Statistical Mechanics
Chirality-sensitive mobility and dissipation of Brownian motion on a helical landscape
Debankur Bhattacharyya and Abraham Nitzan
We study the Brownian dynamics and linear response of a particle with inertia moving in a 2-dimensional helical landscape imprinted on a cylindrical surface. In the harmonic well approximation, the deterministic motion separates into free propagation along the screw direction and harmonic motion in the transverse screw-normal direction. We show that for isotropic damping this simplification survives in the Langevin description, whereas anisotropic damping along the axial and angular directions couples the
stochastic dynamics and destroys separability. The resulting anisotropic model is formulated as a linear Ornstein-Uhlenbeck process in phase space with a zero mode associated with
diffusion along the screw coordinate, so that in an infinite system the full phase-space dynamics does not relax to a stationary distribution. To treat transport in this setting, we construct the stationary dynamics in the stable subspace obtained after projecting out the zero mode. This leads to a linear response theory for this system and yields closed analytical expressions for stationary time-correlation functions and the dynamical mobility tensor in both the time and frequency domains. The off-diagonal elements of the mobility tensor describe cross-response between axial forcing and angular motion, and between applied torque and axial transport. Consistent with time reversal
symmetry, these cross mobilities are equal and provide a direct dynamical signature of the helical geometry. In addition, a simultaneous application of driving in both the axial and angular direction reveals asymmetry in energy dissipation rate due the helical landscape.
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Statistical Mechanics
Orientable Surfactants on Thin Liquid Films: A Dynamic Density-Functional Theory Approach
Toby Kay and Serafim Kalliadasis
Thin
liquid films are ubiquitous across many natural and engineering systems, including films which are laden with surface active molecules, i.e. surfactants. The presence of surfactants may have a destabilising effect on the film owing to their influence on surface tension, the so-called Marangoni effect, which in turn can induce flows in the film. Classical thin-film models for surfactant-laden films lead to paradigmatic gradient dynamics equations governing the film height and surfactant concentration and have been widely studied. However, in all these works, which are based on fluid dynamics or nonequilibrium thermodynamics, the shape of surfactants is neglected, and they have been treated as symmetric point-like particles. In general, this is a drastic oversimplification, as surfactants are amphiphilic with a polar head-tail structure. To account for this effect we use elements from the statistical mechanics of classical fluids, namely density-functional theory (DFT), and its dynamic extension (DDFT). Starting from DDFT and under the long-wave approximation, we derive the pertinent thin-film equations with the surfactants treated as polar uniaxial particles. These are equations which govern the film height, as well as the surfactant concentration and polarisation field. They preserve the gradient dynamics form by appropriately defining the free energy, which contains the usual interfacial contributions, as well as further contributions from the polarisation field. In doing so, we uncover a novel form of a generalised surface tension that is dependent on the surfactant polarisation as well as concentration, and show that it arises in a thermodynamically consistent way.
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Statistical Mechanics
Multi-field Return Point Memory
Nathaniel Croce, Hossein Salahshoor and D. Zeb Rocklin
Non-
equilibrium systems display
memory, a dependence not merely on their present environment but on previously applied fields. Multistable systems such as
spin glasses, martensites and granular matter have exponentially many microstates consistent with an applied field, making their rich dynamics difficult to control. Control and order can be achieved through the concept of partial ordering, which we here generalize to systems subject to multiple control fields. We demonstrate, within the model system of the zero-temperature Ising model, that this leads to return-point
memory, in which an applied sequence of fields restores the hysteretic system not only to a previous magnetization, but to a previous exact microstate. The multiplicity of fields grants more precise and complex control of the system, with different classes of operations displaying commutative and noncommutative behavior. This grants new insight into how physical systems can remember, learn, and be trained.
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Strongly Correlated Electrons
Interaction-Split Edge Spectral Flow in an Integer Hubbard Thouless Pump
Yong-Feng Yang, Zhao-Rui Tian, Chen Cheng et al. (+1 more)
We show that local Hubbard correlations can split the open-boundary spectral flow of an otherwise integer Thouless pump. Using density-
matrix renormalization group calculations for a period-three Hubbard chain with a sliding onsite-repulsion modulation, we identify at filling $\rho=2/3$ a correlated insulating pump with many-body Chern number $C=+2$, corresponding to two units of charge transported per cycle. A
spin-degenerate Hartree/Aubry–André–Harper reference model realizes the same integer response through simultaneous edge spectral flow in two independent
spin channels. In the full Hubbard model, however, this integer bulk pump is implemented differently at an open boundary: the edge flow separates into two boundary events, and the intervening low-energy state is a nearly charge-neutral triplet edge excitation. Thus the quantized bulk charge transport remains integer, but local correlations reorganize its boundary realization into separated charged and neutral spinful sectors.
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All authors Yong-Feng Yang, Zhao-Rui Tian, Chen Cheng, Hong-Gang Luo
Materials Science
Evidence of a Hybridized Topological State in Weyl Semimetal/Topological Insulator Mn$_{3+x}$Sn$_{1-x}$/Bi$_{0.85}$Sb$_{0.15}$ Heterostructures
Ryan T. Van Haren, Gillian P Boyce, Gregory M. Stephen et al. (+4 more)
We report magnetotransport evidence of a hybridized Weyl semimetal (WSM)
Fermi arc/topological
insulator (TI) surface state at the interface of a
ferromagnetic Mn$_{3+x}$Sn$_{1-x}$/Bi$_{0.85}$Sb$_{0.15}$
heterostructure. High target utilization sputtering (HiTUS) was used to grow polycrystalline Mn$_{3+x}$Sn$_{1-x}$ films and Mn$_{3+x}$Sn$_{1-x}$/Bi$_{0.85}$Sb$_{0.15}$
heterostructures on thermally oxidized Si/SiO$_2$ (100) substrates that exhibit the negative coefficient anomalous
Hall effect (AHE) resulting from topological Weyl node transport. When various defects and
impurities are introduced into these Mn$_{3+x}$Sn$_{1-x}$ films, a
ferromagnetic (FM) phase develops that practically eliminates the topological Weyl node conduction. These FM Mn$_{3+x}$Sn$_{1-x}$ films exhibit large exchange bias effects below T=200 K that we attribute to the coexistence of a FM phase and the triangular
antiferromagnetic (AFM) WSM phase. When Bi$_{0.85}$Sb$_{0.15}$ overlayers are grown on the FM Mn$_{3+x}$Sn$_{1-x}$, the magnetotransport signal of Weyl node topological transport is restored, an effect we do not observe when replacing the Bi$_{0.85}$Sb$_{0.15}$ TI with heavy metal overlayers. We attribute the restoration of the Weyl node topological transport to the formation of a hybridized topological state at the WSM/TI interface.
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All authors Ryan T. Van Haren, Gillian P Boyce, Gregory M. Stephen, Vinay Sharma, Don Heiman, Aubrey T. Hanbicki, Adam L. Friedman
Statistical Mechanics
Order-Disorder Tricriticality in $\mathrm{A}_n \mathrm{B}_n$ Star Polymer Melts
Minhoon Kim, Wonjun Kang, Daeseong Yong et al. (+2 more)
Tricriticality usually requires tuning an additional thermodynamic parameter. Here we show that, in symmetric $\mathrm{A}_n\mathrm{B}_n$
star-
polymer melts, the arm number $n$ itself plays this role and drives the order–disorder transition (ODT) from second order to first order. By developing a sixth-order free-energy expansion within the random phase approximation and comparing it with self-consistent field theory (SCFT) calculations, we analytically identify a tricritical arm number, $n_{\mathrm{tc}}\approx 5.4475$. For $n<n_{\mathrm{tc}}$, the lamellar ordering transition remains continuous and occurs at the spinodal point, $(\chi N)_{\mathrm{s}}\approx 10.495$. For $n>n_{\mathrm{tc}}$, the transition becomes first order, and $(\chi N)_{\mathrm{ODT}}$ shifts below $(\chi N)_{\mathrm{s}}$ with a quadratic dependence near the tricritical point. SCFT calculations confirm the predicted transition character and phase-boundary shift. The origin of this behavior is traced to inter-arm correlations generated by the common junction. We further show that the noninteger tricritical arm number can be effectively realized in binary mixtures of
star polymers. This provides a rare analytically tractable example of architecture-induced tricriticality in a microphase-separating
polymer system.
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All authors Minhoon Kim, Wonjun Kang, Daeseong Yong, Junhan Cho, Jaeup U. Kim
Strongly Correlated Electrons
Kondo singlet from ferromagnetic coupling: an analog of Anderson-Morel superconductivity in the magnetic channel
Ewan Scott, Yaqi Chen, Michael Turaev et al. (+4 more)
We consider
magnetic impurities coupled to a conduction sea via a fully isotropic
ferromagnetic spin-exchange term, the strength of which depends on the conduction-electron modes involved in the scattering. In the single-
impurity case we show both analytically and numerically that there exists a parameter regime in which the conventional Kondo effect develops at low temperatures, leading to a singlet ground state. In the case of a
lattice of
impurities, we show that this leads to a heavy
Fermi liquid state that is energetically favored over
magnetic ordering in a broad parameter range. We argue that these effects are analogs of Anderson-Morel superconductivity, and discuss routes to their experimental realization.
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All authors Ewan Scott, Yaqi Chen, Michael Turaev, Tarkan Yzeiri, Chris Hooley, Krzysztof P. Wójcik, Michał P. Kwasigroch
Soft Condensed Matter · Cross-listed from physics.optics
Real time monitoring of pressure-induced deformation of PDMS to evaluate pressure distribution in microfluidic channels
Kiran Acharya, Serge Monneret, Martin Brandenbourger et al. (+1 more)
Accurate pressure measurements in micrometric channels are essential for a wide range of microfluidic applications. Existing approaches rely on a variety of sensing mechanisms, but generally require the integration of additional probes or sensing elements during or after chip fabrication. Here, we introduce a pressure sensing approach based on quantitative phase imaging of the deformation of compliant microfluidic channels. We demonstrate real-time measurements of channel deformation over a large field of view with high sensitivity, without the need for embedded components or modifications of the microfluidic device.
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All authors Kiran Acharya, Serge Monneret, Martin Brandenbourger, Thomas Chaigne
Mesoscale and Nanoscale Physics
Nonreciprocal conductance in uniformly dissipative devices
Oliver Solow, Emil J. Bergholtz and Karsten Flensberg
When studying non-Hermitian electronic systems, an obvious question is how various non-Hermitian effects affect measurable quantities like the conductance. Here, we show that uniformly dissipative
circuits can exhibit nonreciprocal conductance, meaning that the two nonlocal conductances are different. We describe how this happens through a difference in transmission times between left-moving and right-moving electrons. We consider a specific case of a dissipative Rashba nanowire with a skewed
magnetic field, and show how this difference in transmission times comes about through interference inside the
circuit, and how this is modified as the dissipation strength changes.
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Mesoscale and Nanoscale Physics
Graphene-based Photodetector with Engineered Hot Carrier Cooling Dynamics
Yishu Huang, Anand Nivedan, Florian Ludwig et al. (+9 more)
Graphene has emerged as a promising material for integration into silicon photonics, owing to its ultrafast and broadband photoresponse without the need for an external bias voltage. This photoresponse relies on the photo-thermoelectric effect created by hot carriers. A key factor underlying the performance of
graphene photodetectors is the cooling dynamics of these hot carriers. In this work, we engineer these dynamics in a WSe2-
graphene-WSe2 waveguide-integrated photodetector. In particular, by introducing proximity screening by a nearby graphite layer to this structure, we prolong the hot-carrier cooling time, leading to an enhanced photoresponse. We characterize the cooling dynamics under continuous-wave
laser excitation by employing a photomixing technique, revealing an increase in the cooling time by up to a factor of four. Direct photoresponse measurements show that the internal photoresponsivity improves by approximately 50%. Together, these results demonstrate the potential of proximity screening to enhance the performance of
graphene-based photodetectors on an integrated photonics platform.
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All authors Yishu Huang, Anand Nivedan, Florian Ludwig, Bohai Liu, Michiel Debaets, Steven Brems, Hai I. Wang, Alessandro Principi, Dries Van Thourhout, Christian Haffner, Aron W. Cummings, Klaas-Jan Tielrooij
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Atom-Photon Bound States in Fractal Photonic Lattices: Localization Length and Anomalous Diffusion
Florian Bönsel, Flore K. Kunst and Federico Roccati
We study atom-
photon bound states seeded by two-level emitters coupled to self-similar photonic
lattices. By expressing the photonic Green's function through the heat kernel, we show that the far-field localization length obeys $\xi \sim \Delta^{-1/d_w}$, with the detuning $\Delta$ from the lower spectral edge and the walk dimension $d_w$ of the underlying fractal. This scaling is controlled by anomalous
diffusion and does not rely on translational invariance or a band-edge effective-mass approximation. Exact diagonalization on Sierpiński gaskets, pyramids, Vicsek graphs, and Sierpiński carpets confirms the far-field prediction once the bath
Hamiltonian is rendered
Laplacian-like by compensating the local inhomogeneity in the connectivities with on-site potentials. In the near field, the bound-state amplitude exhibits an additional algebraic variation. For nested finitely ramified fractals, the corresponding exponent agrees with the classical resistance/ first-passage scaling, whereas Sierpiński carpets display clear deviations from this simple law. Our results extend structured-bath waveguide QED to self-similar non-periodic geometries and connect bound-state profiles to transport exponents of the underlying fractal
lattice.
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Materials Science
Close correlation between giant magnetostriction and the microstructure in Fe-Ga melt-spun ribbons
Likun Chen, Mitsutaka Sato, Jiahao Han et al. (+1 more)
Magnetoelastic anisotropy and <100> texture are crucial for promoting magnetostriction in Galfenol (Fe-Ga). Given that single-
crystal Fe-Ga remains technically demanding and low magnetostriction of polycrystal, we explore melt-spun as an alternative, where rare-earth (RE) doping and cooling rates optimizing enable controllable <100> textured with required magnetoelastic anisotropy. Fe81Ga19 binary ribbons and RE-doped ribbons (0.2 at.% Pr, 1 at.% Pr and 1 at.% Ce) were fabricated at various cooling rates. Microstructural analyses reveal that RE elements preferentially dissolve into the
matrix, while second-phase formation is suppressed at higher cooling rates. RE substitution increases the magnetostriction by enhancing magnetocrystalline distortion energy, while cooling rates act as an effective tuning knob to maximize the <100> texture. Notably, the decreased melting temperature associated with 1 at.% RE doping shifts the optimum texture to lower cooling rate compared with the binary
alloy and 0.2 at.% RE doping sample. The magnetostriction as high as 688 ppm is achieved for the 1 at.% Ce doped ribbon fabricated at the speed of 1000 rotation per minute. These results demonstrate that RE-doped melt-spun ribbons are promising candidates for
giant magnetostriction and establish a practical processing-texture-property guideline for designing highly magnetostrictive
alloys.
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All authors Likun Chen, Mitsutaka Sato, Jiahao Han, Rie Y. Umetsu
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Non-Hermitian Landau Levels
Anton Montag and Tomoki Ozawa
We formulate non-Hermitian Landau levels in two-dimensional systems under a complex perpendicular
magnetic field. In the symmetric gauge, we derive their discretely spaced, highly degenerate complex spectra and biorthogonal eigenstates, and clarify the role of non-
unitary gauge transformations. A non-Hermitian Harper-Hofstadter
lattice model confirms the continuum theory and reveals Gaussian wave packet dynamics governed by semiclassical equations with a complex Lorentz force, pointing to possible experimental realizations of complex
magnetic fields.
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Materials Science
First-principles transition-state tensorial cluster expansion of vacancy diffusion in Ta-W beyond the kinetically-resolved activation approximation
Jacob Jeffries, Brianna Sebastian-Olazabal and Enrique Martinez
Predicting
diffusion in chemically complex
alloys remains challenging due to the strong dependence of migration barriers on local atomic environments. Migration barriers computed using density functional theory and nudged elastic band calculations are represented via a tensorial cluster expansion including transition states and deployed in on-
lattice kinetic Monte Carlo simulations. Applied to the Ta-W system, the framework captures nontrivial composition-dependent
diffusion behavior arising from a crossover between solute trapping and percolated low-barrier transport pathways, yielding a maximum in the apparent activation energy near intermediate compositions. This approach establishes a general and scalable route for integrating first-principles transition-state energetics into mesoscale kinetic simulations, enabling predictive multiscale modeling of
diffusion in chemically complex materials and providing a pathway for uncovering emergent transport phenomena.
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Superconductivity
Phase Transitions in Disordered Altermagnetic Josephson Junctions
Chang-An Li
Altermagnetic Josephson junctions (AMJJs) can host unconventional $\pi$ phase and $\varphi$ phase despite vanishing net magnetizations. Whether these phases are stable against disorder existing in real materials remains an open question. Here, we investigate impact of disorder on exotic phases in two-dimensional AMJJs consisting of two conventional
superconductors mediated by a $d$-wave altermagnet. We show that disorder is able to drive phase transitions from the exotic $\pi$ phase to conventional $0$ phase, accompanied by a substantial suppression of critical current. This behavior is attributed to modifications of the tunneling Cooper-pair phase shift and superconducting decoherence. Remarkably, the anomalous $\varphi$ phase is highly fragile in presence of disorder and can be driven to either a $\pi$ phase or $0$ phase in a nonreciprocal way. Across such transitions, the first harmonic of current-phase relation changes its sign, while the higher-order harmonics are rapidly suppressed. Our findings reveal the crucial role of disorder in tailoring distinct phases of AMJJs and shed new light on their potential functionalities.
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Disordered Systems and Neural Networks · Cross-listed from cs.LG
Preisach Attention: A Hysteretic Model of Sequential Memory
Piotr Frydrych
We introduce the Preisach
Attention Layer (PAL), a novel sequence modelling architecture grounded in the classical Preisach hysteresis operator from mathematical physics. PAL replaces the softmax
attention mechanism with a binary relay operator parameterised by learned activation and deactivation thresholds, maintaining a stack of local extrema as its internal state. A single-layer PAL-
Transformer with O(1) depth is Turing-complete under arbitrary precision arithmetic, achievable through simulation of a two-stack pushdown automaton – in contrast to the O(log n) depth required by standard hard-
attention transformers. Second, we prove that the function classes computable by PAL and by the
transformer are incomparable: PAL computes historical range statistics in O(1) layers that require O(log n) layers for
transformers, while
transformers support random-access
retrieval that PAL cannot perform without auxiliary state. The separating property is rate-independence – PAL responds only to the sequence of local extrema, not to absolute
token positions or temporal spacing. Third, we show that the extremum stack constitutes a minimal sufficient statistic of the input history for all rate-independent functionals, providing a formal analogue of the wiping property in classical hysteresis theory. PAL is thus an efficient architecture for tasks with long episodic
memory and weak positional dependence, with O(n log n) total inference cost versus O(n^2) for standard
attention.
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Other Condensed Matter · Cross-listed from quant-ph
Entanglement entropy across the dynamical phase transition in the quantum $\mathcal{O}(N)$ model
Frederick del Pozo, Tangi Morvan, Irénée Frérot et al. (+1 more)
We demonstrate that the dynamical phase transition of the
quantum $\mathcal{O}(N)$ model at large $N$ leaves universal fingerprints in the infrared structure of the
entanglement spectrum. While the leading contribution to the
entanglement entropy at long time follows the conventional volume law associated with ballistic
entanglement spreading, its subleading behavior sharply distinguishes the different dynamical regimes. Specifically, quenches at and below the critical point generate gapless low-energy
entanglement modes together with logarithmic corrections to the long-time
entanglement entropy, whose scaling is governed by the scaling exponent of the transition. Using an infinite-slab bipartition geometry and exact numerical correlation functions in the large-$N$ limit, we characterize these scaling laws across the dynamical phase diagram and relate them to the emergence of long-range correlations during the post-quench dynamics. We further show that the
entanglement eigenmodes themselves reveal characteristic signatures of the dynamical phase transition through their spatio-temporal structure and degeneracy properties.
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All authors Frederick del Pozo, Tangi Morvan, Irénée Frérot, Nicolas Cherroret
Disordered Systems and Neural Networks · Cross-listed from stat.ML
Asymmetric Scaling Laws from Sparse Features
John Sous and Michael Winer
We introduce a model for neural scaling laws under sparse activations. In the model, test loss is often dominated by rare coordinates that are never observed in the training input. This mechanism induces a novel bottleneck absent from dense models. We derive the asymptotic population loss in both the underparameterized and overparameterized regimes, and show that the loss exhibits a double-descent peak near the interpolation threshold – where the number of parameters is just sufficient to fit the training data – resulting in a loss curve governed by two distinct scaling exponents – one for the overparameterized regime and one for the underparameterized regime – with a gap determined by the degree of sparsity. Additionally, we derive a compute-optimal frontier that favors increasing
dataset size over model capacity under fixed compute budgets. We also analyze gradient-descent dynamics and identify a scaling law for the probability that fixed-step gradient descent becomes unstable. We further show that the sparsity-induced effect persists under nonlinear activations.
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Mesoscale and Nanoscale Physics
Momentum-Resolved Tunneling Modulation Induced Giant Multistate Resistance in Antiferroelectric Multiferroic Junction
Wei Yang, Yibo Xu, Shen Li et al. (+6 more)
Multiferroic tunnel junctions (MFTJs), integrating ferroelectric and
ferromagnetic functionalities within a single nanoscale device, hold significant promise for non-volatile, multi-state
memory and innovative computing paradigms. In conventional MFTJs, tunneling resistance modulation relies primarily on ferroelectric (FE) polarization switching, which alters interfacial electric fields and shifts the
Fermi level of adjacent
ferromagnetic electrodes. However, achieving high tunnelelectroresistance (TER) through this approach demands strong built-in electric fields, which simultaneously hinder FE polarization switching, creating an intrinsic trade-off between reliable data reading and efficient writing. Here, we propose a dual mechanism that combines antiferroelectric (AFE) phase-transition modulation of the evanescent decay states with interfacial
spin filtering based on $Fe_3GaTe_2$/
bilayer-$In_2Se_3$/$Fe_3GaTe_2$
heterostructure. Beyond altering the electrostatic potential as in AFE-FE switching, the transitions between head-type and tail-type AFE states preserve the centrosymmetric potential profile yet fundamentally modulate the momentum-resolved distribution of evanescent decay rates across the Brillouin zone. When integrated with perfect
spin filtering at the $Fe_3GaTe_2$/$\alpha$-$In_2Se_3$ interface, this mechanism yields a
giant TER (~$7.6\times10^3\%$), over 4 times that of conventional FE-based MFTJs, and a TMR exceeding $6.8\times10^5\%$, enhanced by two orders of magnitude over typical MFTJs. These mechanisms resolve the performance trade-off in MFTJs, enabling six distinct non-volatile resistance states at room temperature.
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All authors Wei Yang, Yibo Xu, Shen Li, Jiangchao Han, Jiayou Chen, Juan-Carlos Rojas-Sánchez, Stéphane Mangin, Xiaoyang Lin, Weisheng Zhao
Superconductivity
Phase diagram of the vortex state in an amorphous Re6Zr thin film exhibiting inverse melting
Pritam Das, Subhamita Sengupta, Anjan Jana et al. (+6 more)
In Type II
superconductors, the
vortex lattice can exhibit "inverse melting," transitioning from a
liquid to a crystalline solid as temperature increases. While recently observed via scanning tunneling microscopy in a 20 nm thick amorphous Re6Zr thin film, this work investigates the corresponding d.c. transport and low-frequency
magnetic screening responses. By identifying distinct signatures of these transitions and integrating scanning tunneling spectroscopy imaging, we construct a comprehensive
vortex-state phase diagram in the
magnetic field-temperature parameter space. Furthermore, we demonstrate that inverse melting is thickness-dependent: a 5 nm film retains an inhomogeneous
liquid state, while a 50 nm film maintains a crystalline solid structure except near the upper critical field.
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All authors Pritam Das, Subhamita Sengupta, Anjan Jana, Rishabh Duhan, Sulagna Dutta, Arghya Dutta, John Jesudasan, Vivas Bagwe, Pratap Raychaudhuri
Materials Science
Ambiguity in B-Site cation ordering: A Case study of the double perovskite Ca$_2$CoNbO$_6$
Svetlana A. Artiukova, Ivan V. Yatsyk, Ruslan G. Batulin et al. (+6 more)
The ordering of cations in the B sublattice remains a challenging issue in double perovskites. In this work, a combined experimental and theoretical approach was employed to investigate the Co/Nb distribution in Ca$_2$CoNbO$_6$ and its influence on
magnetic and transport properties. The density functional theory, supported by
magnetic susceptibility measurements, indicates that Co adopts a high-
spin Co$^{3+}$ state. No long-range
magnetic ordering was observed down to low temperatures; however, the presence of short-range correlations points to the partial disorder in the Co/Nb sublattice. This interpretation is further supported by electron paramagnetic resonance, which also reveals slight oxygen nonstoichiometry. Electrical transport follows a small-polaron hopping mechanism with an activation energy of 0.25 eV. The Seebeck coefficient reaches 0.4 mV/K at 600 K.
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All authors Svetlana A. Artiukova, Ivan V. Yatsyk, Ruslan G. Batulin, Yulia A. Deeva, Tatiana I. Chupakhina, Vladislav V. Bazhal, Alexandr I. Balitskiy, Rushana M. Eremina, Dina I. Fazlizhanova
Mesoscale and Nanoscale Physics
Commensuration torques in double-moiré twisted trilayer hexagonal boron nitride and graphene heterostructures
Youngju Park, Nicolas Leconte, Prathap Kumar Jharapla et al. (+3 more)
We study commensuration-driven torques and angle locking in double-moiré trilayer hexagonal boron nitride (hBN) and
graphene heterostructures using large-scale atomistic relaxations. In twisted trilayer hBN (t3BN) homostructures, double-moiré commensuration ($\theta_{12} = -\theta_{23}$) give rise to local energy minima accompanied by torque sign reversals, signaling a restoring tendency toward the commensurate configuration. The corresponding binding energies are $\sim$0.2-0.3 meV/atom, originating from enhanced overlap of low-energy stacking domains, although the system is globally stable at zero twist. In contrast, in
graphene/hBN heterolayers systems the global energy minimum can coincide with the double-moiré commensuration angle, particularly near $\sim$0.6$^{\circ}$, reflecting competition between
lattice mismatch and interfacial relaxation. Incommensurate atomic structures have reduced stabilization due to suppressed overlap of low-energy stacking and have enhanced superlubricity due to spatial averaging of interfacial energies. These results establish double-moiré commensuration as a general, system-dependent mechanism for twist-angle stabilization, whose angular stability is characterized by the torque magnitude and binding energy. Coulomb electrostatic interactions further enhance the stabilization energy without changing the underlying physics.
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All authors Youngju Park, Nicolas Leconte, Prathap Kumar Jharapla, Md Shaifullah, E. H. Hwang, Jeil Jung
Materials Science
Nonequilibrium dynamics of high energy transitions in monolayer WSe$_{2}$
Oleg Dogadov, Jorge Cervantes-Villanueva, Nicholas Olsen et al. (+6 more)
High-energy optical transitions in monolayer transition-metal dichalcogenides exhibit characteristics that are markedly distinct from those of lower-lying band-edge
excitons. These differences arise from the involvement of electronic states located at regions of the Brillouin zone that are displaced from the $K$ valleys. In this work, we investigate the ultrafast dynamics of these high-energy excitations by employing broadband ultrafast transient absorption spectroscopy spanning the visible to ultraviolet spectral range. We observe that the formation and relaxation dynamics of one of the high energy transitions display a distinct behavior compared to the lower-energy excitonic resonances, developing on a significantly slower timescale. First-principles calculations of the excitonic landscape allow us to account for this delayed response and attribute it to the
phonon-mediated formation of momentum-dark
excitons.
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All authors Oleg Dogadov, Jorge Cervantes-Villanueva, Nicholas Olsen, Chiara Trovatello, Xiaoyang Zhu, Giulio Cerullo, Alejandro Molina-Sánchez, Davide Sangalli, Stefano Dal Conte
Materials Science
Intrinsic Point Defects and Frenkel Pair Formation in Photovoltaic Absorber Zn$_3$P$_2$: Regulating $p$-type Conductivity through Growth and Annealing Conditions
Nico Kawashima and Silvana Botti
This study investigates the ground-state energetics and thermodynamics of intrinsic point defects in zinc phosphide Zn$_3$P$_2$ using ab initio density functional theory combined with an extensive potential energy landscape search. Our analysis reveals that the defect chemistry is dominated by zinc vacancies $V_\mathrm{Zn}$ and zinc interstitials Zn$_i$, with
equilibrium concentrations significantly surpassing those of other intrinsic species. Notably, we find that phosphorus interstitials P$_i$, previously suggested to be significant, possess high formation energies and likely exist only in negligible quantities. The characteristic $p$-type conductivity of undoped Zn$_3$P$_2$ is shown to be a direct consequence of zinc vacancies, which act as shallow acceptors and pull the
Fermi level toward the valence band. Furthermore, we identify a positive binding energy between $V_\mathrm{Zn}$ and Zn$_i$, leading to the formation of electrically benign Frenkel pairs that partially compensate the intrinsic p-type conductivity. Our results suggest that achieving $n$-type conductivity is fundamentally limited by these thermodynamic constraints. We conclude that hole densities can be optimized through phosphorus-rich growth conditions and high-temperature annealing, and suggest that future photovoltaic strategies should prioritize interface engineering over bulk $n$-type doping.
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Disordered Systems and Neural Networks · Cross-listed from cs.LG
Non-normal spectral signatures of instability in neural network training dynamics
Souvik Ghosh
Training instabilities in deep networks - loss spikes, oscillatory convergence, and gradient pathologies - are empirically prevalent but lack a rigorous operator-theoretic explanation. We show that the linearized update operators for practically used optimizers are generically non-normal: for Adam, non-normality is controlled by the commutator [H, M] between the Hessian and the diagonal adaptive preconditioner, while for SGD with momentum it arises from the augmented state-space structure of the update map. Applying non-normal stability theory to these operators, we derive a conservative pseudospectral precursor bound in which \kappa(V) serves as an early-warning indicator of transient amplification even when the spectral radius remains below one, and we establish that exceptional points of the update operator appear as the \kappa(V) -> \infty limiting case of this framework. Numerical experiments on two-layer networks confirm that the spectral radius \rho(J) provides no separation between stable and unstable training phases while \kappa(V) separates them by approximately one order of magnitude, complementing the classical sharpness criterion with a continuous severity measure of non-normal amplification. These results establish non-Hermitian operator theory as a useful and underexplored framework for neural network optimization stability, offering a diagnostic
language and proof-of-concept
benchmark for understanding adaptive optimization stability.
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Materials Science
Self-organized formation of step-terrace structure in SrRuO3 thin films grown on mixed-terminated SrTiO3 (100) substrates
Ryotaro Arakawa, Sachio Komori, Kotaro Tomita et al. (+3 more)
Surface morphology of the substrate and bottom layers plays a critical role in the epitaxial growth of
oxide thin films. Here, we report on the self-organized formation of a step-terrace structure in SrRuO3 (SRO) thin films grown using pulsed
laser deposition on mixed-terminated SrTiO3 (100) substrates without any prior surface treatment. Atomic force microscopy observations reveal that SRO films initially grow in a three-dimensional island mode and subsequently undergo a transition to a step-flow growth mode through island coalescence as the film thickness increases, resulting in a well-defined step-terrace morphology with a step height consistent with the SRO unit-
cell parameter. The average terrace width of the self-organized structure can be systematically tuned by varying the substrate temperature and the target-substrate distance, which we attribute to changes in the critical island radius that governs the nucleation behavior. To demonstrate the utility of this self-organized morphology, we show that BiFeO3 thin films grown on SRO films with such a step-terrace structure exhibit improved surface flatness and crystalline quality compared to those grown directly on bare SrTiO3 substrates. These findings provide a clear understanding of the mechanism of thickness-driven growth-mode transitions in perovskite
oxide thin films under various growth conditions.
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All authors Ryotaro Arakawa, Sachio Komori, Kotaro Tomita, Shunsei Komori, Masaaki A. Tanaka, Tomoyasu Taniyama
Materials Science
Symmetry-protected nodal planes and accidental nodal surfaces in mixed odd-even wave spin-momentum locking of relativistic altermagnets
Xujia Gong, Amar Fakhredine, Sahar Izadi Vishkayi et al. (+1 more)
Non-relativistic
spin–momentum locking in altermagnets exhibits an even number of nodal planes. In the relativistic limit, the number of nodal planes can be lowered by
symmetry reduction due to the Néel vector and
spin–orbit coupling in noncentrosymmetric systems. Therefore, an analysis of the
evolution of the nodal planes in relativistic altermagnets is required. While $g$-wave
spin–momentum locking is straightforward to realize in non-relativistic altermagnets, this $g$-wave does not necessarily survive in the relativistic case. In this work, we investigate the relativistic
spin–momentum locking of the centrosymmetric CrSb and the noncentrosymmetric wurtzite MnTe. As a first result, we show that in both systems the dominant
spin component retains its $g$-wave character in the relativistic regime only when the Néel vector is oriented along the $z$-axis, while the subdominant components exhibit $d$-wave
symmetry in CrSb and $p$-wave
symmetry in ferroelectric wurtzite MnTe. More generally, the $g$-wave character is preserved in the relativistic limit only when both the Néel vector and the electric field associated with inversion-
symmetry breaking are oriented along the $z$-axis. As a second result, we show that relativistic
spin–momentum locking of ferroelectric altermagnets can exhibit $p$-wave magnetism with one
symmetry-protected nodal plane and an accidental nodal surface not protected by
symmetry, or can have two accidental nodal surfaces. With the Néel vector aligned along the $x$-axis, selected bands of ferroelectric altermagnet wurtzite MnTe exhibit $p$-wave magnetism. Our results establish that altermagnets can host distinct
spin components that realize a mixture of angular-momentum wave
symmetries in momentum space in the relativistic limit.
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All authors Xujia Gong, Amar Fakhredine, Sahar Izadi Vishkayi, Carmine Autieri
Disordered Systems and Neural Networks
Analysis of spin avalanches due to interplay of disorder and temperature
Niharika Bhuyan and Diana Thongjaomayum
The nonequilibrium zero-temperature Random Field Ising Model (RFIM) has been extensively studied to understand critical response and avalanches in disordered driven systems. The emergence of power-law behaviour is observed over a wide region around the critical point. These studies however, are confined to zero-temperature dynamics. We study the role of temperature, which is inevitable in real experiments, in the context of RFIM on triangular
lattices. We explore the interplay of different parameters: temperature, random field strength, and relaxation time which affect the prevalence of power-law behaviour on the
lattice. The results indicate that power-law survives only in the regime of low temperature or small and intermediate disorder. Variations in temperature and disorder have similar affects on the avalanche-size distribution, indicating their strong correspondence. We also discuss the process of blurring out of the power law on increasing temperature or disorder.
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Mesoscale and Nanoscale Physics
Generalized Shift Vector as the Intrinsic Dipole of Many-Body Correlated Electronic States
Jiaming Hu, Sudipta Kundu, Zhichao Guo et al. (+4 more)
Shift vectors play a central role in nonlinear optics and transport phenomena, where they are usually understood as charge-center shifts associated with transitions between
quantum states. Here we show that the same
geometric structure can be more fundamentally understood as the intrinsic dipole moment of a single correlated state. Our derivation clarifies the local and global aspects of gauge invariance, the origin of the phase-gradient term, and its connection to the internal coherence structure of many-body correlations. The single-state shift character appears both as a displacement of the real-space joint probability density and as a linear electric-field modification in energy space. Applying this framework to optically induced correlations, electron-
phonon-mediated processes, and excitonic electron-hole states, we recover previously proposed shift vectors and the standard expression for the shift current as special cases. Our results establish a common physical foundation for shift vectors as intrinsic dipolar properties of correlated electronic states.
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All authors Jiaming Hu, Sudipta Kundu, Zhichao Guo, Joshua J.P. Thompson, Wenbin Li, Hua Wang, Bartomeu Monserrat
Soft Condensed Matter
Nonreciprocal surface tension: anisotropy-induced defect motility and organization
Laya Parkavousi and Suropriya Saha
We show that interfacial nonreciprocity transforms defect dynamics in conserved scalar fields within the framework of the Nonreciprocal Cahn-Hilliard model. Nonreciprocal surface tension alone produces intermittently stable defects: system-spanning target patterns form, lose stability, self-destruct, and nucleate again from a defect-chaotic state. When bulk and interfacial contributions interplay in a particular way, the system forms a distinct mosaic-wave state: traveling waves remain coherent within finite domains demarcated by linear arrangements of motile dislocations, which act as lines of phase slip. Mosaic-waves exhibit scale-free fluctuations at length scales much larger than the average wavelength of the traveling patterns. To explain the wide range of emergent dynamics, we construct the dynamics of the Goldstone-mode. The nonlinearities governing its large-scale fluctuations belong to the anisotropic Kardar-Parisi-Zhang universality class, with the sign of the nonlinear anisotropy controlling the nature of the out-of-
equilibrium dynamics.
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