Statistical Mechanics · Cross-listed from q-bio.PE
A master equation approach to the n-coalescent problem
Bahram Houchmandzadeh
Given an evolutionary model, such as Wright–Fisher (WF) or Moran, the n-coalescent problem consists of going backward in time to find for example the time to the most recent common ancestor (MRCA) and the
topology of the tree. In the literature, this problem is tackled mostly by computing directly the random variable t, time to reach the MRCA. I show here that by shifting the focus from the random variable t to the joined variable (n,t), where n is the number of ancestors at time t, the problem is greatly simplified. Indeed, P(n,t), the probability of this variable, obeys a simpler master equation that can be solved in a straightforward way for the most general model. This probability can then be used to compute relevant information of the n-coalescent, for both random variables $t_{n}$ (random time to reach a given state n) and $n_{t}$ (random number of ancestors at a given time t). The cumulative distribution function for $t_{1}$ for example is $P(1,t)$. I give in this article the general solution for continuous time models such as Moran and discrete time ones such as WF.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Quantum Frustration as a Protection Mechanism in Non-Topological Majorana Qubits
E. Novais
I analyze the decoherence of a $\pi$-junction
qubit encoded by two co-located Majorana modes. Although not topologically protected, the
qubit leverages distinct spatial profiles to couple to two independent environmental baths, realizing the phenomenon of
quantum frustration. This mechanism is tested against the threat of quasiparticle poisoning (QP). I show that frustration is effective against Ohmic noise ($s=1$) and has some protection for $0.76<s<1$ sub-Ohmic noise. However, the experimentally prevalent $1/f$ noise ($s\to0$) falls deep within the model's localized phase, where frustration is insufficient. This causes spontaneous
symmetry breaking and catastrophic decoherence. The
qubit's viability depends on what the effective environment is that these local Majorana wave functions experience.
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Mesoscale and Nanoscale Physics
Probing the features of electron dispersion by tunneling between slightly twisted bilayer graphene sheets
Alexey A. Sokolik, Azat F. Aminov, Evgenii E. Vdovin et al. (+6 more)
Tunneling conductance between two
bilayer graphene (BLG) sheets separated by 2 nm-thick insulating barrier was measured in two devices with the twist angles between BLGs less than 1{\deg}. At small bias voltages, the tunneling occurs with conservation of energy and momentum at the points of intersection between two relatively shifted
Fermi circles. Here, we experimentally found and theoretically described signatures of electron-hole asymmetric band structure of BLG: since holes are heavier, the tunneling conductance is enhanced at the hole doping due to the higher density of states. Another key feature of BLG that we explore is gap opening in a vertical electric field with a strong polarization of electron wave function at van Hove singularities near the gap edges. This polarization, by shifting electron wave function in one BLG closer to or father from the other BLG, gives rise to asymmetric tunneling resonances in the conductance around charge neutrality points, which result in strong sensitivity of the tunneling current to minor changes of the gate voltages. The observed phenomena are reproduced by our theoretical model taking into account electrostatics of the dual-gated structure,
quantum capacitance effects, and self-consistent gap openings in both BLGs.
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All authors Alexey A. Sokolik, Azat F. Aminov, Evgenii E. Vdovin, Yurii N. Khanin, Mikhail A. Kashchenko, Denis A. Bandurin, Davit A. Ghazaryan, Sergey V. Morozov, Kostya S. Novoselov
Strongly Correlated Electrons
Accelerating two-dimensional tensor network optimization by preconditioning
Xing-Yu Zhang, Qi Yang, Philippe Corboz et al. (+2 more)
We revisit gradient-based optimization for infinite projected entangled pair states (iPEPS), a tensor network ansatz for simulating many-body
quantum systems. This approach is hindered by two major challenges: the high computational cost of evaluating energies and gradients, and an ill-conditioned optimization landscape that slows convergence. To reduce the number of optimization steps, we introduce an efficient preconditioner derived from the leading term of the metric tensor. We
benchmark our method against standard optimization techniques on the Heisenberg and Kitaev models, demonstrating substantial improvements in overall computational efficiency. Our approach is broadly applicable across various contraction schemes, unit
cell sizes, and
Hamiltonians, highlighting the potential of preconditioned optimization to advance tensor network
algorithms for strongly correlated systems.
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All authors Xing-Yu Zhang, Qi Yang, Philippe Corboz, Jutho Haegeman, Wei Tang
Statistical Mechanics
Typicality of thermal states in isolated quantum systems corresponds to ubiquity of global minima in wide artificial neural networks
Takaaki Monnai
The Neural Tangent Kernel theory theoretically guarantees the existence of global minima of the cost functional in the neighborhood of an arbitrary random initialized parameters in wide artificial neural networks. In this paper, we show that the ubiquity of the global minima directly corresponds to the typicality of pure thermal states in isolated
quantum systems by identifying a common underlying mechanism characterized by the restriction to a few observables and the role of a Wishart-type
matrix. Moreover, we demonstrate that the increase in distinguishability of the reduced density
matrices of typical pure states with subsystem size corresponds to the double descent phenomenon observed by varying the width of layers in finite-width artificial neural networks. Thereby, the threshold for the reduced state become thermal is determined by essentially the same condition as the fitting threshold. In this manner, we reveal a structural correspondence between thermalization in isolated
quantum systems and wide neural network.
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Materials Science
Role of Wadsley Defects and Cation Disorder to Enhance MoNb12O33 Diffusion
CJ Sturgill, Manish Kumar, Nima Karimitari et al. (+8 more)
Wadsley-Roth (WR) niobates have emerged as high-rate anode materials that can combine rapid ionic
diffusion with good electronic conductivity. WR compounds have been defect-enhanced by limited annealing, however, such materials often contain multiple types of defects. In particular, both Wadsley defects (variable block size) and transition metal disorder have the potential to modify transport rates, however the corresponding effects are not well understood mechanistically. Here, MoNb12O33 (MNO) was calcined at two different temperatures to compare a defect-rich condition (MNO-800) with a proximal order-rich condition (MNO-900) as assessed through XRD, XANES, EXAFS, and STEM characterizations. Galvanostatically cycled lithium half
cells of MNO-800 exhibited additional capacity (307 mAh/g at 0.1C, 4.66% higher) and improved high-rate capacity of 200 mAhg-1 at 10C. ICI-based overpotential analysis identified solid state
diffusion as the dominant rate limiting process where MNO-800 correspondingly exhibited ~3X faster capacity-weighted diffusivity. A machine-learning interatomic potential was trained to density functional theory and then applied with molecular dynamics (MLIP-MD) to examine the possible roles of Wadsley defects and transition metal disorder. For both defect-types, Li was found to populate and activate fast
diffusion paths from window sites at lower extents of lithiation as compared to the order-rich model.
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All authors CJ Sturgill, Manish Kumar, Nima Karimitari, Iva Milisavljevic, Coby S. Collins, Aaron Hegler, Hsin-Yun Joy Chao, Santosh Kiran Balijepalli, Scott Misture, Christopher Sutton, Morgan Stefik
Soft Condensed Matter
Photo-Switchable Cross-Linking in Polymer Gels: Effects on Surface Creasing and Network Relaxation during Swelling
Alyssa VanZanten, Surbhi Punhani-Schillinger, M. Reed Blocksome et al. (+6 more)
Polymer gels with photo-responsive cross-links enable tunable mechanics and surface morphologies, making them promising for adaptive materials. While prior work on coumarin cross-linked gels has focused on photo-mediated events in dilute solution, their network-level mechanical responses remain unclear. Here, we design PEG hydrogels with both permanent covalent and dynamic coumarin cross-links, allowing in situ modulation of cross-linking under wavelength specific UV light. Real-time FTIR and dynamic mechanical analysis show that post-cure 365 nm irradiation drives rapid dimerization, increasing storage modulus by up to 69%, whereas cleavage of coumarin cross-links via 254 nm post-cure irradiation has a more limited effect due to attenuation in bulk samples. Surface imaging reveals that dynamic cross-linking governs swelling-induced crease formation and
evolution. Together, these results establish design principles for hydrogels with programmable mechanics and adaptive surface topographies, advancing application in smart coatings, actuators, and responsive biomaterials.
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All authors Alyssa VanZanten, Surbhi Punhani-Schillinger, M. Reed Blocksome, Aditya Ketkar, Shih-Yuan Chen, Michelle M. Driscoll, Robert C. Ferrier, Jr., Caroline R. Szczepanski
Soft Condensed Matter
Force-induced Elastic Softening and Conformational Transitions in a Polyampholyte Chain
Rakesh Palariya and Sunil P. Singh
The mechanical response of intrinsically disordered
proteins (IDPs) and polyampholyte (PA) chains is vital for understanding their biological functions and designing functional materials. We investigate the force-extension behavior of a PA chain with distinct charge sequences using molecular dynamics simulations and a theoretical approach based on the generalized random-phase approximation (GRPA). A diblock PA chain under extensional force undergoes a continuous coil-to-stretch transition at weak electrostatic coupling, which sharpens into a globule-coil-like transition at stronger coupling. The GRPA theory quantitatively captures these behaviors, including the sharp conformational transition and its dependence on electrostatic strength. Simulations reveal pronounced hysteresis during the force-extension and relaxation processes. Additionally, the elastic modulus exhibits four regimes: an initial plateau, stress stiffening, an exponential stress-softening behavior, and a stress stiffening regime. Using the theoretical model and structural input of the PA chain, we have demonstrated that the elastic modulus in the elastic softening regime decreases exponentially, $E\sim \exp(-\alpha_0 f/\Gamma_e)$, as a function of $f$, which aligns with the simulation results. The elastic response of the PA chain is further examined across different charge sequences, where both elastic softening and sharp transitions are absent at smaller block lengths. Finally, coarse-grained models of IDPs such as LAF-1 and DDX4 exhibit similar nonlinear elasticity, highlighting the universality of these mechanisms. Our results establish a fundamental link between electrostatic correlations, charge sequence, and nonlinear elasticity, bridging molecular interactions and macroscopic mechanics.
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Mesoscale and Nanoscale Physics
Spin and orbital-to-charge conversion in noncentrosymmetric materials: Hall versus Rashba-Edelstein effects
Diego Garcia Ovalle and Aurelien Manchon
We investigate
spin- and orbital-to-charge conversion phenomena in nonmagnetic materials with broken inversion
symmetry, treating the contributions from the
Hall effect and the Rashba-Edelstein effect on an equal footing. We develop a general formalism for this interconversion based solely on macroscopic observables. The theory is validated through a case study of ferroelectric GeTe, where we find that the effective Rashba parameter obtained is smaller than previously reported values for the same material. Incorporating these parameters into a drift-
diffusion model, we show that the generated charge current is primarily governed by the Rashba-Edelstein effect, rather than by the
spin or orbital
Hall effects.
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Soft Condensed Matter
Dynamic Permeability in Metastable Droplet Interfacial Bilayers
Nivedina A. Sarma, David A. King, Xuefei Wu et al. (+4 more)
Membrane pores are implicated in several critical functions, including
cell fusion and the transport of signaling molecules for intercellular communication. However, these structural features are often difficult to probe directly. Droplet interfacial
bilayers offer a synthetic platform to study such membrane properties. We develop a theory that links size-selective transport across a metastable membrane with its transient structural properties. The central quantity of our theory is a dynamic permeability that depends on the mechanism of pore growth, which controls the transient distribution of pore sizes in the membrane. We present a mechanical perspective to derive pore growth dynamics and the resulting size distribution for growth via Ostwald ripening and discuss how these dynamics compare to other growth mechanisms such as coalescence and growth through surfactant desorption. We find scaling relations between the transported particle size, the pore growth rate, and the time for a given fraction of particles to cross the membrane, from which one may deduce the dominant mechanism of pore growth, as well as material properties and structural features of the membrane. Finally, we suggest experiments using droplet interfacial
bilayers to validate our theoretical predictions.
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All authors Nivedina A. Sarma, David A. King, Xuefei Wu, Brett A. Helms, Paul D. Ashby, Thomas P. Russell, Ahmad K. Omar
Superconductivity · Cross-listed from physics.ins-det
Photothermal resistivity alignment of optical fibers to SNSPD
Martin Baránek, Dušan Lorenc, Tomáš Ščepka et al. (+5 more)
We demonstrate a straightforward optoelectronic fiber
alignment technique for superconducting nanowire single-
photon detectors (SNSPDs) that exploits the temperature-dependent resistance of the nanowire under optical absorption. The target nanowire is illuminated via the fiber, and the local absorption of light heats the wire, causing a change in its resistivity. Scanning the fiber over the nanowire, the change in its resistivity is monitored by lock-in amplifier, mapping the spatial photothermal response correlated to absorption and coupling efficiency. The maximum of the response corresponds to optimal fiber-SNSPD
alignment. This method allows for aligning the fiber to the center of the meander with sub-micron precision. The response is robust to variations in the angle and height of the fiber, providing an alternative or complement to fiber-to-chip
alignment methods based on the back-reflection or transmission measurement.
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All authors Martin Baránek, Dušan Lorenc, Tomáš Ščepka, Ján Šoltýs, Iuliia Vetrova, Štefan Haščík, Miroslav Grajcar, Pavol Neilinger
Statistical Mechanics
Finite size scaling and edge effects in the Takayasu model of aggregation diffusion with input
Rohan Banerjee Ravindran and R. Rajesh
We analytically and numerically study the effect of finite spatial boundaries on the Takayasu model of diffusing and aggregating particles with steady monomer input in one dimension. Exact expressions are derived for the steady-state density profile, two-point correlation functions, and mean-squared density under both open and periodic boundary conditions. The single-site mass distribution exhibits a crossover from a bulk power law $P(m)\sim m^{-4/3}$ to an edge power law $P(m)\sim m^{-5/3}$, occurring near the boundaries or the condensate that forms in periodic systems. The equivalence between the two boundary conditions is shown to break down in the case of multipoint probability distributions near the edge. The exact solution identifies a distinct boundary layer and shows that the edge anomaly arises when spatial mass currents, which scale as $\mathcal{O}(L)$, dominate over the $\mathcal{O}(1)$ constant flux in mass space. We further generalize these results to mass-dependent
diffusion.
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Superconductivity
Impact of Electron Correlations on Infinite-Layer Cuprates and Nickelates
Xunyang Hong, Yuetong Wu, Ying Chan et al. (+18 more)
Optimization of unconventional superconductivity involves a balance of interaction strengths. Precise determination of correlation strength across different material families is therefore important. Here, we present a combined X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) study of infinite-layer PrNiO$_2$ and SrCuO$_2$ that enables fair comparison of their interaction strengths. For both compounds, we study the orbital and
magnetic excitations and extract their dispersions along high-
symmetry directions. Using a single-band Hubbard model and including higher-order exchange interactions, we derive the correlation factor $U/t$ for both compounds. A key finding is that despite a smaller Coulomb repulsion $U$, PrNiO$_2$ exhibits a correlation strength that is 20% stronger than that of its isostructural cuprate counterpart SrCuO$_2$. This indicates that a moderation of the correlation strength may further optimize superconductivity in nickelates.
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All authors Xunyang Hong, Yuetong Wu, Ying Chan, Sze Tung Li, I. Biało, L. Martinelli, A. Drewanowski, Qiang Gao, Xiaolin Ren, Xingjiang Zhou, Zhihai Zhu, A. Galdi, D. G. Schlom, K. M. Shen, J. Choi, M. Garcia Fernandez, Ke-Jin Zhou, N. B. Brookes, H. M. Rønnow, Qisi Wang, J. Chang
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Interferometric Braiding of Anyons in Chern Insulators
Felix A. Palm, Nader Mostaan, Nathan Goldman et al. (+1 more)
Coherent control and braiding of anyons remain central challenges in realizing topologically protected
quantum operations. We propose a Ramsey interferometry protocol to directly access the
geometric phases associated with anyons in fractional Chern
insulators. Our approach employs
impurities with individually addressable internal states that bind to the anyons, allowing their adiabatic motion and exchange under full spatial control. By combining Ramsey and
spin-echo sequences using one and two
impurities, the protocol gives independent access to the Aharonov-Bohm and exchange contributions to the total
geometric phase, thereby providing an unambiguous probe of anyonic statistics. Our scheme can potentially be implemented in cold-atom
quantum simulators as well as in van der Waals
heterostructures. Complementary finite-size simulations in non-interacting Chern
insulators quantify the system sizes required to faithfully extract
geometric phases, highlighting the role of edge effects. Our results establish
impurity-based interferometry as a feasible route toward direct anyon braiding experiments in
quantum simulators and lay the groundwork for future explorations of non-Abelian braiding and topological
quantum control.
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All authors Felix A. Palm, Nader Mostaan, Nathan Goldman, Fabian Grusdt
Statistical Mechanics
Spatial and Temporal Cluster Tomography of Active Matter
Leone V. Luzzatto, Mathias Casiulis, Stefano Martiniani et al. (+1 more)
Critical phase transitions have proven to be a powerful concept to capture the phenomenology of many systems, including deeply non-
equilibrium ones like living systems. The study of these phase transitions has overwhelmingly relied on two-point correlation functions. In this Letter, we show that cluster
tomography – the study of one-dimensional cross-sections of the clusters that emerge near a phase transition – is an alternative higher-order tool that efficiently locates and characterizes phase transitions in active systems. First, using motility-induced phase separation as a paradigmatic example, we show how complex
geometric features of clusters, captured by spatial cluster
tomography, can be used to measure critical exponents in active systems without explicitly introducing system-specific order parameters. Second, we introduce temporal cluster
tomography, an analogous cluster-based measurement that characterizes the dynamical behavior of active systems. We show that cluster dynamics can be captured by a generalization of burstiness analysis in complex temporal networks. Both spatial and temporal cluster
tomography are easy to implement yet powerful approaches to study non-
equilibrium systems, making them useful additions to the standard toolbox of statistical physics.
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All authors Leone V. Luzzatto, Mathias Casiulis, Stefano Martiniani, István A. Kovács
Soft Condensed Matter
Spatio-temporal dynamics of surfactant driven secondary invasion in Gaussian pore networks
Debanik Bhattacharjee, Guy Z. Ramon and Yaniv Edery
Capillarity-dominated two-phase displacement in porous media often continues beyond the initial invasion-percolation (IP) breakthrough, as surfactants alter interfacial properties and reopen pathways once sealed by capillary forces. This study examines such secondary invasion, where adsorption-driven reductions in interfacial tension and contact-angle shifts lower entry thresholds in yet uninvaded throats, enabling further displacement at a fixed inlet pressure. To capture this process, we employ a time-dependent pore-network framework that couples IP with a reduced-order transport-adsorption module. Local fluxes are governed by Poiseuille flow, interfacial adsorption follows a Langmuir isotherm, and wettability
evolution is modeled through a calibrated phenomenological relation. Heterogeneity is prescribed by Gaussian throat-size distributions whose variance controls structural disorder. The resulting invasion trajectories are sigmoidal, consistent with Gaussian cumulative statistics, indicating that surfactant mass-transfer kinetics and network variance primarily rescale invasion timescales while preserving the overall functional form. The framework thus connects interfacial conditioning to time-varying capillary thresholds and reveals how surfactant-mediated processes govern post-breakthrough dynamics in heterogeneous porous systems.
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Materials Science
First-Principles Investigation of Surface-Induced Effects on the Properties of Divacancy Qubits in 3C-SiC
Rosario G. Viglione, Giovanni Castorina, Gaetano Calogero et al. (+4 more)
Neutral silicon-carbon divacancy (V$_{Si}$V$_{C}$) in cubic silicon carbide (3C-SiC) is a promising class of point defects for
quantum technologies based on active crystalline centers. Within the theoretical framework of
spin-polarized Density Functional Theory (DFT), this study examines the structural and electronic characteristics of V$_{Si}$V$_{C}$ centers near a hydrogen-terminated Si-rich (001) surface. A (2x1):H reconstructed slab of 628 atoms represents the near-surface environment, with divacancies located at depths ranging from 0.6 to 1.2 nm in basal and axial orientations. The optimized geometries show localized relaxations, and the electronic structure reveals in-gap defect levels in both
spin channels. Furthermore, examination of the zero-field splitting (ZFS) tensor demonstrates sensitivity to the orientation of the
spin defects and their distance from the surface. The findings of this investigation suggest that surface proximity exerts a substantial influence on the
spin Hamiltonian of divacancies, providing insight for the engineering of SiC-based
qubits and nanoscale
quantum devices.
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All authors Rosario G. Viglione, Giovanni Castorina, Gaetano Calogero, Giuseppe Fisicaro, Damiano Ricciarelli, Ioannis Deretzis, Antonino La Magna
Soft Condensed Matter
Analytical Analysis of the Conformational and Rheological Properties of Flexible Active Polar Linear Polymers under Shear Flow
Arindam Panda, Sunil P Singh and Roland G. Winkler
The conformational and rheological properties of active polar linear
polymers (APLPs) under linear shear flow are studied analytically. We describe a discrete APLP as an inextensible flexible Gaussian bead-spring chain supplemented by active forces along the bonds. The linear, non-Hermitian equations of motion are solved by an eigenfunction expansion in terms of a biorthogonal basis set. The model reveals an intimate coupling between activity and shear flow, which implies activity-enhanced
polymer conformational and rheological properties. Compared to a passive
polymer, we find a significantly enhanced shrinkage transverse to the flow direction with increasing shear rate, with a power-law exponent $-4/3$, compared to the passive values of $-2/3$. This conformational change is tightly linked with a strongly amplified shear-thinning behavior, where the shear viscosity exhibits the same power law. The characteristic shear rate for the onset of these effects is determined by the activity. In the asymptotic limit of large activities, the shear-induced features become independent of activity and equal to those of passive
polymers.
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Materials Science
A scalable kinetic Monte Carlo platform enabling comprehensive simulations of charge transport dynamics in polymer-based memristive systems
Gerliz M. Gutiérrez-Finol, Kirill Zinovjev, Alejandro Gaita-Ariño et al. (+1 more)
Polymer-assisted ion transport underpins both energy storage technologies and emerging neuromorphic computing devices. Efficient modeling of ion migration is essential for understanding the performance of batteries and memristors, but it remains challenging because of the interplay of drift,
diffusion, and electrostatic interactions, as well as the limitations of continuum and molecular dynamics approaches. Addressing these challenges is particularly relevant in the context of the
climate and energy crisis, where high-performance, low-carbon technologies require optimized ion-conducting materials and devices. Here, we introduce a scalable and flexible
stochastic simulation platform that uses Markov chain Monte Carlo methodology to model ion migration in solid-state systems. The platform employs a vectorized, rail-based representation of device geometry, enabling rapid simulation of lateral ion transport and space-charge effects while preserving the
stochastic nature of hopping events. It accommodates a wide range of material systems and can integrate experimental input parameters without code modification. We also provide an implementation of the model that takes advantage of highly energy-efficient GPUs, improving the performance and reducing the carbon footprint of the simulations. Validation using
polymer-based memristive devices demonstrates the simulator's ability to capture key behaviors, including relaxation decay, current-voltage hysteresis, spike-timing-dependent plasticity, and learning/forgetting rates. By balancing computational efficiency with mesoscale physical considerations, the platform provides a versatile tool for exploring ion-driven phenomena in energy storage and neuromorphic devices, supporting exploratory research.
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All authors Gerliz M. Gutiérrez-Finol, Kirill Zinovjev, Alejandro Gaita-Ariño, Salvador Cardona-Serra
Superconductivity
Zihao Huang, Zhan Wang, Hengxing Tan et al. (+8 more)
The kagome metals and
superconductors hosting rich correlated and topological electronic states have captivated
quantum materials research. These states are triggered by an unconventional
chiral charge density wave (CDW) wherein a
chiral superconductivity emerges at low temperatures, yet the origin of this
chiral CDW order, the parent state, is unresolved. Here, we report the discovery of a parent
chiral-nematic
Fermi liquid state in kagome metals and
superconductors. We use spectroscopic-imaging scanning tunneling microscopy to study Ti-doped CsV3Sb5 where the CDW is suppressed, and find that multiorbital
Fermi surfaces break all mirror reflections and exhibit handedness. We observe the
chiral low-energy quasiparticle dispersions, providing direct evidence for a
chiral-nematic electronic structure. We further observe the direct transition from the parent state to a
chiral-nematic superconducting state. Moreover, in the samples with
chiral CDW, we also observe the residual
chiral-nematic QPI features, demonstrating that the CDW-triggered exotic states descend from the
chiral-nematic
Fermi liquid. Our findings not only provide a plausible
chiral-nematic parent state, but also establish a new conceptual framework for exploring the emergence and consequences of such exotic
quantum phases.
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All authors Zihao Huang, Zhan Wang, Hengxing Tan, Zhen Zhao, Chengmin Shen, Haitao Yang, Bent Weber, Binghai Yan, Hui Chen, Ziqiang Wang, Hong-Jun Gao
Statistical Mechanics
Emergent long-tail dynamics in driven magnets with dynamical frustration
Chenyue Guo, Hongzheng Zhao and Zi Cai
In this study, we show that dynamical frustration can spontaneously emerge in frustration-free
magnetic systems under periodic driving. Specifically, we consider a classical
spin system and demonstrate the emergence of
spin-ice physics when drive-induced heating is well suppressed. In particular, we focus on the dynamics of
magnetic monopole excitations, which, in sharp contrast to their
equilibrium counterparts, exhibit a non-ergodic
stochastic random-walk process with long-tailed, power-law distributed waiting times, where the power-law exponent is tunable by the system's effective temperature. Heating is accelerated at intermediate driving frequencies, and the system eventually heats up to an infinite-temperature state. However, the heating time is extremely sensitive to different initial-state realizations and also follows a long-tailed power-law distribution. We show that a drive-induced short-range attractive interaction between monopoles is responsible for the long-tailed distributions observed in both monopole and heating dynamics.
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Statistical Mechanics · Cross-listed from quant-ph
Thermodynamic uncertainty relations for relativistic quantum thermal machines
Dimitris Moustos and Obinna Abah
We investigate a two-
qubit SWAP thermal machine – a streamlined analogue of the four-stroke Otto cycle – whose working medium comprises inertially moving Unruh-DeWitt
qubit detectors, each coupled to a thermal
quantum field bath prepared at a different temperature. In the presence of relative motion between the working medium and the thermal baths, we derive thermodynamic uncertainty relations (TURs) that quantify the trade-off between performance,
entropy production, and power fluctuations. Our analysis identifies regimes where relativistic motion leads to stronger violation of classical TURs, previously observed in static
quantum setups. In addition, we establish generalized performance bounds for the thermal machine operating as either a heat engine or a refrigerator, and discuss how relativistic motion can enhance their performances beyond the standard Carnot limits defined by rest-frame temperatures.
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Statistical Mechanics
Tree-Graph Based Construction of Quantum Spin Models with Exact Ground State
Toshiya Hikihara
We propose a protocol to generate an
antiferromagnetic S=1/2 Heisenberg model with the exact ground state based on a tree graph. The generated model has a correspondence with a tree graph and possesses the product state of singlet dimers as its unique ground state. A procedure for constructing a model with exact, massively degenerate ground states is also introduced.
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Strongly Correlated Electrons
Probing the Critical Behavior of a Sign-Problematic Model with Monte Carlo Simulations
Ye Ling, Yuting Wang, Wenan Guo et al. (+1 more)
The sign-problematic generalized Baxter-Wu (GBW) model with asymmetric complex couplings is mapped onto a one-dimensional
quantum model. Utilizing the model's exactly known critical properties, we study the relation between the conventional and the modified average signs and the phase transitions in the GBW model. We find that the average sign develops a negative peak near the critical point, but it is not a unique indicator of phase transition, as similar features can appear in non-critical regions. While the average modified sign provides a viable probe for the phase transition, the practical effectiveness of this method is limited by the exponential scaling of computational cost with the system's volume. We propose that the universal properties of the original model can be investigated through simulating the related reference model, based on the universality assumption. Using finite-size scaling analysis based on Monte Carlo simulations, we confirm the validity of this method, which thereby provides a novel framework for investigating phase transitions in systems plagued by the sign problem.
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All authors Ye Ling, Yuting Wang, Wenan Guo, Yuhai Liu
Mesoscale and Nanoscale Physics
Interface tuned Enhanced and Low Temperature Quenching of Orbital Hall Currents Induce Torque and magnetoresistance in Light Metal/Nickel Bilayers
Dhananjaya Mahapatra, Harekrishna Bhunia, Manu S Pattelath et al. (+1 more)
We investigate orbital current induced effects arising from the orbital
Hall effect in light-metal/ferromagnet
bilayers. Thin films of Ti in ohmic contact with Ni were studied using second-harmonic longitudinal and transverse voltage measurements under an applied a.c. current. From these signals, we extract the orbital
Hall torque (OHT) efficiency and the unidirectional orbital magnetoresistance (UOMR). Insertion of a Cu interlayer between the Ni/Ti interface leads to an enhancement of both OHT efficiency and UOMR compared to both Ni/Ti and Ni/Cu
bilayers. Furthermore, systematic variation of Ti thickness reveals that both OHT efficiency and UOMR increase with increasing Ti thickness, indicating that the observed phenomena predominantly originate from the bulk orbital
Hall effect rather than purely from interfacial mechanisms and Lowering the temperature leads to a clear reduction in both the orbital
Hall torque (OHT) efficiency and the unidirectional orbital magnetoresistance (UOMR). The nearly linear and correlated temperature dependence of both parameters suggests a common underlying mechanism, namely, the orbital
Hall effect in the light-metal layer, which governs both the generation of orbital current and its subsequent influence on the ferromagnet through orbital torque and orbital magnetoresistance.
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All authors Dhananjaya Mahapatra, Harekrishna Bhunia, Manu S Pattelath, Partha Mitra
Soft Condensed Matter · Cross-listed from physics.flu-dyn
Active bacterial pattern formation in evaporating droplets
Twan J.S. Wilting, Adriana W.B.P. Reijnier, Michiel H.M. Brebels et al. (+3 more)
Bacteria living on surfaces are often confined to droplets. When these droplets evaporate, the motion of the
liquid-air interface and the associated internal capillary flow confine the bacteria. Here we study how E. coli bacteria interact with this capillary confinement and agglomerate at the droplet's contact line. We identify three different types of bacterial pattern formation that depend on the bacterial activity and the environmental conditions imposed by the evaporating droplet. When the evaporation is fast, the bacteria are slow or the suspension is dilute, a uniform contact-line deposit forms. However, when the capillary confinement concentrates the bacteria at the contact line beyond a critical number density, localized collective motion spontaneously emerges. In that case, the bacteria induce a local stirring of the
liquid that allows them to self-organize into periodic patterns and enables them to collectively escape from the contact line. At very high number densities, these periodic patterns get destabilized by bacterial
turbulence in the bulk of the droplet resulting in the formation of mobile bacterial plumes at the contact line. Our results show how the subtle interplay between the bacteria and the capillary flow inside the droplet that surrounds them governs their dispersal.
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All authors Twan J.S. Wilting, Adriana W.B.P. Reijnier, Michiel H.M. Brebels, Alexandre Villie, Remy Colin, Hanneke Gelderblom
Soft Condensed Matter
Broadband Dielectric and THz Spectroscopy on Bio-Related Matter: Water, Amino Acids, Proteins, and Blood
Peter Lunkenheimer, Sebastian Emmert, Martin Wolf et al. (+1 more)
In the present work, we examine the relevance and proper interpretation of broadband-dielectric and THz-spectroscopy data for the investigation of various types of biological matter. We provide an overview of the rich variety of different dynamic processes that can be detected by these experimental methods. Several experimental examples are discussed in detail, helping to understand the information that can be drawn from such studies. This includes dielectric spectra, extending well into the GHz region, for pure water, which can be considered as a simple but highly important biological molecule. We also discuss results for a prototypical aqueous solution of a
protein, belonging to one of the most important classes of biological macromolecules. Moreover, we examine broadband dielectric spectra on blood as an example of functional biological matter in organisms. To demonstrate the relevance of THz spectroscopy for the investigation of biological molecules, we finally treat such experiments applied to different amino acids.
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All authors Peter Lunkenheimer, Sebastian Emmert, Martin Wolf, Alois Loidl
Materials Science
Low-Temperature Heat Capacity and Phonon Dynamics in Expanded Graphite and EG–MWCNTs Composites
A.I. Krivchikov, A. Jeżowski, M.S. Barabashko et al. (+11 more)
The specific heat of expanded graphite (EG) and EG–multiwalled carbon nanotube (MWCNT) composites (1.0 and 3.0 wt.% MWCNTs) was measured between 2 and 300~K. The low-temperature heat capacity is dominated by out-of-plane flexural
phonons with quadratic dispersion, characteristic of two-dimensional layered systems. Compared with crystalline graphite, EG exhibits enhanced heat capacity due to increased defect density and reduced interlayer coupling. Structural characterization (XRD, Raman, EDS) confirmed variations in stacking order and defect concentration. The data were fitted using a three-term model ($C_{1}T + C_{3}T^{3} + C_{5}T^{5}$), where the negative $C_{5}$ term indicates quadratic
phonon dispersion. The results demonstrate the influence of MWCNT integration and structural disorder on
phonon dynamics and anisotropic heat capacity in EG-based composites.
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All authors A.I. Krivchikov, A. Jeżowski, M.S. Barabashko, G. Dovbeshko, D.E. Hurova, N.N. Galtsov, V. Boiko, Yu. Sementsov, A. Glamazda, V. Sagan, Yu. Horbatenko, O.A. Korolyuk, O.O. Romantsova, D. Szewczyk
Soft Condensed Matter · Cross-listed from physics.soc-ph
Global Population and Carrying Capacity in the Anthropocene: the Relative Growth Rate Insight
Aleksandra Drozd-Rzoska, Agata Angelika Sojecka and Sylwester J. Rzoska
This report provides insights into global population dynamics since the beginning of the Anthropocene, focusing on empirical data and minimizing a priori the impact of model assumptions. It explores the Relative Growth Rate concept, introduced recently to global population studies by Lehman et al. [PNAS 118, e2024150118 (2021)] and subsequently extended to its analytical counterpart [PLoS ONE 20, eo323165 (2025)]. The analysis reveals a general non-monotonic growth pattern in the Anthropocene, emphasizing the uniqueness of the Industrial Revolution era. For the first 290 years, the Doomsday critical scaling provides a superior description for population changes, with a singularity at 2026. This is followed by the crossover to an exceptional 'reversed criticality' patterm, which has held over the last six decades, to the present day. The analysis suggests that the
evolution of the human Real Intelligence (RI) population during the Innovations-driven Industrial Revolution times - a period of rapidly increasing connectivity and complexity - can serve as a model counterpart for the puzzling dynamics of Artificial Intelligence (AI) growth. The final conclusion is positive: the catastropic Doomsday singularity can be avoided due to the generic system constraints , both for RI and AI.
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Materials Science
Laser-generated GHz surface acoustic waves with tunable amplitude during the magnetostructural phase transition in FeRh thin films
Ia. A. Mogunov, A. Yu. Klokov, N. Yu. Frolov et al. (+6 more)
Laser-generated surface acoustic waves (SAW) facilitate efficient information processing in modern spintronics and magnonics. The ability to tune the SAW parameters such as amplitude is crucial to achieve acoustic control over magnonic properties. Such tunability can be achieved in phasechanging
magnetic materials that accommodate both
spin waves and SAWs. A promising material is the FeRh
alloy, a metallic antiferromagnet at room temperature that undergoes a phase transition to the
ferromagnetic state accompanied by a
crystal lattice expansion at 370 K. This transition can also be induced by femtosecond
laser pulses. In this paper, we use the phase transition in a 60 nm Fe49Rh51 film to optically generate pulses of Gigahertz quasi-Rayleigh SAWs. We detect them via the photoelastic effect and show that the
lattice transformation during the phase transition is a dominant strain-generation mechanism for above-threshold excitation. The weight of this contribution rises as the sample is heated closer to the AFM-FM transition temperature and 'switches off' when heated above it, allowing for control of the SAW amplitude. A model based on thermodynamic parameters of Fe49Rh51 shows that the
lattice transformation occurring within 95 ps effectively contributes to SAW generation happening on a comparable timescale, while non-
equilibrium fast kinetics of the phase transition does not.
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All authors Ia. A. Mogunov, A. Yu. Klokov, N. Yu. Frolov, A. V. Protasov, G. E. Zhezlyaev, D. I. Devyaterikov, R. R. Gimaev, V. I. Zverev, A. M. Kalashnikova