our study to monolayer TMDCs, we show that inspite of large band gap, the
spin-valley coupling provides a powerful tool for tailoring M-O absorption features across wide range of 2D materials. Collectively, these results underscore modified-Haldane-model materials as an ideal testbed for engineering
quantum transport, with promising applications in topological photonics, valleytronic devices, and next-generation optoelectronics.
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All authors A. Uzair, Muzamil Shah, Imtiaz Khan, Kashif Sabeeh