Soft Condensed Matter
Data Driven Equation Discovery for Phase-Ordering Dynamics : From Allen Cahn to the Ising Model
Partha Sarathi Mondal, Manav Kumar Jalan, Anish Kumar et al. (+1 more)
Data-driven discovery of governing equations from spatiotemporal data offers a promising route to obtaining coarse-grained descriptions of complex dynamical systems. Here, we investigate the performance of PDE-SINDy for discovering phase-ordering dynamics using the Allen–Cahn equation as a
benchmark and the Ising model with Glauber
spin-flip dynamics as a microscopic system. We systematically analyze the effects of data availability, size of the candidate library, and noise on the efficiency of the equation discovery. We find that stability-selection PDE-SINDy can robustly identify the relevant terms in the governing dynamics even under limited or noisy data, while the recovered coefficient values are substantially more sensitive to these factors. We further show that enlarging the candidate library can strongly affect both term identification and coefficient recovery. Incorporating library bagging with stability selection reduces this sensitivity and improves the efficiency of equation discovery. For the Glauber
spin flip Ising model dynamics, the resulting coarse-grained equation reproduces the characteristic phase-separation and coarsening dynamics of the underlying microscopic system. Overall, our results demonstrate the potential of PDE-SINDy for phase-ordering systems while highlighting the importance of carefully assessing the factors that influence the efficiency of equation discovery.
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All authors Partha Sarathi Mondal, Manav Kumar Jalan, Anish Kumar, Shradha Mishra
Materials Science
Electron correlation in semiconductors and insulators via symbolic regression
Nick Pant, Viet-Anh Ha, Donghwan Kim et al. (+1 more)
Predicting quasiparticle energies in materials requires expensive numerical evaluations of the electron self-energy. This limits calculations to ordered systems with small unit
cells. Here, using symbolic regression, we show that the GW self-energy can be accurately approximated with compact analytical functions of physically motivated Kohn-Sham descriptors. These expressions can be learned from a single GW calculation in the ordered phase and remain accurate under
symmetry breaking induced by
quantum and thermal fluctuations, elastic deformations, and amorphous disorder. This development enables routine GW calculations of complex materials with thousands of atoms at a computational cost comparable to semi-local density functional theory. We demonstrate the accuracy of this approach for covalent
semiconductors, ionic
insulators, and two-dimensional materials. These results establish symbolic regression as a viable route to predictive, interpretable, and transferable many-body electronic structure models.
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All authors Nick Pant, Viet-Anh Ha, Donghwan Kim, Feliciano Giustino
Mesoscale and Nanoscale Physics
Magnetic Field Reorganization of Electronic States in Moiré Bilayer Graphene
Milan Sharma Mandigo-Stoba, William Wang, Jackson Kuklin et al. (+4 more)
Magnetic fields are widely used to diagnose
quantum phases in two-dimensional systems through
quantum oscillations or by tuning
spin and valley polarizations, but
magnetic fields can also reshape the underlying electronic structure. Here, using a
bilayer graphene/hBN moiré system, we reveal a rich
magnetic field induced
evolution of the semiclassical orbit network, encompassing Lifshitz transitions,
magnetic breakdown, and scattering between coexisting electron and hole pockets. At
magnetic fields of 1 T to 2 T,
quantum oscillation frequencies and the
Hall density change markedly over a broad carrier density range, signaling
magnetic breakdown and
magnetic Lifshitz transitions. This
evolution is valley contrasting: Berry
curvature hot spots near the breakdown junctions enhance
magnetic breakdown in the K valley while suppressing it in the K$^\prime$ valley, whereas valley-antisymmetric orbital
magnetic moments split the corresponding Lifshitz transitions. The resulting valley-selective trajectories manifest at higher fields as valley-
symmetry-breaking Hofstadter gaps. At elevated temperatures and low
magnetic fields, scattering between coexisting electron and hole pockets produces nearly density-independent resistance oscillations whose frequency tracks the sum of their
Fermi surface areas, persisting after conventional Onsager oscillations are thermally washed out. Our results provide a unified picture of how modest
magnetic fields reorganize moiré electronic states as the system evolves from semiclassical transport toward the Hofstadter regime.
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All authors Milan Sharma Mandigo-Stoba, William Wang, Jackson Kuklin, Jack Lichterman, Kenji Watanabe, Takashi Taniguchi, Qianhui Shi
Strongly Correlated Electrons · Cross-listed from hep-th
Tilts from 2-Groups
Sven Harder, Theodore Jacobson and Zhengdi Sun
2-group global
symmetries intertwine 0-form and 1-form
symmetries in an interesting way. We analyze universal constraints on lines which are charged under the 1-form subgroup of a 2-group, and show that they generically break the 0-form
symmetry explicitly. This gives rise to a family of line defects parameterized by the broken
symmetry generators, whose existence is invariant under the renormalization group flow of the bulk-defect system. The
symmetry breaking is enforced by a `family anomaly,' which is a topological obstruction to a symmetric defect. Our main tools are the Wess-Zumino consistency condition and a generalized anomaly inflow formalism for defects and boundaries. For nonabelian continuous 2-groups the family anomaly stems from a higher Berry connection on the
moduli space of defects, and constrains response functions that probe the local action of the broken
symmetry. In the continuous abelian case we apply differential
cohomology to the 2-group background fields to uncover a subtle generalization of the Wess-Zumino condition, while in the discrete case we recast it in terms of the associativity of
symmetry defects. We give numerous illustrative examples, computing when possible explicit forms of the tilt operator (which probes the linear response of the defect to the broken
symmetry) and its higher analogs, which are crucial for matching the family anomaly. We also discuss universal features such as how
symmetry violation by charged line defects is related to
symmetry breaking hierarchies in the bulk, and highlight a number of subtle points including the distinction between simple and non-simple lines, and Postnikov class resolution.
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Materials Science
First-principles predictions of carrier mobility with record accuracy using GW perturbation theory
Nick Pant, Sabyasachi Tiwari, Steven G. Louie et al. (+2 more)
Accurate prediction of carrier mobility is critical for the discovery and design of next-generation electronic materials. Despite sustained progress, state-of-the-art ab initio methods remain limited by the approximate treatment of electron-
phonon interactions at the density functional theory level. Here, we demonstrate that incorporating many-body GW corrections to both the electronic band structure and electron–
phonon couplings when solving the ab initio Boltzmann transport equation yields a mean absolute relative error of just 11% for electron mobilities across
benchmark semiconductors, including Si, GaAs, GaP, diamond, and SiC. The common practice of neglecting GW corrections to the electron–
phonon interaction can lead to mobility errors exceeding 50%. The present findings highlight the importance of many-body GW self-energy effects in carrier transport simulations, and provides fundamental insights into how many-body electron–
phonon interactions govern charge transport in crystalline solids.
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All authors Nick Pant, Sabyasachi Tiwari, Steven G. Louie, Zhenglu Li, Feliciano Giustino
Materials Science
Computational Microstructure Analysis of Sintered Ceramics
Nikhil Dhanankam, Deeksha Kodangal, Rajendra K. Bordia et al. (+1 more)
Characterizing materials through manual extraction of physical properties from microstructure images is a laborious process. This work presents a workflow to extract porosity, solid fraction, grain size distribution, and pore size distribution from scanning electron microscopy (SEM) images of sintered ceramic samples using an automated pipeline. The primary challenge for extracting physical properties from SEM images is the presence of unimodal histograms in SEM images as a result of the overlapping intensity ranges for the grain and pore phases. We evaluated several different methods for noise reduction and local thresholding of SEM images. We find that topological filtering in combination with Sauvola thresholding enables
segmentation and extraction of physical property data from SEM images. We validated the automated pipeline by comparing our results with the results of manual analyses performed for samples sintered at 1200$^o$C and 1400$^o$C and achieved an Intersection over Union (IoU) score of 95.14% and 99.85%, respectively. The workflow provides an efficient means to automatically extract microstructure properties from SEM images as a crucial step in generating materials
datasets for machine learning.
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All authors Nikhil Dhanankam, Deeksha Kodangal, Rajendra K. Bordia, Ulf D. Schiller
Materials Science
Green's function theory of magnetism in Bi$_2$CuO$_4$: anisotropic Heisenberg XYZ model
R.O. Kuzian and E.E. Krasovskii
The Green's function theory of Lymar' and Rudoi [Theor. Math. Phys. vol. 21, 990 (1974)] is generalized to the case of multiple intra- and inter-sublattice
magnetic interactions in a collinear
spin-half antiferromagnet and applied to
magnetic excitations in Bi$_2$CuO$_4$. The
spin Hamiltonian includes both the out-of-plane and in-plane symmetric anisotropy terms and the Zeeman term, which describes the interaction with an external
magnetic field $B$ applied along the Néel vector. Within the
spin-wave approximation we calculate
spin excitation dispersion,
antiferromagnetic resonance frequencies and the critical field $B_c$ of the
spin-flop metamagnetic transition. A weak in-plane anisotropy is shown to result in a gap in the acoustic-like branch of the excitations. The gap nonlinearly depends on the external field and closes at $B=B_c$. An expression for the Neel temperature in the Tyablikov random phase approximation at zero field is derived. For the parameters derived from the recent inelastic
neutron scattering study by Yuan et al. [Phys. Rev. B vol. 103, 134436 (2021)] it gives $T_{\rm N}\approx 52$~K.
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Materials Science
Pressure-Tunable Electronic and Magnonic Transport in Altermagnet La$_2$O$_3$Mn$_2$Se$_2$
Nafise Rezaei, Alireza Qaiumzadeh, Artem R. Oganov et al. (+1 more)
Hydrostatic pressure provides a
symmetry-preserving route to engineer electronic and magnonic transport in the correlated insulating altermagnet La$_2$O$_3$Mn$_2$Se$_2$. Using first-principles calculations combined with
spin-
Hamiltonian modeling, we show that compression from 0 to 40 GPa markedly enhances the inequivalence between the competing second-neighbor exchange interactions, increasing $|J_{2a}-J_{2b}|$ from 1.97 to 9.38 meV while preserving the compensated
antiferromagnetic ground state. The resulting exchange anisotropy amplifies the momentum-dependent splitting between the two
chiral magnon branches, yielding a nearly fourfold enhancement of the longitudinal
magnon-driven
spin Seebeck response at 100 K, from $3.68\times10^{-1}$ to $1.36$ meV/K. In contrast, hydrostatic pressure preserves the
magnetic-
symmetry selection rules governing the anomalous
Hall effect while redistributing the electronic Berry
curvature, producing pronounced energy-dependent sign reversals in the anomalous
Hall conductivity. These results identify exchange anisotropy as the microscopic mechanism underlying the pressure-enhanced
magnon response and establish hydrostatic pressure as an effective means of simultaneously controlling electronic and magnonic transport in insulating altermagnets.
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All authors Nafise Rezaei, Alireza Qaiumzadeh, Artem R. Oganov, Mojtaba Alaei
Soft Condensed Matter
Role of Particle Shape in Strain-Controlled Resuspension of Dense Suspensions
Mohammadreza Mahmoudian, Mahdi Vaezi and Parisa Mirbod
> Dense non-Brownian suspensions exhibit complex resuspension dynamics governed by hydrodynamic interactions, particle microstructure, and
gravity. While viscous resuspension in spherical suspensions is known to be strain-controlled, its applicability to anisotropic particles remains unclear. Here, we investigate dense suspensions of spherical and rod-shaped particles under steady and oscillatory shear. The results reveal two strain-controlled transitions: particle detachment from the sediment bed and the transition to a fully suspended state. Both particle shapes exhibit the same critical strain for detachment, approximately 6, indicating that resuspension onset is largely independent of particle morphology. In contrast, complete resuspension requires a critical strain of approximately 120 for spheres and 180 for rods, demonstrating a strong effect of particle anisotropy. A fluid-strain scaling collapses the onset of resuspension across particle concentrations and shapes while highlighting the distinct influence of particle shape on complete suspension. These findings establish a unified strain-based framework for viscous resuspension and clarify the role of particle anisotropy in dense suspension dynamics.
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Mesoscale and Nanoscale Physics
Valley polarization of moiré interlayer exciton complexes driven by many-body interactions
Adrian O. Paulus, Mikołaj J. Metelski, Alain Dijkstra et al. (+4 more)
Localized interlayer
excitons (IX) in moiré transition metal dichalcogenide
heterostructures can both probe and participate in many-body states hosted by the moiré superlattice. When the IX density is small compared to the moiré
lattice density, the formation of incompressible charge
crystals at fractional electronic moiré fillings modifies
exciton-charge scattering, leading to enhanced lifetimes in photoluminescence (PL) measurements. At high IX densities, the
exciton dynamics are altered by the emergence of an excitonic Mott
insulator and the formation of doubly-occupied sites (IXX). Here, we investigate the IX PL lifetime and valley polarization in an R-type $\mathrm{WSe_2}$/$\mathrm{WS_2}$
bilayer across a wide range of IX and charge densities. While previous studies reported a decrease of polarization in time-integrated measurements in charge-incompressible phases, our results show that this arises not from enhanced intervalley scattering, but from a dilution of the valley polarization by the dramatic enhancement of IX lifetimes. At high excitation densities, we probe the dynamics of the IXX and show that despite the nominal antiparallel valley configuration of the two constituent
excitons, a strong, anomalous valley polarization develops as the IXX population decays. Our results shed light on the complex
exciton and valley dynamics of IX and demonstrate that they are strongly modified by the rich many-body physics of moiré heterobilayers.
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All authors Adrian O. Paulus, Mikołaj J. Metelski, Alain Dijkstra, Kenji Watanabe, Takashi Taniguchi, Nathan P. Wilson, Jonathan J. Finley
Soft Condensed Matter
Fractional Viscoelasticity in Transient Unentangled Polymer Networks
Sachin Shanbhag and Ralm G. Ricarte
Stress relaxation in transient
polymer networks often shows extended power-law behavior, $G(t) \sim t^{-\beta}$, where the exponent $\beta$ frequently departs from the value $1/2$ predicted by the sticky Rouse model and its variants. We introduce the fractional inhomogeneous Rouse model (FIRM), which uses a generalized Langevin equation driven by fractional Gaussian noise of exponent $\alpha$, while retaining heterogeneous bead friction to represent sticky cross-links. Thus, FIRM unifies subdiffusive sticker dynamics and chain heterogeneity within a single framework. We show that the relaxation modulus $G(t)$ can be represented as a linear combination of Mittag-Leffler functions. For homogeneous chains, it recovers two power-law regimes, $t^{-\alpha/2}$ and $t^{-2\alpha}$, on either side of the terminal relaxation time. Fitting FIRM to stress relaxation data for an imine-based polystyrene vitrimer shows that $\alpha < 1$ is required to capture the shape of the terminal relaxation. It also accommodates both Arrhenius and non-Arrhenius temperature dependence in the rheological activation energy. We derive expressions for dynamic properties such as mean-squared displacement and dielectric response and outline how generalized
memory kernels extend the framework to real materials. Together, these results suggest novel ways in which data from rheology, dielectric spectroscopy, scattering, and other experimental methods may be incorporated into a chemistry-specific molecular model.
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Statistical Mechanics
Gibbs variational principles and Boltzmann irreversible theorem
Mário J. de Oliveira and Silvio R. Salinas
We analyze the Gibbs variational principles associated with the probability distributions of (i) an isolated system and (ii) a system at constant temperature. We give an example of using the Gibbs inequality to obtain the free energy and analyze the phase diagram of an Ising model with competing interactions. We also review the Boltzmann irreversible
theorem, and show how it is connected to the Gibbs variational principles. This connection is established by using the Kolmogorov equation for the
evolution of the probability distribution, which predicts a monotonic increase of
entropy for an isolated system, and a decrease of the free energy for a system in contact with a thermal reservoir.
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Statistical Mechanics
Experimental Kuramoto Platform
Leandro Freitas, Victor H. S. Bittencourt, Débora D. Superbi et al. (+3 more)
Phase reduction provides a principled relationship between high-dimensional oscillator dynamics and low-dimensional phase models such as the Kuramoto model. However, connecting analytical results derived from the Kuramoto framework to experimental data remains an open challenge, as many physical platforms violate the weak-coupling assumptions underlying standard phase reduction. Here, we present an experimental platform based on electronic quadrature oscillators whose phase dynamics are, under certain assumptions, equivalent to the Kuramoto model. Crucially, this equivalence is achieved for both weak and strong coupling through a non-diffusive coupling
circuit, and the design approach supports arbitrary (weighted and directed) network
topologies while preserving scalability and oscillation regularity. Across different coupling schemes, we demonstrate that the platform reliably reproduces phase transitions predicted by the Kuramoto model for both global and cluster synchronization. This cost-effective platform enables systematic experimental validation of analytical and numerical results for synchronization in complex networks.
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All authors Leandro Freitas, Victor H. S. Bittencourt, Débora D. Superbi, Wanderley C. Silva Junior, Arthur N. Montanari, Luis A. Aguirre
Strongly Correlated Electrons · Cross-listed from quant-ph
Reconstructing non-Abelian braiding and fusion without anyon transport
Lucy Byles, Matthew D. Horner, Benjamin T. H. Varcoe et al. (+1 more)
Non-Abelian anyons offer a route to fault-tolerant and universal
quantum computing, but experimental access to their defining braiding and fusion data remains limited by the resource overhead of implementing extended anyonic processes on
quantum hardware. Here we introduce and experimentally realise a measurement-only protocol based on temporally ordered ribbon operations that reconstructs the non-Abelian braiding and fusion primitives of the
quantum double model $D(S_3)$ without physical anyon transport. We implement a reduced two-qutrit version of the protocol on Quantinuum's H2 trapped-ion processors, realising ancilla-assisted ribbon operations and anyonic charge projections in a
qubit encoding. We reconstruct the squared braiding phases and fusion amplitudes using an adapted Hadamard test and post-selected measurements, respectively. The associated braiding and fusion transformations reproduce their ideal actions with average normalised output-state fidelities of $\overline{\mathcal{F}}_{R}=0.9988$ and $\overline{\mathcal{F}}_{F}=0.9987$. Combining these primitives produces a non-Clifford braid and a non-stabilizer resource state, supporting measurement-only anyonic encodings as building blocks for larger topologically encoded
quantum processors.
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All authors Lucy Byles, Matthew D. Horner, Benjamin T. H. Varcoe, Jiannis K. Pachos
Strongly Correlated Electrons
Santanu Singh, Surajit Bera, Chenyuan Li et al. (+2 more)
The subsystem-size dependence of ground-state
entanglement entropy and its crossover to thermal
entropy as a function of temperature are well understood for one-dimensional (1D) gapless systems described by conformal field theory (CFT), and for free
fermions with a
Fermi surface in any dimension. However, little is known about the
entanglement entropy for gapless fermionic systems without quasi-particles, such as a strange metal. Here we study the
entanglement entropy of
fermions in a solvable large-$N$ 1D
lattice model akin to the Yukawa-Sachdev-Ye-Kitaev (Yukawa SYK) model. In this model, two
Fermi points are coupled to scalar
bosons via spatially random Yukawa interactions, providing a solvable model of a strange metal when the
bosons become critical at a
quantum critical point. We exactly compute the second Rényi
entropy of
fermions in a spatial subregion in this model. Our results unravel crucial role of intra-subregion
entanglement between fermionic and bosonic degrees of freedom along with the inter-subregion
entanglement in understanding the ground states of such strongly coupled
fermion-
boson systems. We show that the crossover from thermal
entropy to
entanglement entropy, is captured by a single scaling ansatz, that collapses the second Rényi
entropy of
fermions for different subregion sizes and temperatures into a single universal curve. We further show that the universal scaling curve for the critical strange metal is well described by the standard CFT formula, albeit with an effective central charge substantially larger than the non-interacting value.
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All authors Santanu Singh, Surajit Bera, Chenyuan Li, Subir Sachdev, Sumilan Banerjee
Strongly Correlated Electrons
Magnetic-Field-Driven Dimensional Reduction in a Quantum Antiferromagnet
Subhankar Khatua, Marcin Raczkowski, Jeroen van den Brink et al. (+1 more)
Low dimensionality enhances
quantum fluctuations, triggering novel states of
quantum matter to emerge. In real materials, low dimensionality usually arises from spatially strongly anisotropic couplings. Here, we demonstrate a different mechanism: in two-dimensional systems with coupled alternating
ferromagnetic (FM) and
antiferromagnetic (AFM)
spin-$1/2$ chains, an applied
magnetic field may drive a dimensional reduction. Under
magnetic field, the FM chains polarize and stiffen, suppressing the propagation of transverse AFM fluctuations from one chain to another, and effectively induce one-dimensional behavior at low energies. For a model describing botallackite, Cu$_2$(OH)$_3$Br,
quantum Monte Carlo dynamics show that beyond a critical
magnetic field, the low-energy spectrum reduces to that of a one-dimensional AFM Heisenberg
spin-$1/2$ chain with field-dependent incommensurate two-spinon fluctuations, providing clear signatures for inelastic
neutron scattering.
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All authors Subhankar Khatua, Marcin Raczkowski, Jeroen van den Brink, Fakher F. Assaad
Strongly Correlated Electrons
Breakdown of Monotonic Impurity Entropy Flow in $\mathscr{PT}$-Symmetric Multichannel Kondo Systems
Pradip Kattel, Abay Zhakenov and Natan Andrei
We study a $\mathscr{PT}$-symmetric non-Hermitian multichannel Kondo model consisting of a pair of
spin-$\frac12$
impurities coupled to $n$ conduction-electron channels through complex-conjugate Kondo couplings. The
impurity renormalization-group (RG) flow is characterized by the Kondo scale $T_K$ and a dimensionless non-Hermiticity parameter $\alpha$. As $\alpha$ increases, the exact Bethe Ansatz solution exhibits four
impurity phases: overscreened Kondo, zero mode, Yu–Shiba–Rusinov (YSR), and local moment. The Kondo, zero-mode, and local-moment phases are $\mathscr{PT}$-unbroken, whereas the YSR phase spontaneously breaks $\mathscr{PT}$
symmetry. Using a generalized thermodynamic Bethe Ansatz, we determine the
impurity free energy and Affleck–Ludwig $g$-function throughout the $\mathscr{PT}$-unbroken phases. In the Kondo phase, the defect RG flow connects the ultraviolet and infrared conformal fixed points, with the
impurity entropy flowing from $2\ln2$ to $2\ln\left[2\cos\left(\frac{\pi}{n+2}\right)\right]$, in agreement with defect conformal field theory. In the zero-mode phase, zero-energy
impurity strings reorganize the spectrum into multiple excitation towers, while in the local-moment phase, the RG flow becomes cyclic, returning to the unscreened local-moment fixed point. We
conjecture that RG irreversibility, and hence a generalized Affleck–Ludwig $g$-
theorem, survives throughout the Kondo phase. Our exact solution nevertheless shows that a real spectrum and defect
entropies consistent with defect CFT do not guarantee RG irreversibility: the
impurity entropy is non-monotonic in both the zero-mode and local-moment phases.
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Quantum Gases · Cross-listed from quant-ph
Quantum Many-Body Metrology of Rotation Sensing with Strong Interactions
Yong-Jun Baak and Uwe R. Fischer
We study the ultimate
quantum limit of rotation sensing with a few strongly interacting
bosons confined in a quasi-one-dimensional ring trap with two weak links. It is demonstrated that a self-consistent many-body solution of the problem is required to correctly predict the ultimate sensitivity of this strongly correlated many-body gyroscope to rotation. For both small rotation velocities and small particle numbers, the many-body
quantum Fisher information becomes maximal for large interaction couplings, showing the potential of strongly interacting miniaturized many-body sensors to precisely estimate slow rotations with high spatial resolution.
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Statistical Mechanics · Cross-listed from quant-ph
Unifying quantum measurement constructions via a relative-entropy minimum change principle
Nana Liu and Mark M. Wilde
The minimum change principle provides an information-theoretic characterization of the Bayes reversal channel in classical probability theory and has recently been proposed as a framework for extending Bayes' rule to
quantum information theory. Using
quantum relative
entropy, we investigate a minimum change principle for the setting of
quantum statistical inference. Specifically, we consider a forward process based on a classical-to-
quantum preparation channel and a reverse process based on a
quantum-to-classical measurement channel. We establish a closed-form characterization of measurements that are optimal for this principle, and this optimal measurement can be found via a dual formulation involving a single unconstrained Hermitian variable. This perspective allows us to recover some notable measurements within the same framework, including pretty good measurements and
Fermi-Dirac thermal measurements, and we use it to discover a novel family that we call softmin thermal measurements. We further show that softmin thermal measurements arise as optimal solutions to
entropy-regularized semidefinite optimization problems, demonstrating that they play a role for measurements analogous to that of thermal states in statistical mechanics. Finally, we prove an additivity property for the relative-
entropy minimum change principle and investigate the performance of
Fermi-Dirac thermal measurements for
quantum hypothesis testing.
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Soft Condensed Matter
Localizing polymers promotes the centre-mode elastic instability
Shailendra Kumar Yadav and Jason R. Picardo
The centre-mode elastic instability, prevalent in rectilinear flows of dilute
polymer solutions, allows dynamic states to emerge even in the absence of inertia. Here, we show that this instability can be significantly enhanced when the base flow has a nonuniform spatial-distribution of
polymers. In such a flow, the polymeric stress not only depends on the conformation tensor (determined here by the Oldroyd-B equation) but also varies proportionally with the
polymer concentration field, which satisfies a scalar transport equation. Focusing on the inertialess Stokes limit, we first consider the simple setting of periodic Kolmogorov flow. A linear stability analysis shows that a nonuniform
polymer concentration either promotes or suppresses the centre-mode instability, depending on whether the
polymers are localised near or away from the maximum of the base-velocity profile. We then focus on a base flow with a distinct
polymer-laden stream overlying the base-velocity maximum, and sandwiched between
polymer-free streams. We find that the critical Weissenberg number (product of the elastic relaxation time and the typical strain-rate) at the onset of instability undergoes a sudden decrease as the total
polymer loading is increased. An energy analysis attributes this destabilisation to elastic feedback forces that arise from concentration gradients near the interfaces between the
polymer-laden and
polymer-free streams. The insights from Kolmogorov flow are shown to carry over to channel flow, where localising
polymers about the centreline strongly promotes the centre-mode instability.
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Statistical Mechanics
Kappa distributions as asymptotic marginals of exponential family ensembles
Sergio Davis
Recently (Physica A 660 130370, 2025), emergence of superstatistical behavior in driven classical systems has been shown for systems with constant microcanonical heat capacity. As an application of this result, in this work we show that a system of $N$ particles with inverse gamma distribution of total kinetic energies must have kappa-distributed single-particle velocities in the limit $N \rightarrow \infty$. Our results provide insight into the nature of kappa distributions outside the theory of nonextensive statistical mechanics, while also bringing forward a practical method for the generation of kappa velocities via Monte Carlo Metropolis simulation in the inverse gamma ensemble.
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Materials Science · Cross-listed from physics.ins-det
A table-top few-femtosecond broadband extreme-ultraviolet absorption spectrometer with cryogenic cooling
Sheng-Chih Lin, Alfred Zong, Emma Berger et al. (+5 more)
We present a table-top cryogenic ultrafast broadband XUV absorption spectroscopy (c-UBXAS) beamline designed for temperature-dependent and time-resolved investigations of
quantum materials. The instrument combines a broadband high-harmonic generation source spanning 22-73 eV with automated image registration and cryogenic sample control down to 20 K, enabling element-specific measurements under both
equilibrium and nonequilibrium conditions. The beamline provides sub-50-meV energy resolution and a sub-10-fs instrument response function, while maintaining long-term stability suitable for extended, ultrasensitive measurements.
Benchmark experiments on NiI$_2$, a van der Waals multiferroic, reveal temperature-dependent spectral
evolution across its
magnetic phase transitions, demonstrating the ability to identify element-specific contributions to the ground state and its transient response in the femtosecond regime. This instrument introduces the opportunity to unravel the very first response of a solid-state material during light-matter interaction, paving the way for understanding the complex phase space of nonequilibrium dynamics and emergent states in strongly correlated systems where the few-femtosecond electronic response has eluded most other types of time-resolved techniques.
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All authors Sheng-Chih Lin, Alfred Zong, Emma Berger, Bailey R. Nebgen, Marcus Hui, Shuaiwei Pan, Jackson McClellan, Michael W. Zuerch
Materials Science
Emergence of Chiral Helimagnetic Order in Chromium-intercalated Tantalum Disulfide CrTa$_3$S$_6$ Powders with Controlled Intercalation
Ayaka Toshima, Keigo Mizutani, Yusuke Kousaka et al. (+3 more)
We report a highly sensitive change in
magnetic properties of a
chiral Cr-intercalated transition-metal dichalcogenide Cr$_{x}$Ta$_{3}$S$_{6}$. Magnetization curves and small-angle
neutron scattering data revealed that the powder samples exhibit
chiral helimagnetism with a Cr intercalation quantity $x$ below 0.996, while they show ferromagnetism above 1.000. The emergence and temperature-dependent
evolution of the helimagnetic period are argued in terms of sample dimensions of powders and microfabricated
crystals.
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All authors Ayaka Toshima, Keigo Mizutani, Yusuke Kousaka, Kazuki Ohishi, Hiroaki Shishido, Yoshihiko Togawa
Materials Science
Understanding and Designing Phase Change Materials: Insights from Atom Probe Tomography
Jan Köttgen, Nils von den Driesch, Alexander Pawlis et al. (+1 more)
Phase Change Materials (PCMs) can be rapidly and reversibly switched between their amorphous and crystalline state; a transition which is accompanied by a pronounced change of optoelectronic properties. Here progress is reviewed to explain these property changes, focusing on advances by atom probe
tomography (APT). This technique classifies bonding by providing two crucial bonding descriptors. Most important is the Probability of Multiple Events (PME), which is related to the likelihood that more than one ion is dislodged per successful
laser pulse in
laser assisted field evaporation. Crystalline PCMs are characterized by a PME above 55%, not found for metals or iono-covalent solids. This confirms that crystalline PCMs employ a unique bonding mechanism coined metavalent bonding (MVB). While crystalline PCMs employ MVB, amorphous PCMs behave as covalent solids characterized by a much lower PME. PCMs thus change their bonding upon crystallization, consistent with
quantum-chemical calculations of bonding. Crystalline solids with a high PME lie in a narrow conductivity range between metals and iono-covalent solids, indicative for a competition between electron localization and delocalization. A map quantifying chemical bonding locates metavalent solids in a region where approximately one electron is shared between adjacent atoms and bonding is not too ionic. This
quantum chemical bonding map is now used to find and explain property trends relevant for PCMs in various application domains.
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All authors Jan Köttgen, Nils von den Driesch, Alexander Pawlis, Matthias Wuttig
Superconductivity
Thermodynamic phase transition, pairing symmetry and Fermi surface topology in Ruddlesden-Popper nickelate films
Yu Miao, Zhiwei Wang, Hongxu Sun et al. (+9 more)
Ruddlesden-Popper (RP) nickelates provide an uncharted territory to explore high-transition-temperature (high-$T_C$) superconductivity and superconducting mechanism. Here, we investigate the electronic structure of a new type of high-$T_C$ superconducting RP nickelate
heterostructure $\mathrm{La_2PrNi_2O_7/NdAlO_3}$ by angle-resolved photoemission spectroscopy. A superconducting state is observed without a pseudogap state, enabling a direct measurement of the superconducting order parameter and a microscopic extraction of the electronic specific heat. The observed superconducting gap opens at $T_C$ with prominent coherence peaks, illustrating the emergence of nonzero order parameter upon entering the superconducting state. An electronic specific heat jump appears at $T_C$, further demonstrating a thermodynamic phase transition. The magnitude of the superconducting order parameter is quantified by the observed superconducting coherence peaks, and a nodeless behavior is unambiguously established in the absence of pseudogap. The underlying
Fermi surface consists of $\alpha$, $\beta$ and $\gamma$ pockets, exhibiting a multi-orbital nature. Strain dependent measurements further reveal the $\gamma$ pocket in all superconducting and non-superconducting films with different epitaxial strain. Our results establish the missing thermodynamic evidence for superconducting phase transition in nickelates. They also provide direct evidence for the
symmetry of the superconducting order parameter and illustrate the relationship between
Fermi surface
topology and the emergence of superconductivity in RP nickelate films.
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All authors Yu Miao, Zhiwei Wang, Hongxu Sun, Jianchang Shen, Runqing Luan, Zhipeng Ou, Xinru Yong, Zhenyu Wang, Tao Wu, Haoyu Hu, Junfeng He, Xianhui Chen
Soft Condensed Matter
Reaction Delays Boost, Rectify, and Reverse Motility in Activity Landscapes
Constantin Rein, Klaus Kroy and Viktor Holubec
Local detailed balance restricts transport by self-propulsion in static activity landscapes. We show that a delayed speed adaptation (“delayed motility/kinesis”) generically breaks this symme try, inducing directed transport in asymmetric periodic motility profiles and enhancing
diffusion in symmetric ones. Both effects occur for run-and-tumble, active Brownian, and inertial active particles, without requiring potential interactions, walls, imposed gradients, higher dimensions, or translational
diffusion. Their magnitudes, and even current reversals, are conveniently tuned via the delay time, which establishes delayed motility as a versatile generic and experimentally accessible mechanism for autonomous self-steering and transport control in motile-active-matter circuity.
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Materials Science
Phonon-based determination of elastic coefficients in the Weyl semimetal TaAs
Fabián Jofré-Parra, Debankita Ghosh and Enrique Muñoz
Reliable determination of elastic properties in topological semimetals is essential for understanding strain-related effects, but is often hindered by methodological and computational limitations. In this work, we combine first-principles
phonon calculations with an elastic continuum model to determine the elastic properties of Weyl semimetal TaAs. The sound velocities extracted from the acoustic
phonon branches are used to obtain the full set of elastic
moduli, which show good agreement with previously reported values in the literature. From these results, we derive standard elastic parameters such as bulk, shear, and Young's
moduli, and the Poisson ratio. This approach highlights a computationally efficient alternative to conventional strain-based methods, avoiding the need for large supercells when shear is applied, and thus possible strain-induced inconsistencies in the electronic basis, while providing a possibility to connect with experimental characterizations (e.g. Raman or Brillouin-Mandelstam scattering) of the
lattice dynamics in Weyl semimetals.
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Materials Science
Optical centers in cubic boron nitride and diamond: remarkable similarities
Konstantin Iakoubovskii, Andrey Katrusha, Weihua Peng et al. (+1 more)
We present a comparative study of optical absorption and luminescence from cubic boron nitride (cBN) and diamond grown by the high-pressure high-temperature technique in the same cubic press. We note remarkable similarities in spectral and spatial dependences for these two materials. Using the previous identification of defects in diamond, we tentatively assign the optical center responsible for yellow color in some cBN
crystals to substitutional oxygen at the nitrogen site, the RC1 and RC3 centers to a defect comprising substitutional oxygen and a boron vacancy in the neutral and negative charge states, respectively, the GC1 center to a nickel-related defect, the 1.816 eV (683 nm) luminescence peak to a Si-vacancy complex, and the BN1 center to an interstitial-related defect.
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All authors Konstantin Iakoubovskii, Andrey Katrusha, Weihua Peng, Jianguo Peng
Strongly Correlated Electrons
Accelerated quantum Monte Carlo simulations of the attractive Hubbard model on the kagome lattice
Jie Zhang, Xiang Li and Yu Wang
The recent discovery of several families of kagome materials and experimental realization of optical kagome
lattices have stimulated growing numerical studies of interaction-driven correlated states on the kagome
lattice. Among the available numerical approaches, determinant
quantum Monte Carlo (DQMC) is a powerful method for investigating such strongly correlated states. However, the accessible system sizes of existing DQMC simulations remain limited, preventing reliable finite-size scaling analyses. Here we develop a general acceleration scheme based on fast
Fourier transform (FFT) for propagator multiplications on composite
lattices and combine it with the delay-update
algorithm, enabling simulations on system sizes twice as large as those of previous DQMC studies, allowing reliable finite-size scaling analyses of the attractive kagome-
lattice Hubbard model. Our large-scale simulations reveal the interaction-driven zero-temperature superfluid
quantum criticality at the Dirac filling and provide reliable estimates of the associated critical exponents. Besides, we find no evidence that the previously proposed triangle-rule charge-density-wave order survives in the thermodynamic limit, suggesting that it is likely a finite-size effect. Moreover, for system sizes accessible in current two-dimensional optical
lattice experiments, the combined FFT and delay-update scheme exhibits an effective computational cost scaling as $N^{2.49}$, substantially below the $\mathcal{O}(N^3)$ computational cost of conventional DQMC simulations.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Few-photon degenerate parametric resonance in a two-tone driven microwave resonator
Orjan Ameye, Jakob Koenig, Clinton Potts et al. (+2 more)
Multi-tone external driving offers a route to parametric physics without directly modulating the device. However, the validity of the parametric response in the few-
photon regime remains underexplored. Here, we apply two coherent microwave tones to a Josephson-junction Kerr oscillator and stimulate degenerate parametric downconversion via four-wave mixing. Using transmission spectroscopy, we observe that the response retains the qualitative semiclassical Kerr parametric oscillator structure, including its instability lobe and bistable phase-space
topology. Interestingly, we demonstrate that a conventional single-mode reduction fails to capture the system quantitatively: the predicted AC Stark shift is severely underestimated, and the reported distributions might not be fully physical when the single-
photon Kerr shift $K$ exceeds the cavity linewidth $\kappa$. Instead, we show that a full three-tone
quantum description accurately reproduces the experimental observables. There,
quantum fluctuations of the drive tones become dynamically dominant over dissipation, and all three interacting tones operate in a deep few-
photon limit where the expected semiclassical macroscopic lobes undergo fundamental renormalization due to profound mixing with
quantum variance. Our results establish two-tone-driven Kerr oscillators as potential parametric amplifiers and open new horizons to explore the
quantum-to-classical crossover in driven-dissipative
circuits.
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All authors Orjan Ameye, Jakob Koenig, Clinton Potts, Oded Zilberberg, Gary Steele