Statistical Mechanics
Invariant measures of exclusion processes with a look-ahead rule
Lam Thi Nhung, Ngo Phuoc Nguyen Ngoc and Huynh Anh Thi
We study a one-dimensional exclusion process with a fixed jump length $I \ge 1$ in which a particle may advance or retreat $I$ sites provided all intermediate sites are vacant, with hopping rates of Arrhenius type depending on the local headway. We identify the class of rates admitting an explicit Ising-Gibbs invariant measure, with stationarity governed by pairwise balance rather than detailed balance. In the thermodynamic limit, we derive a closed-form stationary current that recovers the mean-field prediction for look-ahead traffic-flow models exactly when particles are uncorrelated, and quantifies the correlation-induced correction otherwise. The resulting fundamental diagram is right-skewed and non-concave, as in empirical traffic data, and for a preferred-spacing interaction-where the stationary headway distribution becomes bimodal-the mean-field approximation fails qualitatively. The closed-form current is confirmed by kinetic Monte Carlo simulations and by exact finite-size computations.
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Materials Science
Understanding ultrafast x-ray 'echoes' diffracted from single crystals
Angel Rodriguez-Fernandez, Dmitry Karpov, Steven Leake et al. (+2 more)
Multiple x-ray beams generated by interference processes in perfect
crystals were imaged with a resolution of about 100nm using tele-ptychography in the diffraction direction. These multiple wave-fields, also known as x-ray diffraction echoes, are related to the process known as the Pendelloesung effect and are described by dynamical diffraction theory. The echoes are produced by the constructive interference of diffracted x-rays at the exit surface of the
crystal sample. In the imaged diffraction peak, we observed 10 echoes maxima with a total signal length of 78 um. Which translates into a total temporal delay in the signal of less than 108 fs.This makes the echoes of high importance for x-ray optics at x-ray Free Electron
Laser sources, as the effect could be used for future ultrafast x-ray beam splitters. In addition to this application, echoes can be exploited to follow ultrafast processes in single
crystal micro-structures such as melting or strain propagation.
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All authors Angel Rodriguez-Fernandez, Dmitry Karpov, Steven Leake, Dina Carbone, Ana Diaz
Quantum Gases
Sound propagation in striped supersolid cold gases at zero temperature
Elena Poli, Giovanni I. Martone, Sandro Stringari et al. (+1 more)
We present a unified hydrodynamic approach for the sound propagation in the stripe phases realized in ultracold dipolar gas and
spin-orbit-coupled BEC platforms at zero temperature. Despite the deep difference of the two platforms at a microscopic level, a similar hydrodynamic description can be formulated at a macroscopic level. The main difference between the two platforms is the lack of Galilean invariance in the
spin-orbit case, resulting in a different identification of the normal (nonsuperfluid) component of the density, which leads to new terms in the equation for the current. In both cases the spectrum comprises two sounds, reflecting the spontaneous breaking of the U(1) and translational
symmetries. Both sounds exhibit an anisotropic behavior. A comparison with the first and second sounds of the smectic-A
liquid crystal is also presented.
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All authors Elena Poli, Giovanni I. Martone, Sandro Stringari, Alessio Recati
Mesoscale and Nanoscale Physics
Coupled dynamical Boltzmann transport equations with long-range electron-phonon and electron-electron interactions in 2D materials
Francesco Macheda and Thibault Sohier
We study the interplay between long-range electron-
phonon and electron-electron interactions in electrostatically doped two-dimensional
semiconductors, including interlayer couplings in van der Waals
heterostructures. We evaluate the effects of those interactions on transport properties by writing dynamically coupled Boltzmann equations for the electrons and for the electrodynamically active excitations. We develop a theory with a general validity, and apply it both to simplified
parabolic models, and to the realistic BN-encapsulated
graphene system which we present in an accompanying paper [arXiv:2604.00678]. We show that dynamical screening effects are of fundamental importance in order to correctly describe the electronic transport properties of two-dimensional materials, and in particular the scattering from polar
phonons, whether those come from the
semiconductor itself or the surrounding layers.
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Strongly Correlated Electrons
Sign-Free Evidence for a d-Wave Superfluid Stiffness Dome in the Doped Hubbard Model
Xidi Wang and H. Q. Lin
We construct an effective single-particle
Hamiltonian $K_{\mathrm{eff}}$ from Monte Carlo–averaged
matrix logarithms of the imaginary-time propagator in determinant
quantum Monte Carlo (DQMC). The logarithm maps the multiplicative sign problem into an additive framework where the central limit
theorem guarantees convergence, rendering $K_{\mathrm{eff}}$ sign-problem-free: both sign sectors yield identical dispersions to $<1\%$. $K_{\mathrm{eff}}$ captures the exact correlated single-particle spectrum, incorporating all self-energy effects non-perturbatively. Applied to the Hubbard model ($t'/t = -0.30$, $U/t = 4$), $K_{\mathrm{eff}}$ reveals a $d$-wave pseudogap with strong nodal-antinodal dichotomy below a computational phase transition at $T^*$. Three sign-free observables provide evidence consistent with
spin-fluctuation pairing: (i) the gap ratio $R_g > 1$ confirms $d$-wave
symmetry – a temperature-independent property of the correlated band structure that provides the medium for pairing; (ii) the superfluid stiffness $\rho_s$ forms a dome across doping at $L = 8$, $10$, and $12$, exceeding the Berezinskii-Kosterlitz-Thouless threshold by $5$-$7\times$ at the dome peak; (iii) $S(\pi,\pi)$ is approximately flat across doping, establishing that the dome originates from
Fermi-surface geometry responding to uniform
spin-fluctuation glue. The pseudogap grows monotonically toward half-filling while $\rho_s$ forms a dome, mirroring cuprate phenomenology where $T_c$ is limited by the superfluid density (Uemura relation). Vertex corrections remain to be quantified.
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Materials Science
Electrochemical doping in H-terminated diamond films: Impact of O-functionalization and insights from in-situ Raman spectro electrochemistry
N. Mohasin Sulthana, P.K. Ajikumar and K. Ganesan
The p-type surface conductivity of H-terminated diamond (HD, H-diamond) has created new path ways for developing diamond based electronic devices as well as chemical and bio-sensors. However, the hydrophobic nature of the HD surface can negatively impact device performance due to its low wettability. Herein, we report the study on
polymer electrolyte-gated field effect transistors (EGFETs) fabricated using pristine and partially O-terminated HD films. The HD surface is transformed from hydrophobic to moderate hydrophilic by partial O-termination. Also, the sheet resistance of the HD surface increases from 7.6 to 18.7 k-Ohms per sq. while the sheet hole density decreases from 10.5 to 4.8 x 10^12 cm^-2 upon partial O-termination. Consequently, the ON - OFF ratio of the EGFET devices decreases from ~ 40 to 14 and the maximum transconductance declines from of -150 to -7.9 micro-seimens per V, but the areal capacitance increases from ~ 7.8 to 27.1 microFarad per cm^2 with partial ozonation on HD surface. In addition the in situ Raman measurements in HD EGFET provide direct experimental evidence of a gating-induced blue shift and linewidth broadening of the diamond Raman band which are associated with strong electron
phonon coupling. This work highlights the significant impact of the partial O-termination on the performance of the HD EGFET devices and effect of electrochemical gating on the
phonon behaviour of the H-diamond.
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Strongly Correlated Electrons
Bond-density-wave orders induced by geometric frustration in the kagome metal CeRu3Si2
Ryo Misawa, Shunsuke Kitou, Rinsuke Yamada et al. (+9 more)
Geometric frustration gives rise to vast
manifolds of degenerate ground states and competing orders in
spin and charge systems. Typically, classical ground states are governed by a local “zero-sum constraint" that relieves frustrated
antiferromagnetic interactions or Coulomb repulsion. To date, the paradigm of
geometric frustration has yielded a rich landscape of emergent phases, from
spin ices and
quantum spin liquids to charge glasses. However, an analogous phase rooted in chemical bonding has yet to be firmly demonstrated. Here we report the discovery of bond-density-wave orders induced by
geometric frustration in the kagome metal CeRu$_3$Si$_2$ above room temperature. Through synchrotron X-ray diffraction, real-space transmission electron microscopy, and model calculations, we observe two distinct long-period superlattices with harmonic and anharmonic structural modulations. Crucially, interlayer bonds between kagome planes modulate in a sublattice-selective manner to fulfill the zero-sum constraint on the kagome
lattice. We demonstrate the potential of kagome metals to host complex bond-ordered states constrained by
geometric frustration and establish chemical bonding as a distinct pathway to frustration physics in
quantum materials even above room temperature.
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All authors Ryo Misawa, Shunsuke Kitou, Rinsuke Yamada, Xiaolong Feng, Ryota Nakano, Priya Ranjan Baral, Yuiga Nakamura, Leslie M. Schoop, Yukitoshi Motome, Taka-hisa Arima, Xiuzhen Yu, Max Hirschberger
Materials Science
Quantum-Information Measure of Electron Localization
Stefano Pittalis, Filippo Troiani, Celestino Angeli et al. (+2 more)
Understanding electron localization in molecules and materials plays a central role in electronic structure theory, and will increase in importance with the rise of data-driven approaches. The electron localization function (ELF) is widely used to visualize electron organization in molecules and materials, and it remains a central ingredient in modern density-functional approximations. Yet its formulation retains highly empirical elements. Here we introduce a
quantum-information measure of electron localization derived from the concurrence of a correlated two-
spin mixed state. This construction yields a genuine two-point localization indicator grounded in
quantum-information theory, avoiding the heuristic normalization and chosen nonlinear remapping of the ELF. We show that atomic shells, covalent and ionic bonds, lone pairs, molecular dissociation, and charge-transfer processes are captured. The method is straightforward to evaluate numerically.
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All authors Stefano Pittalis, Filippo Troiani, Celestino Angeli, Irene D'Amico, Tim Gould
Disordered Systems and Neural Networks
Isolated extended states and anomalous critical behavior in the generalized SSH model
Jia-Ming Zhang
We investigate the localization properties of a generalized SSH model. Numerical and analytical results indicate the emergence of extended states protected by unbounded hopping in this model. Moreover, this protection effect is disrupted by the appearance of generalized incommensurate zeros, causing the extended phase in the system to transition into a multifractal phase. However, at the boundaries of the phase region, we still observe the existence of extended states. These extended states coincide with multifractality-enriched mobility edges, separating the multifractal phase from the localized phase. Further analysis reveals that this extended states originates from the band edge states of SSH model. In addition, these isolated extended states also influence eigenstates with nearby energies, giving rise to an anomalous extended-to-multifractal critical transition. These findings not only enrich the behavioral repertoire of eigenstates at critical points, but also offer new insights for further understanding Anderson localization and the induction of multifractal phases.
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Materials Science
Benchmarking Chemically Scalable Machine-Learning Interatomic Potentials for Large-Scale Simulations of Multicomponent Alloys
Fei Shuang, Penghua Ying, Kai Liu et al. (+5 more)
Machine learning interatomic potentials (MLIPs) with broad chemical flexibility are essential for atomistic simulations of compositionally complex
alloys, but their deployment in large-scale molecular dynamics requires a balance among accuracy, efficiency, stability, transferability, and uncertainty quantification. Here, we
benchmark two chemically scalable MLIP frameworks, neuroevolution potential (NEP) and graph atomic cluster expansion (GRACE), for 16 elemental metals and their multicomponent
alloys. GRACE-FS shows higher training efficiency and generally better average accuracy, chemical transferability, and finite-temperature robustness, whereas UNEP-v1 provides substantially higher inference speed and remains competitive in selected stress and large-error metrics. We further show that chemical transferability is closely linked to high-temperature MD stability in highly multicomponent environments and that ensemble-based uncertainty provides a more reliable error indicator than D-optimality for the heterogeneous systems considered here. Finally, three-million-atom shock simulations demonstrate that UNEP-v1, combined with ensemble uncertainty, enables uncertainty-aware simulations under extreme dynamic conditions, yielding robust global spall-strength predictions while revealing model sensitivity in local damage pathways. These results provide practical guidelines for selecting and deploying MLIPs in large-scale simulations of multicomponent
alloys.
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All authors Fei Shuang, Penghua Ying, Kai Liu, Zixiong Wei, Fengxian Liu, Zheyong Fan, Minqiang Jiang, Poulumi Dey
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Mechanism for scale-free skin effect in one-dimensional systems
Shu-Xuan Wang and Zhongbo Yan
Non-Hermitian skin effect is one of the most captivating phenomena in non-Hermitian systems, characterized by the extensive localization of eigenstates near open boundaries. In its conventional form, the localization length of a skin mode is independent of the system size. Remarkably, however, when the open boundaries are coupled to realize a generalized boundary condition, the localization length can undergo a drastic transformation, becoming proportional to the system length. This intriguing regime is known as the scale-free skin effect (SFSE). Although SFSE has been observed in numerous one-dimensional non-Hermitian models through case-by-case studies, a unified theoret ical framework capable of predicting its emergence and characteristic properties remains absent. In this work, we take a firm step forward by establishing a model-independent framework for the ana lytic determination of scale-free localization lengths in certain regimes. Our key insight is to treat generalized boundary conditions as a perturbation of the periodic boundary conditions, thereby circumventing the singular response-to-perturbation that would otherwise arise if they were pertur batively treated relative to open boundary conditions. Our work sheds new light on understanding SFSE in non-Hermitian systems.
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Superconductivity
Xiaolin Wan, Zheng Qin, Fangyang Zhan et al. (+3 more)
The interplay between magnetism and superconductivity provides a fertile ground for engineering exotic topological phases, while dynamical control via periodic driving offers a unique avenue to access
quantum states that are inaccessible in static
equilibrium. Here, we propose a strategy to achieve the Floquet
chiral topological superconductivity in an altermagnet-
superconductor heterostructure driven by elliptically polarized light. We show that for $s$-wave pairing, the system undergoes a transition from a trivial to a
chiral topological superconducting phase. More strikingly, with the introduction of mixed $s+d$-wave pairing, we find that the system can access Floquet
chiral topological superconducting phases with highly tunable Chern numbers up to N=4. These exotic phases are attributed to the intertwining of altermagnetism, superconducting pairing, and the periodic driving field. Our work establishes the light-driven altermagnetic
heterostructure as a versatile platform for exploring and manipulating high-Chern-number
chiral topological superconductivity.
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All authors Xiaolin Wan, Zheng Qin, Fangyang Zhan, Junjie Zeng, Dong-Hui Xu, Rui Wang
Statistical Mechanics
Symmetry-Informed Term Filtering for Continuum Equation Discovery
Junya Yokokura and Kazumasa A. Takeuchi
Discovering governing equations, whether manually or by data-driven methods, has been central in physics and related areas. Since governing equations are typically constrained by a set of
symmetries, using
symmetry constraints to restrict terms is usually the first step in manually formulating a governing equation, but it often becomes intractable for complex systems with high-order derivatives or multiple fields. When a data-driven method is used, on the other hand, imposing physical constraints such as
symmetries typically requires manual preprocessing or computationally expensive iterative procedures. Here, we propose an algebraic filtering method that enumerates all
symmetry-allowed terms for continuum equations within a finite candidate space. By treating
symmetry generators as linear operators on the candidate space, we reduce the problem of enforcing both discrete and continuous
symmetries to solving a set of linear kernel equations. The solution yields a provably complete list of permitted terms. We demonstrate the method's effectiveness by identifying invariant terms for systems with dihedral
symmetry and recovering the governing equations for the Toner–Tu and Kardar–Parisi–Zhang systems, including higher-order terms useful for extending known models. The method provides a systematic way to obtain a
symmetry-allowed search space for data-driven equation discovery, e.g., the sparse identification of nonlinear dynamics method.
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Materials Science
Abrupt crystallization from shock-compressed CaSiO3 glass
A. Amouretti, K. Nonaka, X. Liu et al. (+13 more)
We have performed in situ time-resolved X-ray diffraction at ~100 GPa on
laser-shocked CaSiO3 glass to investigate the glass-to-
crystal transition. At this extreme pressure, we observe the ultrafast crystallization of the CaSiO3 perovskite structure from the compressed amorphous phase, with a typical nucleation time of 1.69 +/- 0.10 ns and a final grainsize of ~20 nm. The grain size temporal
evolution suggest a
diffusion controlled transformation. Moreover, the observed concomitant explosive grain growth together with the release wave arrival into shocked CaSiO3 also suggests a role of the release in the nucleation process.
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All authors A. Amouretti, K. Nonaka, X. Liu, Y. Hironaka, H. Huang, R. Kodama, K. Lawler, K. Miyanishi, H. Nakamura, C. Schwartz, Y. Seto, K. Sueda, Y. Wu, M. Yabashi, T. Yabuuchi, N. Ozaki
Materials Science
Magnetoelectric Coupling in Nickel-Cobalt Ferrite and Lanthanum Ferrite Heterostructure Composites: Experimental Evidence and Simulation-Driven Insights
Manjeet Seth
A material that reveals two or more ferroelectric properties at the same time is called multiferroic materials. The most commonly multiferroic materials shows ferroelectricity and ferromagnetism property within a single phase. Accordingly such materials can possess both spontaneous magnetization and electric polarization and which can be individually controlled through external electric or
magnetic fields. The unique coexistence of ferroelectric properties opens up possibilities for innovative applications, including
memory devices, sensors, and actuators that are responsive to both electrical and magneticstimuli. Multiferroics gives a valuable foundation for generating cutting edge multifunctional devices, exhibiting their versatility across broad area technological fields. Applications include sensors, transducers, spintronics, terahertz emitters, miniature
antennas, energy harvesting, multiple state
memory storage, electric field controlled
ferromagnetic resonance devices, and nanoscale electronics. Researchers are continually work to discovering new materials exploring the fundamental mechanisms involved, and optimizing their performance for various application. Multiferroic materials has a promising area for innovation and exploration the field of advance technologies this type of materials contributing not only to device development but also enhancing the understanding of the interactions between ferroelectric and
magnetic orders. Additionally, magnetoelectric composites is a type of multifunctional material showing strong coupling between
magnetic and electric properties for further technological applications. Magnetoelectric (ME) composites are synthesis by combining separate
magnetic and electric materials to produce unique functionalities through their interactions. This coupling enables the control of
magnetic properties by an electric field and vice versa.
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Mesoscale and Nanoscale Physics
Time-evolving matrix product operators for off-diagonal system-bath coupling
Chu Guo, Wei Wu, Xiansong Xu et al. (+3 more)
The time-evolving
matrix product operator (TEMPO) method has proven to be a powerful method to study the long-time dynamics of bosonic
impurity problems where a small system is linearly coupled to a noninteracting bosonic bath. However, current developments of TEMPO have mostly focused on the case of diagonal system-bath coupling, i.e., $\sum_k \Aop(V_k \bdop_k + \hc)$, with $\Aop$ a Hermitian operator of the system. Based on the process tensor framework, we extend TEMPO to the more general case of off-diagonal system-bath coupling in the form $\sum_k (V_k\Aop\bdop_k + \hc)$, where $\Aop$ could be non-Hermitian. As applications, we study the real-time dynamics of a
spin that is coupled to a sub-ohmic bath via the Jaynes-Cummings-type system-bath coupling and compare it against the standard
spin-
boson model, where we show that the commonly used rotating-wave approximation could be very poor for this bath. We also study the imaginary-time
evolution of a bosonic
impurity with nonzero on-site interaction that is coupled to a sub-ohmic bath, to illustrate the flexibility of our method. Our method provides a unified framework to understand different variants of TEMPO, and is a promising building block for an
impurity solver in the bosonic dynamical mean field theory for the normal phase with a scalar hybridization function.
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All authors Chu Guo, Wei Wu, Xiansong Xu, Tian Jiang, Ping-Xing Chen, Ruofan Chen
Statistical Mechanics
The Mpemba effect likes to hit a wall
Yue Liu, Tan Van Vu, Raphaël Chétrite et al. (+2 more)
The original Mpemba effect refers to the idea that a macroscopic hot system cools faster than an initially colder one. In its microscopic and classical versions, the system is modeled as an overdamped particle in an external potential, and the corresponding Mpemba effect has been observed experimentally and explored theoretically. We establish that the existence of the one-dimensional Mpemba effect for a particle in an asymmetric
polynomial double-well potential is driven solely by the presence of a hard wall, irrespective of the potential's double-well shape and metastability. The Mpemba effect disappears if we consider an infinitely large system. We then show that the underlying mechanism of the Mpemba effect is governed by the fine structure of the initial statistical population as it probes the tails of the potential, which also explains the Mpemba effect in single-well and symmetric double-well potentials.
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All authors Yue Liu, Tan Van Vu, Raphaël Chétrite, Frédéric van Wijland, Hisao Hayakawa
Statistical Mechanics
Anomalous scaling in redirection networks
Harrison Hartle, P. L. Krapivsky, S. Redner et al. (+1 more)
In networks that grow by isotropic redirection (IR), a new node selects an initial target node uniformly at random and attaches to a randomly chosen neighbor of the target. The emerging networks exhibit leaf proliferation, in which the number of nonleaves scales sublinearly as $N^\mu$ and the degree distribution has an algebraic tail with exponent $1+\mu$. To understand these mysterious properties, we introduce a class of models with redirection to leaves whenever possible. The resulting networks exhibit qualitatively similar phenomenology to IR networks, but avoid the inherent non-locality of the IR growth rule. These networks admit an analytical description of the leaf degree distribution, from which we extract the exponent $\mu$.
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All authors Harrison Hartle, P. L. Krapivsky, S. Redner, Yuanzhao Zhang
Materials Science
Precipitate-Induced Dynamic Strain Aging and Its Effect on the Strain Rate Sensitivity of Precipitation Hardened Aluminum Alloys
Sahar Choukir and Derek Warner
We examine precipitate-induced dynamic strain aging in precipitation-hardened Al-Cu
alloys by combining atomistic simulations, kinetic Monte Carlo, and analytical rate theory. Atomistic simulations were used to characterize (1) the energetics of nearest neighbour Cu<->Al exchanges at dislocation - precipitate junctions and (2) the subsequent change in obstacle strength. For robustness, the simulations were performed with two distinct interatomic potentials. The resulting catalog of local Cu-Al exchange events was used as input for a kinetic Monte Carlo model of the time-dependent
evolution of obstacle strength during dislocation pinning at the precipitate. The predicted strengthening kinetics were then embedded in an analytical dynamic strain aging model to predict the strain-rate sensitivity parameter. On the whole, the modeling predicts a low strain-rate sensitivity across a broad range of intermediate quasi-static strain rates, consistent with experimental observations for precipitate-strengthened
alloys. The results therefore identify a mechanistic origin of the low strain-rate sensitivity in precipitation hardened aluminum
alloys, emerging directly from the kinetics of dislocation-precipitate interactions when nearest neighbour Cu<->Al exchanges are considered.
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Materials Science
Insulator-to-Metal Transitions Driven by Quantized Formal Polarization Mismatch
Hongsheng Pang and Lixin He
We propose a mechanism for
insulator-to-metal (IM) transitions driven by the mismatch of quantized formal polarization (QFP), a
symmetry-protected bulk invariant. For a material with a low-
symmetry insulating phase and a high-
symmetry phase that allow distinct QFPs, any continuous path connecting them while preserving the
symmetry of the low-
symmetry phase must inevitably pass through an IM transition. The reason is that QFP remains invariant along any gapped
symmetry-preserving
evolution, whereas the high-
symmetry phase requires a different QFP, which can only be accommodated by gap closing. First-principles calculations on two representative systems, two-dimensional InPS$_3$ and three-dimensional CdBiO$_3$, confirm this mechanism. Our results establish QFP mismatch as a general
symmetry constraint on phase
evolution and reveal a new route to
symmetry-driven IM transitions in high-
symmetry materials.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.ins-det
Electrostatic transfer of sub-micron magnetic particles onto cantilevers using a focused ion beam system
Peter Sun, George R. Du Laney, Tim M. Fuchs et al. (+3 more)
In this paper, we present a focused-ion-beam-assisted method for preparing magnet tips for
magnetic resonance force microscopy measurements. The method electrostatically transfers prefabricated
magnetic nanoparticles to microcantilevers, achieving precise control over the magnet overhang past the cantilever leading edge while minimizing the fabrication damage to the leading edge of the tip magnet. We demonstrate successful fabrication of magnets ranging in size from 460 nm to 2.8 um. These magnets were affixed to two types of cantilevers: silicon cantilevers with a spring constant of 800 uN/m, and single-
crystal silicon cantilevers with a spring constant of 30 uN/m. We show that the electrostatic transfer method enables a wide variety of tip shapes, sizes, and materials that were previously not possible with conventional fabrication methods. The transfer procedure allows us to prefabricate the desired particle geometry with minimal ion-beam damage, as confirmed by Monte Carlo simulations. We show that the technique is versatile and can be used to fabricate custom-tipped cantilevers for a broader range of scanning probe techniques.
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All authors Peter Sun, George R. Du Laney, Tim M. Fuchs, Tjerk H. Oosterkamp, Malcolm G. Thomas, John A. Marohn
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Codimension-controlled universality of quantum Fisher information singularities at topological band-touching defects
C. A. S. Almeida
Topological phase transitions in generic multiband systems are mediated by band-touching defects whose codimension – the number of momentum directions along which the gap closes linearly – varies across universality classes. Although singular behavior of fidelity susceptibilities and
quantum Fisher information (QFI) has been computed for specific models, no unifying principle connecting these results has been identified: it has remained unclear whether the controlling variable is spatial dimensionality, band structure, or an intrinsic
geometric property of the defect. We resolve this question by showing that the singular contribution to the QFI with respect to the tuning parameter $m$ obeys a universal power-law scaling $\sim |m|^{p-2}$ for $p \neq 2$, with a logarithmic divergence $\sim \ln(1/|m|)$ at the marginal codimension $p = 2$, where $p$ denotes the codimension of the band-touching defect. This exponent is independent of spatial dimensionality, anisotropies, ultraviolet regularization, and additional gapped bands, and is protected by renormalization-group arguments at the linearized fixed point. The result unifies previously isolated observations for SSH chains ($p=1$), Chern
insulators ($p=2$), and Weyl semimetals ($p=3$) as instances of a single codimension-dependent universality class, and reveals that only defects with $p \leq 2$ generate divergent information-
geometric responses. This establishes a direct and previously missing link between topological classification in momentum space and
quantum distinguishability in parameter space, with implications for metrological sensitivity near topological transitions and for the experimental detection of topological criticality via
quantum geometric observables.
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Disordered Systems and Neural Networks
Variational Iterative Rotation Algorithm: Combinatorial Optimization with Classical Kicked Tops
Flaviano Morone, Andrew D. Kent and Dries Sels
We investigate a classical formulation of the
Quantum Approximate Optimization
Algorithm (QAOA), realized as a
Hamiltonian dynamical system of classical kicked tops, which we call the Variational Iterative Rotation
Algorithm (VIRAL). The protocol is fully variational, with the initial tilt of the
spins and the $2p$ rotation angles of a depth-$p$
circuit optimized by backpropagation through the deterministic dynamics. On the canonical Sherrington-Kirkpatrick (SK)
spin glass and on Max-Cut for random 3-regular graphs, VIRAL systematically outperforms QAOA, with the SK energy density converging to the Parisi ground state value and the cut fraction to the 2RSB upper bound, both linearly in $1/p$. The optimization is driven by a pitchfork bifurcation of the Floquet map, which destabilizes the equatorial fixed point and adiabatically transports the
spins to the polar Ising configurations encoding the solution. These results indicate that the effectiveness of QAOA-like protocols is rooted in their iterative rotation structure. To explain the classical advantage, we exactly reduce the many-body
evolution to an effective Landau-Lifshitz dynamics of a single
spin in a self-consistent
stochastic magnetic field. The temporal covariance of the effective field is nearly rank-one, so that a single noise mode governs the
stochastic dynamics and the system remains highly controllable.
Quantum fluctuations, by contrast, increase the rank of the noise covariance, hampering control and slowing convergence to the solution. Finally, we propose nanometer-scale
magnetic tunnel junctions, where
spin rotations are naturally realized by
magnetic fields and
spin torques, as a physical platform for implementing VIRAL.
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