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Physics Condensed Matter

cond-mat Watch Condensed Matter

Wednesday 3 June

Mesoscale and Nanoscale Physics

Tunable Resonator Integrated Magnetometry

Colin Stack, Brian Sears, Aruna Ramanayaka et al. (+3 more)

The quantum-technology revolution is reshaping computing, sensing, and communication. In magnetometry, recent advances leverage precise control of spin qubits and color centers in solid-state crystals for mesoscopic-scale sensing. Yet at very low temperatures, superconducting sensing technology remains unrivaled because of its non-invasiveness and higher sensitivity. Here we describe a class of superconducting sensors that offers low loss and quantum non-demolition measurement characteristics. We designed and fabricated a superconducting flux-tunable resonator (tRes) in a superconducting chip foundry and matured it to a level that combines the speed of an inductor-capacitor circuit with the flux sensitivity of a superconducting quantum interference device (SQUID) to perform magnetometry at milli-kelvin temperature to investigate targets. We introduce its fundamental functionality readily at MHz magnetic sampling rate, showcase two measurement modalities, and investigate three circuits with gradually increasing complexity to extract target-specific information. The combination of high sensitivity and fast readout characteristics make tRes an attractive and versatile magnetometer.
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All authors Colin Stack, Brian Sears, Aruna Ramanayaka, David Ferguson, Ilya Sochnikov, Tony X. Zhou

Disordered Systems and Neural Networks

Experimental observation of three-dimensional Anderson localization of electromagnetic waves

Antton Goïcoechea, Alexey Yamilov, Clément Ferise et al. (+3 more)

A prominent phenomenon in contemporary condensed matter physics is Anderson localization – suppression of wave propagation in disordered systems as a result of interference effects. Despite being observed with various types of waves over the years, all prior attempts to reach Anderson localization of light in three-dimensional systems have been hampered by experimental artifacts. Here, we report an unambiguous experimental proof of three-dimensional Anderson localization of microwaves in disordered metal aggregates. By studying samples with different metal volume fractions, we show a clear difference between diffusive and localized behaviors, and the latter is confirmed by a scaling analysis of transmitted beam width in excellent agreement with theoretical and numerical results. Our demonstration opens avenues for both fundamental studies and practical applications of this extraordinary phenomenon.
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All authors Antton Goïcoechea, Alexey Yamilov, Clément Ferise, Sergey E. Skipetrov, Hui Cao, Matthieu Davy

Materials Science · Cross-listed from physics.med-ph

Influence of anisotropy on the expansion performance of auxetic skin meshing geometries: a finite element study

Masoumeh Razaghi Pey Ghaleh, Denis O'Mahoney and Kevin Moerman

This study investigates the combined effects of anisotropy and auxetic mesh geometry on the performance of skin graft expansion. Finite element models of auxetic slit-based geometries were developed and subjected to 25 percent tensile strain. Skin was modelled using an anisotropic constitutive formulation. Langer's line orientations were varied relative to the load direction. Results showed anisotropy strongly influenced expansion behaviour. The effect was observed to be complex and highly dependent on mesh type. Anisotropy was observed to enhance or inhibit the auxetic expansion behaviour. In all mesh types studied, the expansion performance is lowest when Langer's lines align with the transverse direction. Greatest expansion was typically observed when Langer's lines were close to the loading direction. Isotropic models overpredicted stress relative to the anisotropic models. These findings support the use of auxetic structures for skin mesh expansion applications and show that anisotropy is an important factor in both deformation and stress prediction.
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Mesoscale and Nanoscale Physics

Coulomb-mediated interactions of charge-transfer excitons in TMD lateral heterostructures

Kabyashree Sonowal, Daniel Erkensten, Ermin Malic et al. (+1 more)

Lateral heterostructures of transition-metal dichalcogenides (TMDs) host spatially separated charge-transfer (CT) excitons. While analogous to interlayer excitons in vertical TMD heterostructures, these interfacial excitons possess much larger in-plane dipoles of several nanometers and an additional center-of-mass quantization. Here, we study the mutual interactions between these highly dipolar CT excitons on a microscopic footing. Accounting for the dipolar and quantum exchange interactions, we evaluate the experimentally accessible density-dependent energy renormalization and predict a net energy blueshift of a few meV for bound CT excitons. Interestingly, for small dipole moments, the energy renormalization displays a quadratic dependence with respect to the dipole moment, in contrast to the linear dependence found in vertical TMD heterostructures. We show that spatial energy offset and temperature are the key tuning knobs for controlling the density-dependent excitonic response. Overall, our results contribute to a better microscopic understanding of CT excitons and their interactions in lateral TMD heterostructures.
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All authors Kabyashree Sonowal, Daniel Erkensten, Ermin Malic, Roberto Rosati

Materials Science

Variational approach to determine the properties of dislocations at finite deformation

István Groma

A generalised version of the continuum theory of curved dislocations describing the spatial and temporal evolution of the fields: statistically stored dislocation density, geometrically necessary dislocation density, and curvature has recently been proposed. The dynamics of the system are derived from a scalar functional of the relevant fields that cannot increase during the evolution of the system. However, the framework was established only for small deformations. The aim of the present paper is to discuss the fundamentals of the elasticity theory of finite deformation in cases where individual dislocations are present in the system. The equilibrium equations are derived within a variational formalism. It is shown that introducing dislocations into the finite deformation framework is a nontrivial task. Moreover, if the deformation is large, the force acting on a dislocation segment is not the well-known Peach-Koehler force. In a forthcoming paper, the results obtained will be applied to the generalisation of the dislocation continuum theory of curved dislocations.
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Mesoscale and Nanoscale Physics

Topography-based navigation in a millikelvin scanning tunneling microscope using binary-encoded position markers

R. Fischer-Süßlin, R. Hartmann, T. Kandra et al. (+1 more)

We present a compact millikelvin scanning tunneling microscope (STM) operating at 270mK with topographic navigation to micron-scale targets. Two piezoelectric low-temperature nanopositioners extend the accessible sample area, while a multi-stage copper powder and capillary filter scheme preserves millikelvin energy resolution, verified by BCS spectroscopy on aluminium thin films. A lithographically fabricated binary-encoded gold pattern encodes unique 16-bit coordinates in 4x4 pixels of 200nm$\times$200nm each. We demonstrate absolute positioning across a 350$\times$350$\mathrm{\mu}$m$^2$ area from a single STM scan. Requiring only a single lithography step and no hardware modifications to existing STM setups, the navigation system provides a versatile platform for scanning tunneling spectroscopy of nanoflakes and nanoscale devices.
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All authors R. Fischer-Süßlin, R. Hartmann, T. Kandra, E. Scheer

Materials Science

Barrier-channel intermixing and 2-dimensional electron gas degradation in Al-rich Al(Ga)N/AlGaN high electron mobility transistor heterostructures

Pietro Pampili, Vitaly Z. Zubialevich, Badal Mondal et al. (+4 more)

In this work, we report on recent results in understanding and addressing the issue of interface smearing in high-aluminium content AlGaN/AlGaN heterostructures. On the one hand, the growth of high-crystal quality AlGaN by metal-organic vapour phase epitaxy (MOVPE) requires the use of high temperatures, but on the other hand this may lead to alloy intermixing between barrier and channel layers, which smoothens out the polarization contrast and severely degrades or even completely destroys the 2-dimensional electron gas (2DEG). We show that X-Ray Diffraction (XRD) analysis can be used as a non-destructive way to assess the sharpness of the interface, and that improved growth schemes can be successfully used to achieve high-quality 2DEG, as confirmed by contactless resistivity measurements. In particular, sheet resistivities around 2,500 $\Omega/\Box$ were demonstrated for AlN/Al$_{0.75}$Ga$_{0.25}$N, consistent with the best-reported values in the literature.
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All authors Pietro Pampili, Vitaly Z. Zubialevich, Badal Mondal, Jayjit Mukherjee, Stefan Schulz, David A. J. Moran, Peter J. Parbrook

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

Chiral Transport in Metric-Affine Geometries

Miguel A. Vazquez-Mozo

Anomalous transport in equilibrium fermionic fluids chirally coupled to background Weyl-type nonmetricity is studied. A formal descent analysis is carried out in which the dependence of the anomaly polynomial on the nonmetricity tensor is encoded in a Weyl invariant four-form. The constitutive relation of the axial-vector current is evaluated from the equilibrium partition function obtained using transgression techniques, showing the existence of nonmetricity-mediated chiral separation effects driven by the fluid's vorticity and the Weyl magnetic field. A second nonminimal coupling of fermionic matter to metric-affine geometries proposed in the literature is also discussed.
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Statistical Mechanics · Cross-listed from math-ph

Maximal Minimal Spacing for Random Points

Fabio Deelan Cunden, Noemi Cuppone, Giovanni Gramegna et al. (+1 more)

From $N+1$ random points on a line we wish to select $M+1$ points so as to maximize the minimal spacing between them. We consider an initial configuration with independent and identically distributed spacings. Equivalently, the points are arrival times of a generic renewal process. For general spacing distributions, and for all $M\leq N$, we derive exact distributional identities for the maximal minimal spacing and obtain its asymptotic behavior. The problem admits a reformulation in terms of a threshold-resetting random walk. The walk advances by successive random increments and is reset to the origin upon exceeding a fixed threshold. The probability that the optimal spacing exceeds a given value coincides with the probability that the walk completes at least $M$ reset cycles within $N$ steps. This yields an exact representation in terms of first-passage functionals of the walk. The same mapping suggests a numerical scheme for the max-min spacing problem in the regime of large $N$ and $M$, whose accuracy is tested against the exact results obtained here.
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All authors Fabio Deelan Cunden, Noemi Cuppone, Giovanni Gramegna, Pierpaolo Vivo

Superconductivity

A unified theory of thin film and bulk bilayer nickelates

Jiangfan Wang and Yi-feng Yang

The discovery of bilayer nickelate superconductivity in both pressurized bulk and thin films has drawn enormous attention on their similarity and distinction. Here we provide a unified explanation based on the two-component scenario for a number of key experimental observations reported recently. Our theory predicts two superconducting domes upon electron or hole doping, separated by a valence bond state near $d_{z^2}$ half filling for strong interlayer superexchange coupling $J$, and a single dome across half filling with a lower $T_c$ for weak or moderate $J$. Increasing doping drives the normal state from a Fermi liquid to non-Fermi liquid or weak insulating behaviors, with quasi-linear-in-$T$ scattering rate near optimal $T_c$, while breaking the interlayer valence bonds by oxygen vacancies or chemical substitution simultaneously suppresses the superconductivity and causes local Kondo scattering of $d_{x^2-y^2}$ electrons. These explain the different superconducting transitions and normal states in bulk and thin films, the effect of $d_{z^2}$ hole or electron doping, and the Kondo effect in non-superconducting samples. We propose bulk superconductivity at ambient pressure by doping or reducing the interlayer magnetic coupling and predict even higher $T_c$ upon electron doping.
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Materials Science

Machine learning via artificial neural networks coupled with density functional theory and experiments for thermodynamic optimization of high-entropy alloys for hydrogen storage at room temperature

Shivam Dangwal, Pranav Kumar, Yuji Ikeda et al. (+2 more)

High-entropy alloys (HEAs) have received considerable attention for hydrogen storage because of their compositional flexibility; however, designing HEAs with optimal thermodynamics is critical. This study employs machine learning via artificial neural networks (ANN) and density functional theory (DFT) to design a novel AB-type TixNb2-xVCrMnFe (x = 0.5-2.0) high-entropy system for hydrogen storage at ambient temperature (A: Ti, V and Nb, and B: Cr, Mn and Fe). Both ANN and DFT predict that the hydride formation enthalpy decreases to negative values with increasing the titanium content. Two alloys with x > 1.5 are predicted to achieve enthalpies within the -25 to -39 kJ/mol range, making them appropriate for room-temperature hydrogen storage. Experiments demonstrate good agreement with the enthalpy predictions, with the Ti-rich alloys showing reversible hydrogen storage with fast kinetics at room temperature. These results provide a framework for reliable use of data analysis and ab initio calculations to explore high-entropy hydrides as hydrogen storage materials.
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All authors Shivam Dangwal, Pranav Kumar, Yuji Ikeda, Blazej Grabowski, Kaveh Edalati

Strongly Correlated Electrons

Magnetic structure of diamond EuTi2Al20

S. W. Lovesey and D. D. Khalyavin

We study magnetic symmetries of EuTi2Al20 that appear in the ordering of Eu ions below a temperature at 3.3 K, according to an analysis of substantial experimental evidence [M. Kawamata et al., J. Phys. Soc. Japan. 95, 024701 (2026)]. They descend from the cubic diamond structure that depends explicitly on mixing of orbitals with different spatial parities, e.g., diamond s- and p-atomic states. The lattice (2, 2, 2) Bragg spot, for example, is lattice forbidden in the absence of parity mixing. Our work is motivated by a first proposal that low-temperature magnetic symmetries of diamond EuTi2Al20 are orthorhombic and belong to magnetic crystal classes that include time reversal. Body-centre anti-translation in diffraction patterns is a corollary. Current experimental data does not distinguish between two orthorhombic candidates, even though one is centrosymmetric and the other one is polar. We demonstrate that magnetic parity-odd entities (multipoles) formed by mixing d-and f-atomic states at europium positions, with discreet symmetries that match those of a Dirac monopole, are different for the two candidates. Moreover, the multipoles are visible in resonant x-ray diffraction patterns. The selection rule from parity mixing in the parent cubic diamond structure is preserved, and permitted Bragg spots are different for the two candidate magnetic symmetries. Our symmetry informed diffraction patterns include rotation of the crystal about the reflection vector (an azimuthal angle scan) and a full account of x-ray polarization to spur more revealing experiments.
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Mesoscale and Nanoscale Physics

Monitored chaotic scattering

C.W.J. Beenakker, J. Sánchez Fernán and J. Tworzydło

We extend the random-matrix theory of chaotic scattering to quantum dots whose dynamics is monitored by time-resolved measurements. Starting from a scattering matrix drawn from a circular ensemble, we construct the corresponding ensemble of Kraus operators for the monitored evolution of the many-body density matrix. In the single-particle sector the sum over measurement outcomes can be carried out algebraically, giving a discrete-time quantum master equation for the transferred charge. We solve this equation numerically and compare the resulting charge-transfer statistics with closed-form random-matrix predictions. The latter rely on an equipartition rule for monitored particles, which we formulate as a conjecture and test against the master equation.
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Soft Condensed Matter

Surface Charge Doping for Ion-Pairing Criticality in Confined Electrolytes

Na Shen, Yabei Wu and Wenqing Zhang

Dielectric confinement strengthens Coulomb correlations in quasi-two-dimensional electrolytes and can promote Bjerrum pairing in charge-neutral slits. Here we use a generalized Debye-Huckel-Bjerrum theory to show that weak surface charge changes this picture by stoichiometrically doping the slit with mobile counterions. These counterions maintain a finite screening floor, decouple microscopic pairing from macroscopic ionicity, and shift association-driven criticality to lower temperatures. The critical-temperature suppression collapses onto a single scaled perturbation variable, revealing how surface charge and dielectric confinement jointly control charged nanofluidic slits. Brownian-dynamics tests further show that the same counterions are not always fully bulk-like diffusive: at low intrinsic salt density, explicit wall charge slows in-plane diffusion, whereas at higher intrinsic density the wall-induced diffusion penalty decreases and the mobile-counterion description becomes dynamically accurate. These results identify surface charge as a thermodynamic doping field that tunes both correlated ionic stability and the diffusion mechanism in nanofluidic confinement.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Pushing the Classical Frontier of 1D Fermi-Hubbard Quench Dynamics Beyond Current Quantum Simulations

Roman Rausch, Sukhbinder Singh, Saeed S. Jahromi et al. (+2 more)

Establishing quantum advantage requires comparison against the best achievable classical simulation. The Q-CTRL team recently simulated quench dynamics of the one-dimensional Fermi-Hubbard model on an IBM processor, completing a $L=60$ evolution to time $t=6$ in under three minutes and claiming a $3000\times$ speedup over classical Time-Dependent Variational Principle (TDVP) simulation at bond dimension $\chi=4096$. Their classical benchmark required over 160 hours on a CPU cluster, failed to converge in the high-entanglement regime $t\in[5.2,6]$, and left the most challenging window of the experiment unverified. Here, we push the boundaries of classical simulation by exploiting the full $\mathrm{U}(1)\times\mathrm{SU}(2)$ symmetry of the Fermi-Hubbard Hamiltonian combined with GPU-accelerated tensor contractions. Reaching bond dimensions up to $\chi\approx62{,}000$ on four NVIDIA H200 GPUs – among the largest ever achieved in TDVP simulations and fifteen times larger than Q-CTRL's classical baseline – we achieve fully converged results across the entire simulation window, including rigorous certification of the previously unresolved high-entanglement regime $t\in[5.2,6]$. We further advance the classical frontier to $t=7$, which lies beyond the quantum hardware experiment and any previously verified classical evolution of the full wavefunction. At the bond dimension comparable to Q-CTRL's best classical run, our GPU implementation completes in $\sim\!100$ minutes, directly reducing the claimed $3000\times$ quantum advantage to $\sim\!36\times$. These results substantially narrow the quantum-classical performance gap and establish a new standard for tensor-network benchmarking of large-scale quantum simulations.
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All authors Roman Rausch, Sukhbinder Singh, Saeed S. Jahromi, Augustine Kshetrimayum, Roman Orus

Soft Condensed Matter

Tunable supramolecular polymerization from protein charge heterogeneity and architecture

Nynke Marije Hettema, Meng Shen, Frank van Opstal et al. (+2 more)

Multidomain proteins with flexible unstructured sequence regions are abundant in cellular signaling. This protein architecture enables self-assembly into supramolecular structures, but how structured interaction domains and overall protein architecture jointly regulate the assembly size, structure and kinetics remains unclear. Here we use the budding yeast protein Bem1 as a model multidomain system to show that supramolecular polymerization can be tuned by charge heterogeneity and protein architecture. We experimentally demonstrate that Bem1's isolated PB1 domain forms extended filaments, whereas full-length Bem1 forms substantially shorter assemblies, indicating that the PB1 domain drives assembly while the remaining protein architecture tunes filament length. To understand these observations, we develop minimal coarse-grained models approximating the PB1 as a polar 5-bead domain and the full-length Bem1 as a 6-bead model with an additional bead representing the remainder of Bem1. The weight distribution of supramolecular filaments assembled by the 5-bead model quantitatively follows reversible Flory-like polymerization theory, which is tunable within a narrow charge polarity regime. In contrast, the 6-bead model shifts chain-length distributions towards shorter polymers despite retaining the same driving domain. We show that this deviation arises from steric and geometric constraints imposed by the appended unstructured regions, where the rotational flexibility between the charge-polar structured domain and the unstructured region emerges as key physical parameter governing self-limited self-assembly. Together, our results establish charge polarity, protein architecture, and conformational flexibility as programmable control knobs for supramolecular polymerization and suggest a general framework for understanding how multidomain proteins assemble into tunable biomolecular structures.
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All authors Nynke Marije Hettema, Meng Shen, Frank van Opstal, Emmett Lim, Liedewij Laan

Disordered Systems and Neural Networks

Percolation Criticality of Amorphous-Amorphous Transitions in Compressed Glasses

Julien Perradin, Simona Ispas, Ricardo V. Paredes et al. (+2 more)

The low-to-high-density transition in compressed silica glass is investigated using percolation theory. Large-scale molecular dynamics simulations of SiO$_2$ glasses, with system sizes of up to 10$^6$ atoms and pressures ranging from 0 to 35 GPa, were carried out to investigate the emergence of structural motifs and their growth to system-spanning length scales under compression. On this basis, we introduced long-range descriptors that complement conventional local and medium-range structural measures. The results reveal critical percolation transitions of SiO$_Z$-SiO$_Z$ clusters with increasing coordination number $Z$. The critical exponents slightly deviate from the standard (random) correlation, a behavior that seems to be more pronounced for higher coordinated polyhedra than for tetrahedra, suggesting a possible rigidity percolation mechanism. SiSi$_z$-SiSi$_z$ clusters were also analyzed using the non-bonded approach. Bonded and non-bonded approaches complement each other in a particularly illuminating way for describing pressure-induced structural transformations and common mechanisms shared by bonded glasses, such as SiO$_2$, and non-bonded glasses, such as amorphous ice.
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All authors Julien Perradin, Simona Ispas, Ricardo V. Paredes, Anwar Hasmy, Bernard Hehlen

Materials Science

Strain-induced suppression of thermochromism in divalent cobalt molybdate thin films

Kiri Van Koughnet, Joey Williamson, Kane Hill et al. (+10 more)

Thermochromic oxides provide a platform for coupling lattice, electronic, and magnetic degrees of freedom, with divalent cobalt molybdate $CoMoO_4$ as a prototypical example. Despite extensive studies on powders, its thin-film behaviour - critical for device applications - remains largely unexplored. Here, we present a combined experimental and theoretical investigation of $CoMoO_4$ thin films, including the first THz-VIS-UV spectra of $\beta - CoMoO_4$ in thin-film form. In contrast to bulk powders, which undergo a first order $\beta \rightarrow \alpha$ structural transition near 230K, the thin films retain the high-temperature $\beta-$phase across the entire temperature range. We show that microstrain fundamentally reshapes the phase landscape, suppressing thermochromism and stabilizing the $\beta-$phase. The optical spectra reveal pronounced phonon and electronic anomalies, including a softening of a low-energy, cation-dominated phonon (42cm$^{-1}$) upon cooling, in contrast to conventional mode-hardening. This behaviour indicates incipient atomic displacements analogous to those driving the bulk $\beta \rightarrow \alpha$ transition, despite the absence of a structural phase change, and saturates between 200 and 150K. Temperature-dependent X-ray diffraction confirms persistent \beta-phase symmetry with increasing microstrain, consistent with strain-induced frustration of the first-order transition. At higher energies (0.12-3.7eV), the optical response exhibits a blue-shift of Co$^{2+}$crystal-field transitions and charge-transfer excitations, indicating a strain-enhanced ligand field. Supported by structural refinements and theoretical calculations, we identify crystal field strengthening as the key mechanism stabilizing the $\beta$-phase. These results establish strain as a thermodynamic lever to control phase stability and functional properties in thermochromic oxides.
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All authors Kiri Van Koughnet, Joey Williamson, Kane Hill, Robert G. Buckley, Yurii G. Pashkevich, Sergej M. Orel, Eric Lebraud, Manuel Gaudon, Peter P. Murmu, Sarah Spenser, Dominique Appadoo, Shen V. Chong, Fryderyk Lyzwa

Superconductivity

Orientation-resolved tunnel spectroscopy of a triplet superconductor

Marcel Strohmeier, Andriy Smolyanyuk, Karsten Held et al. (+5 more)

Although Cooper pairs in superconductors generally condense into a spin-singlet state, spin-triplet superconductivity has attracted sustained interest since it enables dissipationless spin transport and is an important requirement for manifold topological quantum technologies. Noncentrosymmetric superconductors provide a promising route to triplet pairing because their intrinsic antisymmetric spin-orbit coupling (ASOC) can mix spin-singlet and spin-triplet states. To date, the strongest evidence for such mixed-parity superconductivity has been reported in heavy-fermion systems, where strong electronic correlations obscure the role of ASOC. Whether ASOC alone can support a substantial spin-triplet component remains unresolved. Here we show that the weakly correlated noncentrosymmetric superconductor Nb$_{18}$Re$_{82}$ (Nb-Re) hosts a mixed-parity superconducting state with a substantial spin-triplet contribution. Orientation-resolved low-temperature scanning tunneling spectroscopy reveals a strongly anisotropic quasiparticle density of states. A symmetry-constrained analysis shows that the spectroscopic dataset is explained by an order parameter combining a nodal spin-singlet component with a spin-triplet contribution reaching up to half of the singlet amplitude. These findings provide a momentum-resolved determination of a mixed-parity superconducting order parameter and demonstrate that ASOC alone can foster a substantial spin-triplet component. More broadly, they establish orientation-resolved tunneling spectroscopy as a route to identifying mixed-parity superconducting states and suggest that triplet superconductivity may be more widespread among noncentrosymmetric materials than previously recognized.
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All authors Marcel Strohmeier, Andriy Smolyanyuk, Karsten Held, Michael Smidman, Geetha Balakrishnan, Wolfgang Belzig, Elke Scheer, Angelo Di Bernardo

Materials Science

Local multiferroic ordering at room temperature in collinear magnetoelectric antiferromagnets induced by flexo-Zeeman coupling

Paulina J. Prusik, Igor Veremchuk, Florin Radu et al. (+11 more)

Spin-driven multiferroicity attracts significant interest due to its tunability and inherently strong magnetoelectric coupling. While this mechanism induces sizeable electric polarization, it typically occurs at low temperatures and in complex materials. In the simple oxide, the magnetoelectric antiferromagnet Cr$_2$O$_3$, we experimentally demonstrate the existence of specific domain walls that act as room-temperature multiferroic regions. This behavior stems from an anisotropic crystal-symmetry-dependent mechanism of exchange origin, which is applicable to a broad class of bipartite antiferromagnets. The key signature is the magnetization occurring at antiferromagnetic textures, driven by the flexo-Zeeman interaction. These findings establish a foundation for exploring high-temperature spin-driven multiferroicity for magnetoelectric spin-orbit memory and logic applications.
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All authors Paulina J. Prusik, Igor Veremchuk, Florin Radu, Andrey N. Anisimov, Pavlo Makushko, Georgy V. Astakhov, René Hübner, Alexander Edström, Jürgen Fassbender, Fabian Ganss, Massimiliano Stengel, Kirill D. Belashchenko, Denys Makarov, Oleksandr V. Pylypovskyi

Materials Science

Curvature-driven revival of charge density waves in non-Euclidean space

Zhipeng Song, Zeyu Liu, Junde Liu et al. (+12 more)

Strongly correlated quantum states, such as charge density waves (CDWs), are exquisitely sensitive to Fermi surface topology and lattice symmetry, and are typically quenched by heavy carrier doping. In two-dimensional (2D) systems, however, macroscopic geometric curvature emerges as a novel structural degree of freedom to modulate microscopic quantum coherence. This raises a compelling physical question: can non-Euclidean geometric deformations compete with extreme electronic perturbations to reshape, or even revive, a quenched macroscopic quantum order? Here, by constructing monolayer TiSe$_2$-NbSe$_2$ heterostructure on a BLG/SiC substrate for the first time, we report the curvature-driven revival of a frustrated charge order in a non-Euclidean space. Low-temperature angle-resolved photoemission spectroscopy (ARPES) reveals a massive interfacial charge transfer, which destroys the global Fermi surface nesting and completely suppresses the long-range CDW order in Euclidean flat regions. Strikingly, high-resolution scanning tunneling microscopy (STM) reveals that a novel, non-linear CDW state miraculously survives, remaining strictly localized within morphologically distorted, non-Euclidean nanoscale curved regions. Atomistic simulations unravel the structural origin of this phenomenon, demonstrating that interfacial twist and lattice mismatch spontaneously generate a corrugated superlattice.
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All authors Zhipeng Song, Zeyu Liu, Junde Liu, Yi Biao, Anning Yang, Qian Fang, Mojun Pan, Chen Liu, Jiaou Wang, Tian Qian, Chenmin shen, Hongliang Lu, Wei Ji, Hong-Jun Gao, Xiao Lin

Mesoscale and Nanoscale Physics

Local-to-global heating crossover in chains of nanomagnets: A two-scale analytical framework

H. Kachkachi

We develop a two-scale analytical formalism to study heat generation and thermal transport in one-dimensional systems of nanomagnets subjected to a uniform alternating magnetic field. At the nanoscale, each nanomagnet acts as a localized, temperature-dependent heat source governed by its magnetic response, dipolar interactions, and interfacial coupling to the matrix, characterized by a nanoscale volumetric loss coefficient $L_m$. After spatial and temporal averaging, we obtain a coarse-grained assembly-scale equation with effective heating terms and a macroscopic loss coefficient $L_N$. Using modal decomposition, we solve both equations exactly under Dirichlet and Neumann boundary conditions and establish explicit conditions for a local-to-global heating crossover; this is governed by the competition between heat generation, diffusion, dipolar coupling, and hierarchical losses. The crossover is quantified through the spatial correlation length and temperature variance, with stability criteria incorporating both diffusion and nanoscale losses. The coarse-graining procedure is derived rigorously, and its systematic approximation errors are quantified. For prototypical magnetic hyperthermia systems, such as magnetite nanomagnets in water, our formalism reveals that realistic parameters place these systems firmly in the collective heating regime, with local temperature variations at the $\sim\mu$K level, which is currently unresolvable experimentally. The continuum Fourier description used here is validated by a Knudsen-number analysis ($\mathrm{Kn} \ll 1$ for amorphous polymer and aqueous matrices).
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Disordered Systems and Neural Networks

Finite-Basis Duality Estimate for the Surface-Code Threshold under Correlated Bit-Flip Errors

Masayuki Ohzeki

We apply finite-basis duality to a statistical-mechanical model introduced by Chubb and Flammia for the surface code under spatially correlated bit-flip noise. Their mapping gives a random-bond Ising model with both two-body edge interactions and four-body face interactions. The single-equation estimate based on the duality analysis is slightly deviated from their Monte Carlo estimate \(p_c=0.1004(6)\). Following the finite-basis and graph-polynomial strategy, we instead use periodic and twisted-periodic sectors on toroidal bases. We have obtained an improved estimate of the critical point, $p_c = 0.10348$.
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Materials Science

Tunable Rashba Splitting in Janus InXPbP (X = S, Se, Te) Monolayers for Enhanced Photocatalytic Water Splitting

Vuong Van Thanh, Nguyen Minh Quan and Nguyen Tuan Hung

Janus two-dimensional (2D) materials exhibiting Rashba spin splitting have recently attracted considerable attention owing to their potential applications in spintronic devices and photocatalytic water splitting. In this work, we investigate, using first-principles calculations, the structural, mechanical, electronic, optical, and photocatalytic properties of Janus InXPbP (X = S, Se, Te) monolayers that exhibit significant Rashba effects. Our results demonstrate that all three monolayers are energetically, dynamically, and mechanically stable, as evidenced by cohesive energy calculations, phonon dispersion analysis, and elastic constants. By varying the chalcogen atom (X = S, Se, Te), the Rashba effect in InXPbP can be effectively tuned. Rashba parameters of 0.16 and 0.20 eVÅ are obtained near the conduction-band minimum (CBM) for InSPbP and InSePbP, respectively, whereas InTePbP exhibits giant Rashba spin splitting near both the CBM and valence-band maximum (VBM), with corresponding Rashba parameters of 0.90 and 0.87 eVÅ. Furthermore, the Janus InXPbP monolayers exhibit suitable band gaps of 1.21, 1.27, and 0.76 eV for InSPbP, InSePbP, and InTePbP, respectively, which are favorable for photocatalytic applications. All three monolayers possess suitable band-edge alignments for overall water splitting, yielding solar-to-hydrogen (STH) conversion efficiencies of 21.67%, 26.03%, and 29.83% for InSPbP, InSePbP, and InTePbP, respectively. Our findings not only enrich the family of Janus materials but also suggest that the Janus InXPbP monolayers are promising candidates for spintronic devices and high-performance photocatalytic water-splitting applications.
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Materials Science

Why Fe doping kills photoluminescence in CsPbCl$_3$ but not in CsPbBr$_3$: Role of midgap Fe 3$d$ states and electron-phonon coupling

Arpan Das, Saptarshi Chakraborty, Ranjani Viswanatha et al. (+1 more)

Understanding the impact of transition-metal doping on the optoelectronic properties of halide perovskite nanocrystals is essential for their rational design in photonic applications. We establish the microscopic origin of photoluminescence (PL) quenching in Fe-doped CsPbCl$_3$ using spin-polarized density functional theory calculations. The emergence of Fe 3$d$ midgap states creates efficient electron-trapping centres that drive nonradiative recombination, accounting for the reduced PL intensity. Extending this analysis to Fe-doped CsPbX$_3$ (X = Cl, Br), we show experimentally that although PL intensity is suppressed in both systems relative to their pristine counterparts, their high-doping behaviour diverges: CsPbCl$_3$ becomes completely non-emissive, whereas CsPbBr$_3$ retains a finite, saturated PL intensity. Despite this contrast, electronic structure calculations reveal nearly identical midgap states in both materials, indicating that electronic effects alone cannot explain the distinct PL responses. Phonon calculations likewise fail to capture this difference. In contrast, electron-phonon coupling calculations based on the deformation potential approach reveal significantly stronger coupling in Fe-doped CsPbCl$_3$, enabling efficient dissipation of electronic excitation energy into lattice vibrations and leading to complete PL quenching. These results identify electron-phonon coupling as the key factor governing halide-dependent PL quenching and provide a unified microscopic framework for dopant-induced nonradiative processes in halide perovskites.
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All authors Arpan Das, Saptarshi Chakraborty, Ranjani Viswanatha, Shobhana Narasimhan

Mesoscale and Nanoscale Physics

Dipolar interlayer excitons in transition metal dichalcogenide alloy heterobilayers

E. Katsipoulaki, N.G. Chatzarakis, E. Rigoutsou et al. (+6 more)

Interlayer excitons in transition metal dichalcogenide (TMD) heterobilayers possess a permanent electric dipole moment and long recombination lifetimes, making them a promising platform for exploring excitonic many-body physics. Here, we report dipolar interlayer excitons in a MoS$_{1.4}$Se$_{0.6}$/MoSe$_2$ heterobilayer encapsulated in hexagonal boron nitride. Low-temperature photoluminescence measurements reveal a distinct emission peak at $\sim1.4$ eV, attributed to radiative recombination of interlayer excitons. The emission exhibits a blueshift with increasing excitation power, indicating repulsive dipole-dipole interactions. Time-resolved photoluminescence measurements uncover nanosecond-scale lifetimes, consistent with the spatial separation of electrons and holes across the two layers. These findings establish chalcogen-alloyed TMD heterobilayers as a versatile platform for engineering dipolar excitons and tuning excitonic interactions in van der Waals materials.
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All authors E. Katsipoulaki, N.G. Chatzarakis, E. Rigoutsou, D. Katrisioti, T. Taniguchi, K. Watanabe, S. Psilodimitrakopoulos, N.T. Pelekanos, I. Paradisanos

Statistical Mechanics

Hydrodynamics of the Fermi-Pasta-Ulam-Tsingou chain

Shubhadeep Chakraborty and Abhishek Dhar

We provide a pedagogical review of the hydrodynamics of the FPUT chain. There are three hydrodynamic fields corresponding to the conservation of mass, momentum and energy. We provide physically motivated derivations of the hydrodynamic equations at the levels of Euler and then Navier-Stokes-Fourier. Next we consider examples to test as to how successful the hydrodynamic description is in predicting the observed time evolution of nonequilibrium initial conditions such as domain walls and blasts. We find that in some cases there is good agreement of microscopic simulations with predictions from the Euler equations while, in several other cases, there is significant departure from the Euler predictions suggesting that the role of dissipation and noise is important in general.
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Materials Science

Electron-Ion Path Integral Monte Carlo with Hard Core

Riccardo Fantoni

We performed numerical (restricted) path integral Monte Carlo experiments on metallic Hydrogen from first principles. We study a quantum two component plasma where one component is made of pointwise particles of negative unitary charge and the other is made of charged hard spheres of positive unitary charge. We study both the additive mixture and a nonadditive mixture where we only keep a hard core between unlike species. We specialize to the case of the electron-proton plasma with a 1:1 ratios between the molar fraction of the two species. We measured thermodynamic and structural properties of the plasma. From an analysis of the structure we see a transition from a metallic Hydrogen phase, to a molecular Hydrogen phase as the temperature is lowered. As expected at high density the correlations are diminished.
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Disordered Systems and Neural Networks

Spectral criteria for generalization in unsupervised Hebbian nets

Elena Agliari, Paulo Duarte Mourão, Alberto Fachechi et al. (+1 more)

We consider an unsupervised Hebbian network where the pairwise interactions among neurons are built on noisy realizations of hidden ground-truth vectors. Unlike classical Hopfield models, designed as memory devices, this class of networks can be employed to extract latent structure and generalize beyond the "training" set. By combining random matrix theory and replica methods, we derive the asymptotic spectrum of the corresponding interaction matrix and show that the onset of generalization is controlled by a sharp spectral transition. Depending on the quality and the size of the accessible dataset, the spectrum displays either two separated bulks, encoding informative and noisy directions, or a merged single-bulk phase where such distinction is lost. We show that, when coupled with regularization, the emergence of such a spectral split predicts the network's capability to reconstruct the ground-truth vectors from corrupted samples.
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All authors Elena Agliari, Paulo Duarte Mourão, Alberto Fachechi, Pierpaolo Vivo

Soft Condensed Matter

Functional trends and rheological evaluation of polyurethane microcapsules in dermato-cosmetic applications

Sharadwata Pan, Andreas Wierschem, Natalie Germann et al. (+1 more)

To date, natural and synthetic polymer-based microcapsules have been used extensively in various dermato-cosmetic applications, with an emphasis on the targeted delivery of active ingredients, including therapeutic and aesthetic interventions. Although numerous polymer candidates have been comprehensively investigated, polyurethane based microcapsules have received comparatively minor attention, despite possessing a multitude of intrinsic benefits. However, in recent years, although there has been an upsurge of studies involving polyurethane, predominantly as a capsule wall or shell component, towards tangible dermato-cosmetic applications, these are only intermittently documented. In the current review, we target this lacuna, explore, and collate only the most contemporary trends and advances (2017-to date) in the field. In addition, despite the significance, and pertaining to the acute deficiency of rheological studies targeting polyurethane-based microcapsules in dermato-cosmetic applications, we critically examine and lay a comprehensive interpretation, based on the current state-of-the-art, inevitability for systematic inquiries, and identification of several target domains that need urgent attention. Finally, we deliberate on the challenges and the impending projections from a diverse outlook. We focus on a steady and more sustainable path forward via incorporation of green raw materials, cumulative domain-optimized and customer-focused applications, and a significantly improved understanding of the microcapsule mechanical behavior via implementation of novel rheological characterization procedures.
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All authors Sharadwata Pan, Andreas Wierschem, Natalie Germann, Thomas Becker