Materials Science · Cross-listed from math.DS
Effective Dynamics of Disclination Pairs
Nicolò Briatico, Pierluigi Cesana and Marco Morandotti
The dissipative dynamics of a pair of wedge disclinations with opposite Frank angles confined to a circular domain is investigated under the assumption of radially symmetric motion. Two distinct dynamical regimes are identified: a diverging-disclination regime and an annihilating-dipole regime. The stationary points associated with both regimes are determined and fully characterized in terms of their stability, and quantitative estimates for the characteristic
evolution times in their vicinity are derived. In the annihilating-dipole regime, it is shown that, after a suitable time rescaling, the resulting law of motion for the dipole coincides with that of an edge dislocation.
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Mesoscale and Nanoscale Physics
Second-Chern Bounds in Non-Abelian Quantum Geometry
Junwen Zhao, Zhiming Pan, Kang Yang et al. (+1 more)
We study the
quantum geometry of doubly degenerate energy levels in a four-dimensional parameter space. We find that the scalar
quantum metric $g$ and the Berry
curvature $F$ obey $(\operatorname{tr} g)^2/16\geq\sqrt{\det g}\geq |2\Tr(F\wedge F)-\Tr F\wedge \Tr F|/24$. The first inequality characterizes the anisotropy in the metric. The second determinant inequality measures the self-duality of the traceless $SU(2)$ part of the
curvature under Hodge
star operation and the algebraic closedness of the inter-level polarization amplitudes under $SU(2)$ rotations in the doubly degenerate levels. The saturation of the determinant bound imposes a quaternionic Cauchy-Riemann equation, analogous to the complex analyticity imposed by ideal-band conditions in two-dimensional Chern
insulators. As examples, four-band Dirac
Hamiltonians automatically saturate the determinant bound and possess a topological zero in $\operatorname{tr}(F\wedge F)$. We compare the differences between Kramers degeneracy and ordinary $U(2)$ degeneracy. In addition to the non-Abelian
geometric bound, the latter also obeys an independent first-Chern bound.
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All authors Junwen Zhao, Zhiming Pan, Kang Yang, Congjun Wu
Mesoscale and Nanoscale Physics
Electron transport in a 1.6~nm-thick double-gated (100) silicon nanosheet: A theoretical study accounting for phonon confinement and remote-phonon scattering
Shoaib Mansoori, Bimin Cai, Edward Chen et al. (+2 more)
We study theoretically electron transport in an top-and bottom-gated (100) 1.6 nm-thin silicon nanosheet with SiO2/HfO2 gate stacks, focusing on the intrinsic physical processes that affect transport: the confinement of
phonons and the presence of interface hybrid plasmon-
phonon excitations (IPPs or `remote
phonons'). The band structure is calculated using local empirical pseudopotentials; an approximated elastic continuum model is used to consider the confinement of acoustic
phonons; the dielectric continuum limit is used to deal with the IPPs. We find that the electron mobility is affected significantly by the boundary conditions chosen to deal with
phonon confinement. The more realistic assumption of
phonons clamped at the SiO2/HfO2 interfaces and optical
phonons at the Si/SiO2 interfaces results in a room temperature mobility much smaller than what is obtained using the common assumption of bulk
phonons in the elastic, high-temperature approximation. We also find that, as a result of the complicated structure of the primed subbands, the high-field saturated velocity is significantly lower than its bulk value, as it had been measured in the past in the case of Si inversion layers but never explained theoretically. Finally, we find that IPP scattering does depress the low-field mobility but to a small extent, thanks to the presence of the interfacial SiO2 layers and to the proximity of the metal gates. Moreover, by keeping electrons `cooler', IPP scattering results in a higher saturated velocity. Therefore, the presence of high-kappa materials in the gate-
insulator stacks should not affect negatively the performance of field effect transistors based on Si nanosheets.
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All authors Shoaib Mansoori, Bimin Cai, Edward Chen, Dallin O. Nielsen, Massimo V. Fischetti
Statistical Mechanics · Cross-listed from q-bio.PE
Dynamics of fluctuating populations in multi-state switching environments
Mauro Mobilia
Microbial populations generally evolve in fluctuating environments under time-varying conditions. These are often described by binary switching models, sometimes seen as coarse-grained feast-famine cycles, in which resource availability switches abruptly between abundant and scarce conditions. However, experimental studies suggest that feast-famine environments actually exhibit more complex temporal dynamics. Here, we study how two strains, one growing slightly slower than the other, compete for the same resources in fluctuating environments comprising a finite number of intermediate states, each having its own carrying capacity. Environmental switching between these states and their carrying capacities represents gradual changes in nutrient availability. This class of multi-state
stochastic switching models can be interpreted as a coarse-grained description of feast–famine cycles and allows us to investigate strain competition under the gradual recovery and depletion of resources. By computational and analytical means, we characterise the population dynamics in these multi-state fluctuating environments. In particular, we study how the switching rates and distribution of carrying capacities affect the population-size statistics, fixation probability, and mean fixation time. By comparing these results with their counterparts in binary environments, we clarify how the frequency and amplitude of environmental fluctuations influence population dynamics in coarse-grained feast-famine cycles.
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Mesoscale and Nanoscale Physics
Lectures on ultrathin film ferromagnetism
D. Pescia
In these Lecture Notes we review some of the fundamental principles that have emerged from research on the ferromagnetism of ultrathin films consisting of 3d transition-metal overlayers. Their growth is often layer-by-layer. This growth mode produces
quantum wells along the vertical direction that profoundly impact any physical property of the materials. In addition, the vertical confinement establishes
spin ensembles that extend to macroscopic distances along the in-plane directions and are finite along the vertical (perpendicular) direction, i.e. they are two-dimensional. Accordingly, they display ground state properties that originate from the two-dimensionality, such as “dead”
magnetic layers or “enhanced
magnetic moments”, an oscillatory interlayer
magnetic coupling and an anomalous perpendicular versus in-plane
magnetic anisotropy that produces, in some specific situations, a perpendicular collective orientation of the
spins. At finite temperatures,
ferromagnetic order is observed to persist and an analysis of the
magnetic order of ultrathin films in terms of the renormalization group provides a suitable framework for explaining this observation. The
ferromagnetic order is lost at a phase transition which follows closely the two-dimensional Ising universality class, as shown by an accurate analysis of data in the vicinity of the critical point. The perpendicular
spin orientation is often observed to turn in-plane by a reorientation phase transition which is also properly described by a renormalization group argument. Finally, the perpendicular
spin orientation introduces topological excitations of the
ferromagnetic order, consisting of stripes of reversed perpendicular
spin direction. The stripe order undergoes a phase transition to the paramagnetic state that is not yet completely understood.
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Materials Science · Cross-listed from eess.SP
Effects of Tool Wear on the Surface Texture in Turning: A Feature Characterization Approach Based on ISO 21920-2
Alexander Müller, Maximilian Berndt, Hagen Schmidt et al. (+5 more)
The surface texture of a turned component acts as a fingerprint of both the process parameters and the tool wear condition, imaging the cutting edge. Classical surface parameters such as $R_\mathrm{a}$ or $R_\mathrm{q}$ describe the topography only globally and allow no spatially resolved evaluation of the process-induced deterministic structures. This work investigates how far the feature characterization standardized in ISO 21920-2 makes this wear information accessible and physically interpretable. The database comprises roughness profiles of twelve AlTiN-coated carbide inserts (CNMG120408) machining normalized AISI 1045 steel, measured at nine wear states over the entire tool life, with three replicate profiles per state. The correlation of standardized field and feature parameters with crater wear, flank wear, and cutting time is first examined. Watershed
segmentation is then adapted to extract the rotational tool grooves and evaluate their geometry statistically. A newly developed mean-feature approach decomposes the profile into a deterministic and a
stochastic component. Wear-induced changes are almost entirely carried by the deterministic component, and within it by the trailing flank of the cutting groove. A comparison with confocal measurements confirms that the mean feature reconstructs the engaged cutting edge geometry, with the trailing-flank steepening attributable to notch wear on the secondary cutting edge. An exhaustive evaluation of more than 920,000 feature characterization combinations and multivariate models reveals that the groove-level mean maximum absolute gradient $\overline{R_\mathrm{dt}}_\mathrm{groove}$ alone explains 83-90% of the variance of the wear indicators, so that a single, physically motivated parameter suffices for robust wear estimation. A follow-up study will investigate inline monitoring using scattered light sensors.
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All authors Alexander Müller, Maximilian Berndt, Hagen Schmidt, Lars Müller, Matthias Eifler, Eberhard Kerscher, Benjamin Kirsch, Jörg Seewig
Strongly Correlated Electrons · Cross-listed from hep-th
Fortuity and fragility in supersymmetric SYK
James Chryssanthacopoulos and David Vegh
In the $\mathcal N=2$ supersymmetric SYK model with $N$
fermions, every BPS state is fortuitous: at fixed charge, it exists only over a finite range of $N$. Allowing the charge to increase, we show that BPS classes may instead be uplifted along
lattice walks inside the fortuity window, potentially to arbitrarily large $N$. However, there is no canonical uplift. We therefore introduce a decoder $D$ whose spectrum quantifies exact uplift and its metric cost. We call the failure or increasing cost of uplifting harmonic representatives metric fragility. Chen's single-
matrix model and the protected tower of the two-
flavor SYK model realize bare and genuinely dressed mechanisms of perfect uplift, respectively. In the generic one-
flavor model, exact uplift eventually fails. The Lin-Maldacena-Rozenberg-Shan-type operator $D^\dagger D$ exhibits level statistics consistent with the Gaussian
unitary ensemble. Its
eigenvalues quantify the metric continuity of BPS states across system size, while their correlations probe chaos within the BPS sector. Metric fragility and BPS chaos are thus encoded in complementary observables of the same operator, distinguishing chaotic BPS sectors from exactly solvable towers.
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Strongly Correlated Electrons
Connecting the tensor-categorical formulation of anyon condensation with operator algebras and entropic order parameters
Hua-Chen Zhang
Anyon condensation that describes the transition between topological
quantum field theories can be formulated in the
language of tensor categories or that of operator
algebras. We deploy the formalism of Doplicher-Haag-Roberts bimodules over quasi-local $\mathrm{C}^{*}$-
algebras recently developed in [1] to investigate anyon condensation, which is associated with an extension of a certain operator
algebra. The connection between notions in the two formulations is thereby made manifest in an intuitive, diagrammatic manner. An entropic order parameter, as the
quantum information-theoretic measure characterising a condensation, is naturally defined, and we give a very simple proof of a bound on it.
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Statistical Mechanics
Shortcuts to Parameter Sweeps
Chi Xiang, Guodong Cheng and Geng Li
Efficient evaluation of stationary parametric sensitivities over broad parameter ranges is important for identifying influential training data, fitting force fields, and predicting material responses, but standard pointwise approaches require repeated relaxation and sampling. Here we introduce Shortcuts to Parameter Sweeps (STPS), an engineered control strategy that uses an auxiliary control to transport the probability density along a prescribed family of instantaneous stationary states during a finite-time parameter sweep. This enables the continuous response curve over the full parameter interval to be estimated from a single controlled sweep using covariance-based response relations. STPS applies to both
equilibrium and nonequilibrium steady-state systems, including those with unknown stationary distributions, and can be implemented directly using stationary samples in high-dimensional settings. Numerical tests on single-particle and interacting many-body systems show that STPS yields response curves in close agreement with reference results. These findings establish STPS as an efficient, sample-based framework for continuous sensitivity analysis in
stochastic simulations.
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Materials Science
The electrical transport of intrinsic two-dimensional ferroelectric metal PtBi2
Dan Li, Liu Yang and Lei Li
Breaking the conventional stereotype that ferroelectrics are necessarily insulating, two-dimensional (2D) ferroelectric metals combine seemingly incompatible switchable electric polarization and metallic conductivity, providing a fertile ground for the discovery of novel electrical transport phenomena and the development of innovative electronic devices. Using the semiclassical Boltzmann equation and first-principles calculations, we systematically investigate the linear and nonlinear transport responses of the intrinsic 2D ferroelectric metal \ch{PtBi2} to an applied electric field. Our ab initio molecular dynamics simulations reveal that it possesses a high Curie temperature reaching $800~\text{K}$. We propose that the
crystal structure of its high-temperature paraelectric phase can be explicitly distinguished through simple measurements of the in-plane electrical conductivity. Quantitative calculations of the Edelstein effect and the intrinsic
spin Hall effect demonstrate a sizable charge-to-
spin conversion efficiency, highlighting its potential in spintronics. We also find that a Berry
curvature dipole-induced nonlinear
Hall effect emerges in uniaxially strained \ch{PtBi2}. Furthermore, we highlight the unique advantages of 2D ferroelectric metals in gate-controlled transport applications. Based on the domain wall scattering mechanism, we conceptually design a novel ferroelectric metal field-effect transistor (FEM-FET) capable of nonvolatile switching between high-resistance and low-resistance states under a gate voltage. Our work not only unveils the rich transport physics in 2D ferroelectric metals but also provides valuable insights into the design of next-generation nonvolatile
memory and spintronic devices.
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Strongly Correlated Electrons
Two routes to quantum anomalous Hall states in altermagnets
Makoto Naka, Shuntaro Sumita, Yukitoshi Motome et al. (+1 more)
We theoretically propose two possible routes to realizing
quantum anomalous
Hall states in altermagnetic materials. We consider a minimal square-
lattice Hubbard model with antisymmetric
spin-orbit coupling associated with an orthorhombic
crystal structure, which supports a topologically trivial altermagnetic state. By incorporating Rashba-type
spin-orbit coupling and external perturbations, we demonstrate that this trivial state can be turned into topological altermagnetic phases in two distinct ways. The first route is driven by a staggered potential that breaks the
symmetry connecting crystallographically equivalent sublattices, leading to a topological altermagnetic ground state characterized by a quantized
Hall conductivity $\left| \sigma_{xy} \right|=e^2/h$ and a Chern number $C=1$. The second route is realized by applying a
magnetic field perpendicular to the two-dimensional plane. The resulting topological state appears as a metastable state in the
magnetic hysteresis loop, exhibiting a quantized
Hall conductivity $\left| \sigma_{xy} \right|=2e^2/h$ associated with a Chern number $C=2$. We show that these topological transitions are accompanied by characteristic gap closings at the Brillouin-zone boundary, with the number of gap-closing points determining the Chern number. Ribbon-geometry calculations reveal
chiral edge states consistent with the bulk topological invariants and demonstrate distinct
spin polarizations between the $C=1$ and $C=2$ states. Our results establish experimentally accessible routes to quantized anomalous
Hall responses in altermagnets.
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All authors Makoto Naka, Shuntaro Sumita, Yukitoshi Motome, Hitoshi Seo
Statistical Mechanics
Macroscopic fluctuation theory for the multi-time statistics of current in non-stationary diffusive systems
Sabyasachi Chowdhury, Kapil Sharma, Sandeep Jangid et al. (+1 more)
The statistics of current fluctuations has long been a central object of study in non-
equilibrium physics. Most existing work has focused on one-time statistics, while their multi-time generalisation remains comparatively less explored. We address this gap by extending the fluctuating hydrodynamics framework of Macroscopic Fluctuation Theory (MFT) to study multi-time statistics in the non-stationary state of a diffusive system on an infinite line. For the simplest cases of a non-interacting
lattice gas and hard-core Brownian point particles, we present explicit solution of the MFT leading to multi-time large-deviation statistics. For generic systems, the MFT is solved perturbatively, yielding explicit results for two-time correlations. These reveal that the connection between current fluctuations and fractional Brownian motion, previously observed for flat initial conditions, does not persist for step initial conditions. We independently verify these hydrodynamic results by solving the corresponding microscopic dynamics for the non-interacting gas and for the symmetric simple exclusion process. Additional confirmation comes from numerical simulations.
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All authors Sabyasachi Chowdhury, Kapil Sharma, Sandeep Jangid, Tridib Sadhu
Materials Science
Beam Routing through Excitons in Transition Metal Dichalcogenide Monolayers
Yonas Lebsir, Jacob Terndrup Heiden, Jorge Barcia Rodríguez et al. (+6 more)
Routing light at the nanoscale typically relies on nanostructured surfaces to imprint directionality on the emission. Using low-temperature, angle-resolved cathodoluminescence spectroscopy, we show that the intrinsic excitonic transitions of a
semiconductor can themselves produce routed emission. We probe monolayers of WSe$_2$, MoSe$_2$, and MoTe$_2$ and resolve the excitonic species of monolayer WSe$_2$ – the bright
exciton, the trion, and the
spin-forbidden dark
exciton – through their distinct angular emission profiles. While the in-plane transition dipoles of the bright
exciton and trion radiate predominantly toward the surface normal, the out-of-plane dipole of the dark
exciton, inaccessible under normal-incidence optical excitation, produces a directional emission channel at large angles. We further tune the balance between neutral and charged
exciton emission through the local dielectric environment. Our results establish dark
excitons in TMD monolayers as a platform for directional light emission in compact photonic architectures without additional nanostructuring.
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All authors Yonas Lebsir, Jacob Terndrup Heiden, Jorge Barcia Rodríguez, Maria Papadopoulou, Kenji Watanabe, Takashi Taniguchi, N. Asger Mortensen, Sergii Morozov, Nicolas Ubrig
Materials Science
Thermoelastic Harvesting Outperforming Thermoelectric Generators Below 100 {\deg}C
Bruno Neumann, Andreas Henschke, Morik Nikolic et al. (+1 more)
Low-grade waste heat below 100 degC is one of the largest untapped opportunities in solid-state energy conversion. Three ferroic routes are candidates for recovering this resource: thermomagnetic, pyroelectric, and thermoelastic harvesters. The last has remained the most unexplored, despite decades of progress on the underlying NiTi shape-
memory alloy wires. Three system-design changes close this gap: a protagonist-antagonist architecture that recovers the energy for prestraining, a continuously tunable prestrain mechanism that sets the force-strain balance, and transversal water flow that decouples cycle frequency from wire length. The resulting harvester delivers a directly measured power density of 366 mW/cm^3 with respect to the active material, about 1.7 times the next-best thermoelastic device, ahead of every reported thermomagnetic and pyroelectric generator, and outperforming the best thermoelectric generators in this temperature range also with respect to power per material cost. The system maps the parameter space directly through force and displacement measurements, without using material-property estimates, giving a quantitative picture of how the
alloy responds while the device is doing work.
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All authors Bruno Neumann, Andreas Henschke, Morik Nikolic, Sebastian Fähler
Strongly Correlated Electrons
Phonon anomalies and critical scaling in the spin-$1/2$ trimer chain Na$_2$Cu$_3$Ge$_4$O$_{12}$
P. Srikanth Patnaik, A. K. Bera, Srishti Bhardwaj et al. (+4 more)
Low-dimensional
quantum magnets provide an ideal platform to explore
spin-
lattice coupling-mediated
quantum correlations, which give rise to emergent quasiparticle excitations. The antiferromagnetically coupled
spin-1/2 trimer chain of copper ions in Na$_2$Cu$_3$Ge$_4$O$_{12}$ (NCGO) hosts high-energy
spin excitations of different species, whose energy scales overlap with those of
lattice vibrations. Here, we report a comprehensive temperature-dependent Raman spectroscopic study performed between 80 and 400 K. The dynamic
spin susceptibility, as obtained from the analysis of the broad spectral background, reveals the emergence of quasiparticle excitations below 170 K. We further identify an unusual crossover of
phonon dynamics when the material transits from a normal paramagnetic state to a correlated
quantum magnetic state. A power law dependence of the integrated Raman susceptibility of the
phonon modes, $I_{\chi^{\prime\prime}}^{i}\sim|T-T_{c}|^\beta$, is observed with the critical temperature $T_c$=167$\pm$1 K, and critical exponent $\beta = 0.24\pm 0.02$. The combined results obtained from the broad spectral background and sharp
phonon peaks further indicate that the
phonon renormalization observed across the crossover is driven by dynamic
spin states. Additionally, statistical correlations among
phonon energy
eigenvalues, quantified through
matrix-norm and power-test analyses of 200 spectra recorded at 80 K, reveal an unexpected linear correlation among
phonon modes, also indicating that the collective
lattice response is governed by
spin correlations. These findings establish NCGO as a model system for investigating cooperative
spin-
lattice coupling and critical scaling behavior of
phonon dynamics in low-dimensional
magnetic materials.
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All authors P. Srikanth Patnaik, A. K. Bera, Srishti Bhardwaj, Tulika Maitra, Buddhananda Banerjee, Anushree Roy, S.M. Yusuf
Quantum Gases · Cross-listed from quant-ph
Enhancing the power of a quantum heat engine via control of the system–reservoir coupling
Sabrina Burgardt, Julian Feß, Silvia Hiebel et al. (+2 more)
The non-
equilibrium properties of open
quantum systems are determined by the microscopic laws governing energy exchange with their environment. In particular, an enhancement of the performance of
quantum heat engines has been predicted by speeding up the dynamics through control of the system–bath interaction. However, direct microscopic control of heat transfer between the machine and the reservoir has remained elusive so far. Here, we experimentally demonstrate such control in a
quantum Otto engine realized with ultracold Cs-133 atoms coupled to an atomic reservoir of ultracold Rb-87 atoms. Heat exchange between the two is mediated by inelastic s-wave collisions whose energy-dependent scattering cross sections lead to an asymmetric equilibration dynamics in the isochoric heating and cooling strokes. By tuning the kinetic temperature of the atomic reservoir, we modify the associated microscopic scattering rates, and thereby the heat transfer law, giving control over the time allocation within the engine cycle through control over the microscopic, multi-exponential relaxation dynamics. This enables power output optimization at fixed efficiency. Our results establish microscopic control of system-reservoir interactions as a tool for manipulating heat flow at the nanoscale and engineering the finite-time performance of
quantum thermal machines.
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All authors Sabrina Burgardt, Julian Feß, Silvia Hiebel, Eric Lutz, Artur Widera
Other Condensed Matter
Formally Verified Lock-Free Software Transactional Memory for Scientific Measurement
Kentaro Kitagawa
Automated measurement of condensed-matter experiments requires instrument-control, data-acquisition, and user-interface threads to access shared, naturally hierarchical state concurrently. Coarse-grained locking can delay acquisition and cause sample loss, whereas fine-grained locking requires deadlock-prone lock ordering across instruments. Instead, we describe the lock-free software transactional
memory (STM) that has been at the core of an open-source measurement platform for 16 years, in
nuclear magnetic resonance experiments and, more recently, in optically detected
magnetic resonance experiments. The STM organizes this state as a tree and provides atomic subtree updates and consistent subtree snapshots. After initial bundling, an unchanged subtree snapshot is acquired in $O(1)$ time through a custom lock-free atomic shared pointer. Within each bounded TLA+ configuration, TLC exhaustively checks the state space, establishing the safety and livelock-freedom properties specified for that configuration. Bounded executions of the atomic shared-pointer implementation are separately checked under the C11 weak-
memory model. The same Snapshot and Transaction interfaces are exposed to Python scripting and AI-assisted automation.
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Materials Science
Energy-Dependent Dechanneling in Cu: Insights from Monte Carlo Channeling Simulations
Przemyslaw Jozwik, Cyprian Mieszczynski, Renata Ratajczak et al. (+1 more)
Ion channeling and backscattering techniques are powerful tools for studying
crystal lattice disorders and defect structures in crystalline materials. However, the accurate interpretation of channeling phenomena necessitates the utilization of simulation models that account for the intricate interactions between point defects, dislocations, and extended defect clusters. The present paper introduces a Monte Carlo method that reproduces experimental spectra over a wide range of analyzing beam energies and enables quantitative identification of defect types and distributions. The simulations reveal characteristic energy dependencies that distinguish point defects from extended defects, offering a novel perspective on disturbances caused, for example, by ion implantation in metals and
semiconductors. To this end, the McChasy code has been developed as a flexible and accessible tool for scientists, enabling the modeling of various
crystal systems, including complex
semiconductors, multilayer epitaxial films, and
oxide crystals. The program's integration of experimental data on ion channeling with defect modeling establishes a robust framework for defect analysis in materials science. The present article expounds upon the simulation capabilities of the program by reproducing the characteristic "elbows" in channeling spectra that were previously observed in experiments conducted on Cu
crystals.
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All authors Przemyslaw Jozwik, Cyprian Mieszczynski, Renata Ratajczak, Andrzej Turos
Statistical Mechanics · Cross-listed from physics.plasm-ph
The Ambipolar electric field in multispecies plasma atmospheres: effects of alpha particles and stochastic heating
Luca Barbieri, Pascal Démoulin and Daniel Verscharen
We investigate stationary states of a collisionless, gravitationally stratified
plasma atmosphere composed of electrons,
protons, and alpha particles by extending Pannekoek–Rosseland theory to multispecies and multi-temperature
plasmas. Starting from Liouville's
theorem, we derive the self-consistent ambipolar electric field from kinetic
equilibrium and charge neutrality. For a single-temperature atmosphere, we obtain analytical expressions for the ambipolar field, show its dependence on alpha-particle abundance, and determine the relative stratification of the three species. A first-order analytical approximation to the electrostatic potential accurately reproduces the numerical solution. We then generalize the formalism to multi-temperature
plasmas generated by
stochastic boundary heating, representing the stationary distribution as a superposition of Maxwellian populations. Gravitational filtering produces non-exponential density profiles and increasing temperatures with altitude, while preserving the relative species stratification, with alpha particles most strongly stratified and
protons least. The ambipolar electric field contains a dominant gravitational contribution, corresponding to the generalized Pannekoek–Rosseland field, and a thermoelectric contribution arising from species-dependent temperature gradients, which accounts for its non-monotonic structure. These results provide a framework for studying the combined effects of
plasma composition and
stochastic heating in gravitationally stratified astrophysical
plasmas.
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Soft Condensed Matter
A Multi-scale Investigation of Aqueous Foams Stabilised by PNIPAM Microgels
Joanne Zimmer, Luca Mirau, Kevin Gräff et al. (+5 more)
Aqueous foams possess multiple structural motifs across different length scales: macroscopic foam, bubbles, foam films and the air/water interface. In this study, macroscopic foams are generated by sparging gas through an aqueous dispersion of PNIPAM microgels which act as foam stabilisers due to their surface activity. The stiffness of the microgels and thus their interfacial activity are tuned by variation of the cross-linker density. The effect of the cross-linker density and the microgel concentration on the resulting foam formation properties (foamability) and the foam stability are investigated. A lower cross-linker density and a higher microgel concentration enhance the foamability, generate foams with smaller bubbles and higher
liquid fractions, and increase the foam stability. These observations are correlated with the microgel behavior at the single air/water interface examined by pendant drop tensiometry and Langmuir compression experiments as well as the mobility in single free-standing foam films determined using a Thin Film Pressure Balance. Our findings highlight good agreement across all length scales: increased foamability correlates with a faster decrease in surface tension, and higher foam stability with a higher surface elastic modulus of a microgel-covered single air/water interface and decreasing mobility in foam films.
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All authors Joanne Zimmer, Luca Mirau, Kevin Gräff, Gaëtan Barth, Carina Schneider, Vera A. N. A. Hesse, Hayden Robertson, Regine von Klitzing
Mesoscale and Nanoscale Physics
Mismatch between Raman shear modes and ferroelectric polarization in 3R-MoS$_{2}$
Johannes Schwandt-Krause, Jan-Niklas Heidkamp, Mohammed El Amine Miloudi et al. (+7 more)
Sliding ferroelectricity in parallel-stacked two-dimensional van der Waals materials enables a broad range of novel device concepts, but exploiting it requires reliable, non-destructive assignment of the underlying stacking order and polarization state. Here, we combine Kelvin-probe force microscopy (KPFM) with low-frequency Raman spectroscopy to probe the polarization domains and stacking configurations of a exfoliated trilayer 3R-MoS$_{2}$ flake on a hBN substrate. We find that ABA and BAB - both stackings with zero net polarization - are indistinguishable in KPFM, yet show drastically different low-frequency shear modes. This observation is reproduced across multiple flakes and is corroborated by low-temperature photoluminescence. Notably, the standard bond-polarizability model does not account for the difference in shear-mode activity between the ABA and BAB configurations, indicating that the interlayer Raman response of these stackings is governed by physics beyond a simple polarizability picture. Our results show that none of the here-used individual techniques alone is sufficient to assign sliding-ferroelectric stacking order and motivate a combined spectroscopic-scanning-probe approach.
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All authors Johannes Schwandt-Krause, Jan-Niklas Heidkamp, Mohammed El Amine Miloudi, Swarup Deb, Elisabeth Grune, Kenji Watanabe, Takashi Taniguchi, Rico Schwartz, Oliver Kühn, Tobias Korn
Soft Condensed Matter
Universality of the deswelling of tangentially active polymer chains in dilute solutions
Suryansh Tripathi and Aritra Santra
Dilute solutions of linear
polymer chains with tangentially active monomeric beads are simulated using a Brownian dynamics (BD)
algorithm over a range of solvent quality in the thermal crossover regime between $\theta$ and athermal solvents. The conformational changes with increasing Péclet number ($Pe$) (which is proportional to the strength of activity) suggest deswelling of the chains resulting in a collapse of the radius of gyration data to a random walk (RW) statistics at a unique value of $Pe$, independent of the solvent quality. The swelling behaviour of active
polymers in the crossover regime relative to their size at the $\theta$ state is found to follow the same universal characteristics as that of passive
polymer chains. Furthermore, based on
polymer blob theory we present a novel scaling of the thermal blob size with tangential activity of the monomeric beads which leads to the definition of a renormalized solvent quality parameter for active
polymers. Altogether, this work establishes a connection between the configurational properties of active
polymers and scaling laws in
polymer physics, which provides a useful framework to study the dynamics of activity induced motion of polymeric molecules for various applications in biophysics and other related areas.
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Statistical Mechanics
Ideal heat engine cycles at maximal efficiency – the ideal gas and beyond
Gregory Behrendt and Sebastian Deffner
Given a particular heat engine cycle, what is the optimal working medium that results in the highest efficiency? While one might jump to the conclusion that it must surely be the ideal gas, the situation is actually more intricate. Starting with a general Helmholtz potential that depends polynomially on molar volume and temperature we derive exact expressions for the ideal Stirling, Otto, and Brayton cycles. We find that for the thermodynamic systems described by our ansatz for the Helmholtz potential the maximal efficiency is achieved, if the working medium is described by a fundamental relation linear in temperature. This includes the ideal gas, but also classical harmonic oscillators and phenomenological models of the rubber band.
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Materials Science
Electronic structure, band offset, and interface electron population of the LaInO$_3$/BaSnO$_3$ system
G. Hoffmann, A. A. Riaz, C. Kalha et al. (+7 more)
Perovskite
oxides and their
heterostructures exhibit a wide range of functional properties. Among these materials, BaSnO$_3$/LaInO$_3$
heterostructures form high-mobility two-dimensional electron gases (2DEGs) at their interfaces. In particular, room-temperature electron mobilities exceeding 100~cm$^2$/Vs were enabled by recent advances in thin-film growth. This work presents a combined experimental and theoretical study of the electronic structure of BaSnO$_3$, LaInO$_3$, and BaSnO$_3$/LaInO$_3$
heterostructures with varying LaInO$_3$ overlayer thicknesses. Soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) measurements are combined with densities of states (DOS) derived from hybrid density functional theory (DFT) calculations. The analysis of core, semi-core, and valence states allows to arrive at a comprehensive understanding of the chemical bonding and electronic structure in the parent
oxides as well as the formed
heterostructures. For the BaSnO$_3$/LaInO$_3$
heterostructure, the band offset and population of 2DEG states at the interface is directly probed using HAXPES.
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All authors G. Hoffmann, A. A. Riaz, C. Kalha, A. Gloskovskii, C. Schlueter, O. Brandt, W. Aggoune, C. Draxl, O. Bierwagen, A. Regoutz
Mesoscale and Nanoscale Physics
Dimensional crossover and local strain induced deflection of the spin spiral state in multiferroic NiI2
Tianxing Jiang, Lianchuang Li, Haiyan Zhu et al. (+10 more)
Low-dimensional multiferroics hold great promise for integrated magnetoelectric devices.
Spin spiral state has recently been shown to induce ferroelectricity in single-layer van der Waals (vdW) material NiI2. However, how this state evolves and can be tuned towards the two-dimensional limit remain unclear. Here, we combine
spin-polarized scanning tunneling microscopy, layer-by-layer film growth, and multi-scale theoretical modeling to investigate the
spin spirals in NiI2 thin films. As the film thickness increases from 1 to 7 monolayers, we observed a continuous increase of
spin-spiral wavelength and a rotation of wavevector from near [110] to [1-10] direction, which evidences a dimensional crossover primarily driven by enhanced interlayer exchange energy. Moreover, we find that the film wrinkles can cause deflection of the
spin spiral wavevector, which is caused by local
curvature induced modification of exchange interactions. Our findings establish thickness and local strain as two tuning methods for engineering non-collinear helical magnetism and accompanied electric polarization in vdW multiferroics.
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All authors Tianxing Jiang, Lianchuang Li, Haiyan Zhu, Hongyu Wang, Junchao Tian, Wenzhao Wang, Weiyi Pan, Haitao Wang, Changlin Zheng, Hongjun Xiang, Changsong Xu, Donglai Feng, Tong Zhang
Materials Science
Resonant Raman signatures of bright and momentum-dark exciton coupled by intervalley phonon scattering in monolayer WSe2
Hendrik Lambers, Nihit Saigal, Lara Blinov et al. (+3 more)
Exciton-
phonon coupling in atomically thin transition metal dichalcogenides governs key processes such as
exciton thermalization and intervalley scattering and remains challenging to access directly by optical spectroscopy. Here, we employ resonance Raman spectroscopy at cryogenic temperatures to probe
exciton-
phonon coupling in hBN-encapsulated WSe_2 monolayers. Tuning the excitation
laser across the bright
exciton X_KK resonance, we observe rich Raman spectra and focus on the resonance profile of the degenerate A_1'/E' optical
phonon mode. The profile exhibits two asymmetric resonance peaks whose energetic separation significantly exceeds the
phonon energy - a feature that cannot be explained by first-order Raman scattering alone. We demonstrate that this discrepancy is resolved by including third-order Raman scattering, in which intervalley scattering enabled by a finite-momentum
phonon couples the bright
exciton X_KK to a momentum-dark
exciton X_d. Fitting the experimental resonance profiles of two independent samples with a model comprising coherent first- and third-order scattering yields consistent
exciton energies and linewidths, with a momentum-dark
exciton X_d approximately 45meV to 55meV below the bright
exciton X_KK. The results indicate efficient bright-to-dark
exciton coupling. Our findings provide a microscopic framework for understanding the anomalously bright emission spectra of WSe_2 monolayers despite its
spin-forbidden lowest
exciton transition and highlight the role of momentum-dark
excitons in resonant light-matter interaction.
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All authors Hendrik Lambers, Nihit Saigal, Lara Blinov, Jonas Kiemle, Alexander W. Holleitner, Ursula Wurstbauer
Disordered Systems and Neural Networks · Cross-listed from quant-ph
Hamilton-Zero: A Neural Tensor-Network Foundation Model for Ground States of Arbitrary Quadratic Qubit Hamiltonians
Timothy Heightman, Elena Orlova, Philip Mantrov et al. (+1 more)
A central promise of useful
quantum advantage is the ability to compute ground states of
Hamiltonian systems beyond the reach of classical simulation methods. Here we demonstrate that this problem can be effectively amortized across an arbitrary and universal set of
Hamiltonians by a foundation model with $\sim0.5$B variational parameters, trained with contemporary techniques from large
language models and deep reinforcement learning. To do this, we formulate $\text{spin-}1/2$
quantum ground-state learning as
manifold variational optimisation over centrally odd scalar functions on $\mathrm{SU}(2)^N$. This replaces explicit
Hilbert-space vector amplitudes with
manifold functions on which the
Hamiltonian acts through Lie derivatives, evaluated by custom automatic differentiation primitives. We prove that the resulting variational principle on this
manifold preserves the $\text{spin-}1/2$ sector's ground-state upper bound using the Peter-Weyl
theorem and justify the choice of such a representation with a no-go
theorem for pure state foundation NQS. We then pre-train our foundation model on a
dataset of hundreds of thousands of different
Hamiltonian systems, varying the connection
topology, system size, interaction types and strengths, bringing together a century of many-body literature. Using a novel $\mathrm{SU}(2)$ replica-exchange Langevin sampler and sharded natural-gradient optimisation, we train our model with our own extension of the Kronecker-Factored Approximate
Curvature (KFAC) optimiser on system sizes up to 64
qubits. On a held-out generalisation
dataset, we fine-tune our model on system sizes of up to 1024
qubits, and evaluate on systems up to 8100
qubits.
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All authors Timothy Heightman, Elena Orlova, Philip Mantrov, Aleksei Ustimenko
Materials Science
Magneto-optical magnetoelectric voltage sensor
Michael P. Path and Jeffrey McCord
Applications in high-voltage and electromagnetically harsh environments require reliable galvanically isolated voltage sensing, which can be achieved using optical readout. While established optical voltage sensors rely on electro-optic effects or piezoelectric strain with direct optical detection, strain-mediated magnetoelectric coupling combined with magneto-optical readout offers an alternative voltage sensing principle that remains largely unexplored. Here, such a sensor based on a bismuth-substituted yttrium iron garnet magneto-optical indicator film mechanically coupled to a piezoelectric actuator is presented. Voltage induced stress results in changes of the out-of-plane magnetization via magnetoelastic coupling which is detected through magneto-optical Faraday rotation. A critical state of the domain structure is set via an applied bias field, in which voltage-induced nucleation and domain-wall motion dominates the response. In this high sensitivity regime, both AC and DC voltage readout modes are demonstrated, based on either voltage-driven magnetization reversal or voltage-induced modifications of the magnetization loop shape. Equivalent voltage noise densities in the millivolt per root hertz range are achieved. The results establish strain-mediated magneto-optical voltage sensing as a distinct approach to optically isolated voltage measurement.
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Superconductivity
Spin-polarized supercurrents and Josephson diode effect in altermagnets
Janus F. Niebuhr, Matthias Eschrig and Danilo Nikolić
We present a systematic theoretical study of the Josephson effect in junctions consisting of a d-wave altermagnet (AM) placed between two BCS
superconductors (SC). In general, the SC/AM interfaces are
spin-active and modeled by
spin-dependent $\delta$ potentials, allowing for an arbitrary direction of the local exchange field vector. The model is formulated within the fully
quantum (Gor'kov) and quasiclassical (Eilenberger) Green's function technique, applied to two distinct cases of (i) a weakly
spin-polarized AM (exchange field much smaller compared to the
Fermi energy) and (ii) a strongly
spin-polarized AM (exchange field comparable to the
Fermi energy). We apply our model to the SC/AM/SC geometry, accounting for the Josephson current-phase relation (CPR). In the weakly
spin-polarized regime, the CPR displays the normal Josephson effect. Irrespective of the orientation of the altermagnet, the junction undergoes the $0-\pi$ transition. Depending on the orientation, the system displays the features similar to those of a
ferromagnetic or an
antiferromagnetic junction. To investigate the
spin-polarized currents and nonreciprocal transport as the central results of the present work, we put the main focus on the strongly
spin-polarized regime. Within this regime, we distinguish two cases. A coplanar exchange field profile across the junctions displays the normal Josephson effect; however, with a pure and stable long-range second harmonic in the CPR. In contrast, a noncoplanar exchange field profile gives rise to the so-called
quantum geometric phases across the junction, leading to the absence of the phase-inversion center in the Josephson CPR. As a result, a Josephson diode effect emerges with a significant charge diode efficiency larger than 30% and a perfect
spin diode efficiency of 100%.
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Materials Science
Strain-controlled sign reversal of the anomalous Hall effect in Ru/[Co/Ni]$_N$ multilayers
Jingying Zhang, Sigang Wang, Yue Xiang et al. (+4 more)
The anomalous
Hall effect (AHE) is a hallmark transport phenomenon in ferromagnets arising from relativistic
spin-orbit interaction. Here, we report an unexpected sign reversal of the AHE in Ru/[Co/Ni]$_N$ multilayers controlled by the stacking sequence of the Ru layer. When Ru is placed beneath, rather than atop, the Co/Ni multilayers, the anomalous
Hall signal switches from positive to negative. By systematically varying the multilayer repeat number N and combining transport measurements with first-principles calculations, we show that this reversal originates from in-plane tensile strain imposed by the Ru underlayer, which reshapes the electronic structure and redistributes Berry
curvature near the
Fermi level. Our findings establish interfacial strain as an effective knob for tuning Berry-
curvature-driven transport and suggest a pathway toward strain-controlled topological transport phenomena in
magnetic multilayers.
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All authors Jingying Zhang, Sigang Wang, Yue Xiang, Wenhui Xie, Zhe Yuan, Yi Liu, Zongzhi Zhang