Materials Science
Generative and multimodal AI for materials prediction and design: Progress, challenges, and perspectives
Xianyuan Liu, Charles Anjah, Benjamin E. Jolly et al. (+9 more)
Artificial intelligence (AI) is accelerating materials prediction and design by enabling efficient exploration of chemical and structural spaces, with particular promise for novel materials discovery. However, novelty in materials discovery encompasses chemical plausibility, structural distinctiveness, property relevance and experimental realisability, making AI-driven novelty claims difficult to substantiate. We introduce a materials property hierarchy, from intrinsic, composition-determined properties to extrinsic, processing-dependent performance, to clarify deployment constraints and distinguish structural, physical and deployment novelty. This framework motivates an evidence-based view of
multimodal materials data spanning chemical composition, microstructure, processing, and testing and characterisation, showing that current evidence remains concentrated in composition and idealised structure while heterogeneous, under-represented and weakly integrated modalities limit support for physical and deployment novelty. It also highlights the limitations of
benchmarks based mainly on computational labels and proxy novelty criteria. Community-wide standards for data collection, modality
alignment and evidence synthesis are needed to support
multimodal data construction, process-aware
multimodal modelling, feasibility-first generative modelling and deployment-aware benchmarking, so that generative and
multimodal AI can design experimentally realisable materials with defensible scientific and practical novelty.
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All authors Xianyuan Liu, Charles Anjah, Benjamin E. Jolly, Jonathon F. S. Markanday, Joshua Berry, Haolin Wang, Nicola A. Morley, Robert D. J. Oliver, Alexandra J. Ramadan, Delvin Ce Zhang, Katerina A. Christofidou, Haiping Lu
Superconductivity
Geometric Superconducting Diode Effect in an NbN Nanoring
Tianyu Li, Peiyuan Huang, Jiong Li et al. (+14 more)
Superconducting diodes, which exhibit nonreciprocal critical currents, are promising building blocks for low-power cryogenic electronics and superconducting
circuits. Existing superconducting diode platforms commonly rely on Josephson junctions, multilayer
heterostructures,
ferromagnetic elements, gate-difined structures. Here, we demonstrate a geometrically induced superconducting diode effect realized in a structurally minimal, single-materials NbN nanoring, where inversion-
symmetry breaking is introduced solely by the asymmetric geometry. The device exhibits pronounced and polarity-switchable critical-current nonreciprocity. Systematic
magnetic-field and temperature-dependent measurements reveal that, at low fields, the applied
magnetic field redistributes the critical current asymmetrically between opposite bias directions without significantly reducing the overall superconducting current-carrying capability. Moreover, the maximal nonreciprocity and diode efficiency exhibit distinct temperature dependence: the maximal diode efficiency follows the
evolution of the energy gap, whereas the maximal nonreciprocity is more closely associated with the superfluid density. These results establish asymmetric superconducting nanorings as a minimal
geometric platform for studying nonreciprocal superconducting transport and provide a simple design principle for future superconducting electronics.
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All authors Tianyu Li, Peiyuan Huang, Jiong Li, Jiyao Shang, Nuo-Zhou Yang, Wuyue Xu, Wen-Cheng Yue, Yang-Yang Lyu, Chong Li, Yihuang Xiong, Xuecou Tu, Tao Tao, Xiaoqing Jia, Qing-Hu Chen, Huabing Wang, Peiheng Wu, Yong-Lei Wang
Materials Science
First-principles calculation of electron-phonon spectral functions for defects using phonon interpolation
Zoltan Santha and Gergo Thiering
Point defects in wide-band-gap
semiconductors exhibit optical spectra strongly shaped by electron-
phonon coupling, but direct first-principles calculation of the corresponding
phonon sidebands is often limited by the coarse vibrational spectrum of the largest defect supercells accessible by ab-initio electronic structure codes. In this work, we present a
phonon-interpolation method for Huang-Rhys spectral densities and optical lineshape functions on hypercells created by extending the defect-containing supercells on arbitrarily dense
phonon $q$-point grids. The method is based on reconstructing the transition-induced force associated with the optical excitation and using this localized force source to couple the defect transition to a densely sampled vibrational continuum. In this formulation, the local defect physics is obtained from ab initio supercell calculations, while the long-wavelength acoustic modes and the detailed structure of the host
phonon spectrum are recovered by diagonalizing interpolated dynamical
matrices in large hypercells. We demonstrate the method on the negatively charged nitrogen-vacancy centre in diamond between its ground $^{3}A_{2}$ and excited $^{3}E$ states. The transition-force is shown to be strongly localized around the defect, with converged localization measures obtained in a $4\times4\times4$ supercell accessible by density functional theory calculations. We interpolate the electron-
phonon coupling on hypercells up to $32\times32\times32$ corresponding to approximately 17 million atoms, thereby recovering smooth, continuous Huang-Rhys spectral densities with ultrafine spectral resolution. The dominant coupling band is found near 63~meV; the low-energy acoustic contribution follows the expected linear scaling; and we recover the finer van-Hove-related structures in the optical
phonon regime observed in experiments.
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Materials Science
Analytical Forces from the Bethe-Salpeter Equation for Large-Scale Excited-State Relaxation
Yu Jin, Victor Wen-zhe Yu, Marco Govoni et al. (+1 more)
We present an efficient plane-wave implementation of analytical
nuclear forces for electronic excited states described by the Bethe-Salpeter equation (BSE). The formulation combines density-
matrix perturbation theory with a Lagrangian approach, and avoids both explicit empty-state summations and the response calculations for each atomic displacement, required by conventional approaches based on density functional perturbation theory. Together with GPU acceleration, these advances make BSE forces calculations tractable for solid-state systems containing hundreds of atoms. We demonstrate the method on two point defects with distinct dielectric environments: the nitrogen-vacancy center in diamond, where BSE and time-dependent density functional theory (TDDFT) yield consistent excited-state relaxations, and the carbon-dimer defect in two-dimensional hexagonal boron nitride, where the screened electron-hole interaction included in the BSE stabilizes the localized defect excitation and corrects the relaxation pattern predicted by semilocal TDDFT. These results establish a scalable framework for BSE-level studies of excited-state relaxation and vibronic coupling in heterogeneous condensed systems.
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All authors Yu Jin, Victor Wen-zhe Yu, Marco Govoni, Giulia Galli
Strongly Correlated Electrons
Exact Nagaoka-to-spiral transition in the doped infinite-U triangular lattice
Yang Zhang and Cristian D. Batista
We study the single-hole infinite-$U$ Hubbard model on the triangular
lattice with nearest- and next-nearest-neighbor hoppings $t_1$ and $t_2$. A frustrating $t_2$ destabilizes Nagaoka ferromagnetism through a Lifshitz transition to a long-wavelength coplanar spiral at $t_{2,c}/t_1=-0.182$. We determine the critical point analytically by reducing the many-body problem to an effective two-body hole–
magnon scattering problem in which the hard-core hole–
magnon constraint is treated exactly. Numerical calculations confirm the spiral ground state and reveal a coherent
spin polaron with quasiparticle weight $Z\simeq0.92$ at the Lifshitz-shifted momentum $\mathbf{K}=-\mathbf{Q}^*$. These results establish the first
quantum instability of the triangular-
lattice Nagaoka ferromagnet and provide the
magnetic baseline for
magnon-mediated pairing at finite doping.
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Statistical Mechanics · Cross-listed from cs.SI
Component structure and percolation in block models
Riccardo Franchi and M. E. J. Newman
The
stochastic block model is a widely studied model of community structure in networks. Here we study the component structure and percolation properties of networks generated from this model and its variants, using exact methods based on probability generating functions. In particular, we derive expressions for the size of the
giant component and the distribution of small components in such networks and for the size of the percolating cluster and position of the percolation threshold for both node and edge percolation, for the original
stochastic block model and for its degree-corrected versions. In passing, we also develop a mapping between generating functions for microcanonical and canonical block models that allows us to generalize results for the former to the latter with minimal effort.
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Strongly Correlated Electrons · Cross-listed from quant-ph
Fermionic Genuine Multiparty Entanglement
James Allen, Liuke Lyu and William Witczak-Krempa
Entanglement can show fundamentally different behavior in fermionic systems. However, while bipartite measures of fermionic
entanglement have been established, genuine multiparty
entanglement (GME) in fermionic systems is much less understood. We introduce an efficiently computable measure (via semi-definite programming) of fermionic GME,
fermion genuine multiparty negativity (fGMN), and show that it is an
entanglement monotone and a natural multipartite extension of the fermionic negativity. Using this measure, we find many states which have fGMN but no non-fermionic GME, including large classes of fermionic stabilizer states. The fGMN also displays some major phenomenological differences to the bipartite fermionic negativity, such as finite sudden death points over separation and temperature.
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Mesoscale and Nanoscale Physics
High Order Geometric Channels for Nonlinear Transport in Bloch Bands
Sami Farrag, Eugene Mele and Tony Low
We develop a
geometric perturbation theory of Bloch states for perturbations coupling via the interband Berry connection. The gauge-invariant Bargmann trace builds the dressed dispersion and connection order by order as a hierarchy \(Q^{(N)}\), starting with the
quantum geometric tensor. Higher members encode multiband geometry beyond the
quantum metric and Berry
curvature. Connected amplitudes control vertex-order corrections, while strict-order corrections reduce to disconnected products. For a uniform electric field we find the fully coherent, purely
geometric sector of the third-order response.
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Materials Science
A high-dimensional neural network potential for finite-temperature phenomena in NiTi martensite
Petr Jaroš, Petr Sedlák, Petr Šesták et al. (+3 more)
We present a high-dimensional neural network potential (HDNNP) for the martensitic phase of the NiTi shape-
memory alloy trained to density functional theory (DFT) data. A central aspect of this work is the systematic validation of the potential with respect to the underlying DFT reference method for key properties governing structural
evolution, including
equilibrium crystal structures, elastic constants, generalized-stacking fault energies, and vibrational spectra. The HDNNP accurately describes the relative stability of the B19$^\prime$ and B33 phases, including subtle energy differences on the order of meV/atom. The predicted stacking-fault energy landscape is strongly anisotropic and reveals a preferential shear pathway, providing atomistic insight into deformation and twinning mechanisms. Finite-temperature molecular dynamics simulations further enable the investigation of unconstrained structural
evolution as a function of temperature. Overall, the developed HDNNP provides a robust basis for atomistic simulations of the complex structural and functional behavior of martensitic NiTi systems containing hundreds of thousands of atoms on nanosecond time scales.
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All authors Petr Jaroš, Petr Sedlák, Petr Šesták, Miroslav Černý, Jörg Behler, Hanuš Seiner
Strongly Correlated Electrons
Doping-induced Ferromagnetic order and its unusual evolution to Helical Antiferromagnetic Order in Sr(Ni$_{1-x}$Co$_x$)$_2$P$_2$
A. Sapkota, J. Schmidt, J. M. Wilde et al. (+6 more)
SrNi$_2$P$_2$ represents a unique case of a collapsed structural phase (one-third collapsed, where one out of every three P-P pairs forms a bond) in the A(TM)$_2$X$_2$ family of compounds (A = alkali metal, alkaline-earth metal, or rare earth; TM = transition metal; X = pnictogen). Furthermore, Co doping studies aimed at understanding the interrelationship between this unusual bonding motif and the resulting physical properties produced a magnetically rich phase diagram, specifically on the Co-rich side of the phase diagram. However, important questions remained regarding the detailed nature of the
magnetic ground states. To address these issues, we performed single-
crystal neutron diffraction measurements on Sr(Ni$_{1-x}$Co$_x$)$_2$P$_2$ with compositions $x = 0.88$, 0.94, and 0.97. For $x = 0.88$ and 0.94, the measurements reveal incommensurate helical
magnetic order with a doping-dependent propagation vector $(0,0,\tau)$, similar to that observed in Sr(Ni$_{1-x}$Co$_x$)$_2$As$_2$. In contrast, the $x = 0.97$ composition shows clear signatures of a ferromagnetically ordered ground state, resolving the earlier ambiguity regarding the nature of the low-temperature phase. Furthermore, our results highlight the subtle balance between these competing ground states, whose
evolution does not appear to be fully captured by the conventional frameworks of either itinerant or local-moment Heisenberg models typically applied to related 122 systems.
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All authors A. Sapkota, J. Schmidt, J. M. Wilde, L.-L. Wang, W. Tian, M. Matsuda, A. Kreyssig, S. L. Bud'ko, P. C. Canfield
Materials Science · Cross-listed from cs.RO
Scalable Low-Cost Laboratory Automation: A Digital Twin-Integrated Robotic Platform for Autonomous Liquid Handling (RAINBOT)
Mohamed Rami Ayeche, Souhil Sid, Ahyen Mostofa et al. (+3 more)
Laboratory automation accelerates discovery, yet its adoption is constrained by the high cost, proprietary design, and limited remote supervisability of commercial
liquid-handling systems. This work presents RAINBOT\textsuperscript{TM}, a low-cost, openly reproducible
liquid-handling
robot built by converting a consumer-grade Cartesian 3D printer (Elegoo Neptune 4 Max). The printer extruder is replaced by a precision single-channel pipette actuated through the printer's own G-code-driven X–Y–Z gantry, with plunger and tip-eject motions effected by two compact linear actuators under Python control. To make experiments transparent and remotely supervisable, a browser-based digital twin is implemented to synchronise bidirectionally with the physical platform, mirroring kinematics and pipetting states in real time and exposing remote monitoring, intervention, and an emergency stop from any web browser. As a proof of concept, RAINBOT\textsuperscript{TM} performed sequential exchanges of differently coloured aqueous solutions while an integrated colour sensor quantified the resulting mixtures; measured red, yellow, and blue (RYB) responses agreed with expected mixing behaviour to within a mean absolute error of two percentage points, validating correct execution and real-time tracking. Closing the loop, the platform is coupled to the CEID\textsuperscript{TM} (Cooperative Explorer for Inverse Design) framework, which recasts experimentation from iterative manual guessing into a goal-directed inverse-design search while keeping a human in the loop. The complete hardware costs under US\$1300, which is roughly an order of magnitude below entry-level commercial handlers, thereby establishing an accessible physical–virtual framework for self-driving laboratory automation.
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All authors Mohamed Rami Ayeche, Souhil Sid, Ahyen Mostofa, Rehaan Hussain, Ali Shayesteh, Fadwa El Mellouhi
Soft Condensed Matter · Cross-listed from math.GT
Writhe-Based Polymer Link Classification Using Machine Learning
Jack Beda, Djordje Mihajlovic, Kasturi Barkataki et al. (+1 more)
Unique and rapid classification of
knots and links is an open mathematical problem that is relevant to a range of (bio)physical systems, including
polymer melts, DNA, and
proteins. In this paper, we explore a data-driven approach to the classification problem of link
topology. Extending the framework introduced in Ref. 1 (Sleiman et al, 2024 Soft Matter, 20(1), pp.71-78), we show that a feedforward neural network trained on the writhe density
matrix classifies thermally equilibrated configurations of the first six
prime links with 97% accuracy. We demonstrate that this accuracy remains high across a range of temperatures and lengths of link components, while rapidly deteriorating with the addition of
topology-altering Gaussian noise; a result consistent with the writhe density
matrix containing features sensitive to
topology. Our results show that neural networks based on the writhe density
matrix efficiently classify two-component links, establishing machine learning as a promising tool for rapid classification of more complex link
topologies, e.g. Borromean rings and multi-component links, as the computational cost of exact numerical calculation of topological invariants becomes prohibitive.
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All authors Jack Beda, Djordje Mihajlovic, Kasturi Barkataki, Davide Michieletto
Quantum Gases · Cross-listed from quant-ph
Unitary $k$-designs without independent Hamiltonian quenches
Pratik Nandy
Unitary $k$-designs provide a resource-efficient framework for emulating Haar randomness up to the $k$-th order. Quench-based protocols have recently been shown to generate such designs, but achieving this typically requires multiple
Hamiltonian realizations, even when using temporal ensembles. Here, we show that no independent
Hamiltonians are required: a single chaotic
Hamiltonian is sufficient to generate approximate
unitary $k$-designs, even when that
Hamiltonian is spatially local. We introduce a two-Pauli-kick (2PK) protocol, in which
unitary evolution under a fixed
Hamiltonian is interspersed with two Pauli operator insertions (kicks). We find that the resulting frame potential approaches the Haar value at long times, with deviations suppressed by the inverse
Hilbert-space dimension. We verify this for single realizations of Gaussian random
matrices, the Majorana and
Spin Sachdev-Ye-Kitaev models, and deterministic local
quantum spin chains. Remarkably, in deterministic
spin systems, the 2PK protocol generates approximate
unitary designs in regimes where quench-based protocols are either inapplicable or fail to converge. Furthermore, our protocol provides a finite-temperature extension of the frame potential and establishes an analytic bound in terms of the
equilibrium partition function. We discuss a
holographic perspective on this mechanism.
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Materials Science
Magnetic ground states of CrPS$_4$ and NiPS$_3$ monolayers from long-range exchange interactions
Balazs Nagyfalusi, Ritesh Das, Alvaro Bermejillo-Seco et al. (+4 more)
We investigate the
magnetic properties of monolayer CrPS$_4$ and NiPS$_3$ by combining first-principles calculations, second-principles
spin models, and Monte Carlo simulations. Unlike conventional approaches that truncate exchange interactions after only a few shells and determine them by fitting total energies, we extract the
magnetic exchange tensors directly from density functional theory using the LKAG formalism and include interactions until numerical convergence is achieved. We show that long-range exchange interactions qualitatively modify the
magnetic behavior of both materials. In CrPS$_4$, they destabilize the previously predicted
ferromagnetic ground state and stabilize a
spin-spiral phase, reducing the critical temperature to about 21\,K, in agreement with available experiments. The resulting
magnetic phase diagram contains multiple collinear and non-collinear phases that can be tuned by temperature and external
magnetic fields. In NiPS$_3$, the experimentally observed zigzag
antiferromagnetic order only emerges when exchange interactions up to the fifth shell are included. These results demonstrate that quantitatively predictive
spin models for thiophosphate monolayers require long-range exchange interactions and provide a predictive framework for accurately describing two-dimensional van der Waals magnets.
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All authors Balazs Nagyfalusi, Ritesh Das, Alvaro Bermejillo-Seco, Herr S. J. van der Zant, Yaroslav M. Blanter, Amador Garcia-Fuente, Jaime Ferrer
Strongly Correlated Electrons
Yuan Xiao, Rimika Jaiswal, Oleg A. Starykh et al. (+1 more)
We study the zero-temperature properties of a
spin-$1/2$ Heisenberg chain with an additional three-site
chiral term of strength $g$. The model is known to be integrable and previous work using Bethe ansatz has identified a phase transition from the usual critical state for $g< g_c = 2/\pi$ to a
chiral phase for $g>g_c$. Here we combine analytical and computational approaches to demonstrate that the transition point comprises an unusual Lifshitz transition with dynamical critical exponent $z=3$, and the
chiral phase is a partially
spin-polarized phase which spontaneously breaks SU(2)
symmetry. We derive a strongly interacting
chiral boson field theory for the critical point, and show that even its small-momentum, low-energy response is non-trivial. We also determine the universal properties of the
chiral phase, and specifically show that it has a simultaneous quadratically dispersing
ferromagnetic "
magnon" mode coexisting with the power-law singularities and linearly dispersing excitations typical of a Luttinger
liquid. Our conclusions are validated using numerics based on
matrix product state methods and exact diagonalization.
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All authors Yuan Xiao, Rimika Jaiswal, Oleg A. Starykh, Leon Balents
Strongly Correlated Electrons
Colloquium: Semi-Dirac Fermions in Quantum Matter
Bruno Uchoa, Mohamed M. Elsayed, Valeri N. Kotov et al. (+2 more)
This article reviews the recent progress on the subject of semi-Dirac
fermions, two dimensional quasiparticles that disperse quadratically, as Galilean invariant particles, or linearly, as massless relativistic particles, depending on their direction of motion. These particles exist at a phase boundary set by the continuous change in the connectivity of
Fermi surfaces, known as topological Lifshitz transitions. The basic properties and the current experimental evidence of the existence of these particles in both synthetic
lattices and
quantum materials are presented. The many-body problem of semi-Dirac
fermions is discussed from a theoretical perspective with an eye to physical observables of relevance to experiments.
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All authors Bruno Uchoa, Mohamed M. Elsayed, Valeri N. Kotov, Yinming Shao, Dmitri N. Basov
Mesoscale and Nanoscale Physics
Ideal Bands in Tight-Binding Models
Lexu Zhao, Yang Ge, Shinsei Ryu et al. (+1 more)
A band is called ideal when its Dirichlet functional saturates the topological lower bound. We study ideal bands in finite-band tight-binding models with conventional two-dimensional
lattice translation
symmetries, allowing the bands to have non-flat dispersion. We first provide an analytic construction of isolated Chern-ideal bands with Chern number $|\mathrm{Ch}|=1$ in finite-band models with exponentially decaying hopping. This construction applies only when at least two orbitals have different embedded positions (modulo
lattice vectors), complementing the previously known construction for $|\mathrm{Ch}|>1$. We then show that isolated Chern-ideal bands with any nonzero Chern number cannot exist in finite-band models with finite-range hopping, regardless of the embedded orbital positions. The conclusion holds even if there are isolated band touching points, as long as the Berry
curvature does not diverge anywhere in the Brillouin zone. We finally generalize the conclusions to Wilson-loop-ideal bands with zero total Chern number, such as Kane-Mele $\mathbb{Z}_2$-ideal bands.
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All authors Lexu Zhao, Yang Ge, Shinsei Ryu, Jiabin Yu
Disordered Systems and Neural Networks · Cross-listed from quant-ph
Learning the closest Slater determinant
Nisarga Paul, Haimeng Zhao and David D. Dai
Learning compact, interpretable descriptions of
quantum many-body states is an important task in
quantum science. We study the task of learning the Slater determinant with maximum fidelity to an arbitrary fermionic many-body state, with motivation from both Hartree-Fock methods and agnostic
tomography. Given an $n$-
fermion wavefunction built from $m$ fermionic modes, we provide classical and
quantum algorithms returning a Slater determinant with fidelity within $\varepsilon$ of maximal in time $m^{\text{poly}(n,1/\varepsilon)}$. We prove matching hardness lower bounds, assuming standard complexity
conjectures, along some parameter axes. Given access to
quantum copies, we prove this can be accomplished with $\text{poly}(m,n,1/\varepsilon)$ copies of $\rho$. We also show that above a fidelity of $2/3$ any stationary point is the unique global maximum while below $2/3$ the optimization landscape can have spurious stationary points, and hence $2/3$ marks a transition point in the optimization landscape for this problem. We apply the
algorithm to the
Fermi-Hubbard model, extracting the closest Slater determinant from neural
quantum state solutions. Together, our results provide algorithmic tools with provable guarantees in understanding fermionic many-body systems with classical or
quantum simulation.
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Strongly Correlated Electrons · Cross-listed from hep-th
Sp(4,Z) actions on 3d U(1)^2 symmetric theories: Order-five duality and bilayer quantum Hall hierarchies
Yasin F. Alam, Andreas Karch, Da-Chuan Lu et al. (+1 more)
The $\mathrm{SL}(2,\mathbb{Z})$ electromagnetic duality of 4d Maxwell theory induces theory-generating operations on 3d theories with $U(1)$ global
symmetry. For theories with $U(1)^n$
symmetry, this structure generalizes to $\mathrm{Sp}(2n,\mathbb{Z})$. Focusing on the $U(1)^2$ case, we formulate the bulk $\mathrm{Sp}(4,\mathbb{Z})$ action and derive the corresponding boundary operations. We identify an intrinsically two-component element of order five, which generalizes the order-three $ST$ element of the single-$U(1)$ theory. Although its fifth power acts trivially in the bulk, the corresponding boundary operation closes only up to a decoupled $U(1)_1$ invertible phase, suggesting a mixed duality-gravitational anomaly. We realize the resulting theory-generating web for Abelian Chern-Simons theories within the $K$-
matrix formalism and apply it to
bilayer fractional
quantum Hall systems. Recasting the Haldane–Halperin hierarchy construction as a sequence of $\mathrm{SL}(2,\mathbb{Z})$ operations, we generalize it to systems with charge $U(1)_c$ and pseudospin $U(1)_s$
symmetries. The resulting
bilayer hierarchies contain branches terminating in an interlayer-correlated bosonic $(221)$ daughter sector, yielding candidate Abelian states at the even-denominator equal-layer fillings $3/8+3/8$ and $5/12+5/12$. Integral changes of anyon basis establish the equivalence of these states to their corresponding Abelian composite-
fermion descriptions. We further discuss the
spin-charge constraints that arise when the electromagnetic background field is treated as a
spin-$c$ connection.
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All authors Yasin F. Alam, Andreas Karch, Da-Chuan Lu, Ryan C. Spieler
Strongly Correlated Electrons
Monte Carlo Studies of Twisted Bilayer Graphene: Strain and Thermal Fluctuations
Johannes S. Hofmann, Patrick Ledwith, Ashvin Vishwanath et al. (+2 more)
We study the phase diagram of twisted
bilayer graphene at charge neutrality as a function of twist angle $\theta$, uniaxial heterostrain $\varepsilon$, and temperature $T$ using sign-problem-free
quantum Monte Carlo simulations. At $T=0$ and zero strain, we find a continuous transition from a Dirac semimetal to a gapped Kramers inter-valley coherent (KIVC) phase as $\theta$ decreases toward the magic angle. With finite strain, the KIVC phase undergoes a further continuous transition at smaller $\theta$ into an anisotropic semimetal with gapless excitations near the center of the moiré Brillouin zone. In the KIVC regime, the
entropy rises sharply with temperature and plateaus at $15\,\text{K} \lesssim T \lesssim 40\,\text{K}$ near the value expected from a Mott-like regime of localized electrons with nearly uncorrelated
spin, valley, and orbital degrees of freedom, despite the topological obstruction preventing a localized tight-binding description of the active bands. The spectral function evolves continuously with $\theta$: at low $T$, a gap opens at the $K$ points and the minimal gap shifts to $\Gamma$ as $\theta$ decreases; at intermediate $T$, the spectral function smoothly interpolates between a Dirac semimetal spectrum with coherent $K$-point quasiparticles and a spectrum with gapless $\Gamma$-centered quasiparticles near the magic angle. The later can be a anisotropic or a Mott semimetal and we discuss how to distiguish them in experiment.
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All authors Johannes S. Hofmann, Patrick Ledwith, Ashvin Vishwanath, Jong Yeon Lee, Erez Berg
Statistical Mechanics · Cross-listed from quant-ph
Perturbatively Stable Self-Correcting Classical Memory from Gauge Averaging
Ryan Thorngren
We show that the self-correcting
memory in 3d Wegner gauge theory is stable to arbitrary small enough perturbations of the
Hamiltonian. Our proof relies on a new method we dub “gauge averaging”, which gives conditions under which explicitly broken gauge
symmetries are effectively restored by fluctuations. These conditions show that the self-correcting
memory phase is fluctuation stabilized, with its robustness to perturbations increasing with increasing temperature, up to some $T_c > 0$.
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Statistical Mechanics · Cross-listed from quant-ph
Autonomization of Quantum Systems and the Emergence of the Work Operator
Carlo Cepollaro, Danial Chughtai, Alberto Spalvieri et al. (+2 more)
Estimation of work at the
quantum scale remains a central challenge in
quantum thermodynamics. Canonical approaches have a fundamental drawback: they assume classical control of the
quantum system, as implicit in a time-dependent
Hamiltonian. Yet the energetic cost of implementing this control is omitted and can exceed the system's energy scale by orders of magnitude, calling into question the operational significance of work values. We address this by autonomizing the controlled energy transfer,
embedding the driven dynamics into an energy-conserving
evolution on a larger
quantum system. Work is then unambiguously identified with the energy transferred between the two systems, singling out a unique observable on the driven system: the work operator. We show that the
quantum work operator evades a no-go
theorem by deriving a
quantum fluctuation
theorem that recovers the Jarzynski equality in classical scenarios. We incorporate imperfect control and quantify corrections to work statistics. Extending the framework to open
quantum systems, we obtain an operatorial first law in which work, heat, and internal-energy changes are represented by distinct operators on the reduced system. Together, these results settle the long-standing debate over whether work is a
quantum observable: it is, once the controlling system is included in the description rather than treated as external.
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All authors Carlo Cepollaro, Danial Chughtai, Alberto Spalvieri, Marcus Huber, Alberto Rolandi
Materials Science
Lithium-Projected Phonon Spectral Distributions as Robust Descriptors of Ionic Conductivity in Solid Electrolytes
Gajendra Bohara and Ramakrishna Podila
Lattice dynamics are widely invoked in the design of solid electrolytes, yet
phonon information is commonly compressed into a band center or another scalar softness measure. Here we test whether the complete lithium-projected
phonon density of states (Li-PDOS) provides a reproducible descriptor of experimental room-temperature ionic conductivity. MatterSim forces and Phonopy were used to generate harmonic total and Li-projected spectra for crystallographically resolved entries in the OBELiX
dataset. A composition and structure audit defined a primary cohort of 260 materials (212 train and 48 test), a strict cohort of 241, and an exact-composition cohort of 168. Across 20 independently generated
phonon-calculation database comparisons, the mean Wasserstein-1 distance was 0.542 THz for total DOS and 0.731 THz for Li-PDOS, revealing broad agreement but systematic, projection-dependent softening. Higher conductivity was associated with redistribution of normalized Li spectral weight toward low frequencies: in the untouched test set, the Li fractions below 2 and 5 THz had Spearman coefficients of 0.333 and 0.374, while the 5$\%$ cumulative-frequency
quantile had a coefficient of $-0.393$. A Wasserstein kernel on the full Li-PDOS achieved held-out $R^2=0.444$, compared with 0.012 for total DOS and 0.181 for a static composition–structure kernel. The Li model remained stable in the strict ($R^2=0.462$) and exact ($R^2=0.451$) cohorts. Family adjustment attenuated scalar associations, and Li-versus-total whole-spectrum dependence was cohort sensitive. The results therefore support mobile-ion-resolved spectral distributions as useful comparative screening descriptors, not as a universal causal softness law.
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Strongly Correlated Electrons
Quantum Fisher Information in semiclassical magnon systems
Wolfgang Simeth, Pontus Laurell and Allen Scheie
Quantum Fisher Information (QFI) is a powerful spectroscopic tool to witness many-body
quantum entanglement in solid state materials, but it is not always obvious how to relate it to other characteristics of condensed matter systems. In this study we elaborate on the meaning and interpretation of QFI in condensed matter by examining simple theoretical
spin systems. We use finite sized
spin systems to illustrate that QFI quantifies the momentum- dependent degree of
quantum entanglement (that is the
entanglement depth at a given wave vector) within a wave function. We subsequently use semiclassical frustrated
spin models to show that, in the context of linear
spin wave theory (LSWT), QFI quantifies the momentum-dependent degree of
magnon squeezing in the ground state. In antiferromagnets with a zero-energy Goldstone mode, LSWT breaks down and QFI (and
entanglement depth) diverges at the
magnetic ordering wave vector. We also show examples of emergent
quantum phases in frustrated
spin systems that do not appear in classical phase diagrams. When these emergent phases are approached, QFI diverges across multiple wave vectors in momentum space. Taken together, QFI is not only helpful as a lower bound for
entanglement depth, but serves as a momentum-resolved probe of
entanglement that offers a novel perspective on
quantum critical phenomena.
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Statistical Mechanics
Stable valleys in the glassy landscape of a low-density parity-check (LDPC) code
Grace M. Sommers and David A. Huse
Classical low-density parity-check (cLDPC) codes defined on expander graphs are a fundamental ingredient in the construction of good
quantum LDPC codes, a recent milestone in
quantum error correction. They also define interesting statistical mechanics models in their own right, as they include examples of
spin glass order without quenched randomness or frustration. We investigate this via a case study of a cLDPC code on a locally tree-like expander graph. Recursive techniques on trees, made possible by the locally tree-like property, probe a menagerie of stable, incongruent valleys induced by imposing different boundary conditions at low temperature. Analysis of stable and unstable fixed points within the cavity method, complemented by a numerical study of the valleys on closed finite graphs, reveals the inequivalence of the microcanonical and canonical ensemble for certain valleys.
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Statistical Mechanics
Optimal Finite-Time Control of Nonreciprocal Brownian Dimers: Thermodynamic Anomaly and Multiple Transitions
Ruicheng Bao
We solve exactly a finite-time thermodynamic optimal control problem for two nonreciprocally interacting Brownian particles translated by two harmonic traps. The controller manipulates both the center and separation of the pair. Nonreciprocal interactions generate an internal active force that couples these two channels. The optimal protocol is oscillatory, deliberately opens the dimer even when the target separation is unchanged, and can extract work during transport. A central finding is a finite critical time beyond which the external-work infimum is $-\infty$: at any prescribed duration beyond this threshold, both extractable work and output power are unbounded. Physical regularizations such as finite trap range and force saturation restore a finite optimum and convert the anomaly into optimal-protocol transitions: in the zero-target case, a hard finite range produces a first-order-like jump from the zero protocol to a maximum-range protocol, whereas smooth force saturation gives a continuous, second-order-like onset. Under finite-range constraints, the optimal protocol can further undergo multiple finite-time transitions, producing multiple work-duration kinks with no qualitative analog in prior studies.
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Quantum Gases
Fermionic pairs, from the surface to the bulk
Sandra Brandstetter, Carl Heintze, Fabian Brauneis et al. (+4 more)
Fermion pairing underlies collective
quantum phenomena across widely different forms of matter. In extended systems such as ultracold
Fermi gases, pairing is commonly understood through the BCS–BEC crossover, where the pair size evolves from large, overlapping Cooper pairs to tightly bound dimers. In finite systems such as atomic
nuclei, superconducting grains and
quantum dots, however, the same pairing tendency competes with confinement, shell filling and spatial inhomogeneity, making the microscopic structure of pairs much harder to access. Here, we image pair correlations in a finite, tunable system of few fermionic atoms with single-particle resolution and full counting statistics. We observe that confinement and shell structure re-organize pairing in real space: In the weakly interacting, confinement-dominated regime, closed-shell configurations suppress correlations in the high-density trap center. Pairing is mainly observed toward the low-density surface. Open-shell systems, however, support substantially stronger central pairing. Already for surprisingly small systems, increasing either interaction strength or particle number restores a locally bulk-like Cooper-pair profile in the trap center, whereas the edge retains dimer-like correlations. By resolving where pairs form and how their character changes from localized dimers to overlapping Cooper pairs, our measurements provide a microscopic view of pairing in finite fermionic matter and connect the physics of mesoscopic cold atoms to pairing phenomena in
nuclei and superconducting nanostructures.
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All authors Sandra Brandstetter, Carl Heintze, Fabian Brauneis, Stephanie M. Reimann, Georg Bruun, Maciej Gałka, Selim Jochim
Superconductivity
The exact universal exchange-only local field factor of the uniform electron gas and its near-cancellation by parallel-spin correlation near 2kF
Carl A. Kukkonen
The exchange-only local field factor Gx(q,omega) of the uniform electron gas is exact and universal, a single curve at every density when q is measured in units of kF and omega in units of the
Fermi energy. This paper assembles the known pieces of the static Gx(q,0) into one expression, evaluates it exactly at every wavevector, and releases it as a short Python module. It rises as (q/kF)^2/4, has a large peak of 1.99 just below 2kF, passes through the exact value pi^2/6 at 2kF, and tends to 1/3. Used alone in static linear response it predicts a
spin density wave at rs = 4.96 and a charge density wave at rs = 10.62.
Quantum Monte Carlo shows the gas has neither. Exchange enters the measured density and
spin factors G+ and G- identically, so their difference, twice the antiparallel-
spin factor, contains no exchange at all and is measured to be small and smooth through 2kF. Whatever removed the exchange peak is therefore in the parallel-
spin channel, where everything beyond first-order exchange is large, negative, and mirrors the exchange peak. In the measured G+ the net removal is 40 to 48 percent of the peak at rs = 1 to 10. The stability of the metallic electron gas against exchange-driven density waves is a parallel-
spin effect, and the decomposition shows where microscopic calculations of each channel should go.
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Materials Science · Cross-listed from physics.optics
Probing the temperature dependence of dielectric function of ternary transition metal dichalcogenides: towards thermo-driven ultrathin photonic components
Artsruni Margaryan, Maria Levonyan, Maksim Sargsyan et al. (+4 more)
Transition metal dichalcogenides (TMDs), along with their ternary derivatives, have attracted considerable
attention mostly due to pronounced excitonic resonances emerging in visible (Vis) and near-infrared (NIR) spectral regions, enabling strong light-matter interaction. Nevertheless, a comprehensive insight of the temperature-dependent optical dispersions for the most of representatives of the family remains yet unrevealed. Here, we report on systematic studies of dielectric permittivity functions of uniaxial ternary MoSSe and WSSe across 430-1000 nm spectral region over a broad temperature window of 80-670 K. We show that the temperature
evolution of their dielectric responses is governed by Varshni's formalism in Vis spectral region further affecting their high refractive index properties at the lossless NIR spectral tails. Furthermore, we exploit the measured optical dispersion of ternary WSSe designing ultrathin plano-convex NIR photonic lenses that demonstrate continuous modulation of performance with temperature variation. Our work provides critical insights for the creation of next-generation thermo-driven nanophotonic and optoelectronic devices.
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All authors Artsruni Margaryan, Maria Levonyan, Maksim Sargsyan, David Karakhanyan, Meri Hayrapetyan, Kostya S. Novoselov, Davit A. Ghazaryan
Superconductivity
Stacking-tuned superconductivity and competing charge-density-wave states in NbSe$_2$
Sandra Sajan, Xinze Yang, Haojie Guo et al. (+9 more)
Layer stacking provides a powerful yet underexplored route for reshaping collective
quantum order in van der Waals materials. Here we use high-resolution scanning tunneling microscopy and spectroscopy to show that the stacking sequence alone can qualitatively transform the charge density and superconducting orders in NbSe$_2$, while preserving the same in-plane atomic structure. Comparing the 4Ha and 2H polytypes, we find that, unlike the ubiquitous triangular incommensurate $3Q^\mathrm{I}$ order of 2H-NbSe$_2$, 4Ha-NbSe$_2$ hosts two competing CDW states with no measurable correlation with local strain: a unidirectional commensurate $1Q^\mathrm{C}$ phase and a triangular incommensurate $3Q^\mathrm{I}$ phase, with $Q^\mathrm{I}=Q^\mathrm{C}+\delta$. We introduce a phase-resolved analysis that directly maps the gradient of the CDW phases and reveals
vortices bound to the $1Q^\mathrm{C}$ - $3Q^\mathrm{I}$ phase boundaries. These
vortices accommodate the momentum mismatch $\delta$ through abrupt $2\pi$ phase slips, providing a mechanism by which distinct charge orders coexist. Superconductivity is also reshaped by stacking, while both polytypes exhibit multiband pairing.
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All authors Sandra Sajan, Xinze Yang, Haojie Guo, Tarushi Agarwal, Samuel Mañas-Valero, Carla Boix-Constant, Eugenio Coronado, Fernando de Juan, Eduardo H. da Silva Neto, Ravi P. Singh, Maria N. Gastiasoro, Miguel M. Ugeda