Mesoscale and Nanoscale Physics
Anisotropic wavevector-dependent damping of thickness-quantized magnons
Tamara Azevedo, Rostyslav O. Serha, Yannik Kunz et al. (+7 more)
Magnon damping is a key factor governing
spin-wave transport and nonlinear dynamics of multimode
magnon systems. However, many descriptions rely on the assumption of an effective mode-independent parameter, which can mask wavelength-dependent relaxation processes that depend on propagation geometry and mode profile. Here, we employ high-resolution parametric-instability spectroscopy to probe thickness-quantized
spin waves with wavelengths down to about a hundred nanometers in micrometer-thick yttrium iron garnet films. The instability threshold exhibits a regular sawtooth dependence on the
magnetic field, arising from switching between discrete thickness modes. Comparison with dipole–exchange theory reveals anisotropic wavevector-dependent damping that increases with mode number and depends differently on the in-plane and out-of-plane wavevector components.
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All authors Tamara Azevedo, Rostyslav O. Serha, Yannik Kunz, Matthias R. Schweizer, Vitaliy I. Vasyuchka, Mathias Weiler, Andrii V. Chumak, Burkard Hillebrands, Mikhail Kostylev, Alexander A. Serga
Disordered Systems and Neural Networks · Cross-listed from physics.optics
How Shaped Waves Propagate in Scattering Media
David Gaspard and Arthur Goetschy
The radiative transport equation provides a powerful framework for describing wave propagation in scattering media. However, it cannot describe coherent waves whose incident wavefront is deliberately tailored. Here we establish a transport theory for wavefront-controlled waves based on a
matrix transport equation. Unlike conventional radiative transport, which propagates a scalar radiance, our theory propagates a complex two-by-two
matrix that retains phase information and obeys a nonlinear transport equation, despite the underlying wave dynamics being linear. We use this
matrix transport equation to derive the spatial profiles of the energy and current densities of transmission eigenchannels, from closed to open channels, in arbitrary diffusive systems. Remarkably, these profiles can be expressed analytically in terms of the solution of the conventional radiative transport equation for a random wave in the same medium. The theory further extends to energy loss, including absorption and leakage into uncontrolled channels, revealing
symmetry breaking of transmission eigenchannels in complex structures. Validated against numerical solutions of the wave equation, we establish
matrix transport as a framework for describing and controlling coherent wave propagation in complex media.
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Soft Condensed Matter · Cross-listed from physics.bio-ph
Axonemal bending stiffness of $\mathit{Chlamydomonas}$ cilia implies single-motor forces above $5\,\mathrm{pN}$
Maximilian Kotz, Veikko F. Geyer and Benjamin M. Friedrich
The regular bending waves of cilia and flagella provide an iconic model system for the collective dynamics of molecular motors. The known regular arrangement of dynein motors in a cilium's axoneme allows us to connect mesoscopic cilia properties, such as axonemal bending stiffness, to microscopic motor properties, such as the force exerted by an individual motor. We estimate the active force generated by the collection of dynein molecular motors in $\mathit{Chlamydomonas}$ axonemes using previous estimates of its bending stiffness. Divided by the maximal possible number of active motor heads, this provides a lower bound of ${>}10\,\mathrm{pN}$ for the peak force generated by a motor head, which exceeds typical stall forces ${<}5\,\mathrm{pN}$ of molecular motors. This discrepancy suggests that either the bending stiffness of microtubules and axonemes was previously overestimated, or that collective force generation in dense motor arrays can surpass the sum of expected contributions of individual motors.
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Statistical Mechanics · Cross-listed from quant-ph
Exact Maximum Likelihood Decoding beyond Treewidth via Rank-Decomposition Dynamic Programming
Bin Cheng and Feng Pan
Maximum-likelihood decoding provides an optimal decoding strategy for
quantum error correction under
stochastic Pauli noise. However, computing logical-class probabilities is challenging, and the leading exact tensor-network contraction requires time exponential in treewidth. In this work, we introduce a new decoding
algorithm based on rank-decomposition dynamic programming (Rank DP). We express decoding partition functions with independent local fault factors as quadratic sums of powers and apply Rank DP. The resulting exact evaluator has arithmetic complexity
polynomial in the input size and exponential in the Tanner graph's rank-width with respect to the fault partition, including decomposition construction. After Gaussian elimination, it gives
polynomial-time decoding for punctured
quantum Reed-Muller codes and a Steane-concatenated family with growing distance under independent single-
qubit Pauli noise. Standard contraction of the corresponding tensor networks requires superpolynomial time. A nonnegative realization gives relative floating-point error bounds under explicit arithmetic assumptions. Numerical experiments demonstrate runtime advantages over the tested tensor-network implementations on selected code-capacity and
circuit-level instances, with full likelihood evaluation for Reed-Muller codes up to $1{,}023$
qubits. We further use these likelihoods to learn
circuit noise parameters from syndromes, evaluate rare postselection probabilities, and quantify decoder optimality gaps. Our work opens new avenues for exploiting algebraic structure in
quantum decoding and noise characterization.
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Disordered Systems and Neural Networks
On the Relation Between the Boson Peak and the Vibrational Phases in Glasses
Philip Rasmussen and Søren S. Sørensen
The
boson peak is a vibrational feature commonly found in glassy and disordered systems described as the deviation from the scaling of the Debye model ($g({E}){\propto}{E}^2$). Despite much interest, the microscopic nature of the vibrations is only anecdotally discussed. Here we report how the
boson peak appears in conjunction with a crossover from localized to extended but non-wave-like and coincide with a maximum in phase coherence and spatial periodicity. Our analyses provide new insights into the microscopic origin of the
boson peak across a wide range of glasses.
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Materials Science · Cross-listed from quant-ph
Boron vacancies in bulk h-BN created by high-energy He+ irradiation
P.G. Vilyuzhanina, S.V. Bolshedvorskii, R. Isayev et al. (+11 more)
While color centers in diamond and other three-dimensional
crystals are nowadays key elements of several types of sensors, color centers in two-dimensional materials are rapidly developing and promise high-tech applications. One of the interesting centers in 2D materials is the negatively charged boron vacancy in hexagonal boron nitride (h-BN), which has already shown some potential for magnetometry, but the reliable creation of boron vacancies remains challenging. Here, we demonstrate the fabrication of negatively charged boron vacancy color centers in bulk h-BN via implantation of helium ions at ~1 MeV, as confirmed by characteristic photoluminescence and optically detected
magnetic resonance. The resonance has a width of 180 MHz and exhibits the expected Zeeman shift of the resonance lines. The depth of the color center along the c-axis of the h-BN
crystal was measured and compared with predictions from Stopping and Range of Ions in Matter modeling. The experimental value and the modeled prediction are consistent within the reported uncertainties, as their 1{\sigma} intervals overlap, although the experiment shows a greater depth and a wider distribution.
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All authors P.G. Vilyuzhanina, S.V. Bolshedvorskii, R. Isayev, A.S. Doroshkevich, A.A. Tatarinova, V.V. Soshenko, I.S. Cojocaru, A.M. Kozodaev, S.M. Drofa, A. Chernyavskiy, N.I. Salangin, A.N. Smolyaninov, A.Y. Kuntsevich, A.V. Akimov
Materials Science
First-Principles Study of I$_2$ and CH$_3$I Adsorption on Transition Metal Decorated 2D-Material substrates : Insights from Electronic Structure and Reaction Kinetics
M. Meena and P. Anees
Radioactive iodine species, particularly I$_2$ and CH$_3$I, pose significant environmental and technological hazards owing to their high
volatility, chemical stability, and relatively weak interaction with traditional substrate and sorption materials. In this work, we proposed a series of transition-metal (TM) (Fe, Ni, Cu, Zn) decorated boron-doped
graphene (BDG) and 2D-MoTe2 substrates for efficient adsorptive capture and mitigation of such volatile Iodine species. Using systematic first-principles density functional theory (DFT) calculations, we elucidate the microscopic origin of the enhanced adsorption by analyzing the changes in the electronic structure upon adsorption. More importantly, we analyzed the thermodynamic and kinetic feasibility of adsorption on these newly designed substrates using Climbing-Image Nudged Elastic band (CI-NEB) calculations and found that TM decoration serves as an effective catalytic center, thereby making the reaction thermodynamically and kinetically feasible. Conversely, the reaction becomes thermodynamically and kinetically unfavorable on pristine substrates in the absence of a TM atom as a catalytic center. This work deepens our understanding of the electronic origin of the enhanced adsorption and reaction kinetics, and the predictions made will be useful for experimental realization.
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Statistical Mechanics · Cross-listed from quant-ph
Optimal local recovery cannot alter critical orthogonality exponents in quantum spin environments
Zhixuan Zhao and Jun Li
We prove a local amplitude-ratio bound for positive ground states and an explicit reverse comparison for fidelity under finite-region erasure. An exact
lattice calculation for a connected critical Ising chain gives amplitude exponent $1/8$, preserved by arbitrary optimal control on any fixed
spin set and, at the level of logarithmic exponents, on $o(\log N)$
spins. We derive the finite Cauchy product and its uniform strip asymptotics, and document exact two-
spin optimization and an interferometric protocol for measuring the recovery. A separate finite-mediator
theorem quantifies local compression and a decoder while retaining the rest of the environment exactly. We also give a ground-preparation Ramsey bound, counterexamples to stronger projected-state inferences, and independent full-
Hamiltonian calculations.
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Materials Science
Realization of ZnSe-based Field-Effect Transistors operating at Cryogenic Temperatures as a Platform for Future Spin-Qubit Applications
Christine Falter, Yurii Kutovyi, Nils von den Driesch et al. (+12 more)
The wide-bandgap compound
semiconductor ZnSe is a promising host material for the realization of electron
spin-
qubits. Its non-degenerate conduction band and the potential for isotopic nuclear
spin purification promise long
spin coherence times. Key requirements for such devices include reliable electrostatic control of electrons in ZnSe and low-resistance ohmic contacts that remain functional at cryogenic temperatures. In this work, we utilize a novel Shadow Wall technique for molecular-beam epitaxy combined with in-situ deposition of Al ohmic contacts to realize normally-off ZnSe-based field-effect transistors. The devices exhibit linear output characteristics and effective gate control of the channel from room temperature down to 5 K, confirming low-resistance ohmic contacts to the undoped ZnSe channel. The drain current can be modulated by several orders of magnitude through electrostatic gating, with threshold voltages of approximately 3 V and field-effect mobilities exceeding 100 cm2/Vs over the investigated temperature range. Self-consistent Schrödinger-Poisson and drift-
diffusion simulations reproduce the measured transfer characteristics and provide insight into the role of interface electrostatics in determining the channel formation and threshold voltage. These results demonstrate the potential of gated ZnSe
heterostructures for future
spin-
qubit applications.
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All authors Christine Falter, Yurii Kutovyi, Nils von den Driesch, Denny Dütz, Nataliya Demarina, Prateek Kaul, Anjana Rajan, Jiayuan Zhang, Thomas J. Smart, Leqi Zhou, Lidia Kibkalo, András Kovács, Qing-Tai Zhao, Lars R. Schreiber, Alexander Pawlis
Mesoscale and Nanoscale Physics
Pablo Canteli, Yuriko Baba, Juan Salvador-Sánchez et al. (+11 more)
In
graphene and other two-dimensional materials, periodic modulations of the electron density can significantly alter the energy spectrum and transport properties. Here, we report magnetotransport measurements in encapsulated monolayer
graphene with ultra-high-quality patterned periodic antidot
lattices that preserve the intrinsic electronic properties of the material. This lithographically defined platform enables controlled access to commensurability and superlattice phenomena at length scales otherwise difficult to achieve. By systematically tuning the
lattice dimensions, we reveal a hierarchy of classical commensurability features arising from cyclotron orbits with comparable radii that follow multiple classical trajectories, resulting in broadened resistance peaks beyond the conventional single-orbit picture. Superimposed on these features, we observe pronounced Brown-Zak oscillations arising from the
quantum commensurability between the
magnetic flux
quantum and the unit
cell of the engineered Bravais
lattices. We demonstrate that the intrinsic
geometric phase of our system is directly measurable and show a precise matching of the
magnetic field periodicity to the lithographic periodic patterning, where moiré-like electronic spectra can be geometrically generated in single-layer
graphene without the need for twist,
lattice mismatch, or multilayer stacking. Our results establish nanopatterned
graphene as a clean, tunable, and scalable platform for realizing and exploring moiré physics through on-demand real-space design
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All authors Pablo Canteli, Yuriko Baba, Juan Salvador-Sánchez, Jorge Estrada-Álvarez, Ana Pérez-Rodríguez, Carlos Sánchez-Sánchez, Vito Clericò, Takashi Taniguchi, Kenji Watanabe, Francisco Domínguez-Adame, Rafael A Molina, Enrique Diez, Elena Díaz, Mario Amado
Strongly Correlated Electrons · Cross-listed from quant-ph
Excitation gap of a blockade structure with $\mathbb{Z}_2$ topological order
Simon Fell, Tobias F. Maier, Hans Peter Büchler et al. (+1 more)
Mathematically rigorous statements on the spectral gap of
quantum many-body systems in the thermodynamic limit are notoriously difficult to prove – yet they are of fundamental importance for classifying
quantum phases of matter. Here we prove the existence of a finite excitation gap for a particular
Hamiltonian which was proposed in [T. F. Maier et al., PRX
Quantum 6, 030340 (2025)] and is motivated by the Rydberg platform. The
Hamiltonian exhibits only two-body blockade interactions between two-level systems and has a topologically ordered ground state in the toric code phase. We show that our result also applies to a broader class of blockade
Hamiltonians which realize non-Abelian
quantum double phases and were proposed in [H. P. Büchler et al., Phys. Rev. B 114, 065113 (2026)]. The proof builds on known gap stability results and exploits the local
symmetry of the studied models.
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All authors Simon Fell, Tobias F. Maier, Hans Peter Büchler, Nicolai Lang
Disordered Systems and Neural Networks · Cross-listed from math-ph
Many-body localization: an introduction for mathematicians
Francois Huveneers
These notes are based on a mini-course delivered at the Les Houches Summer School,
Quantum Theory on All Scales, in August 2026. They introduce many-body localization to mathematicians, emphasizing the physical mechanisms that support or destabilize it. The mathematical discussion deals with how perturbative rotations, renormalization, and probabilistic estimates are used in a rigorous approach to many-body localization.
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Statistical Mechanics
Debasish Sarker
Topology and geometry are inseparable in physical networks. Here we show that, after factoring out the
topology-dependent number of eligible edge pairs, exchangeability makes the mean crossing propensity of those pairs exactly graph independent while preserving
topology in fluctuations. We derive a microscopic, parameter-free theory for the response when
topology and geometry are coupled. Synthetic and real three-dimensional networks confirm that structure hidden from the normalized mean survives in correlations that determine response.
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Statistical Mechanics · Cross-listed from quant-ph
Optimal Scaling of Unitary Design Formation in $U(1)$-Symmetric Random Circuits: A Bottleneck Slower than Charge Transport
Toshihiro Yada
Understanding randomness formation under conservation laws is important both for characterizing chaotic phenomena in isolated
quantum systems and for
symmetry-constrained
quantum information processing. $U(1)$-symmetric random
circuits on an $N$-
qubit system provide a minimal tractable setting for this problem, in which the formation rate of
unitary designs was conjectured to be governed by charge transport in the underlying
circuit geometry. However, whether charge transport indeed sets the slowest relaxation mode has remained an open problem, due to the difficulty in deriving a tight lower bound on the symmetric design formation rate. In this work, we resolve this problem by determining the optimal scaling of the
unitary $k$-design formation rate for $k\leq O(\sqrt{\log N})$ across a wide range of
circuit geometries, including
lattices, expander graphs, and all-to-all interactions. In particular, for $k\geq 2$, we find that the formation rate is governed by a two-particle encounter rate—the rate at which two random walkers encounter each other in a given
circuit geometry. This encounter rate is parametrically smaller than the single-particle transport rate in many geometries, thereby disproving the previous
conjecture. We further establish that this encounter mechanism indeed sets the design formation rate by deriving its matching lower bound with a novel proof technique based on an auxiliary subgroup ensemble. Although this encounter mode imposes a robust bottleneck on design formation in $U(1)$-symmetric random
circuits, we also show that this bottleneck can be circumvented by using
symmetry-breaking local gates and provide an explicit protocol that achieves faster design formation. These results establish a new picture of randomness formation under conservation laws and provide guiding principles for characterizing and efficiently generating
symmetry-constrained randomness.
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Soft Condensed Matter
Active spinners drive passive matter into chiral rotors
Dennis Schorn, Anpuj Nair S, Stijn van der Ham et al. (+2 more)
Active spinners inject angular momentum into their surroundings without persistent translation, providing a route to chiral active matter distinct from self-propelled particles moving along curved trajectories. Recent studies have shown that spinner fluids can self-organize into rotating clusters with circulating edge currents characteristic of odd-viscous fluids. Whether angular-momentum injection alone can transmit chirality and edge-current dynamics to ordinary passive matter - and thus serve as a generic mechanism for imparting chiral functionality to undriven material components - remains an open question. Here, we show experimentally and numerically that purely rotational
magnetic spinners can transfer chirality and edge-current dynamics to passive colloidal matter. At high passive-particle fraction, active spinners assemble into rotating clusters surrounded by passive matter. Remarkably, upon changing composition and spinner attraction, this organization turns inside out: passive colloids form rotating clusters surrounded by a chiral spinner fluid. These passive rotors exhibit edge currents and the same angular-velocity scaling, $|\Omega|\sim R^{-2}$, as their active counterparts. A minimal model shows that spinner-mediated transverse and non-reciprocal interactions are sufficient to reproduce these complementary chiral states and their boundary-driven dynamics. Our results establish angular-momentum injection by active spinners as a route to endow otherwise passive matter with chirality, collective rotation and boundary transport.
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All authors Dennis Schorn, Anpuj Nair S, Stijn van der Ham, Benno Liebchen, Hanumantha Rao Vutukuri
Mesoscale and Nanoscale Physics
Giant PhotoMagnetoDiode Effect
V. L. Korenev and S. A. Tarasenko
We report the observation of a photomagnetodiode effect with a giant rectification ratio of 10 at a
magnetic field of 0.1 T. The effect consists of a diode-like dependence of the photocurrent on the applied voltage that emerges in an external
magnetic field. Such a pronounced nonreciprocity is observed in high-quality VPE-grown GaAs samples at low temperatures at interband photoexcitation. The photoconductivity is invariant upon reversing the polarity of both electric and
magnetic fields. Similar behavior with even higher asymmetry ratio is observed in photoluminescence. The findings are well described by the theory of ambipolar drift of carriers in electric and
magnetic fields together with fast surface recombination.
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Strongly Correlated Electrons
Weyl superconductivity from Feshbach resonance in the three-dimensional repulsive Hubbard model
Taekyoung Kim, Pit Bermes, Giorgia A. Busin et al. (+1 more)
Motivated by the first experimental realization of the antiferromagnetic phase transition in the three-dimensional Fermi-Hubbard model, we present a theoretical study of the model's 3D superconducting phase. By formulating a 3D extension of the Feshbach mechanism, we provide a unified microscopic picture at strong coupling in which pairing is driven by near-resonant bound states of dopants. These long-lived bound states acquire a qualitatively different internal structure in three dimensions compared to their two-dimensional counterparts, giving rise to a distinct superconducting state, namely a time-reversal
symmetry breaking $d_{x^2-y^2}+id_{z^2}$ pairing state. We also provide an estimate of the corresponding critical temperature, and provide evidence that the resulting superconducting phase hosts gapless Weyl points. Our results represent a new milestone for the field of
quantum simulation, challenging experiments and large-scale numerics alike to test our predictions.
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All authors Taekyoung Kim, Pit Bermes, Giorgia A. Busin, Fabian Grusdt
Strongly Correlated Electrons
Quantum Well Resonant Tunneling Diode Probe of Correlated States in Twisted Bilayer MoS$_2$
Chengjie Zhou, Jin-Xin Hu, Sihong Xu et al. (+4 more)
Moiré superlattices formed in transition metal dichalcogenides (TMDs) offer a versatile platform for exploring emergent
quantum phases arising from strong electronic correlations. In this work, we develop a new experimental platform, the
quantum well resonant tunneling diode (QWRTD), to probe the electronic landscape of $\approx 57^\circ$ twisted bilayer MoS$_2$ (tMoS$_2$). By measuring the differential conductance ($dI/dV_{\text{Probe}}$) as a function of filling factor $\nu$ and displacement field $D$, we observe a robust integer correlated insulating state at $\nu = 1$ that persists across the measured displacement field range. Furthermore, we identify a displacement-field-induced fractional insulating state at $\nu = 3/4$ for $D < -75$~mV/nm. Temperature- and
magnetic-field-dependent measurements characterize this $\nu = 3/4$ state as a potential generalized Wigner
crystal. These correlated states are also observed in another device with a similar twist angle ($\approx 56.5^\circ$). Our results provide direct evidence of correlated states in near-AB-stacked tMoS$_2$ and establish QWRTD as a powerful experimental tool for investigating strong correlations and
topology in van der Waals
heterostructures.
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All authors Chengjie Zhou, Jin-Xin Hu, Sihong Xu, Zijing Jin, Hui Li, Kam Tuen Law, Jiannong Wang
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Reservoir- and Measurement-free Microwave Initialization of Semiconductor Spin Qubits
Leon C. Camenzind, Ik Kyeong Jin, Akito Noiri et al. (+4 more)
Scalable
quantum processors require repeated
qubit initialization throughout large arrays. In
semiconductor spin qubits, fast initialization commonly relies on local reservoir access or measurement-based feedback, requiring dedicated infrastructure that becomes increasingly difficult to distribute as processors scale. Here, we demonstrate reservoir- and measurement-free initialization of a silicon
spin-
qubit pair in an industrially fabricated Si/SiGe
quantum-dot device using a fixed sequence of microwave and baseband pulses. Odd
spin-parity states relax to the singlet charge state, whereas blocked even
spin-parity states are microwave-driven through the triplet
manifold and subsequently converted to the singlet by singlet-triplet mixing and charge hybridization. Repeated cycles produce the singlet-associated charge outcome with a median probability of 99.4% across the sampled preparation states, while exchange spectroscopy independently verifies mapping to the target $|\uparrow\downarrow\rangle$ operational state. Microwave spectroscopy and time-domain measurements identify the dark-state-limited single-cycle transfer and the subsequent blockade-lifting dynamics that set the initialization time scale. The demonstrated pumping sequence uses approximately $12\,\mu\mathrm{s}$ of microwave bursts and mixing dwells, while we project sub-microsecond initialization under improved device conditions. These results establish fixed-sequence microwave initialization as a scalable control primitive for
semiconductor spin-
qubit processors, based on singlet-triplet physics that can be adapted to platforms with suitable Pauli-blockade transitions.
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All authors Leon C. Camenzind, Ik Kyeong Jin, Akito Noiri, Kenta Takeda, Takashi Nakajima, Takashi Kobayashi, Seigo Tarucha
Soft Condensed Matter
The hidden side of paintbrushes: a geometry shaped by power laws
Raphaelle Taub, Jérôme Crassous, Thomas Salez et al. (+2 more)
Since prehistoric times, painters have used brush-like tools made from animal and plant materials to apply pigments to rock surfaces. Over time, techniques for creating brushes evolved, and converged towards optimized shapes. Our study performed over hundreds of brushes reveals universal scaling laws. In particular, we uncover a relation between bristle length and diameter linked to the shape of the bristle ends, which governs their mechanical response with and without paint. For round brushes, we found that the size of the paint stain scales with the compression distance, meaning it only depends on the force applied by the painter and not on the mechanical responses of the brushes. These results shed light on the underlying universal physical principles governing brush design and their impact on artistic techniques.
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All authors Raphaelle Taub, Jérôme Crassous, Thomas Salez, Frédéric Restagno, Christophe Poulard
Strongly Correlated Electrons
Chiral quantum chaos around exponentially many zero modes in the quantum breakdown model
Kohei Kawabata, Kinya Guan and Hosho Katsura
The
quantum breakdown model is a model of randomly interacting
fermions, motivated by the physics of dielectric breakdown. Here, we establish classification of
symmetry and
quantum chaos in a zero-dimensional, all-to-all-interacting version of the
quantum breakdown model. It exhibits the $\mathbb{Z}_4$ periodicity with respect to the number of fermionic modes, reminiscent of the
symmetry classification of the Sachdev-Ye-Kitaev model. A unique feature of the
quantum breakdown model is that it realizes all the five classes with chiral
symmetry and hosts an exponentially large number of many-body zero modes protected by the chiral index. We elucidate the separation and scaling of the spectral gap and demonstrate the
symmetry-enriched hard-edge spectral statistics as signatures of
quantum chaos in the chiral
symmetry classes.
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Statistical Mechanics
Square-root growth of operator entanglement in an integrable brickwork circuit
Balázs Pozsgay
It is widely expected that the von Neumann
entanglement of a local operator grows at most logarithmically in integrable many-body systems in infinite volume. We give a counterexample in a four-state brickwork
circuit whose gate is a permutation
matrix solving the constant Yang–Baxter equation. The von Neumann operator
entropy grows as $(\log2)\sqrt{t/\pi}+O(\log t)$, whereas fixed-index Rényi
entropies grow linearly below index one and logarithmically above it. At fixed relative
Hilbert–Schmidt error below one,
matrix product operator simulations require a bond dimension growing at least as $\exp(c\sqrt t)$ for some $c>0$.
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Statistical Mechanics · Cross-listed from stat.ML
A Dynamical Theory of LoRA in Continual Learning
Théo Marchetta, Filippo Alessandroni, Alessandro Breccia et al. (+2 more)
Despite the widespread use of Low-Rank Adaptation (LoRA), little is known about its dynamics in continual learning and the mechanisms by which low-rank updates affect catastrophic forgetting. We provide an asymptotically exact dynamical characterization of LoRA in a solvable two-task teacher-student model. In the high-dimensional online-learning limit, we derive a closed system of ordinary differential equations for a finite set of macroscopic order parameters, yielding exact expressions for the generalization errors throughout both the initial Task 1 learning phase and the subsequent LoRA fine-tuning on Task 2. The theory quantitatively matches finite-dimensional simulations and exposes two characteristic effects of LoRA: low-rank adaptation reduces interference with features learned on the first task, but its initialization slows adaptation to the second task. Building on this mechanistic picture, we analyze a state-dependent masking strategy that freezes hidden units carrying the strongest first-task representations and restricts adaptation to the complementary subspace. This structural partitioning markedly reduces forgetting, while preserving plasticity on the new task. Our framework further clarifies the role of adapter rank: transfer improves only up to the intrinsic dimensionality of the target task and saturates beyond it, while forgetting continues to grow with rank. These results provide a dynamical and
geometric account of how low-rank adaptation organizes information across sequential tasks and are qualitatively reproduced on a sequential MNIST
benchmark.
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All authors Théo Marchetta, Filippo Alessandroni, Alessandro Breccia, Alessandro Ingrosso, Federica Gerace
Soft Condensed Matter
Fully developed active turbulence defined through a non-equilibrium phase transition
Lasse Bonn, Tianxiang Ma, Olga Bantysh et al. (+7 more)
Non-
equilibrium systems challenge the standard definitions of phases and phase transitions from
equilibrium statistical physics. For example, in active fluids continuous energy injection at the microscale drives the system intrinsically out of
equilibrium. This process gives rise to collective turbulent-like flows whose onset remains poorly defined. Here we show that the onset corresponds to an activity-driven phase transition, marked by the emergence of a system-spanning critical backbone of vorticity nodal lines. Combining two independent experiments, microtubule kinesin active nematics and dense suspensions of swimming bacteria, with large-scale simulations of active nematics and fluctuating nematohydrodynamics, we show that this transition occurs at a critical activity threshold. Below this threshold, vorticity structures are fragmented and
vortex centres form only finite, mechanically floppy networks. Above this threshold, the nodal lines of the vorticity field percolate and the
vortex centres form a rigid, system-spanning cluster. The
geometric transition is characterised by the emergence of critical percolation statistics for the vorticity nodal lines, while the mechanical transition reflects the emergence of a
vortex configuration that shows rigidity percolation statistics for
vortex centres. We show that both transitions are absent in
equilibrium systems that obey detailed balance, demonstrating that the transition is intrinsically non-
equilibrium. These results establish an experimentally accessible definition of fully developed active
turbulence, linking microscopic activity to macroscopic geometry, and mechanics of living systems.
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All authors Lasse Bonn, Tianxiang Ma, Olga Bantysh, Wei Feng, Martin Cramer Pedersen, Guangyin Jing, Jordi Ignés-Mullol, Francesc Sagués, Nuno A. M. Araujo, Amin Doostmohammadi
Strongly Correlated Electrons
Varying A-site radius and size disorder to tune magnetic ordering in compositionally complex perovskite oxides
Madeleine Geers, Ravi Kiran Dokala, Roland Mathieu et al. (+1 more)
A long standing goal of magnetochemists is to be able to control ordering temperatures through compositional engineering. Here we report the synthesis of a perovskite oxide with twelve cations accommodated within a single phase solid solution, AM7O$_3$, A = La$_{\frac{1}{5}}$Sm$_{\frac{1}{5}}$Gd$_{\frac{1}{5}}$Nd$_{\frac{1}{5}}$Dy$_{\frac{1}{5}}$ (A5), and M7 = Ti$_{\frac{1}{7}}$Cr$_{\frac{1}{7}}$Mn$_{\frac{1}{7}}$Fe$_{\frac{1}{7}}$Co$_{\frac{1}{7}}$Ni$_{\frac{1}{7}}$Cu$_{\frac{1}{7}}$. Bulk magnetometry measurements for A5M7O$_3$, as well as for A = La, Gd and La$_{\frac{1}{2}}$Gd$_{\frac{1}{2}}$, show that all these compounds magnetically order as ferrimagnets between 89 and 115 K. We find that the
magnetic properties are influenced by considering a combination of the size disorder parameter alongside the A-site cationic radii and valence electrons.
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All authors Madeleine Geers, Ravi Kiran Dokala, Roland Mathieu, Rebecca Clulow
Statistical Mechanics
Phase Separation in Fractonic Fluids: Coarsening, Interfaces, Nucleation, and Hyperuniformity in and out of Equilibrium
Raphael Maire
We develop a theory of phase separation in fluids conserving higher-order multipole moments of a scalar density, and investigate the resulting coarsening dynamics, nucleation, and interfacial fluctuations. We find that higher-order conservation laws slow the relaxation of all phenomena investigated, whereas nonequilibrium violations of the fluctuation–dissipation
theorem suppress density and interfacial fluctuations while hindering nucleation. Simulations support these predictions.
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Materials Science
Surface-Dependent Phonon Dynamics in 9-Armchair Graphene Nanoribbon Arrays
Ángel Labordet Álvarez, Gabriela Borin Barin, Michel Calame et al. (+1 more)
Atomically precise
graphene nanoribbons (GNRs) have electronic properties tunable through their width and edge structure, making them promising building blocks for nanoscale electronics and optoelectronics. Their integration into devices also requires understanding how the supporting surface and neighbouring ribbons affect their vibrations. Here, we use temperature-dependent Raman spectroscopy to compare five configurations of 9-armchair GNR arrays differing in substrate,
alignment and coverage. Transferring the same unaligned film from Au to a Raman-optimised Al$_2$O$_3$-coated support reduces the measured $D$- and $G$-mode
redshift rates by about 80%. This reduction is consistent with a substantial thermoelastic contribution. The fitted strain and anharmonic contributions depend on the assumed thermal expansion and strain transfer. Compared with the low-coverage array, the dense aligned Au array has higher extrapolated reference frequencies ($\omega_0$). On first heating, its $G$-mode linewidth decreases and then increases, forming an unusual intermediate-temperature minimum. The $D$-mode linewidth shows a similar, less pronounced variation. These findings establish substrate choice and array arrangement as routes to engineer nanoribbon vibrations, extending control beyond the atomic structure defined during synthesis.
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All authors Ángel Labordet Álvarez, Gabriela Borin Barin, Michel Calame, Mirjana Dimitrievska
Statistical Mechanics
Logarithmic Quasi-Long-Range Order and Novel Continuous Phase Transition in the Two-Dimensional $XY$ Model with $1/r^4$ Interaction
Xianzhi Pan, Zhijie Fan and Youjin Deng
We investigate the phase diagram and critical properties of the two-dimensional classical XY model with interactions decaying as $1/r^{2+\sigma}$. At the marginal case $\sigma=2$, we show that the low-temperature phase is characterized by logarithmic quasi-long-range order (log-QLRO), where
spin correlations decay as a power of the logarithm of distance, $C(r)\sim(\ln r)^{-\eta_\ell}$ with $\eta_\ell\propto T$, and that the transition into this phase is a continuous transition beyond both the Ginzburg-Landau-Wilson paradigm and the Berezinskii-Kosterlitz-Thouless mechanism. Our analysis combines a near-exact Gaussian
spin-wave theory, an adiabatic renormalization-group analysis, and large-scale Monte Carlo simulations. We find that the $1/r^4$ interaction logarithmically modifies the
spin-wave stiffness kernel as $\gamma(k)\simeq\kappa k^2\ln(1/k)$, which suppresses
vortex proliferation and gives rise to a nonuniversal correlation-length exponent $\nu\propto1/\sqrt{2\pi\kappa}$. Crucially, our simulations reveal that the short-range algebraic QLRO phase is unstable against a weak long-range perturbation for $\sigma\le2$. These results establish $\sigma=2$ as the crossover boundary between the long-range and short-range universality classes and provide a critical evaluation of recent theoretical proposals.
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Strongly Correlated Electrons
Multipartite entanglement hidden in vector-chiral correlations of spin-1/2 chains
Tokuro Shimokawa
Characterizing
entanglement in
quantum materials through experimentally accessible observables remains a central challenge.
Quantum Fisher information associated with sums of one-site
spin operators provides one route by linking
entanglement to measurable
magnetic response functions. Extending this approach to local composite observables requires new
entanglement bounds. Here we establish a rigorous bound for the total nearest-neighbour vector chirality of periodic
spin-$1/2$ chains with an even number of sites. The
quantum Fisher information per
spin cannot exceed $k+1$ for any $k$-producible state, so exceeding this bound certifies
entanglement depth of at least $k+1$. Combining infinite density-
matrix renormalization group and thermal pure
quantum-state calculations, we demonstrate multipartite
entanglement encoded in vector-chiral correlations at zero and finite temperatures in an extended $J_1$-$J_2$ XXZ chain. The
spin-current coupling between vector chirality and electric polarization offers, in principle, access to this witness through dielectric loss spectroscopy in multiferroic magnets, extending
entanglement diagnostics beyond dipolar
spin correlations.
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Soft Condensed Matter
Moving against the odds with a cyclic active Brownian ratchet: current enhancement and reversal
Theo Spornhauer, Benoît Mahault and Johannes Zierenberg
We show that periodically modulating the self-propulsion of active particles in an asymmetric potential can both enhance rectified transport and reverse its direction, even when the propulsion speed itself never changes sign. Current enhancement results from the nonlinear response of active transport to propulsion strength, whereas current reversal emerges from the interplay between activity-induced spreading and passive relaxation. Our theoretical analysis identifies the regimes governed by these distinct rectification mechanisms and predicts the finite range of driving periods over which current reversal occurs. These results establish cyclic activity as a means of dynamically controlling rectified active transport, with potential applications to the selective sorting of active particles.
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