We investigate nonequilibrium transport through superconducting nanojunctions using a Liouville space approach. The formalism allows us to study finite-gap effects, and to account for both quasiparticle and Cooper-pair tunneling. With focus on the weak-tunneling limit, we study the stationary dc and ac current up to second order (cotunneling) in the hybridization energy. For the particular case of a strongly interacting quantum dot sandwiched between two superconductors, we identify the characteristic virtual processes that yield the Andreev and Josephson current and obtain the dependence on the gate and bias voltage for the dc current, the critical current, and the phase-dependent dissipative current. In particular, the critical current is characterized by regions in the stability diagram in which its sign changes from positive to negative, resulting in a multitude of $0-\pi$ transitions. The latter signal the interplay between strong interactions and tunneling at finite bias.

Many technologies require stable or metastable surface morphology. In this paper we study the factors that control the metastability of a common feature of rough surfaces: “hillocks.” We use low energy electron microscopy to follow the evolution of the individual atomic steps in hillocks on Pd(111). We show that the uppermost island in the stack often adopts a static, metastable configuration. Modeling this result shows that the degree of the metastability depends on the configuration of steps dozens of atomic layers lower. Our model allows us to link surface metastability to the atomic processes of surface evolution.

Optimizing valley dynamics serves as an effective tool for facilitating the coherent control within the framework of two-dimensional semiconductors. In this study, we develop a comprehensive model that encompasses both intravalley and intervalley exciton channels in monolayer ${\mathrm{WSe}}_2$, and simultaneously takes the light-matter interaction into account. This model allows us to investigate the optimal control of valley dynamics starting from an initial coherent excitonic state. Drawing upon the quantum speed limit (QSL) theory, we propose two optimal control schemes with the goal of reducing the evolution time required for valley dynamics to reach the final state, and of enhancing the evolution speed over a period of time. We emphasize that the implementation of dynamical optimization is associated with the detuning difference, the variance in exciton-laser field detunings between the $K$ and $K^{\prime }$ valleys, dictated by the optical excitation mode and magnetically induced valley splitting. Specifically, to minimize the evolution time, we find that a small detuning difference drives the actual dynamical path to converge toward the geodesic length from the initial state to final state, enabling the system to evolve in the shortest time. In particular, in the presence of valley coherence, the actual evolution time closely aligns with the calculated QSL time, facilitating high-fidelity quantum system evolution. Conversely, we illustrate a remarkable enhancement in the evolution speed of valley dynamics by using a large detuning difference, which induces emerging valley polarization even without initial polarization. Our work has important theoretical implications for facilitating coherent valley manipulations, and could also serve as a guide for searching optimal evolution path in relevant experiments.

Employing large-scale quantum Monte Carlo simulations, we find that in the magnetized interacting Dirac fermion model there emerges a universal collective Larmor-Silin spin wave mode in the transverse dynamical spin susceptibility. Such mode purely originates from the interaction among Dirac fermions and distinguishes itself from the usual particle-hole continuum with finite lifetime and clear dispersion, both at small and large momenta in a large portion of the Brillouin zone. Our unbiased numerical results offer the dynamic signature of this collective excitation in interacting Dirac fermion systems, and provide experimental guidance for inelastic neutron scattering, electron spin resonance, and other spectroscopic approaches in the investigation of such universal collective modes in quantum moiré materials, topological insulators, and quantum spin liquid materials under magnetic field, with quintessential interaction nature beyond the commonly assumed noninteracting Dirac fermion or spinon approximations.

We present a unifying theory for predicting electronic descriptors (e.g., the $d$-band center ${\epsilon }_d$) of transition and noble metal surfaces by interpretable deep learning. Distinct from black-box machine learning models, fundamental insights into underlying physical processes can be obtained without sacrificing prediction accuracy. In addition to the charge transfer, strain, and ligand effects in the conventional wisdom, we identified orbital resonance in $d$-electron hopping as a crucial factor that modulates the shape of the $d$-state distribution, thereby shifting ${\epsilon }_d$ of a $d$-metal site upon perturbation. Our findings reveal the promise of machine learning in advancing domain knowledge, paving the way toward theory-guided, data-driven design of materials beyond brute-force screening.

We provide a theory for atomic resolution phonon spectroscopy in scanning transmission electron microscopy. This formulation goes beyond a recently proposed simple approach to atomic-scale phonon spectroscopy by explicitly including the dependence on probe position in the inelastic scattering cross section itself and considers the contribution to the spectrum from individual atoms. An application is made to existing experimental data that demonstrates how a direct comparison of the data with phonon densities of states projected onto individual atoms can be made and also the importance of contributions to the energy-loss spectrum on a particular atomic site from surrounding atoms due to probe spreading.

We show that, contrary to common belief, the depolarizing electric field generated by bound charges at thin-film surfaces can have a substantial impact on the domain structure of an improper ferroelectric with topological defects. In hexagonal-manganite thin films, we observe in phase-field simulations that through the action of the depolarizing field, (1) the average magnitude of the polarization decreases, (2) the local magnitude of the polarization decreases with increasing distance from the domain walls, and (3) there is a significant alteration of the domain-size distribution and average domain size, which is visualized with the pair-correlation function. We conclude that, in general, it is not appropriate to ignore the effects of the depolarizing field for thin-film ferroelectrics.

We investigate the transport properties of massive Dirac fermions subjected to uncorrelated scalar potential disorder, and mass disorder. Using a finite difference method, the conductance is calculated for a wide variety of combinations of these two disorder strengths. By calculating the scaling of conductivity with system size we find that, depending on the combination, the system can have an insulating, scale invariant, and metallic behavior. We identify the critical values of these disorder strengths where the phase transitions occur. We study both the zero and nonzero average mass cases to examine the effect of scalar potential disorder on band gap. Our results suggest a suppression of the band gap by the scalar potential disorder.

In the iron–chromium system, ${\mathrm{Fe}}_x{\mathrm{Cr}}_{1-x}$, a wide dome of spin-glass behavior emerges when the ferromagnetism of iron is suppressed and the antiferromagnetism of chromium emerges as a function of increasing iron content $x$. As both, the high-temperature state and the characteristic cluster size vary as a function of $x$, different regimes of spin-glass behavior may be compared in a single, isostructural material system. Here, we report a study of the spin dynamics across the freezing process into the spin-glass state for different iron concentrations ($x=0.145$, 0.175, 0.21) using modulation of intensity with zero effort (MIEZE) spectroscopy. In the parameter range studied, the relaxation process observed experimentally may be described well in terms of a stretched exponential. In the reentrant cluster-glass regime, $x=0.145$, this behavior persists up to high temperatures. In comparison, in the superparamagnetic regime, $x=0.175$ and $x=0.21$, a single relaxation time at elevated temperatures is observed. For all samples studied, the spin relaxation exhibits a momentum dependence consistent with a power law, providing evidence of a dispersive character of the spin relaxation.

Two-dimensional (2D) intrinsic superconductors with nontrivial topological band and vertical ferroelectricity exhibit fascinating characteristics for achieving electrostatic control of quantum phases. Only a few such 2D materials have been theoretically predicted. In this paper, based on first-principles calculations, we explore the superconductivity and ferroelectric properties in fluorinated 2D ${\mathrm{Nb}}_2\mathrm{N}$. In the stable ${\mathrm{Nb}}_2{\mathrm{NF}}_2, H_3\text{−}{\mathrm{Nb}}_2{\mathrm{NF}}_2$ breaks the spatial inversion symmetry, exhibiting vertical ferroelectric. More interestingly, it not only possesses intrinsic superconductivity with superconducting transition temperatures ($T_c$) of 10 K, but also exhibits nontrivial band topology. $H_1\text{−}{\mathrm{Nb}}_2{\mathrm{NF}}_2$ shows topological band and superconductivity with $T_c$ of 32 K, surpassing most of the 2D conventional topological superconductors' candidates. Our research has enriched 2D superconducting materials with nontrivial band topology and ferroelectric properties, and provided a theoretical basis for the preparation of devices switching between superconducting and ferroelectric states with external electric field.

The systems without symmetries, e.g., the spatial and chiral symmetries, are generally thought to be improper for topological study and no conventional integral topological invariant can be well defined. In this work, with multiband asymmetric Rice-Mele–type systems as examples, we show that the topology of all gaps can be reconstructed by two general methods and the topological origin of many phenomena are revealed. An integral topological invariant, i.e., the renormalized real-space winding number, can properly characterize the topology and bulk-edge correspondence of such systems. For the first method, a homomorphic mapping relationship between a Rice-Mele–type system and its chiral counterpart is set up, which accounts for the topology reconstruction in the half-filling gaps. For the second method, the Hilbert space of asymmetric systems could be reduced into degenerate subspaces by perturbation approximation, so that the topology in subspaces accounts for the topology reconstruction in the fractional-filling gaps. Surprisingly, the topology reconstructed by perturbation approximation exhibits extraordinary robustness since the topological edge states even exist far beyond the weak perturbation limit. We also show that both methods can be widely used for other asymmetric systems, e.g., the two-dimensional Rice-Mele systems and the superconductor systems. At last, for the asymmetric photonic systems, we predict different topological edge states by our topology-reconstruction theory and experimentally observe them in the laboratory, which agrees with each other very well. Our findings show the possibility that for some types of asymmetric systems the topology can be reconstructed by two methods and the related phenomena can be observed. Various reconstruction methods might be found for other types of asymmetric systems and the category of topology study could be expanded.

Cobalt titanate, ${\mathrm{CoTiO}}_3$, is a honeycomb antiferromagnet recently confirmed experimentally to host Dirac magnons, topological spin-orbit excitons, and chiral phonons. Here, we investigate a magnon gap at the zone center which calls for a refined spin Hamiltonian. We propose a microscopic model for the magnon gap and attribute it to a lattice-distortion (phonon)-induced higher-order spin interaction. Strong magnetoelastic coupling in ${\mathrm{CoTiO}}_3$ is also evident in Raman spectra, in which the magnetic order exerts a stronger influence on phonons corresponding to in-plane ionic motions than those with out-of-plane motions. We further examine the evolution of the zone-center magnons in a high magnetic field up to 18.5 T via THz absorption spectroscopy measurements. Based on this field dependence, we propose a spin Hamiltonian that not only agrees with magnon dispersion measured by inelastic neutron scattering but also includes fewer exchange constants and a realistic anisotropy term. Our work highlights the broad implications of magnetoelastic coupling in the study of topologically protected bosonic excitations.

A zero-magnetic-field Josephson diode effect (JDE) is observed in an asymmetric superconducting quantum interference device (SQUID) mediated by Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$. We argue that a phase coupling between the surface and bulk superconducting channels, a unique phenomenon recently identified in the observations of fractional Josephson effect and Leggett modes in $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$, can break time reversal symmetry and therefore give rise to the zero-field JDE. Our results are anticipated to have important implications in superconducting electronic circuit applications.

High-pressure application is crucial for shaping material properties and revealing novel phenomena. Materials undergo more unpredictable transitions in their crystal and electronic structures under nonhydrostatic/isotropic pressures than under hydrostatic conditions. However, limited knowledge regarding this behavior exists due to a scarcity of relevant studies. In this study, we systematically investigated the evolution of the structure and electronic states of pristine $2H\text{-MoT}{\mathrm{e}}_2$ in anisotropic pressure environments. Surprisingly, we observed anisotropic compression-induced superconductivity in this material, a phenomenon that is absent in quasihydrostatic environments. High-pressure x-ray diffraction and Raman spectroscopy measurements showed no structural phase transitions; however, an anomalous compressive behavior was observed along the $c$ axis. An anisotropic pressure model was proposed based on experimental conditions. First-principles calculations revealed that a strengthened anisotropic compression effect could enhance electron-phonon coupling by increasing the density of states on the Fermi level and softening the phonon modes, contributing to the emergence of superconductivity in $2H\text{-MoT}{\mathrm{e}}_2$. Our findings offer a route for inducing superconductivity, providing insights into the nature of superconductivity in transition metal dichalcogenides and other similar layered compounds under extreme conditions.

In two-dimensional electronic lattices, changes in the topology of the Fermi surface (Lifshitz transitions) lead to Van Hove singularities characterized by a divergence in the electronic density of states. Van Hove singularities can enhance the effect of electronic interactions, providing a platform to explore novel correlated electronic states. In this work, we investigate the emergence of topological Chern bands on the surface of three-dimensional topological insulators, which host higher-order Van Hove singularities that are characterized by the power-law diverging density of states. These singularities can arise from the interplay between a time-reversal-breaking Zeeman field induced by proximity to a ferromagnetic insulator and a time-reversal-invariant moiré potential on the surface electrons, created by quintuple layer misalignment in a family of topological insulators such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, which host a single surface Dirac fermion. We establish the onset of Chern bands near charge neutrality with Chern numbers $C=\pm 1$ that also possess a manifold of higher-order Van Hove singularities on the moiré Brillouin zone valleys controlled by the Zeeman and moiré potential energy scales, unveiling a platform to realize exotic Lifshitz transitions in topological bands. Furthermore, we show that the strong peaks in the density of states in the vicinity of Lifshitz transitions give rise to characteristic features in the low-temperature intrinsic anomalous Hall conductivity, yielding a path to probe Van Hove singularities in Chern bands through anomalous transport measurements.

We present a charge self-consistent density functional theory combined with the ghost rotationally invariant slave-boson ($\mathrm{DFT}+\mathrm{gRISB}$) formalism for studying correlated materials. This method is applied to ${\mathrm{SrVO}}_3$ and NiO, representing prototypical correlated metals and charge-transfer insulators. For ${\mathrm{SrVO}}_3$, we demonstrate that $\mathrm{DFT}+\mathrm{gRISB}$ yields an accurate equilibrium volume and effective mass close to experimentally observed values. Regarding NiO, $\mathrm{DFT}+\mathrm{gRISB}$ enables the simultaneous description of charge-transfer and Mott-Hubbard bands, significantly enhancing the accuracy of the original $\mathrm{DFT}+\mathrm{RISB}$ approach. Furthermore, the calculated equilibrium volume and spectral function reasonably agree with experimental observations.

We study the condensation of Abelian bosons in string-net models by constructing a family of Hamiltonians that can be tuned through any such transition. We show that these Hamiltonians admit two exactly solvable, string-net limits: one deep in the uncondensed phase, described by an initial, uncondensed string-net Hamiltonian, and one deep in the condensed phase, described by a final, condensed string-net model. We give a systematic description of the condensed string-net model in terms of the uncondensed string-net and the data associated with the condensing Abelian bosons. Specifically, if the uncondensed string-net is described by a fusion category $\mathcal{C}$, we show how the string labels and fusion data of the fusion category $\tilde{\mathcal{C}}$ describing the condensed string-net can be obtained from that of $\mathcal{C}$ and the data describing the string operators that create the condensing boson. This construction generalizes previous approaches to anyon condensation in string-nets by allowing the condensation of arbitrary Abelian bosons, including chiral bosons in string-nets constructed from, for example, Chern-Simons theories, which describe time-reversal invariant bilayer states. This gives a method for obtaining the full data for string-nets without explicit time-reversal symmetry from such bilayer models. We illustrate our approach with several examples.

We examine two-dimensional electron transport through a long narrow channel driven by an external electric field in the presence of diffusive boundary scattering. At zero temperature, we derive an analytical solution of the transition from ballistic to diffusive transport if we increase the bulk disorder strength. This crossover yields characteristic current density profiles. Furthermore, we illustrate the current density in the transition from ballistic to hydrodynamic transport. This corresponds to the Gurzhi effect in the resistivity. We also study the influence of finite temperature on current densities and average current in this system. In particular, we analyze how a particular scaling law of electron-electron scattering with respect to temperature affects the current along the channel.

In this work we propose the average spin Chern number (ASCN) as an indicator of the topological significance of the spin degree of freedom within insulating materials. Whenever this number is a nonzero even integer, it distinguishes the material as a spin Chern insulator, and the number is a topological invariant whenever there is a symmetry that commutes with the spin and protects Chern numbers. If this number is not zero, it indicates that the material has nontrivial spin transport properties, and it lies close to the value of the spin Hall conductivity (SHC) within the band gap. For systems where the spin commutes with the Hamiltonian, the ASCN matches the SHC. When the noncommutativity of the spin with the Hamiltonian cannot be neglected, both values are nonzero simultaneously. The ASCN is therefore a good complement for the intrinsic contribution of the SHC, and permits us to detect topological information of the material, which is not possible alone from the value of the SHC.