Disorder and topology compete in one-dimensional systems, yet analytical results beyond perturbative limits remain scarce. We derive a nonperturbative, energy-dependent expression for the Lyapunov exponent of the disordered Su-Schrieffer-Heeger (SSH) chain in the presence of both on-site and hopping disorder. Our approach is based on a closed recurrence relation for the local density whose disorder average yields an effective transfer problem at the unit-cell level. The resulting energy-dependent Lyapunov exponent quantitatively matches numerical results across a broad range of disorder strengths and hopping asymmetries. We find that the magnitude of the Lyapunov exponent is independent of the topological phase for a semi-infinite chain. We further compute the real-space winding number under disorder and show the contrasting roles of hopping disorder and on-site disorder in the stability of the topological phase.

The layer Hall effect is usually revealed by applying out-of-plane electric fields and electrostatic gating in fully compensated A-type antiferromagnetic layered systems. Here, we theoretically demonstrate that the bilayer system of noncompensated A-type antiferromagnetic ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/{\mathrm{NiBi}}_2{\mathrm{Te}}_4$ heterostructures provides an intrinsic platform to host a spontaneously polarized layer Hall effect. By extending to multilayers, we show that different stacking configurations enable the realization of a nearly quantized layer Hall effect and quantum anomalous Hall effect. These findings make ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/{\mathrm{NiBi}}_2{\mathrm{Te}}_4$ heterostructures a versatile platform for engineering various Hall states in antiferromagnetic van der Waals systems.

Intercalation at the graphene/SiC interface provides a controlled route to stabilize atomically thin layers with properties distinct from their bulk counterparts. In this platform, the structure and stability of the intercalated phase depend sensitively on the defect landscape of the starting substrate. For intercalated two-dimensional silver at the graphene/SiC interface, two phases have been observed: a phase epitaxial to the SiC lattice, called ${\mathrm{Ag}}_{\left(1\right)}$, readily obtained following the conventional intercalation method under ultra-high-vacuum conditions and extensively characterized, and a more densely packed phase, called ${\mathrm{Ag}}_{\left(2\right)}$, which has remained largely unexplored. Here we report an in situ ultra-high-vacuum preparation method of the second phase intercalated at the graphene/SiC interface; this phase was previously prepared via high-pressure confinement heteroepitaxy. Low-energy electron diffraction shows that ${\mathrm{Ag}}_{\left(2\right)}$ is rotated by ${30}^{\circ }$ relative to the SiC lattice and forms supercells, in contrast to the $(1\times 1)$ epitaxial relation of ${\mathrm{Ag}}_{\left(1\right)}$ with SiC. High-resolution angle-resolved photoemission spectroscopy reveals a more complex ${\mathrm{Ag}}_{\left(2\right)}$ band dispersion compared to ${\mathrm{Ag}}_{\left(1\right)}$, as a consequence of the modified Ag-Si interaction in the denser phase. In density functional theory calculations, by defining the unfolding entropy, which, in a quantified way, finds that the band structure of ${\mathrm{Ag}}_{\left(2\right)}$ is more suitable to be unfolded to the SiC primitive cell, the resulting unfolded band dispersion is in great agreement with the experimental data. We further show that the different intercalated Ag phases tune the electronic properties of the overlying quasi-free-standing graphene layer differently: compared with ${\mathrm{Ag}}_{\left(1\right)}, {\mathrm{Ag}}_{\left(2\right)}$ yields an $\sim 1.75$ times higher charge carrier density and modifies the charge-plasmon interaction of the graphene layer, indicating a change in effective screening at the interface.

The two-dimensional homogeneous electron gas (2D HEG) is a prototype system of fundamental interest that can be realized experimentally over a wide range of densities. Here, we investigate its collective charge excitations at low densities, using time-dependent density functional theory. We demonstrate that, beyond traditional plasmons, additional collective excitonic modes emerge for a Wigner-Seitz radius larger than $r_{\mathrm{s}}\approx 1$. These excitonic modes leave characteristic fingerprints in experimentally accessible quantities, namely, asymmetric peak structures in the loss function and very strong Friedel-like oscillations in the static linear density response that increase when the system approaches a regime of instability. Indeed, at low enough densities the collective modes cross the zero energy axis, indicating an instability of the paramagnetic 2D HEG toward the formation of a charge-density-wave phase with excitonic origin. These findings provide valuable insights for the experimental detection of excitonic collective modes in tunable 2D electron systems and contribute to the fundamental understanding of many-body effects in low-density electron gases.

Plastic deformation is widely regarded as an intrinsically dissipative phenomenon whose theoretical description relies heavily on phenomenological constitutive assumptions. We argue instead that defect-mediated plasticity possesses a universal, nondissipative backbone: the kinematics and mobility constraints of defects are fixed by symmetry and conservation laws prior to dissipation. Starting from the spontaneous breaking of spacetime symmetries in a crystalline phase, we construct an effective field theory in which elasticity reorganizes into a coupled higher-rank tensor-vector gauge structure emerging directly from stress and defect conservation laws rather than being postulated microscopically. Dislocations, disclinations, and singular frame-geometric configurations appear as gauge sources whose coupled continuity equations and mobility constraints follow from generalized Gauss laws. The construction provides a symmetry-based organization of the kinematic sectors relevant to defect-mediated plasticity and a starting point for a subsequent dissipative theory of defect motion and stress relaxation.

We report a detailed anisotropic study on the magnetic and transport properties of a ${\mathrm{CeRu}}_2{\mathrm{Ge}}_2$ single crystal. A clear ferromagnetic transition is observed at $T_{\mathrm{C}}=7.5\mathrm{K}$ in the magnetic susceptibility, electrical resistivity, and heat capacity measurements. The magnetoresistance (MR) is positive at low temperature within the magnetically ordered state. We confirmed the presence of a magnon excitation gap in the resistivity, MR, and low temperature heat capacity data. The pronounced anisotropy observed in the magnetic susceptibility and magnetization along the [100] and [001] directions is quantitatively explained within the crystal electric field (CEF) framework using the point-charge model. The analysis reveals a CEF level scheme comprising three doublets, with a ground state and two excited states at 491 and 671 K, that was further confirmed from the Schottky effect in the magnetic part of the heat capacity data. Unlike most Kondo lattice ferromagnets of Ce compounds, where the magnetic easy axis deviates from the expected CEF ground state, ${\mathrm{CeRu}}_2{\mathrm{Ge}}_2$ exhibits magnetization, which is fully consistent with the CEF derived ground state wave function, with the easy axis of magnetization aligned along the [001] direction.

A promising application of machine learning is the creation of low-cost surrogate models to mitigate computational bottlenecks in quantum many-body simulations. Here, we explore whether a neural network (NN) can be trained in the low-data regime, with one to two orders of magnitude fewer training examples than previous works, as an efficient substitute for the impurity solver in dynamical mean-field theory simulations of correlated electron models. We show that the NN solver achieves accuracy comparable to popular continuous-time quantum Monte Carlo (CT-QMC) impurity solvers when interpolating between samples within the training set. While the NN's performance decreases notably when extrapolating to lower temperatures outside the training distribution, its output still provides an excellent initial guess for input to more accurate CT-QMC impurity solvers, thus accelerating the time to solution up to a factor of five. We discuss our results in the context of rapid phase-space exploration.

While numerous models exist which explain certain aspects of irradiation-induced nanopatterning on semiconductors, a comprehensive theoretical explanation has remained elusive. However, it is increasingly apparent that such a model will require understanding the dual influence of the collision cascade initiated by ion implantation: first, as a source of material transport by sputtering and atomic displacements occurring over short timescales, and, second, as a source of defects permitting viscous flow within the thin, amorphous layer that results from sustained irradiation over longer timescales. To better understand the latter, we develop several asymptotic approximations for coupling the local ion flux experienced by the amorphous layer to the layer's evolving free interface. From these and the physical hypothesis of irradiation-induced anisotropic plastic flow, or “ion hammering,” we derive a generalized Kuramoto-Sivashinsky–type equation for the evolving free surface. With physically plausible parameters, the present model achieves good quantitative and qualitative agreement with several aspects of experimental observations of nanopattern formation during irradiation of silicon by argon, krypton, and xenon, and with projectile energies from 500 to 2000 eV. Disagreements between model and experiment are discussed, as are implications for future directions.

We introduce the Zitterbewegung areal rate operator to study the relation between the orientation of trembling motion and band topology. Constructed from the oscillatory interband displacement and velocity, this operator distinguishes clockwise from anticlockwise motion. For a generic gapped two-band Hamiltonian in two dimensions, we show that the Zitterbewegung areal rate is proportional to the Berry curvature. The derivation uses only spectral projectors and is therefore gauge invariant. We also demonstrate that the Zitterbewegung areal rate operator is time independent and a constant of motion at each momentum, although the displacement and velocity from which it is constructed oscillate in time. Its sign distinguishes anticlockwise from clockwise Zitterbewegung and coincides with the sign of the Berry curvature. For wave packets narrowly localized around the isolated massive Dirac points, the Zitterbewegung chirality equals the sign of the local Chern contribution. The Chern number can therefore be reconstructed from the orientations of the Zitterbewegung motion around the individual Dirac points.

Electron–hole plasma in two-dimensional solid-state systems has previously been studied mainly in two limiting cases: the degenerate regime in semimetals with parabolic bands, and the nondegenerate symmetric Dirac regime in graphenelike materials. Here we investigate a third, qualitatively different case realized in a gapless HgTe quantum well: a multicomponent plasma with a hybrid spectrum, in which massless Dirac carriers coexist with thermally activated heavy holes from lateral valence-band valleys. Near the charge-neutrality point we observe a pronounced increase of the resistance with temperature, following an approximately $T^2$ law, in sharp contrast to the nearly temperature-independent resistivity expected for a symmetric two-component Dirac plasma. We show that this behavior originates in the charge-neutrality condition, which, once the heavy holes are populated, pins the chemical potential above the Dirac point: the Dirac electrons remain moderately degenerate, while the heavy holes stay nondegenerate and obey Boltzmann statistics. Because the light degenerate electrons scatter almost elastically off the heavy Boltzmann holes, the interparticle transport admits a relaxation-time description and can be treated nonperturbatively. Using carrier densities from self-consistent band structure calculations, we find that the data are well reproduced by a short-range interaction between Dirac electrons and heavy holes in the weakly disordered limit for which the electronic conductivity scales as $T^{-2}$ and the excess resistivity is proportional to the heavy-hole density, whereas an unscreened Coulomb interaction would leave the conductivity temperature independent. The extracted interaction amplitude is of the Coulomb scale expected at the low charge-neutrality density. Our results establish near-critical HgTe quantum wells as a versatile platform for studying interaction-driven transport in multicomponent systems combining degenerate massless and nondegenerate massive carriers in which the linear Dirac spectrum decouples the current from the total momentum.

Recent numerical and theoretical studies on the two-dimensional $J\text{−}{Q}_3$ model suggest that the deconfined quantum critical point is actually a $\mathrm{SO}\left(5\right)$-symmetry-enhanced first-order phase transition that is spontaneously broken to $\mathrm{O}\left(4\right)$. However, the precise nature of the transition remains under active discussion. Despite extensive studies based on static quantities and entanglement properties, its low-energy excitation spectrum has received comparatively less attention. Here we investigate the dynamical spectra of spin and bond operators at the deconfined critical point of the $J\text{−}{Q}_3$ model using large-scale quantum Monte Carlo simulations and contrast them with the well-established $\mathrm{O}\left(3\right)$ Wilson-Fisher criticality in the $J_1\text{−}{J}_2$ columnar dimerized Heisenberg model. Although both models exhibit two gapless magnon modes in the Néel phase, their critical behaviors diverge strikingly. At the $J_1\text{−}{J}_2$ critical point, the Higgs mode becomes gapless, yielding three gapless modes that reflect the complete restoration of the $\mathrm{O}\left(3\right)$ symmetry. In the $J\text{−}{Q}_3$ model, we instead observe transverse excitations associated with the Néel and VBS order-parameter fluctuations becoming gapless at the transition, a behavior consistent with the emergence of $\mathrm{SO}\left(5\right)$ symmetry. While transverse modes become gapless at the transition, the behavior of the longitudinal (Higgs) mode remains unresolved.

The efficiency of optical cooling of nuclear spins in a CdTe/CdZnTe quantum well is investigated as a function of an external magnetic field. Our results confirm that there is indeed an optimal external magnetic field for optical cooling. We associate it with the kinetic local field $B_{\mathrm{KL}}$ defined by the heating rate of the spin-spin reservoir due to the fluctuations of the hyperfine interaction. We also propose an experimental technique for measuring $B_{\mathrm{KL}}$. For our sample we find that $B_{\mathrm{KL}}=1.0\pm 0.4$ G and it is independent of the electron polarization and pump power. The measured values of the kinetic local fields are in good agreement with the theoretical calculation $B_{\mathrm{KL}}=0.7$ G, taking into account indirect spin-spin interactions of Cd and Te nuclear spins and their considerably different hyperfine constants. The hyperfine constants of the magnetic isotopes of Cd and Te in CdTe are estimated.

Topological zero-line modes (ZLMs) based on valley Hall effect induced by inversion symmetry breaking have attracted significant attention in designing topological quantum devices. Compared with the toy model in monolayer graphene with staggered sublattice potentials, ZLMs in bilayer graphene are more accessible in experiments. However, it is still experimentally challenging to design the fine-tuned electric gates to realize ZLMs, and the introduction of electrodes also makes it difficult to further reduce the device size. In this work, we demonstrate the emergence of ZLMs in bilayer graphene from layer-reversing in-plane electric fields, which are induced by chemical dopants. Our proposal provides an experimentally feasible route to realize electrodeless ZLMs in graphene systems.

We construct the tight-binding model Hamiltonians of transition metals on the basis of $s$, $d$, and $p_z$ orbitals by employing a two-center approximation based on Slater-Koster tables. We systematically investigate the electronic structures of $3d$ and $5d$ transition-metal monolayers, and the material-specific parameters are extracted by fitting the first-principles calculations. We find that, compared with bulk systems, the local environments of atoms in these monolayers have lower symmetries, making $d$ orbitals split into more energy levels. The energy splittings are comparable with the $d$-band width and thus have significant influence on the band structure. Moreover, the lattice constants of these monolayer systems are remarkably smaller than those of corresponding bulk systems, indicating a dramatic change of hopping energies, band widths, and magnetisms. By further invoking the spin degree of freedom, we find that most $3d$ transition metals exhibit magnetizations that are closely related to their lattice constants. By fitting spin-minority and spin-majority bands separately, we find that the spin-majority electron exhibits a smaller $d$-state radius and therefore is more localized. In contrast, most $5d$ monolayers are nonmagnetic while exhibiting strong spin-orbit coupling, whose strength increases approximately linearly as the atomic number increases.

Magnetism in transition-metal systems emerges from exchange interactions that depend sensitively on carrier density. Yet leveraging this sensitivity to deliberately engineer exchange frustration and associated topological spin textures remains largely unexplored. Here, combining first-principles calculations with atomistic Monte Carlo simulations, we demonstrate that ferroelectric polarization enables electrostatic control of exchange frustration in the itinerant ferromagnet $\mathrm{SrRu}{\mathrm{O}}_3$. We show that electrostatic hole doping renormalizes competing exchange interactions, driving $\mathrm{SrRu}{\mathrm{O}}_3$ away from its bulk ferromagnetic ground state toward frustrated regimes, whereas electron doping largely preserves ferromagnetism. At $\mathrm{BaTi}{\mathrm{O}}_3/\mathrm{SrRu}{\mathrm{O}}_3$ interfaces, polarization-induced charge depletion modulates layer-dependent exchange couplings, enhancing competition between first, second, and third nearest-neighbor exchanges. The resulting exchange frustration stabilizes a sequence of magnetic phases as a function of thickness and applied magnetic field, including stripe and spiral states, topological meron and bimeron textures, and diverse skyrmionic objects. A minimal spin model identifies exchange frustration as the primary control parameter governing these crossovers, with magnetic anisotropy, Dzyaloshinskii-Moriya interaction, and external field selecting the emergent topology. Our results establish electrostatic doping as a route to engineer frustrated and topological magnetism in itinerant oxide metals.

Using the determinant Quantum Monte Carlo method, we investigate the interplay among doping, interlayer tunneling, and on-site Hund's coupling in stabilizing superconductivity (SC) in a two-orbital model for the bilayer nickelate ${\text{La}}_3{\text{Ni}}_2{\text{O}}_7$. With realistic dispersion and for certain values of the interaction parameters, the auxiliary-field-decoupled fermion Hamiltonian has Kramers antiunitary symmetries which guarantee the absence of a sign problem. The same antiunitary symmetries can also be used to show there is a second instability towards the $(\pi ,\pi )$ interlayer bond order (BO) in the strong interaction limit. We indicate the possible connection between this bond order and the enigmatic density wave state observed in experiments and clarify the decisive role played by the interlayer tunneling in the competition between the SC and the BO. Finally, possible directions on how to enhance the SC transition temperature and stabilize the SC phase are also discussed.