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Physics Condensed Matter

cond-mat Watch Condensed Matter

Wednesday 29 October

Soft Condensed Matter

Associative and Segregative Liquid-Liquid Phase Separation in Macromolecular Solutions

Remco Tuinier and Alvaro Gonzalez Garcia

We investigate liquid-liquid phase separation (LLPS) and interfacial properties of two LLPS modes: associative (ALLPS) and segregative (SLLPS). Analytical expressions for the critical point (CP) and binodal boundaries are derived and show excellent agreement with self-consistent field (SCF) lattice computations. Distinct thermodynamic features differentiate ALLPS from SLLPS: (1) in ALLPS, polymers co-concentrate within a single dense phase coexisting with a solvent-rich phase, whereas in SLLPS each polymer forms a separate phase; (2) the attractive interaction per monomer in ALLPS is strongly dependent on solvent quality, but solvent-independent in SLLPS; and (3) ALLPS binodals exhibit near-universal behavior, largely independent of solvent content. SCF results further show that interfacial tension increases and interfacial width decreases with distance from the CP. We provide scaling relations for both quantities are provided. Compared with SLLPS, ALLPS displays higher interfacial tension and a thinner interface, reflecting distinct molecular organization at the liquid-liquid boundary.
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Mesoscale and Nanoscale Physics

Strongly enhanced lifetime of higher-order bimerons and antibimerons

Shiwei Zhu, Moritz A. Goerzen, Changsheng Song et al. (+2 more)

Magnetic bimerons, similar to skyrmions, are topologically nontrivial spin textures characterized by topological charge $Q$. Most studies so far have focused on low-$Q$ solitons ($|Q| \leq 1$), such as skyrmions, bimerons, and vortices. Here, we present the first calculations of the lifetimes of {ring-like} high-$Q$ bimerons and demonstrate that they are fundamentally more stable than high-$Q$ skyrmions over a wide range of temperature. To obtain realistic results, our chosen system is an experimentally feasible van der Waals interface, Fe$_3$GeTe$_2$/Cr$_2$Ge$_2$Te$_6$. We show that the lifetimes of high-$Q$ (anti)bimerons can exceed the lifetime of those with $|Q|=1$ by 3 orders of magnitude. Remarkably, this trend remains valid even when extrapolated to room temperature (RT), as the lifetimes are dominated by entropy rather than energy barriers. This contrasts with high-$Q$ skyrmions, whose lifetimes fall with $|Q|$ near RT. We attribute this fundamental difference between skyrmions and bimerons to their distinct magnetic texture symmetries, which lead to different entropy-dominated lifetimes.
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All authors Shiwei Zhu, Moritz A. Goerzen, Changsheng Song, Stefan Heinze, Dongzhe Li

Mesoscale and Nanoscale Physics · Cross-listed from hep-ph

Vorticity-induced effects from Wess-Zumino-Witten terms

Geraint W. Evans, Naoki Yamamoto and Di-Lun Yang

We study vorticity-induced effects arising from the Wess-Zumino-Witten terms for Nambu-Goldstone modes in chiral perturbation theory. We first provide an alternative derivation of the Wess-Zumino-Witten terms in the presence of external vector, axial-vector, and pseudoscalar fields using a derivative expansion of the fermion determinant. We then employ the previously found correspondence in which vorticity is treated as an axial-vector field coupled to Dirac fermions in flat spacetime. Using this, we derive vorticity-induced contributions for Nambu-Goldstone modes in the presence of electromagnetic fields at finite baryon and isospin chemical potentials, including a vorticity-induced current, a magnetic-field-induced angular momentum, and a vorticity-modified photon-pion coupling. We also briefly discuss the phenomenological implications of these vorticity-induced effects.
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Statistical Mechanics

Impact of fluctuations on particle systems described by Dean-Kawasaki-type equations

Nathan O. Silvano, Emilio Hernández-García and Cristóbal López

We study the role of fluctuations in particle systems modeled by Dean-Kawasaki-type equations, which describe the evolution of particle densities in systems with Brownian motion. By comparing microscopic simulations, stochastic partial differential equations, and their deterministic counterparts, we analyze four models of increasing complexity. Our results identify macroscopic quantities that can be altered by the conserved multiplicative noise that typically appears in the Dean-Kawasaki-type description. We find that this noise enhances front propagation speed in systems with density-dependent diffusivity, accelerates the onset of pattern formation in particle systems with nonlocal interactions, and reduces hysteresis in systems interacting via repulsive forces. In some cases, it accelerates transitions or induces structures absent in deterministic models. These findings illustrate that (conservative) fluctuations can have constructive and nontrivial effects, emphasizing the importance of stochastic modeling in understanding collective particle dynamics.
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Mesoscale and Nanoscale Physics

Strongly nonlinear Bernstein modes in graphene reveal plasmon-enhanced near-field magnetoabsorption

I. Yahniuk, I.A. Dmitriev, A.L. Shilov et al. (+12 more)

Bernstein modes – hybrid magnetoplasmon excitations arising from the coupling between cyclotron motion and collective oscillations in two-dimensional electron systems – offer direct access to non-local electrodynamics. These modes can exhibit rich nonlinear behavior akin to strong-coupling phenomena in cavity quantum electrodynamics, but reaching nonlinear regime has remained experimentally challenging. Here we report the observation of nonlinear Bernstein modes in graphene using terahertz excitation with near-field enhancement from embedded metallic contacts. Photoresistance spectroscopy reveals sharp resonances at Bc/2 and Bc/3 that saturate at radiation intensities nearly an order of magnitude lower than the cyclotron resonance. We ascribe this to strong local heating of the electron gas due to resonant excitation of high-amplitude Bernstein magnetoplasmons, associated with a combination of the field-concentration effect of the near field and plasmonic amplification that is resonantly enhanced in the region of Bernstein gaps. Polarization-resolved measurements further confirm the near-field origin: Bernstein resonances are insensitive to circular helicity but strongly depend on the angle of linear polarization, in sharp contrast to the cyclotron resonance response. Our results establish graphene as a platform for nonlinear magnetoplasmonics, opening opportunities for strong-field manipulation of collective electron dynamics, out-of-equilibrium electron transport, and solid-state analogues of cavity quantum electrodynamics.
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All authors I. Yahniuk, I.A. Dmitriev, A.L. Shilov, E. Mönch, M. Marocko, J. Eroms, D. Weiss, P. Sadovyi, B. Sadovyi, I. Grzegory, W. Knap, J. Gumenjuk-Sichevska, J. Wunderlich, D. A. Bandurin, S. D. Ganichev

Materials Science

Finite-Temperature Ferroelectric Phase Transitions from Machine-Learned Force Fields

Kristoffer Eggestad, Ida C. Skogvoll, Øystein Gullbrekken et al. (+2 more)

Simulating finite temperature phase transitions from first-principles is computationally challenging. Recently, molecular dynamics (MD) simulations using machine-learned force fields (MLFFs) have opened a new avenue for finite-temperature calculations with near-first-principles accuracy. Here we use MLFFs, generated using on-the-fly training, to investigate structural phase transitions in four of the most well-studied ferroelectric oxides; BaTiO$_3$, PbTiO$_3$, LiNbO$_3$ and BiFeO$_3$. Only using the 0 K ground state structure as input for the training, the resulting MLFFs can qualitatively predict all the main structural phases and phase transitions, while the quantitative results are sensitive to the choice of exchange correlation functional with PBEsol found to be more robust than LDA and r$^2$SCAN. MD simulations also reproduce the experimentally observed order-disorder character of Ti displacements in BaTiO$_3$, the abrupt first order transitions of BiFeO$_3$ and PbTiO$_3$, and the mixed order-disorder and displacive character of the ferroelectric transition in LiNbO$_3$. Finally, we discuss the potential and limitations of using MLFFs for simulating ferroelectric phase transitions.
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All authors Kristoffer Eggestad, Ida C. Skogvoll, Øystein Gullbrekken, Benjamin A. D. Williamson, Sverre M. Selbach

Statistical Mechanics

Schrödinger-invariance in non-equilibrium critical dynamics

Malte Henkel and Stoimen Stoimenov

The scaling functions of single-time and two-time correlators in systems undergoing non-equilibrium critical dynamics with dynamical exponent ${z}=2$ are predicted from a new time-dependent non-equilibrium representation of the Schrödinger algebra. These explicit predictions are tested and confirmed in the ageing of several exactly solvable models.
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Mesoscale and Nanoscale Physics

Toward a Deterministic Nucleation Theory for Chirality-Controlled Nanotube Synthesis

Zhengrong Guo

The electronic properties of carbon nanotubes are governed by their chirality, specified by the integer indices (n,m). While chirality-controlled synthesis has achieved notable successes, theoretical understanding remains predominantly focused on post-nucleation growth. Two fundamental obstacles impede deeper insight: the absence of a clear description of nucleation cap topology and its connection to tube chirality, and an incomplete understanding of atomic-level mechanisms governing templated cap formation. Here we address these challenges directly. First, we develop a mathematically rigorous topological framework for carbon networks that provides both a concise definition of cap structures and a quantitative relationship between cap architecture and chirality-the vector sum rule. Second, contrary to conventional perspectives attributing chirality enrichment to edge matching during growth, we demonstrate that chirality is deterministically encoded during nucleation through selective formation of specific cap structures on catalyst surfaces. For the specific case of (12,6) nanotubes, we show that their enrichment arises from a six-fold symmetric cap with epitaxial matching to catalyst facets. Our deterministic nucleation theory not only provides a coherent explanation for chirality enrichment but also elucidates its pattern in chirality space. This work establishes a theoretical framework that redefines the field, shifting the paradigm from stochastic growth kinetics to deterministic nucleation programming and paving the way toward predictable synthesis.
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Mesoscale and Nanoscale Physics

Effects of interlayer Dzyaloshinskii-Moriya interaction on the shape and dynamics of magnetic twin-skyrmions

Tim Matthies, Levente Rózsa, Roland Wiesendanger et al. (+1 more)

Magnetic skyrmions have been proposed as promising candidates for storing information due to their high stability and easy manipulation by spin-polarized currents. Here, we study how these properties are influenced by the interlayer Dzyaloshinskii–Moriya interaction (IL-DMI), which stabilizes twin-skyrmions in magnetic bilayers. We find that the spin configuration of the twin-skyrmion adapts to the direction of the IL-DMI by elongating or changing the helicities in the two layers. Driving the skyrmions by spin-polarized currents in the current-perpendicular-to-plane configuration, we observe significant changes either in the skyrmion velocity or in the skyrmion Hall angle depending on the current polarization. These findings unravel further prospects for skyrmion manipulation enabled by the IL-DMI.
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All authors Tim Matthies, Levente Rózsa, Roland Wiesendanger, Elena Y. Vedmedenko

Materials Science

The Microscopic Nature of Orbital Disorder in LaMnO$_{3}$

Bodoo Batnaran, Andrew L. Goodwin, Michael A. Hayward et al. (+1 more)

We present a revised atomistic picture of the order-disorder transition in the archetypal orbital-ordered perovskite material, LaMnO$_{3}$. Our study uses machine-learning-driven molecular-dynamics simulations which describe the temperature evolution of pair distribution functions in close agreement with experiment. We find the orbital-disordered phase in LaMnO$_{3}$ to comprise a mixture of differing structural distortions with and without inversion symmetry, implying a mixture of different orbital arrangements. These distortions are highly dynamic with an estimated lifetime of $\sim 40$ fs at 1,000 K, and their fluctuations converge with the timescales of conventional thermal motion in the high-$T$ phase - indicating that the electronic instability responsible for static Jahn-Teller distortions at low temperature instead drives phonon anharmonicity at high temperatures. Beyond LaMnO$_{3}$, our work opens an avenue for studying a wider range of correlated materials.
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All authors Bodoo Batnaran, Andrew L. Goodwin, Michael A. Hayward, Volker L. Deringer

Mesoscale and Nanoscale Physics

Stability of Planar Slits in Multilayer Graphite Crystals

Alexander V. Savin and Artem P. Klinov

Using a two-dimensional coarse-grained chain model, planar slits in multilayer graphite crystals are simulated. It is shown that when covering a linear cavity on the flat surface of a graphite crystal with a multilayer graphene sheet, an open (unfilled slit) can form only if the cavity width does not exceed a critical value L_o (for width L>L_o, only a closed state of the slit is formed, with the cavity space filled by the covering sheet). The critical width of the open slit L_o increases monotonically with the number of layers K in the covering sheet. For a single-layer cavity, there is a finite critical value of its width L_o<3nm, while for two- and three-layer cavities, the maximum width of the open slit increases infinitely with increasing K as a power function K^\alpha with exponent 0<\alpha<1. Inside the crystal, two- and three-layer slits can have stable open states at any width. For a slit with width L>7.6nm, a stationary closed state is also possible, in which its lower and upper surfaces adhere to each other. Simulation of thermal oscillations showed that open states of two-layer slits with width L<15nm are always stable against thermal oscillations, while wider slits at T>400K transition from the open to the closed state. Open states of three-layer slits are always stable against thermal oscillations.
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Statistical Mechanics

Nonreciprocity as a Generic Mechanism for Demixing in Flocking Mixtures

Charlotte Myin and Benoît Mahault

We show that even weak nonreciprocal alignment leads to large-scale structure formation in flocking mixtures. By combining numerical simulations of a binary Vicsek model and the analysis of coarse-grained continuum equations, we demonstrate that nonreciprocity destabilizes the ordered phase formed by mutually aligning or anti-aligning species in a large part of the phase diagram. For aligning populations, this instability results in one species condensing in a single band that travels within a homogeneous liquid of the other species. When interactions are anti-aligning, both species self-assemble into polar clusters with large-scale chaotic dynamics. In both cases, the emergence of structures is accompanied by the demixing of the two species, despite the absence of repulsive interactions. Our theoretical analysis allows us to elucidate the origin of the instability, and show that it is generic to nonreciprocal flocks.
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Soft Condensed Matter

Self-organization, Memory and Learning: From Driven Disordered Systems to Living Matter

Muhittin Mungan, Eric Clement, Damien Vandembroucq et al. (+1 more)

Disordered systems subject to a fluctuating environment can self-organize into a complex history-dependent response, retaining a memory of the driving. In sheared amorphous solids, self-organization is established by the emergence of a persistent system of mechanical instabilities that can repeatedly be triggered by the driving, leading to a state of high mechanical reversibility. As a result of self-organization, the response of the system becomes correlated with the dynamics of its environment, which can be viewed as a sensing mechanism of the system's environment. Such phenomena emerge across a wide variety of soft matter systems, suggesting that they are generic and hence may depend very little on the underlying specifics. We review self-organization in driven amorphous solids, concluding with a discussion of what self-organization in driven disordered systems can teach us about how simple organisms sense and adapt to their changing environments.
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All authors Muhittin Mungan, Eric Clement, Damien Vandembroucq, Srikanth Sastry

Materials Science

Terahertz Time-Domain Spectroscopy and Density Functional Theory Analysis of Low-Frequency Vibrational Modes of a Benzoxazolium-Coumarin Donor-pi-Acceptor Chromophore

Sidhanta Sahu, Phalguna Krishna Das Vana, Anupama Chauhan et al. (+5 more)

To elucidate low-frequency vibrational modes, we investigate a benzoxazolium–coumarin (BCO+) donor-pi-acceptor derivative using transmission terahertz time-domain spectroscopy (THz-TDS). The retrieved complex refractive index reveals distinct modes at 0.62, 0.85, 1.30, 1.81, and 2.07 THz. Gas-phase density functional theory (DFT) agrees well with these features and enables assignment of specific intramolecular motions. Together, THz-TDS and DFT identify the characteristic low-frequency modes of BCO+ and suggest their connection to intramolecular charge transfer-relevant nuclear motions, highlighting that THz-TDS can serve as a sensitive probe of vibrational signatures in donor-pi-acceptor systems.
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All authors Sidhanta Sahu, Phalguna Krishna Das Vana, Anupama Chauhan, Poulami Ghosh, Vijay Sai Krishna Cheerala, Sanyam, C. N. Sundaresan, N. Kamaraju

Statistical Mechanics

Thermodynamics of Biological Switches

Roger D. Jones, Achille Giacometti and Alan M. Jones

We derive a formulation of the First Law of nonequilibrium thermodynamics for biological information-processing systems by partitioning entropy in the Second Law into microscopic and mesoscopic components and by assuming that natural selection promotes optimal information processing and transmission. The resulting framework demonstrates how mesoscopic information-based subsystems can attain nonequilibrium steady states (NESS) sustained by external energy and entropy fluxes, such as those generated by ATP/ADP imbalances in vivo. Moreover, mesoscopic systems may reach NESS before microscopic subsystems, leading to ordered structures in entropy flow analogous to eddies in a moving stream.
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Quantum Gases

Entanglement-enhanced correlation propagation in the one-dimensional SU($N$) Fermi-Hubbard model

Mathias Mikkelsen and Ippei Danshita

We investigate the dynamics of correlation propagation in the one-dimensional Fermi-Hubbard model with SU($N$) symmetry when the repulsive-interaction strength is quenched from a large value, at which the ground state is a Mott-insulator with $1/N$ filling, to an intermediate value. From approximate analytical insights based on a simple model that captures the essential physics of the doublon excitations, we show that entanglement in the initial state leads to collective enhancement of the propagation velocity $v_{\text{SU}(N)}$ when $N>2$, becoming equal to the velocity of the Bose-Hubbard model in the large-$N$ limit. These results are supported by numerical calculations of the density-density correlation in the quench dynamics for $N=2,3,4,$ and $6$.
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Materials Science

Colloidal quasi-2D Cs2AgBiBr6 double perovskite nanosheets: synthesis and application as high-performance photodetectors

Pannan I. Kyesmen, Eugen Klein, Brindhu Malani S et al. (+2 more)

The search for non-toxic lead-free halide perovskites that can compete with the lead-based counterparts has led to the emergence of double perovskites as potential candidates. Among many options, Cs2AgBiBr6 stands out as one of the most suitable eco-friendly materials for numerous optoelectronic applications. In this study, quasi-2D Cs2AgBiBr6 nanosheets (NSs) were prepared via the low-temperature injection colloidal synthesis and used to fabricate high-performance photodetectors in a transport-layer-free architecture. The reaction temperature and ligands played vital roles in the structural purity, shape, and size of the synthesized Cs2AgBiBr6 NSs. The fabricated NSs disclosed lateral sizes of up to 1.4 um and are only a few nanometers thick. The high-performance photodetectors fabricated using the Cs2AgBiBr6 NSs yielded a high detectivity (D) of 1.15*10^12 Jones, responsivity (R) of 121 mA/W, a notable on/off ratio of 2.39*10^4, and a fast rise and decay time of 857 and 829 us, respectively. The device demonstrates remarkable stability. Basically, it sustains its entire photocurrent after storage in ambient conditions for 80 days. This work showcases a pathway for the colloidal synthesis of quasi-2D Cs2AgBiBr6 lead-free double perovskite NSs with suitable properties for high-performance photodetection and other optoelectronic applications.
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All authors Pannan I. Kyesmen, Eugen Klein, Brindhu Malani S, Rostyslav Lesyuk, Christian Klinke

Strongly Correlated Electrons

Immobile and mobile excitations of three-spin interactions on the diamond chain

M. Bayer, M. Vieweg and K.P. Schmidt

We present a solvable one-dimensional spin-1/2 model on the diamond chain featuring three-spin interactions, which displays both, mobile excitations driving a second-order phase transition between an ordered and a $\mathbb{Z}_2$-symmetry broken phase, as well as non-trivial fully immobile excitations. The model is motivated by the physics of fracton excitations, which only possess mobility in a reduced dimension compared to the full model. We provide an exact mapping of this model to an arbitrary number of independent transverse-field Ising chain segments with open boundary conditions. The number and lengths of these segments correspond directly to the number of immobile excitations and their respective distances from one another. Furthermore, we demonstrate that multiple immobile excitations exhibit Casimir-like forces between them, resulting in a non-trivial spectrum.
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Statistical Mechanics · Cross-listed from hep-th

NLIE formulations for the generalized Gibbs ensemble in the sine-Gordon model

Arpad Hegedus

In this paper we propose two sets of nonlinear integral equations (NLIE) for describing the thermodynamics in the sine-Gordon model, when higher Lorentz spin conserved charges are also coupled to the Gibbs ensemble. We call them NLIE I and II. The derivation of the equations, is based on T-Q relations given by the equivalent thermodynamic Bethe ansatz (TBA) formulation of the problem in the repulsive regime. Though the equations are derived in the repulsive regime at discrete values of the coupling constant, a straightforward analytical continuation ensures their validity within the whole repulsive regime of the theory. For the NLIE I formulation, appropriate analytical continuation makes the penetration into the attractive regime also possible. However, the magnitude of this penetration is restricted by the spin of the largest spin conserved charge contained in the Gibbs ensemble. Within their range of validity, these NLIE formulations provide efficient theoretical frameworks for computing expectation values of conserved charge densities, their associated currents, and vertex operators and their descendants, with respect to the generalized Gibbs ensemble.
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Materials Science

Two Orders of Magnitude Enhancement in Oxide Ion Conductivity in Cu2P2O7 via Vanadium Substitution: A Pathway Toward SOFC Electrolytes

Bibhas Ghanta, Kuldeep Singh Chikara, Uttam Kumar Goutam et al. (+2 more)

In the quest of green energy, Solid Oxide Fuel Cells (SOFC) have drawn considerable attention for chemical-to-electric energy conversion. In the present paper, we report an enhancement of ionic conductivity in Cu2P2-xVxO7 by vanadium substitution. The electrical (dc and ac conductivity, diffusivity, hopping rate, electric modulus and dielectric properties) and crystal structural properties of Cu2P2-xVxO7 (x = 0, 0.4, 0.6, 0.8 and 1) are investigated by impedance spectroscopy and neutron diffraction, respectively. X-ray photoelectron spectroscopy (XPS) study confirms the presence of Cu2+, P5+and V5+ mono-valence states. The dc conductivity results reveal a two orders of magnitude enhancement of ionic conductivity from ~3.81x10-5 S cm-1 for x =0 to ~2.08x10-3 S cm-1 for x =1 at 993 K, revealing a possible application in SOFCs. DC transport number studies reveal that the total conductivity is dominated by ionic conduction (> 95%). In addition, the diffusivity and hopping rate of oxide ions increase with increasing x. Besides, ac conductivity, electric modulus and dielectric properties have been investigated to illustrate the microscopic conduction mechanism. The derived results suggest that the mechanism for ionic conduction is the correlated barrier hopping (CBH) process. The soft-bond valence sum (BVS) analysis of the neutron diffraction patterns reveals the three-dimensional (3D) oxide ion conduction pathways within the crystal structure. The present study provides a pathway to enhance the ionic conductivity, as well as understanding of microscopic conduction mechanism, ionic conduction pathways and the role of crystal structure on the ionic conduction.
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All authors Bibhas Ghanta, Kuldeep Singh Chikara, Uttam Kumar Goutam, Anup Kumar Bera, Seikh Mohammad Yusuf

Materials Science

Advanced structural characterization of single-walled carbon nanotubes with 4D-STEM

Antonin Louiset, Daniel Förster, Vincent Jourdain et al. (+5 more)

Single wall carbon nanotubes (SWCNT) exhibit remarkable optical and electrical properties making them one of the most promising materials for next generation electronic and optoelectronic devices. Their electronic properties strongly depend on their chirality, i.e., their structural configuration, as well as on the presence and nature of atomic defects. Currently, the lack of versatile and efficient structural characterization techniques limits SWCNT applications. Here, we report how four-dimensional scanning transmission electron microscopy (4D-STEM) can address critical challenges in SWCNT structural analysis. Using modern fast pixelated electron detectors, we were able to acquire rapidly a large number of low noise electron diffraction patterns of SWCNTs. The resulting 4D-STEM data allow to precisely determine the local chirality of multiple nanotubes at once, with limited electron dose (down to 1750 e-/Å^2) and nanometric spatial resolution (down to 3.1 nm). We also show how this approach enables to track the chirality along a single nanotube, while giving access to the strain distribution. Then, we report how 4D-STEM data enable to reconstruct high-resolution images with electron ptychography. With this second approach, structural information can be obtained with atomic scale spatial resolution allowing atomic defect imaging. Finally, we investigate how multi-slice electron ptychography could provide even further insight on nanotube defect structures thanks to its close to 3D imaging capabilities at atomic resolution.
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All authors Antonin Louiset, Daniel Förster, Vincent Jourdain, Saïd Tahir, Nicola Vigano, Jean-Luc Rouvière, Christophe Bichara, Hanako Okuno

Mesoscale and Nanoscale Physics

Strain Engineering of Correlated Charge-Ordered Phases in 1T-TaS2

Rafael Luque Merino, Felix Carrascoso, Eudomar Henríquez-Guerra et al. (+3 more)

Strain engineering is a powerful strategy for controlling the structural and electronic properties of two-dimensional materials, particularly in systems hosting charge density wave (CDW) order. In this work, we apply uniaxial tensile and compressive strain to thin 1T-TaS2 flakes using a flexible, device-compatible platform, and systematically investigate the strain-dependent behavior of the nearly commensurate (NC) to incommensurate (IC) CDW phase transition. This transition is driven by Joule heating at room temperature. Electrical transport measurements reveal that both the switching threshold voltage and the resistance of the NC-CDW phase exhibit clear, reversible strain dependence. Furthermore, we identify a quadratic dependence between the strain-induced resistance change and the threshold voltage, confirming that piezoresistive modulation governs the strain-tunability of the phase transition. We demonstrate a room-temperature, electrically-readout strain and displacement sensor with threshold-like response in a programmable window. These results highlight the potential of 1T-TaS2 for on-chip sensing of strain and displacement.
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All authors Rafael Luque Merino, Felix Carrascoso, Eudomar Henríquez-Guerra, M. Reyes Calvo, Riccardo Frisenda, Andres Castellanos-Gomez

Disordered Systems and Neural Networks · Cross-listed from quant-ph

Encoding computationally hard problems in triangular Rydberg atom arrays

Xi-Wei Pan, Huan-Hai Zhou, Yi-Ming Lu et al. (+1 more)

Rydberg atom arrays are a promising platform for quantum optimization, encoding computationally hard problems by reducing them to independent set problems with unit-disk graph topology. In Nguyen et al., PRX Quantum 4, 010316 (2023), a systematic and efficient strategy was introduced to encode multiple problems into a special unit-disk graph: the King's subgraph. However, King's subgraphs are not the optimal choice in two dimensions. Due to the power-law decay of Rydberg interaction strengths, the approximation to unit-disk graphs in real devices is poor, necessitating post-processing that lacks physical interpretability. In this work, we develop an encoding scheme that can universally encode computationally hard problems on triangular lattices, based on our innovative automated gadget search strategy. Numerical simulations demonstrate that quantum optimization on triangular lattices reduces independence-constraint violations by approximately two orders of magnitude compared to King's subgraphs, substantially alleviating the need for post-processing in experiments.
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All authors Xi-Wei Pan, Huan-Hai Zhou, Yi-Ming Lu, Jin-Guo Liu

Superconductivity

Low-Gap Hf-HfOx-Hf Josephson Junctions for meV-Scale Particle Detection

Y. Balaji, M. Surendran, X. Li et al. (+7 more)

Superconducting qubits have motivated the exploration of Josephson-junction technologies beyond quantum computing, with emerging applications in low-energy photon and phonon detection for astrophysics and dark matter searches. Achieving sensitivity at the THz (meV) scale requires materials with smaller superconducting gaps than those of conventional aluminum or niobium-based devices. Here, we report the fabrication and characterization of hafnium (Hf)-based Josephson junctions (Hf-HfOx-Hf), demonstrating Hf as a promising low-Tc material platform for ultra-low threshold single THz photon and single-phonon detection. Structural and chemical analyses reveal crystalline films and well-defined oxide barriers, while electrical transport measurements at both room and cryogenic temperatures exhibit clear Josephson behavior, enabling extraction of key junction parameters such as critical current, superconducting gap and normal-state resistance. This work presents the first comprehensive study of Hf-based junctions and their potential for next-generation superconducting detectors and qubit architectures leveraging low superconducting gap energies.
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All authors Y. Balaji, M. Surendran, X. Li, A. Kemelbay, A. Gashi, C. Salemi, A. Suzuki, S. Aloni, A. Tynes Hammack, A. Schwartzberg

Strongly Correlated Electrons

Linear and isotropic magnetoresistance of Co$_{1-x}$Fe$_x$Si at x=0.2; 0.4; 0.65

A.E. Petrova, S.Yu. Gavrilkin, V.A. Stepanov et al. (+3 more)

We studied the magnetoresistance (MR) of well-characterized samples of Co$_{1-x}$Fe$_x$Si at x=0.2, 0.4, and 0.65 at temperatures between 1.8 and 100~K and magnetic fields of 9~T. The quasilinear dependence of MR on the magnetic field at low temperatures and the practically isotropic properties of MR in these compounds are tentatively attributed to the specifics of Weyl electron spectra and general disorder of the materials.
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All authors A.E. Petrova, S.Yu. Gavrilkin, V.A. Stepanov, S.S. Khasanov, Dirk Mensel, S.M. Stishov

Soft Condensed Matter

Temperature-Gradient Effects on Electric Double Layer Screening in Electrolytes

Kazuhiko Seki

Temperature gradients drive asymmetric ion distributions via thermodiffusion (the Soret effect), leading to deviations from the classical Debye–Hückel potential.We introduce the Eastman entropy of transfer, $\hat{S}_\pm = \alpha_\pm k_{\rm B}$ for cations and anions, respectively, where $k_{\rm B}$ is the Boltzmann constant, and analyze non-isothermal electric double layers in terms of the dimensionless Soret coefficients $\alpha_\pm$. Analytical solutions of the generalized Debye–Hückel equation show that, for $\alpha_+ = \alpha_-$, the potential is exactly described by a modified Bessel function, while the marginal case $\alpha_\pm = 1$ exhibits algebraic decay. An effective screening length, $\lambda_{\rm eff}$, characterizes the near-electrode potential and increases with temperature, resulting in weaker screening on the hot side and stronger screening on the cold side for $\alpha_\pm > -1$. The differential capacitance is controlled by $\alpha_\pm$ via $\lambda_{\rm eff}$, with its minimum coinciding with the potential of zero charge (PZC) even in the presence of a temperature gradient. These findings highlight the fundamental coupling between electrostatics and thermodiffusion in non-isothermal electrolytes.
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Strongly Correlated Electrons

Monte Carlo study on critical exponents of the classical Heisenberg model in ferromagnetic icosahedral quasicrystal

Shinji Watanabe, Tsunetomo Yamada, Hiroyuki Takakura et al. (+1 more)

Quasicrystals (QCs) lack three-dimensional periodicity of atomic arrangement but possess long-range structural order, which are distinct from periodic crystals and random systems. Here, we show how the ferromagnetic (FM) order arises in the icosahedral QC (i-QC) on the basis of the Monte Carlo simulation of the Heisenberg model on the Yb lattice of Cd$_{5.7}$Yb composed of regular icosahedrons. By finite-size scaling of the Monte Carlo data, we identified the critical exponents of the magnetization, magnetic susceptibility, and spin correlation length, $\beta=0.508(30)$, $\gamma=1.361(59)$, and $\nu=0.792(17)$, respectively. We confirmed that our data satisfy the hyperscaling relation and estimated the other critical exponents $\alpha=-0.376(51)$, $\delta=3.68(23)$, and $\eta=0.282(65)$. These results show a new universality class inherent in the i-QC, which is different from those in periodic magnets and spin glasses. In the i-QC, each Yb site at vertices of the regular icosahedrons is classified into 8 classes with respect to the coordination numbers of the nearest-neighbor and next-nearest-neighbor bonds. We revealed the FM-transition mechanism by showing that the difference in the local environment of each site is governed by cooperative evolution of spin correlations upon cooling, giving rise to the critical phenomena.
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All authors Shinji Watanabe, Tsunetomo Yamada, Hiroyuki Takakura, Nobuhisa Fujita

Materials Science

A Geometric Pathway for Tuning Ferroelectric Properties via Polar State Reconfiguration

Hao-Cheng Thong, Bo Wu, Fan Hu et al. (+7 more)

We report the discovery of a geometric pathway for tuning ferroelectric properties through thermally driven reconfiguration between coexisting polar states in Li-substituted NaNbO3. Using first-principles density functional theory calculation and 7Li solid-state nuclear magnetic resonance spectroscopy measurement, we reveal that Li substitution creates two distinct polar configurations whose transformation under annealing enhances the Curie temperature and induces piezoelectric hardening. Our findings establish a geometrically-driven polar state reconfiguration mechanism, providing a general design principle for ferroics whereby macroscopic functional properties can be engineered via lattice geometry.
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All authors Hao-Cheng Thong, Bo Wu, Fan Hu, Pedro B. Groszewicz, Chen-Bo-Wen Li, Jun Chen, Mao-Hua Zhang, Dragan Damjanovic, Ben Xu, Ke Wang

Quantum Gases

Polar core vortex dynamics in disc-trapped homogeneous spin-1 Bose-Einstein condensates

Matthew Edmonds, Lewis A. Williamson and Matthew J. Davis

We study the dynamics of polar core vortices in the easy plane phase of an atomic spin-1 Bose-Einstein condensate confined in a two-dimensional disc potential. A single vortex moves radially outward due to its interaction with background flows that arise from boundary effects. Pairs of opposite sign vortices, which tend to attract, move either radially inward or outward, depending on their strength of attraction relative to boundary effects. Pairs of same sign vortices repel. Spiral vortex dynamics are obtained for same-sign pairs in the presence of a finite axial magnetization. We quantify the dynamics for a range of realistic experimental parameters, finding that the vortex dynamics are accelerated with increasing quadratic Zeeman energy, consistent with existing studies in planar systems.
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Soft Condensed Matter

Exotic Acoustic-Edge and Thermal Scaling in Disordered Hyperuniform Networks

Yang Jiao

We develop a first-principles theory for the vibrational density of states (VDOS) and thermal properties of network materials built on stationary correlated disordered point configurations. For scalar (mass–spring) models whose dynamical matrix is a distance-weighted graph Laplacian, we prove that the limiting spectral measure is the pushforward of Lebesgue measure by a Fourier symbol that depends only on the edge kernel \(f\) and the two-point statistics \(g_2\) (equivalently the structure factor \(S\)). For hyperuniform systems with small-$k$ scaling \(S(k)\sim k^\alpha\) and compensated kernels, {the VDOS exhibits an algebraic pseudogap at low frequency, \(g(\omega)\sim \omega^{\,2d/\beta-1}\) with \(\beta=\min\{4,\alpha+2\}\), which implies a low-temperature specific heat \(C(T)\sim T^{\,2d/\beta}\) and a heat-kernel decay \(Z(t)\sim t^{-d/\beta}\), defining a spectral dimension \(d_s=2d/\beta\).} This hyperuniformity-induced algebraic edge depletion could enable novel wave manipulation and low-temperature applications. Generalization to vector mechanical models and implications on material design are also discussed.
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