Statistical Mechanics
A $q$-Caputo Fractional Generalization of Tsallis Entropy
Matias P. Gonzalez and Micolta-Riascos Bayron
In this work we build a $q$-Caputo fractional generalization of the Tsallis
entropy functional $S_q$. The construction starts from the already known Jackson derivative representation of $S_q$ and replaces the Jackson derivative with a Caputo type $q$-fractional operator ${}^{C}D_q^{\alpha}$. The parameter $q$ keeps its standard role as the nonextensive index, while the fractional parameter $\alpha$ gives a nonlocal,
memory like contribution in the
entropy generating procedure. The resulting functional, denoted by $S_q^{\alpha}$, is written as a convergent series of logarithms in the probabilities and is defined for $0<\alpha<1$, $q>0$, and $p_i>0$. We show explicitly that the standard Tsallis
entropy is recovered in the local limit $\alpha\to 1$. We then start rewriting $S_q^{\alpha}$ in trace form and analyze its main structural properties. Non-negativity is tested numerically in a binary and ternary probability distributions. These tests show positive values in the respective domains, but they are interpreted just as numerical evidence rather than as a global/formal proof. Concavity is studied through the second derivative of the trace form contribution yielding to a parameter dependent condition. A global statement requires control over the full probability domain. We also discuss Lesche stability and expansibility. Both properties depend on the behavior of the fully resumed functional near its boundary \(p\to 0^+\), because the
entropy contains powers of \(\ln p_i\). Finally, we derive the composition rule for statistically independent subsystems. The logarithmic structure produces mixed moments, so the standard Tsallis pseudo-additive law is not directly inherited for \(0<\alpha<1\). As an illustration, we evaluate the proposed
entropy in a two-state fractional relaxation process with
memory.
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Materials Science
Virtual materials testing of ASSB cathodes combining AI-based stochastic 3D modeling and numerical simulations
Anina Dufter, Sabrina Weber, Orkun Furat et al. (+7 more)
The performance of all-solid-state battery (ASSB) cathodes strongly depends on their microstructure. Optimizing the cathode morphology can therefore enhance effective macroscopic properties such as ionic and electronic conductivity. The search for optimized microstructures can be facilitated by virtual materials testing: By integrating image analysis and
stochastic microstructure modeling to generate a wide range of realistic 3D microstructures and evaluate their effective macroscopic properties by means of numerical simulations, thereby reducing the need for extensive physical experiments. This approach allows for the investigation of structure-property relationships through parametric regression models that incorporate relevant geometrical descriptors of microstructures such as volume fractions, mean geodesic tortuosities, specific surface areas, and constrictivities. By linking these geometrical descriptors to macroscopic properties, virtual materials testing provides quantitative insight into how microstructure influences material performance. In the present paper, this framework is applied for ASSB cathodes. In addition, by systematically varying model parameters, a broad range of 3D microstructures can be generated, which remain close to the original cathode morphology while inducing targeted changes in selected geometrical descriptors. The resulting database enables the calibration of regression models whose predictive performance is assessed by comparing predicted and simulated effective properties such as the ionic and electronic conductivity, thereby quantifying how accurately combinations of geometrical descriptors can explain and predict variations in effective macroscopic properties.
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All authors Anina Dufter, Sabrina Weber, Orkun Furat, Johannes Schubert, René Rekers, Maximilian Luczak, Erik Glatt, Andreas Wiegmann, Anja Bielefeld, Volker Schmidt
Materials Science · Cross-listed from physics.optics
Free electrons unveil nanolaser physics at the sub-picosecond and nanometer scale
Cléo Santini, Thi Huong Ngo, Luiz H. G. Tizei et al. (+14 more)
Semiconducting nanowire
lasers (NWLs) constitute nanoscale modular and tunable light sources, essential components for integrated optoelectronics. However, advancing NWL operation requires characterization of their near-field and dynamics at the nanometer scale, which is hampered by the diffraction limit of light. Here, we show a non-perturbative measurement of the absolute number of
photons in a GaN/InGaN nanowire nanolaser cavity above the lasing threshold with sub-picosecond resolution and maps of the lasing mode spatial profile at the nanoscale. For this we use
photon-induced near-field electron microscopy while simultaneously measuring the
photons far-field, unraveling the key nanoscale and time-resolved characteristics of a NWL in operation. With this technique, we observed lasing with a pump-power-dependent time offset of 0.5-1 ps that reflects carrier thermalization and spontaneous emission, and a pulse duration of 500 to 700 fs. From the spatial maps of the lasing mode, we observe TM-polarized azimuthal whispering gallery modes along the nanowire circumference, as well as two distinct Fabry-Perot modes along the nanowire length. These spatial maps allow us to understand which of the multiple complex cavity modes available in the nanowire are responsible for lasing paving the way for their practical application into devices.
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All authors Cléo Santini, Thi Huong Ngo, Luiz H. G. Tizei, Aurélie Lloret, Tom Fraysse, Sebastien Weber, Adrien Teurtrie, Albert Polman, Virginie Brändli, Sebastien Chenot, Denis Lefebvre, Stéphane Vézian, Hugo Lourenço-Martins, Christelle Brimont, Benjamin Damilano, Thierry Guillet, Sophie Meuret
Mesoscale and Nanoscale Physics
Enhanced spin-current generation in Dirac altermagnets through Klein tunneling
Tomas T. Osterholt, Lumen Eek, Cristiane Morais Smith et al. (+1 more)
Altermagnets have recently emerged as a new platform for spintronics applications, offering
spin-split electronic bands despite vanishing net magnetization. Here, we investigate
spin-current generation in Dirac altermagnets and identify Klein tunneling as an efficient mechanism for enhancing
spin transport. Using a low-energy Dirac model combined with scattering theory, we demonstrate that Klein tunneling in altermagnets is strongly
spin-dependent and can be used to effectively control the electronic
spin-current polarization by, for instance, adjusting the height, width and orientation of the potential barrier. Finally, we explore how the l-wave
symmetry of the Dirac altermagnet shapes the
spin-current polarization and transmission, focusing especially on the d- and g-wave cases. Particularly promising results are obtained for the g-wave Dirac altermagnet, as it is found that the presence of a potential barrier can significantly boost the
spin-current polarization, even when the intrinsic polarization due to the
spin-split band structure is vanishingly small. For a barrier implemented via electrostatic gating, such a mechanism would in turn allow the
spin-current polarization to be switched on and off via a gate voltage.
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All authors Tomas T. Osterholt, Lumen Eek, Cristiane Morais Smith, Rembert A. Duine
Statistical Mechanics
Dynamics of O(2) excitations in a non-reciprocal medium
Ylann Rouzaire, Daniel JG Pearce, Ignacio Pagonabarraga et al. (+1 more)
We investigate emergent dynamics due to non-reciprocity in the $\mathcal{O}(2)$ model. The
lattice XY model, where non-reciprocity stems from
vision cone like couplings, can be described by a continuum description in which non-reciprocity translates into a new term depending on the rotational of the orientation field. We argue that non-reciprocity is akin to activity and we highlight the connection between our hydrodynamic equation and the constant density Toner-Tu framework. The active force advects and reshapes patterns, a generic feature found in many non-reciprocal systems. We show how $1d$ excitations in the non-reciprocal $\mathcal{O}(2)$ model can be described by a generalized Burgers equation, derived from our continuum model. We then extend the results to $2d$ perturbations. As such, we establish the first principles of excitation trajectory control in a non-reciprocal $\mathcal{O}(2)$ medium. Concretely, we explain how tuning the degree of non-reciprocity and the orientation of the background medium impacts the time
evolution of excitations. We also showcase how initially different excitations lead to very different dynamical behavior. Non-reciprocity also affects the stability of defect-free excitations with non-zero winding numbers and, unlike in its
equilibrium $O(2)$ counterpart, enables the system, above a certain threshold, to relax to its ground state.
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All authors Ylann Rouzaire, Daniel JG Pearce, Ignacio Pagonabarraga, Demian Levis
Disordered Systems and Neural Networks
Generative Inversion of Spectroscopic Data for Amorphous Structure Elucidation
Jiawei Guo and Daniel Schwalbe-Koda
Determining atomistic structures from characterization data is one of the most common yet intricate problems in materials science. Particularly in amorphous materials, proposing structures that balance realism and agreement with experiments requires expert guidance, good interatomic potentials, or both. Here, we introduce GLASS, a generative framework that inverts multi-modal spectroscopic measurements into realistic atomistic structures without knowledge of the potential energy surface. A score-based model learns a structural prior from low-fidelity data and samples out-of-distribution structures conditioned on differentiable spectral targets. Reconstructions using pair distribution functions (PDFs), X-ray absorption spectroscopy, and diffraction measurements quantify the complementarity between spectral modalities and demonstrate that PDFs is the most informative probe for our framework. We use GLASS to rationalize three contested experimental problems: paracrystallinity in amorphous silicon, a
liquid-
liquid phase transition in sulfur, and ball-milled amorphous ice. In each case, generated structures reproduce experimental measurements and reveal mechanisms inaccessible to diffraction analysis alone.
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Disordered Systems and Neural Networks · Cross-listed from math.ST
Gaussian mixtures and non-parametric likelihoods through the lens of statistical mechanics
Subhroshekhar Ghosh, Adityanand Guntuboyina, Satyaki Mukherjee et al. (+1 more)
In this work, we investigate Gaussian Mixture Models (abbrv GMM) and the related problem of non parametric maximum likelihood estimation (abbrv NPMLE) from the perspective of statistical mechanics. In particular, we establish stability guarantees for the NPMLE procedure that extend well beyond the state of the art. Crucially, we obtain guarantees on the Kullback-Leibler divergence between NPMLE
estimators and the ground truth, a type of result which has been known to be challenging in the literature on this problem. In particular, we provide high probability upper bounds on the KL divergence between the NPMLE and the true density that are of the order of $\min\big\{\frac{(\log n)^{d+2}}{n} , \frac{\log n}{\sqrt n}\big\}$, which cover a wide range of scenarios for the comparative sizes of $n$ and $d$. We obtain similar guarantees for approximate solutions to the NPMLE problem, addressing realistic situations wherein optimization
algorithms need to be stopped in finite time, allowing access only to approximations to the true NPMLE. A cornerstone of our approach is an analysis of the function class complexity of logarithms of gaussian mixture densities, which is able to handle their unboundedness, and could be of wider interest. Our methods lead to novel confidence-interval guarantees for
entropy estimation in Gaussian mixtures, demonstrating their wider impact. We also establish correspondences between stability phenomena in the NPMLE problem and concepts such as chaos and multiple valleys in random energy landscapes of statistical mechanics models. While these correspondences are largely of a conceptual nature at this point, we believe that these connections, especially those with concentration phenomena and Langevin dynamics, may be developed into a toolbox for studying a wide variety of random optimization problems in statistics and machine learning.
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All authors Subhroshekhar Ghosh, Adityanand Guntuboyina, Satyaki Mukherjee, Hoang-Son Tran
Statistical Mechanics · Cross-listed from math.PR
On the Golomb-Dickman constant under Ewens sampling
José Ricardo G. Mendonça and Luis Jehiel Negret
We define a generalized Golomb–Dickman constant $\lambda_{\theta}$ as the limiting expected proportion of the longest cycle in random permutations under the Ewens measure with parameter $\theta > 0$. Exploiting the independence properties of Kingman's Poisson process construction of the Poisson–Dirichlet distribution, we obtain an explicit integral representation for $\lambda_{\theta}$ in terms of the exponential integral. The dependence of $\lambda_{\theta}$ on $\theta$ reflects the transition between regimes dominated by long cycles (small $\theta$) and those with many small cycles (large $\theta$). We also derive the asymptotic behavior of $\lambda_{\theta}$ for small and large $\theta$ and illustrate our results with numerical computations, Monte Carlo simulations of the Hoppe urn, and an application.
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Mesoscale and Nanoscale Physics
Symmetry-enabled tunable square-lattice Hubbard models in $\Gamma$-valley moiré bilayers
Rui Shi, Kejie Bao, Huan Wang et al. (+1 more)
Square-
lattice moiré systems provide a promising route for
quantum simulation of Hubbard physics, yet electrically tunable realizations of the hopping ratio $t'/t$ have so far been limited to specific valley configurations. A natural question is whether the same tunability can be extended to the more broadly occurring $\Gamma$-valley systems. Here we identify a
symmetry-controlled route for realizing electrically tunable square-
lattice Hubbard models in $\Gamma$-valley twisted homobilayers. At small twist angles, an emergent layer-exchange
symmetry separates the low-energy states into flat bands localized on two nested square sublattices, suppressing inter-sublattice hopping. An interlayer displacement field breaks this
symmetry and induces controllable hybridization between the sublattices, enabling continuous tuning of the $t'/t$ ratio over a wide range while preserving an effective single-band description. We further establish a formal correspondence between $\Gamma$- and M-valley moiré systems, revealing them as different
symmetry limits of a unified framework for tunable square-
lattice Hubbard models. Our results uncover a general
symmetry principle underlying displacement-field tunability and extend electrically controllable square-
lattice Hubbard physics to a broad family of $\Gamma$-valley materials.
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All authors Rui Shi, Kejie Bao, Huan Wang, Jing Wang
Statistical Mechanics
Amplification based on the noise-induced negative differential resistance in a Zener diode
Alexandre Dumont and Bertrand Reulet
A voltage biased Zener diode always exhibit positive differential resistance, thus cannot be used as an element to provide amplification of a signal. We show how to induce negative differential resistance in the reverse bias regime of a 12V Zener diode by noise feedback. We use this to build a voltage amplifier in the audio frequency range, which we characterize by providing bandwidth, gain, power consumption, gain compression and output noise spectral density.
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Materials Science
Field-induced spin-flip and spin-flop transitions in NdFeO3
Mariana M. Gomes, Rui Vilarinho, E. Miranda et al. (+14 more)
Magnetic control of correlated
spin systems is central to the development of next-generation
spin-based technologies. Rare-earth orthoferrites provide an interesting platform in which exchange coupling between rare-earth 4f and transition-metal 3d moments generates competing
magnetic interactions and multiple metastable states. Here, we show that the orientation of the applied
magnetic field drives different
magnetic phase transition sequences in NdFeO3 across a broad temperature range. Using Raman and polarized terahertz spectroscopies, supported by magnetization and specific-heat measurements, we track the temperature- and field-dependent
evolution of the different
magnetic phases and the successive
spin rearrangements, driven by 4f - 3d
magnetic anisotropic interactions. For fields applied along the crystallographic c-axis, a
spin-reorientation transition is followed by
spin-flop and
spin-flip processes, producing an unexpectedly complex
magnetic phase sequence at low temperatures. Below 8 K, precursor effects associated with ordering of the Nd-sublattice strongly modify the transition pathway. Our results demonstrate how anisotropic 4f-3d coupling enables
magnetic-field control of coupled
spin excitations and provide a route to accessing novel
spin configurations in rare-earth orthoferrites.
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All authors Mariana M. Gomes, Rui Vilarinho, E. Miranda, Ana S. Silva, Christelle Kadlec, Filip Kadlec, Miroslav Lebeda, Petr Proschek, Matus Mihalik jr., Marian Mihalik, Diparkan Jana, Fadi Choueikani, Clement Faugeras, José Antonio Paixão, Ester de Prado, Stanislav Kamba, Joaquim Agostinho Moreira
Strongly Correlated Electrons
Petar Brinić, Hugo U. R. Strand and Jakša Vučičević
The simulation of strongly correlated electron systems remains a formidable challenge. Certain experimentally relevant dynamical response functions are especially difficult to calculate, due to issues of finite-size effects and the ill posed analytic continuation. To address this we propose the
quantum cluster
embedding theory, an embedded cluster method aimed at computing the response of the system following an external perturbation; the frequency dependent dynamical susceptibility is obtained subsequently by means of inverse linear response theory. The embedded clusters, used within the method as representative of short range correlations, are open
quantum systems governed by the Lindblad equation. The short-range correlations extracted from the clusters are used to close the equations of motion for the fermionic bilinear and the local double occupancy on the
lattice. In turn, the clusters' Markovian baths are tuned to keep the bilinear and the double occupancy expectation values on the clusters and the
lattice identical, throughout the concomitant
evolution of the two sets of equations. The theory becomes numerically exact in the non-interacting, atomic and infinite cluster size limits, and it respects the total charge and energy conservation laws. We show that our approach can treat very large
lattices while avoiding analytic continuation through the explicit time
evolution. Finally we compute the charge-charge correlation function in the square
lattice Hubbard model and compare with a recent cold atom experiment, finding good qualitative agreement.
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Strongly Correlated Electrons
From Quantum Dimers to the $\pi$-flux Toric Code via Deconfined Multicriticality
Ankush Chaubey, Sergej Moroz and Subhro Bhattacharjee
Two-dimensional Rokhsar-Kivelson (RK) dimer models on bipartite
lattices are generally limited to translation-
symmetry-broken dimer
crystals. We introduce a tensor-product regularisation of the dimer
Hilbert space that yields a
qubit Hamiltonian interpolating from the RK model to the $\pi$-flux toric code, thereby accessing a deconfined $\mathbb{Z}_2$ topological
liquid. In this framework, the $\mathbb{Z}_2$
liquid descends from a multicritical $U(1)$
spin liquid through condensation of a charge-2
Higgs field, thus avoiding confinement. Using iDMRG together with low-energy field theory, we determine a phase diagram containing two continuous
quantum phase transitions – a $3\mathrm{D}$ XY$^{\ast}$ transition between the $\mathbb{Z}_2$
liquid and the columnar/plaquette-VBS, and a
quantum Lifshitz transition between two dimer
crystals – alongside a first-order transition between the staggered
crystal and the $\mathbb{Z}_2$
liquid. Our field theory suggests a deconfined multicritical point described by an Abelian
Higgs model with dynamical critical exponent, $z=2$, where the three transitions meet, highlighting the interplay of fractionalisation and emergent gauge fluctuations.
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Mesoscale and Nanoscale Physics
Pre-Patterned Superconducting Contacts for Clean Superconductor-Topological Material Interfaces Enabling Long-Range Josephson Coupling
Yong-Bin Choi, Chang-Won Choi, Luke Holtzman et al. (+8 more)
Phase-coherent superconducting proximity in topological materials requires clean
superconductor-topological material (SC-TM) interfaces, yet conventional top-contact fabrication often degrades them through oxidation,
polymer residue, and process-induced disorder. Here we introduce a pre-patterned superconducting bottom-contact architecture in which MoRe/Au electrodes are defined before van der Waals
crystal transfer, thereby avoiding on-flake lithography after transfer. In WTe2- and Bi1.5Sb0.5Te1.7Se1.3-based Josephson junctions, this architecture yields systematically larger I_c R_N and longer-ranged coupling than conventional top contacts. Cross-sectional STEM/EDS reveals atomically abrupt, chemically well-separated interfaces. These results establish pre-patterned SC-TM contacts as a practical route to reproducible, micrometer-scale Josephson platforms in van der Waals topological materials.
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All authors Yong-Bin Choi, Chang-Won Choi, Luke Holtzman, Hoil Kim, Seongwoo Kang, Kenji Watanabe, Takashi Taniguchi, James Hone, Jun Sung Kim, Si-Young Choi, Gil-Ho Lee
Strongly Correlated Electrons
Impurity quadrupole moments as local probes of flux sectors in the Kitaev spin liquid
Masahiro O. Takahashi, Wen-Han Kao, Satoshi Fujimoto et al. (+1 more)
Emergent fluxes play a central role in the low-energy properties of
quantum spin liquids (QSLs), where they encode the underlying gauge structure and fractionalization of
spins. Here, we show that the quadrupole moment of
magnetic impurities provides a direct probe of these flux configurations in QSLs and can be measured by local tunneling spectroscopy. Employing the SO(6) Majorana representation for
spin-3/2
impurity operators in the isotropic Kitaev
spin liquid together with a self-consistent mean-field approximation for
impurity-related terms, we show that the ground-state flux sector can be identified by discontinuous jumps of the
impurity quadrupole moment at the flux sector transition points. We also demonstrate that the quadrupole correlations between
impurities under a
magnetic field exhibit exponential decay, with decay rates that depend sensitively on the flux sector. Furthermore, we discuss the stability of pi fluxes bound to
impurities with respect to model parameters and internal flux configurations, and relate our findings to Lieb's
conjecture on flux configurations. These results establish the quadrupole moments of
magnetic impurities as a sensitive tool to study fractionalized excitations and flux physics in Kitaev magnets.
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All authors Masahiro O. Takahashi, Wen-Han Kao, Satoshi Fujimoto, Natalia B. Perkins
Materials Science
Open Quantum System Theory of Muon Spin Relaxation in Materials
Elvis F. Arguelles and Osamu Sugino
We present a non-Markovian theory of
muon spin relaxation that treats the implanted
muon as an open
quantum spin coupled to a temporally correlated local
magnetic environment. Using a Schwinger-Keldysh influence-functional formulation, we derive a
stochastic equation of motion for the
muon spin, in which the fluctuation kernel is fixed by the local-field correlation tensor, while the retarded
memory torque is introduced through an effective phenomenological backaction kernel. In the appropriate limits, the theory reduces to standard Kubo-Toyabe descriptions. This enables quantitative, global analysis of zero-field (ZF) and weak longitudinal-field (LF) $\mu$SR spectra beyond the strong-collision approximation. Applied to $\mathrm{Li}_{0.73}\mathrm{CoO}_2$, the model supports a decomposition into a quenched width and a Li-driven dynamical component within the adopted parametrization, and yields fluctuation rates approximately consistent with activated behavior over the intermediate-temperature window. The fitted
memory parameter is most visible in the crossover between quasi-static and fast-fluctuation limits.
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Strongly Correlated Electrons
Complex magnetic interactions in geometrically frustrated TbOF
Pim Witte, Denis Sheptyakov, Elsa Lhotel et al. (+5 more)
We have identified TbOF as a unique frustrated and mixed-anion
lattice, hosting unconventional magnetism. By means of magnetization, specific heat and
neutron diffraction measurements down to 90 mK, as well as DFT calculations, we present a comprehensive study of the
magnetic and structural properties of TbOF. We show that at 9.7 K, TbOF undergoes a structural phase transition accompanied by short-range
magnetic correlations, in contrast to previously proposed long-range
antiferromagnetic order. At lower temperatures, we observe two
magnetic ordering transitions, consisting of incommensurate
spin density waves and
antiferromagnetic and
ferromagnetic correlations. Furthermore, we observe metastable and hysteresis behavior below 2.0 K, highlighting the richness of complex
magnetic interactions in TbOF. These results uniquely clarify the
magnetic phase diagram of TbOF and highlight the intricate interplay between structure and magnetism in rare-earth oxyfluorides.
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All authors Pim Witte, Denis Sheptyakov, Elsa Lhotel, Nongnuch Artrith, Robin de Hoogh, Giuditta Perversi, Kim Lefmann, Machteld E. Kamminga
Materials Science
Formation of Ag and Au Plasmonic Nanoparticles by Ion Implantation in Ga$_2$O$_3$ thin films
Inês Freitas, Ana Sofia Sousa, Duarte Magalhães Esteves et al. (+6 more)
Gallium
oxide (Ga$_2$O$_3$) is a wide-bandgap
semiconductor with exceptional electrical and optical properties, making it a promising material for optoelectronic and sensing applications. In this work, we demonstrate for the first time the formation of plasmonic silver (Ag) and gold (Au) nanoparticles embedded in Ga$_2$O$_3$ thin films via ion implantation. Ga$_2$O$_3$ films deposited by RF sputtering on sapphire substrates were implanted with Ag or Au ions at 150 keV and a nominal fluence of 5 $\times$ 10$^{16}$ ions/cm$^2$, followed by thermal annealing between 200 and 700 {\deg}C. Rutherford backscattering spectrometry (RBS) measurements revealed saturation effects during implantation, resulting in lower incorporated fluences, as well as out-
diffusion with post-implantation annealing. Transmission electron microscopy confirmed the formation of metallic nanoparticles with a distribution consistent with the metal profiles measured by RBS. Optical absorption measurements showed a pronounced localized surface plasmon resonance (LSPR) band in the Ag-implanted films, visible even in the as-implanted state and red-shifting with increasing annealing temperature, while Au-implanted films exhibited a distinct LSPR peak only after annealing at $\geq$500 {\deg}C. The observed LSPR shifts with annealing are attributed primarily to changes in the Ga$_2$O$_3$
matrix rather than a change in nanoparticle size. These results establish ion implantation as a viable approach for integrating plasmonic nanostructures into Ga$_2$O$_3$.
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All authors Inês Freitas, Ana Sofia Sousa, Duarte Magalhães Esteves, Mamour Sall, Ângelo Rafael Granadeiro da Costa, Joana Madureira, Sandra Cabo Verde, Katharina Lorenz, Marco Peres
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Metastability, chaos and spectrum tomography for Bose-Hubbard rings and chains
Rajat and Doron Cohen
We analyze the metastability of Bose-Hubbard condensates for finite-size one-dimensional ring
lattices and open chains, using a semiclassical tomographic perspective that emphasizes the relation of the many-body spectrum to the underlying classical phase-space structures. In order to address
quantum ergodicity in far-from-
equilibrium scenarios of experimental interest, we inspect both local aspects (via Bogoliubov analysis) and global aspects (mixed regular-chaotic dynamics). In particular, we highlight the roles of the two parameters that control metastability, clarify the essential differences between low- and high-dimensional chaos, and show how the dynamical instabilities diminish in the limit of the Gross-Pitaevskii equation. It is somewhat frustrating that with more degrees of freedom, the dynamically metastable islands become better distinct from the ergodic sea, while their borders become ill-defined topologically. This stands in opposition to the very structured phase-space of two-degree-of-freedom systems, as reflected in the tomographic
quantum spectrum.
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Soft Condensed Matter
Template-free fabrication of reconfigurable magnetic micropillars and filaments through controlled Nanoflower assembly and actuation
Caterina Landi, Rosa Pérez-Garrido, Julio Marco Cuenca et al. (+4 more)
Magnetic nanoflowers (MNFs), which exhibit large intrinsic
magnetic losses and high specific absorption rates under clinically relevant alternating
magnetic fields, highlight strong potential as efficient mediators for
magnetic hyperthermia. In this work, we provide a versatile platform for creating dynamic, field-responsive microstructures based on MNFs through a flexible, low-cost, and template-free self-assembly strategy driven by tunable interparticle interactions, external
magnetic fields, and spatial confinement. By controlling ionic strength, particle coverage, surface charge, particle concentration, and confinement, MNFs spontaneously assemble in aqueous solution into
magnetic micropillars and microfilaments without predefined scaffolds, with low ionic strength favoring reversible assemblies and intermediate salt concentrations yielding stable, irreversible structures. The size, geometry, and dynamic response of these architectures can be precisely tuned, enabling field-induced behaviors such as cilia-like rotations, oscillations, and torque-driven detachment of micropillars into free-standing, swarming microfilaments. L-dopamine (L-DOPA) was used for surface modification in this work, as it is a biocompatible ligand offering catechol, amine, and carboxylate groups. Resulting MNFs@L-DOPA have negative surface charge and show assembly behavior qualitatively similar to the uncoated system under
magnetic actuation. Together, these results establish practical guidelines for the template-free design of biomimetic, functional
magnetic and elongated microarchitectures, highlighting their potential for microfluidic manipulation and bio-microrobotic applications.
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All authors Caterina Landi, Rosa Pérez-Garrido, Julio Marco Cuenca, Javier Tajuelo, Chantal Valeriani, Helena Gavilán, Fernando Martínez-Pedrero
Statistical Mechanics
Basis dependence of eigenstate thermalization
Lennart Dabelow, Christian Eidecker-Dunkel and Peter Reimann
Eigenstate thermalization refers to the property that an energy eigenstate of a many-body system is indistinguishable from a thermal
equilibrium ensemble at the same energy as far as expectation values of local observables are concerned. In systems with degeneracies, the choice of an energy eigenbasis is not unique and the fraction of basis states exhibiting eigenstate thermalization can vary. We present a simple example where this fraction vanishes in the thermodynamic limit for one basis choice, but remains nonzero for another choice. In other words, the weak eigenstate thermalization hypothesis is satisfied in the first, but violated in the second basis. We furthermore prove that degeneracies must abound whenever a system is simultaneously symmetric under spatial translations and reflection. Finally, we derive general bounds on how strongly eigenstate thermalization may depend on the choice of the basis, and we reveal some interesting implications regarding the temporal relaxation properties of such systems.
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Materials Science
Fine-tuning of universal machine-learning interatomic potentials for high-entropy alloys with application to 2D (Mo,Ta,Nb,W,V)S$_2$
Chun Zhou and Hannu-Pekka Komsa
High-
entropy alloy (HEA) materials and their two-dimensional counterparts (2D-HEAs) have recently attracted
attention due to their tunable properties and catalytic potential, yet their chemical complexity makes direct density functional theory (DFT) calculations computationally prohibitive. The complexity also makes training of machine-learning interatomic potentials (MLIPs) challenging, but this could possibly be overcome by employing universal MLIPs as starting point. In this work, we investigate the applicability of universal MLIP models for 2D transition metal dichalcogenide HEAs and develop effective fine-tuning strategies. Training structures are systematically generated and selected, and the performance of universal and fine-tuned models are benchmarked against DFT. We find that all universal MLIPs employed in this work yield unsatisfactory mixing energies without fine-tuning. Applied to the experimentally synthesized (Mo,Ta,Nb,W,V)S$_2$ system, fine-tuned models based on enumerated structures can achieve near-DFT accuracy in predicting mixing energies while enabling Monte-Carlo simulations and random structure sampling at scales inaccessible to DFT.
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Quantum Gases
Transformation of the Talbot effect in response to phase disorder
Ilia Mosaki and A. V. Turlapov
Bose-Einstein condensates initially arranged in a long chain freely expand and interfere. If the initial phases of the condensates are identical, the initial density distribution is restored periodically during the expansion, giving rise to the Talbot effect. Even a slight disorder in the initial phases leads to a transformation of the interference pattern. In response to the phase disorder, the spectrum of the spatial density distribution acquires peaks that are absent in the case of identical phases. We derive an analytical expression for the spectrum of the spatial density distribution for an arbitrary phase disorder. We show that the new peaks emerging due to the phase disorder originate from pairwise interferences of the condensates. The positions of these peaks coincide with the wave vectors of the density modulations (wavelets) generated by such pairwise interferences. The absence of these peaks, when the initial phases are identical, is explained by the mutual destruction of the overlapping wavelets during their summation.
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Statistical Mechanics
Dynamics of Aligning Active Matter: Mapping to a Schrödinger Equation and Exact Diagonalization
Tara Steinhöfel, Horst-Holger Boltz and Thomas Ihle
There has been recent interest in the relaxational modes of small-scale fully connected systems of aligning self-propelled particles (Spera et al., Phys. Rev. Lett. 132: 078301 (2024)). We revisit the classical connection between Fokker-Planck and Schrödinger equations to address this by means of exact diagonalization, allowing for rigorous analytical insight into the full spectrum. This allows us to extract exact results which we compare to the existing result from linearized statistical field theory. We derive asymptotically correct analytical results that improve upon the prior approximations. We show that this methodology can fruitfully be extended to the case of non-reciprocal interactions which gives rise to a non-Hermitian Schrödinger problem akin to those in open
quantum mechanics. While the non-reciprocity can be chosen such as not to alter the stationary distribution, it fundamentally changes the nature of the steady state which we quantify via the
entropy production. We discuss the case of low particle numbers as well as the emergence of mean-field dynamics at large numbers.
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Mesoscale and Nanoscale Physics
Quantum correlations and dissipative blockade of polaritons in a tunable fiber cavity
Gian-Marco Schnüriger, Martin Kroner, Emre Togan et al. (+5 more)
Cavity
exciton–polaritons are quasiparticles that form when
quantum well
excitons hybridize with a cavity mode. Here, we carry out
photon correlation measurements under continuous wave resonant
laser excitation to demonstrate
quantum correlations between cavity–polaritons. Our experiments reveal an unexpectedly strong dependence of polariton interactions on cavity–
exciton detuning. When the polaritons are predominantly
exciton-like, we observe a transition from
photon antibunching to bunching as the
laser is tuned across the polariton resonance, in agreement with a simple Kerr-nonlinearity model. When the lower-branch polariton energy is tuned to induce a two-polariton Feshbach resonance with the biexciton mode, the degree of polariton antibunching becomes independent of the
laser detuning: we explain our finding by invoking a dissipative blockade mechanism arising from large biexciton broadening. Our experiments demonstrate that the strong polariton blockade regime would be achieved by reducing the polariton decay rate by a factor of 10.
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All authors Gian-Marco Schnüriger, Martin Kroner, Emre Togan, Patrick Knüppel, Aymeric Delteil, Stefan Fält, Werner Wegscheider, Atac Imamoglu
Mesoscale and Nanoscale Physics
Exploring Spectral Singularities in Dirac Semimetals: The Role of Non-Hermitian Physics and Dichroism
Mustafa Sarisaman, Murat Taş and Enes Talha K{\i}rca
In this study, motivated by recent advancements in non-Hermitian physics, we explore new characteristics of Dirac semimetals (DSMs) using the spectral singularities by means of scattering techniques, with the goal of uncovering additional unique properties. To achieve this, we investigate how the
axion texture of a DSM affects its topological properties by analyzing its interaction with electromagnetic waves. We examine the transverse electric (TE) mode configuration, where the magneto-electric effect induces a dichroic property in these materials. This behavior is particularly interesting and commonly seen in potential DSM candidates. Consequently, we report for the first time that a dichroic DSM generates 12 unique topological
laser types. We discover that surface currents are generated by topological terms on the surface of the DSM slab. Furthermore, we examine how the {\theta} term associated with
axions in topological materials contributes to these topological properties. Our study reveals distinct topological role of the {\theta} term more clearly than ever before. Our results confirm that the topological properties of DSMs with a single Dirac cone remain stable under external influences and that a topologically robust DSM
laser can be developed accordingly
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Statistical Mechanics
Genuine and spurious (non-)ergodicity in single particle tracking
Wei Wang, Qing Wei, Igor M. Sokolov et al. (+2 more)
In single-particle tracking experiments measuring anomalous
diffusion dynamics, understanding ergodicity is crucial, as it ensures that the time average of an observable matches the ensemble average, and can thus be fitted with known ensemble-averaged observables. A commonly used criterion for assessing the ergodicity of a
stochastic process is based on the comparison of the mean-squared displacement (MSD) with the time-averaged MSD (TAMSD). This approach has been widely applied and proves effective in cases of weak ergodicity breaking across various systems in both theoretical and experimental studies. However, there is relatively little discussion regarding the theoretical justification and limitations of this definition. Here, we demonstrate that this widely accepted criterion to some extent contradicts the classical definition of ergodicity as well as physical intuition, leading to spurious (non-)ergodicity results when applied to several well-known
stochastic models. To address this limitation, we propose using the mean-squared increment (MSI) instead of the MSD for comparison of ensemble- and time-averaged observables. Several well-established examples demonstrate that our MSI-TAMSD criterion not only effectively reveals weak ergodicity breaking, equivalent to the MSD-TAMSD approach, but also provides a more accurate characterization of the genuine (non-)ergodicity of systems where the MSD-TAMSD method fails. Additionally, for systems exhibiting "ultraweak" ergodicity breaking, the MSI can reveal the asymptotic stationarity and ergodic nature of the process' increments. Our findings emphasize the important role of the MSI observable for SPT experiments and anomalous
diffusion studies.
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All authors Wei Wang, Qing Wei, Igor M. Sokolov, Ralf Metzler, Aleksei Chechkin
Quantum Gases · Cross-listed from quant-ph
A PAC-Bayesian approach to generalization for quantum models
Pablo Rodriguez-Grasa, Matthias C. Caro, Jens Eisert et al. (+3 more)
Generalization is a central concept in machine learning theory, yet for
quantum models, it is predominantly analyzed through uniform bounds that depend on a model's overall capacity rather than the specific function learned. These capacity-based uniform bounds are often too loose and entirely insensitive to the actual training and learning process. Previous theoretical guarantees have failed to provide non-uniform, data-dependent bounds that reflect the specific properties of the learned solution rather than the worst-case behavior of the entire hypothesis class. To address this limitation, we derive the first PAC-
Bayesian generalization bounds for a broad class of
quantum models by analyzing layered
circuits composed of general
quantum channels, which include dissipative operations such as mid-
circuit measurements and feedforward. Through a channel perturbation analysis, we establish non-uniform bounds that depend on the norms of learned parameter
matrices; we extend these results to
symmetry-constrained equivariant
quantum models; and we validate our theoretical framework with numerical experiments. This work provides actionable model design insights and establishes a foundational tool for a more nuanced understanding of generalization in
quantum machine learning.
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All authors Pablo Rodriguez-Grasa, Matthias C. Caro, Jens Eisert, Elies Gil-Fuster, Franz J. Schreiber, Carlos Bravo-Prieto
Mesoscale and Nanoscale Physics
Charge Transport Modeling of CdSe/ZnS core/shell Quantum Nanorod Light-Emitting Diodes
A.G. Melkonyan, G.A. Mantashian and D.B. Hayrapetyan
In this study, we investigate the electronic structure, charge transport dynamics, and optical properties of a
quantum dot light-emitting diode (QD-LED) featuring a double nanorod (NR) emission layer composed of CdSe-ZnS core-shell structures. Utilizing a rigorous self-consistent numerical approach, we solve the coupled Schrodinger-Poisson equations iteratively to obtain accurate wave functions, energy levels, and potential profiles under varying external bias voltages. Detailed analyses reveal voltage-dependent electron localization dynamics, demonstrating a systematic transition of electrons between distinct NR regions via
quantum tunneling. Charge density and electrostatic potential distributions are modeled comprehensively, employing the asymmetric Erlang distribution to characterize interface effects. By calculating current-voltage (I-V) characteristics and photoluminescence spectra, we demonstrate that external voltage serves as a robust tuning parameter for modulating emission energies and intensities, underscoring the potential of these NR-LED systems for tunable optoelectronic and photonic applications.
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Soft Condensed Matter
Cooperative effect of local active stresses on the macroscopic contractility of elastic fiber networks
Abhinav Kumar, David A. Quint and Kinjal Dasbiswas
The collective action of actively contractile units embedded in elastic biopolymer networks plays a crucial role in regulating the network's macroscopic mechanical response. Here, we investigate how the macroscopic boundary stress in model elastic fiber networks depends on the number and nature of embedded contractile units, each exerting an isotropic force dipole, as well as on the bending stiffness of fibers. We find that the macroscopic stress increases nonlinearly with the number of dipoles due to mutual stiffening of initially soft, bending-dominated networks. Using effective medium theory, we relate this enhanced contractility to an increase in the effective average network coordination number due to constraints imposed by the force dipoles. By comparing three distinct force dipole models that differ in their local structures, we demonstrate that the specific manner in which an active unit constrains the network strongly influences the onset and nature of the stiffening transition. Our results highlight that not only the quantity but also the local geometry of force-generating units critically determines the macroscopic mechanical behavior. This framework provides a physical basis for understanding how biological systems-such as molecular motors in the cytoskeleton, or adherent
cells in the extracellular
matrix-can modulate network-scale nonlinear elastic properties through local tuning of active force-generating units.
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