arXane › Physics › Condensed Matter
Wednesday, 4 November 2026: 13 new papers, 8 cross-lists and 14 replacements in Mesoscale and Nanoscale Physics (cond-mat.mes-hall), in arXiv's order, with full abstracts. The same papers as arXiv's own listing, set as a newspaper; every title links to arXiv.
The Kramers theorem generally guarantees the spectral degeneracy of time-reversal-invariant Hamiltonian operators with half-integer spin, underlying a variety of physical phenomena. Despite its fundamental significance, the Kramers degeneracy has been argued to be absent in Liouvillian superoperators acting on mixed states because of the double structure of the Hilbert space. Here, we instead demonstrate that the interplay of time-reversal symmetry$^{\dag}$ and parity symmetry enforces the Kramers degeneracy in Liouvillians. We explicitly construct dissipative many-body models of interacting fermions and spins exhibiting the Kramers degeneracy and show that their complex spectral statistics follow those of non-Hermitian random matrices in the corresponding symplectic symmetry classes. Of the three fundamental universality classes of non-Hermitian random matrices, the symplectic class has never been realized in symmetry-resolved many-body Liouvillians. Our constructions provide its explicit realizations and spectral signatures of nonintegrability in symplectic open quantum systems.
Intense THz pulses, combined with the lightning-rod field enhancement at metal nanotips, can drive field emission of keV-energy, femtosecond-duration electron bunches, yet the dynamics governing this emission remain largely unexplored. Here we probe these dynamics by cross-correlating the energy-resolved emission driven by two independently generated single-cycle THz pulses focused onto a tungsten nanotip, scanning their relative delay over 160 ps. At total bunch charges approaching $10^6$ e$^-$, the delay-energy emission map near zero delay reveals electrons accelerated up to 2.1 keV, with a cross-correlation width of ~210 fs full width at half maximum at the highest energies. Apart from replicas caused by optical reflections in the setup, no emission echoes are observed over the full delay range, ruling out long-lived antenna resonances such as Sommerfeld-wave reflections along the tip shaft. A 1D Fowler-Nordheim model incorporating a low-$Q$, sub-picosecond antenna response for the local field enhancement reproduces the peak electron energies and the asymmetry of the delay-energy map. Our results indicate that each THz pulse produces a single, isolated electron burst, and that energy filtering can select its shortest, highest-energy component.
Optical zeros are momenta where the interband dipole vanishes. The dipole phase winds around each of them by an integer, so they are vortices of the resonant optical response with a conserved charge. We show that in rhombohedral graphene multilayers one integer, the stacking chirality $N$, fixes their constellation near the $K$ valley: the total charge per linear polarization is $N-1$ and is conserved under trigonal warping and interlayer bias; for $N\ge3$ the $K$ point stays an exact optical zero, the Bernal bilayer being the only exception; warping places the remaining charge on satellites whose radii scale linearly with the warping; and the charge pinned at $K$ is bias independent for $N\ge4$. A quantized interband character counts the enclosed charge as an integer staircase with steps at these radii, and an interlayer bias drives a satellite through the protected $K$ vortex in the trilayer. The rules survive the remote hoppings of the full tight-binding model and, being integer charges, the corrections that reshape the bands. The same integer sets the magnitude of the response: the band-edge shift current grows linearly with $N$.
We investigate the anomalous Hall effect (AHE) and magnetization observed in rhombohedral graphene through real-space calculations which lead to new insights. Our calculations reveal no equilibrium loop currents of the type associated with the Haldane model of a quantum anomalous Hall insulator. Instead, the AHE arises in this instance from interference between electronic paths forming oriented closed Wilson loops of current operators in the presence of valley polarization. We show some examples of these Wilson loops. Using the fact that the added-carrier charge density resides dominantly in the top and bottom layers, we develop a real-space expansion of the surface Hamiltonian that reproduces the Hall response of the full model with good accuracy. We also predict that a smooth local potential induces equilibrium circulating currents whose strength depends on both Berry curvature and the orbital moments of states at the Fermi surface.
We propose an altermagnetism-induced route to a two-dimensional higher-order topological Dirac semimetal in a trilayer nonsymmorphic Dirac system. Interlayer hybridization preserves the nonsymmorphic bulk Dirac crossings and generates helical edge states in the trilayer phase. Introducing outer-layer $d$-wave altermagnetic order gaps the helical edge states, while only the symmetry-protected Dirac crossing at high-symmetry point $M$ remains gapless. The resulting phase hosts a surviving bulk Dirac point, gapped edge spectra, and corner states. An exact layer-space decomposition further reveals that these features arise from the coexistence of a gapless Dirac sector and a gapped $\mathbb{Z}_2$ topological sector. Our results establish altermagnetism as an effective mechanism for engineering higher-order topological Dirac semimetals in two-dimensional nonsymmorphic systems.
Superconducting diodes have attracted considerable interest for nonreciprocal device applications owing to their dissipationless transport. Here, we theoretically propose that a tunnel junction based on the Kekul\'e graphene superlattice can host a finite valley supercurrent that exhibits pronounced nonreciprocity even in the absence of superconductivity, giving rise to a dissipationless valley diode effect. This effect originates from the additional reflection phase acquired by electrons in each transverse channel and supports perfect valley diode rectification with $100\%$ efficiency, which can be readily tuned via the gate voltage. This finding provides a platform for designing dissipationless valleytronics in a more versatile manner.
Topological insulators can host chiral, disorder-robust edge states that dissipate energy when coupled to an external bath. We show that nonlinear interactions enable all-optical control over edge-state dissipation in a laser-written Floquet topological insulator periodically coupled to a photonic bath. Although moderate nonlinearity enhances dissipation due to mode mixing, increasing the nonlinear strength to an optimal value dramatically suppresses dissipation by forming highly localized traveling edge solitons. These results establish engineered system-bath coupling and nonlinear interactions as a route to suppress dissipation of chiral topological edge transport.
We theoretically investigate the transport properties of a topological insulator (TI) connected to two-dimensional altermagnetic (AM) leads in the absence of an external magnetic field. Exploiting the intrinsic time-reversal symmetry breaking of altermagnets, we report the emergence of a field-free transverse differential conductivity in the resulting AM-TI-AM heterostructure. Our analysis reveals that this transverse response is highly sensitive to the orientation of the N\'eel vector: a finite transverse conductivity manifests when the N\'eel vector possesses an out-of-plane component, whereas it strictly vanishes for in-plane orientations ($\theta = \pi/2$) under symmetric chemical potentials ($\mu_t = \mu_b$) due to the restoration of symmetry between the $k_y$ and $-k_y$ momentum channels. Conversely, when asymmetric chemical potentials are applied ($\mu_t \neq \mu_b$), this symmetry is broken, yielding a finite transverse response even for an in-plane N\'eel vector. Furthermore, we demonstrate that while the longitudinal differential conductivity exhibits Fabry-P\'erot-type oscillations as a function of the TI's chemical potential, the transverse conductivity displays a distinct antisymmetric dependence. Notably, the oscillatory behavior of the longitudinal conductivity persists even at finite temperatures. Spatially, the transverse current contribution decays exponentially with position inside the TI region. We also find that both the longitudinal and transverse differential conductivity exhibit periodic oscillations with the junction parameter $\chi_t=\chi_b$. These findings provide a fundamental understanding of field-free transverse responses in spin-momentum locked boundary states, highlighting the potential of AM-TI heterojunctions for next-generation spintronic applications.
We investigate the geometric phase acquired by low-energy quasiparticles encircling defects in graphene. Following Anandan, this phase is the holonomy of the Poincare connection; its translational part couples momentum to torsion, giving $P_a\int T^a$. In the Katanaev-Volovich theory, dislocation torsion is confined to the defect core. For graphene, the relevant momentum is $\hbar(\tau_3\KK+\mathbf q)$. We show that an isolated dislocation yields a topological valley factor $e^{\mp2\pi i/3}$ when its Burgers vector $\bb$ lies outside the $\sqrt3\times\sqrt3$ superlattice, together with a dispersive factor $e^{-i\mathbf q\cdot\bb}$ for every nonzero $\bb$. The topological factor recovers the lattice result of Mesaros et al. as a torsion-flux phase; 4-8 dislocations retain only the dispersive contribution. Composing Volterra elements gives the holonomy of defect complexes. At zero net Frank angle, it depends only on the net Burgers vector. The valley flux obeys a three-coloring rule: pentagons and heptagons contribute according to their Kekule site color, whereas squares and octagons do not. Atomic models confirm that ring content alone does not fix the phase: a 5-8-5 core is invisible as a divacancy or as a dislocation with $|\bb|=\sqrt3a$, but carries flux $\pm1/3$ for $\bb=2\mathbf a_1$. The torsion trace produces no geometric phase: its vector coupling violates time-reversal symmetry, while the allowed axial coupling causes only anisotropic short-range scattering.
Quantum geometry, comprising the Berry curvature (BC) and the quantum metric (QM), underpins a wide array of topological and transport phenomena. However, in most solid-state platforms, BC and QM signals are inherently entangled, making their experimental separation highly challenging. Here, we demonstrate that the chiral limit of type-II semi-Dirac semimetals provides a symmetry-protected mechanism to cleanly isolate the QM. At the chiral limit where the spin-orbit gap $m_z=0$, chiral and $\mathcal{PT}$ symmetries strictly suppress the BC, its dipole (BCD), and the shift current, leaving the QM and its dipole (QMD) as the sole sources of the optical and nonlinear Hall responses. This pure-QM regime exhibits distinct experimental fingerprints, including a divergent longitudinal optical conductivity and a QMD-induced nonlinear Hall conductivity that exhibits a pronounced singularity as the chemical potential approaches the nodal point. Turning on a finite $m_z$ reactivates the BC channels, transitioning the system from a pure-QM phase into a mixed geometric phase. By deriving a scaling law for the nonlinear Hall conductivity, we establish a direct experimental strategy to separate the mixed Drude, BCD, and QMD contributions based on their distinct relaxation-time dependencies. Our findings identify chiral type-II semi-Dirac semimetals as highly tunable platforms for isolating and probing intrinsic quantum geometric effects.
Bragg Coherent Diffraction Imaging (BCDI) relies on phase retrieval (PR), a non-convex, ill-posed inverse problem whose conditioning is degraded by factors like noise, partial coherence, and strain or crystallographic defects in the imaged nanocrystal. Conventional approaches, including alternating projections and deep learning methods, represent the object on a dense voxel grid, which is prone to amplitude-phase mixing and becomes increasingly ill-posed under these factors, often preventing reliable three-dimensional mapping of the particle's shape and strain. Here we present ConvexAD, a gradient descent PR model that parametrizes the object shape as a convex polyhedron, built by the intersection of half-spaces and enforces flat amplitude within it by construction. This geometric regularization eliminates amplitude-phase mixing and reduces the number of trainable parameters, hence facilitating the convergence of the PR. Implemented in JAX for native GPU acceleration and forward-model flexibility, ConvexAD is validated on simulated and experimental BCDI data, where it outperforms conventional and deep learning assisted PR at high strain, low signal, and reduced coherence, extending the conditions under which quantitative BCDI reconstruction is achievable.
Periodic deformations flatten electronic bands of graphene which could host strongly correlated electrons similar to those observed in twisted graphene bilayers. The most spectacular effects of strong correlations like superconductivity or magnetism have however not yet been observed despite several experimental attempts. Here, we use tight-binding calculations to investigate the effect of twisting the periodic strain lattice with respect to the atomic lattice. We find that misaligning the two lattices kills the band flattening. We also report on the universal behavior of the band flattening by periodic strain which is controlled by a single parameter, a pseudo-magnetic field flux. Our study strongly suggests that controlling the twist of the strain lattice is essential to unlock the potential of periodic strains in graphene.
Scaling quantum electronic systems such as quantum-dot spin qubits and single-electron devices is increasingly limited by the wiring overhead between cryogenic devices and room-temperature control electronics. Multi-channel quantum-dot architectures that selectively route electrons using a few shared control lines, analogous to demultiplexers in classical networks, offer a promising solution. Here, we propose and theoretically investigate a gate-defined architecture of three parallel quantum dots sharing a common source reservoir and coupled to independent drain channels. Using an analytical electrostatic model and tight-binding quantum transport simulations, we show that the device can act as an on-chip single-electron demultiplexer based on resonant tunneling in the cryogenic regime. Closed-system eigenstate analysis reveals localized orbital states in each dot, and open-system transport calculations show resonant conductance peaks when these levels align with the incident electron energy, consistent with the closed-system eigenenergy evolution. By engineering the barrier-gate spacing and the relative plunger-gate dimensions, we obtain well-separated resonances with minimal overlap between channels. Bond-current density calculations show that transport is confined to the resonant channel with negligible leakage into neighboring outputs, yielding channel selectivity approaching unity for all resonant modes. The architecture is compatible with Si/SiGe and GaAs/AlGaAs platforms and can be extended to more channels, offering a route toward programmable quantum routing and resonant-tunneling-based quantum circuitry.
Monolayer FeSe grown on SrTiO$_3$ (001) exemplifies interface-enhanced superconductivity, exhibiting a transition temperature nearly ten times that of bulk FeSe. Yet, its tunneling spectra lack clearly identifiable superconducting coherence peaks, preventing an unambiguous determination of the gap magnitude. Here, we use ultralow-temperature scanning tunneling spectroscopy as a function of temperature, tip-sample separation, and lateral position to disentangle tunneling contributions by statistical analysis. Decay rates extracted by regression of tip-height-dependent spectroscopy reveal three energy regimes: Andreev-reflection-dominated tunneling below 6 meV, mixed Andreev and quasiparticle tunneling between 6 and 13 meV, and predominantly quasiparticle tunneling above 13 meV. Spatially resolved spectroscopy and correlation analysis further reveal that the tunneling conductance is predominantly localized near defects below 6 meV but becomes spatially extended at higher energies. Together, these results constrain the superconducting gap to lie between 6 and 13 meV. Our findings establish statistical analysis of spectroscopy as a powerful approach to constraining superconducting gap structures when complex spectral features prevent the unambiguous identification of coherence peaks.
A mechanical version of the SSH model, made of equal masses joined by two alternating non-linear springs is studied in the continuum limit. Such a non-linear dimerized mechanical chain generates a nonlinear, dispersive, two-field hyperbolic system of Boussinesq type of differential equations, describing coupled acoustic and gapped optical fields. The corresponding Lagrangian density is found, revealing that the acoustic and optical degrees of freedom are coupled through a structure that is formally analogous to a synthetic spin-orbit coupling, where the spatial motion is locked to the internal sublattice degree of freedom. An emergent non-Abelian Berry connection term also appears. Some particular solitary-wave solutions are found using variational theory, as the effective optical field obeys an undamped and unforced cubic-quintic Duffing equation. Near the Brillouin-zone edge, the system becomes a nonlinear Dirac field theory of the Soler type, providing a direct mechanical realization of interacting relativistic solitons. By factoring out fast spatial oscillations, an effective Jackiw-Rebbi Hamiltonian is obtained, with topological zero modes at domain walls. These modes are akin to the electronic flat bands in moire graphene systems associated with superconductivity and charge fractionalization. Finally, we discuss the mechanical rigidity transition and its effects on the fields as the system evolves toward the fully dimerized limit, in which the zero modes are floppy modes.
Heat transport in solids is usually described in a phenomenological way via Fourier's law. However, at small time and length scales in low thermal conductivity materials, a microscopic description is needed. This is traditionally done by invoking the phonon Boltzmann Transport Equation (BTE), which has been successful in describing transport in high thermal conductivity materials. Recently, a generalization of the BTE, the Wigner Transport Equation, was developed to be able to microscopically describe transport in low thermal conductivity materials by taking into account not only phonon scattering processes but also phonon tunneling processes, so-called coherences. We present a Python package that solves the Wigner Transport Equation (WTE) in spatial and temporal Fourier space for arbitrary source terms, thereby enabling the study of heat transport at small space and time scales. This allows one to compute both static and dynamic thermal conductivities from bulk to nanoscale. Beyond that, it can be used to study the response of phonon systems in materials to arbitrary heat sources.
Effective transition conditions that replace a thin polarizable layer by a zero-thickness sheet are commonly applied in electromagnetic models of two-dimensional (2D) materials and metasurfaces. These conditions usually take the arithmetic mean of the fields on either side as the polarizing field at the boundary, and characterize the layer by the $0^\mathrm{th}$ moment of its susceptibility, but both choices are typically imposed rather than derived. We derive transition conditions for a finite-thickness, in-plane homogeneous, in-plane isotropic layer with a general nonlocal susceptibility, using a single-scale asymptotic expansion in $\eta = l\max(k_0,q)$, where $l$ is the layer thickness, $k_0$ the free-space wavenumber and $q$ the in-plane wavenumber. The sheet limit must be taken after resumming the out-of-plane depolarizing field, as this ensures that the resulting sheet susceptibility is independent of the material thickness. The arithmetic mean then emerges as the leading-order polarizing field for general susceptibility profiles. Asymmetries in the layer profile modify the effective susceptibility appearing in the transition conditions, rather than altering the arithmetic-mean field prescription. If the spatial profile is not even in both out-of-plane coordinates, in-plane and out-of-plane response hybridize, turning the diagonal volumetric susceptibility into a $q$-dependent effective tensor surface susceptibility with off-diagonal components. Mirror symmetry alone does not remove this hybridization. Comparison with exact solutions for local and separable layers shows that the derived conditions remain accurate to $O(\eta)$ across propagating and evanescent excitations, whereas the standard sheet conditions fail once hybridization becomes appreciable.
Nonrelativistic spin splitting in collinear compensated magnets is constrained by spin-group symmetry, with distinct alternating and Zeeman-type splitting typically associated with altermagnets and fully compensated ferrimagnets, respectively. From a spin-density perspective, we propose a class of hybrid compensated magnets in which alternating-like and Zeeman-type spin splitting coexist within a single collinear compensated state. Using a minimal two-sector model, we demonstrate that this coexistence arises from competition between directional sublattice dispersion and the sector-dependent energy biases arising from the same structural inequivalence. First-principles calculations provide a proof of concept in a crossed bilayer of unequal-width Janus graphene nanoribbons, where the structurally inequivalent ribbons retain approximately symmetry-related spin-density distributions. The valence and conduction sectors support direction-selective spin transport and energy-selective spin-polarized states, respectively. These results connect the spatial organization of spin density with the electronic response to structural inequivalence, providing a design principle for hybrid spin splitting in compensated magnets.
The real mechanism of ferromagnetism in III-V diluted magnetic semiconductors such as Ga$_{1-x}$Mn$_x$As is still debated. The experimental results point out the realization of appreciable ferromagnetism at filling of the impurity band close to one-half that can be expressed in terms of compensation degree. Based on our recent results on the existence of two competing mechanisms of pair indirect exchange (with ferro- and antiferro alignment of Mn $d$ shells), a theory of magnetic ordering in (Ga,Mn)As is presented. The molecular-field-like equation is derived in which the effective exchange constant contains all necessary information about material, including the Mn content, the compensation degree, the crystal coordination, and the parameters of two competing pair exchange interactions. The Curie temperature depends crucially on the compensation degree. Depending on Mn content and compensation degree, three possible magnetic phases (paramagnetic, ferromagnetic, and spin-glass-like) are predicted. However, the spin-glass-like phase in (Ga,Mn)As is apparently not realized due to the additional inevitable compensation caused by As antisites. The phase transition into the ferromagnetic state occurs not only with Mn content variation but through the change of the compensation degree as well. The new mechanism of photoferromagnetic effect is predicted and discussed in the context of possible clarification of the magnetic ordering mechanism. It is assumed that the main obstacle to a possible increase in the Curie temperature is the increasing contribution of the antiferromagnetic superexchange between the nearest-neighbor Mn's, rather than compensatory (electric or magnetic) effects from Mn interstitials, as was previously stated. The comparison of our predictions with available experimental data is presented.
Quantum impurity calculations become challenging when multiple correlated orbitals are treated explicitly, particularly in nonequilibrium steady states. We introduce Coupled-Cluster Coupled Lindblad (C$^3$L), a deterministic approach that combines the recently developed coupled-Lindblad compact bath representation with coupled-cluster theory to compute steady states and Green's functions directly in imaginary and real frequency. C$^3$L has favorable polynomial computational complexity and avoids the sign problem of quantum Monte Carlo (QMC). In a real-time multiorbital benchmark, it agrees closely with state-of-the-art QMC while reducing computational cost by four to five orders of magnitude, with a further 10--100-fold speedup on a GPU. Beyond the practical reach of established real-time QMC solvers, four-orbital nonequilibrium impurities with general interactions and biased reservoirs become accessible with C$^3$L. We obtain resonance-resolved transport signatures relevant to scanning tunneling microscopy experiments on molecular junctions.
Minimal model and its linear stability analysis (LSA) for Plateau-Rayleigh (PR) instability of an intermixed, solid binary alloy nanowire is presented. Model accounts only for the surface diffusions of the alloy components without anisotropies of a surface energies, and for a classical thermodynamic surface segregation. Model allows to assess a deviation of a longwave instability cutoff wavelength $\lambda_c$ from the classical value $2\pi r_0$ (a wire circumference) as a function of a surface composition deviation from its equilibrium value, a wire radius, and a bulk concentration of a minority component. Results of LSA are presented for AuNi, PtNi, and CoNi nanowires, where an important distinction is that a surface segregation curve for CoNi is qualitatively different from the one for AuNi and PtNi. For each nanowire there is a sizable region of a parameter space where $\lambda_c<2\pi r_0$ and correspondingly, a fastest growing perturbation wavelength $\lambda_{max}$ is also less than a wire circumference. This is in stark contrast to PR instability of a single-component nanowire transporting mass by surface diffusion - in this case $\lambda_c$ and $\lambda_{max}$ are larger than a wire circumference. Strong surface segregation is primarily responsible for unusually small $\lambda_c$ and $\lambda_{max}$. Results suggest that a large number of small and closely spaced nanoparticles may emerge upon disintegration via annealing of a binary alloy nanowire.