Two-dimensional arrays of superconductors separated by normal metallic regions exhibit rich phenomenology and a high degree of controllability. We establish such systems as platforms for topological phases of matter, and in particular chiral topological superconductivity. We propose and theoretically analyze several minimal models for this chiral phase based on commonly available superconductor-semiconductor heterostructures. The topological transitions can be adjusted using a time-reversal-symmetry breaking knob, which can be activated by controlling the phases in the islands, introducing flux through the system, or applying an in-plane exchange field. We demonstrate transport signatures of the chiral topological phase that are unlikely to be mimicked by local nontopological effects. The flexibility and tunability of our platforms, along with the clear-cut experimental fingerprints, make for a viable playground for exploring chiral superconductivity in two dimensions.

The supercurrent field effect is experimentally realized in various nanoscale devices, based on the superconductivity suppression by external electric fields being effective for confined systems. In spite of intense research, a microscopic theory of this effect is missing. Here, a microscopic theory of phonon-mediated superconductivity in thin films under an external electric field is presented, which accounts for the effect of quantum confinement on the electronic density of states, on the Fermi energy, and on the electron Coulomb repulsion. By accounting for the complex interplay between quantum confinement, the external static electric field, the Thomas-Fermi screening in the electron-phonon matrix element, and the effect of confinement on the Coulomb repulsion parameter, the theory predicts the critical value of the external electric field as a function of the film thickness, above which superconductivity is suppressed. In particular, this critical value of the electric field is exponentially lower the thinner the film, in agreement with recent experimental observations. Crucially, this effect is predicted by the theory when both Thomas-Fermi screening and the Coulomb pseudopotential are taken into account, along with the respective dependence on the thin film thickness. This microscopic theory opens up new possibilities for the supercurrent field effect and for electric-field gated quantum materials.

The layered square-planar nickelates $R_{n+1}\mathrm{N}{\mathrm{i}}_n{\mathrm{O}}_{2n+2}$ (R: rare-earth element) hold great promise in realizing cupratelike superconductors. While the appearance of zero resistivity is extremely sensitive to the concentration of hydrogen($x$) in infinite-layer $\mathrm{S}{\mathrm{r}}_{0.2}\mathrm{N}{\mathrm{d}}_{0.8}\mathrm{Ni}{\mathrm{O}}_2{\mathrm{H}}_x$, the configurations of many other $\mathrm{N}{\mathrm{d}}_{n+1}\mathrm{N}{\mathrm{i}}_n{\mathrm{O}}_{2n+2}{\mathrm{H}}_x$ and the general roles of H in these systems are still unknown. Using first-principles calculations, we find that H atoms prefer to form staggered one-dimensional chains along the $c$ axis, which are obstructed by the fluorite layer of $\mathrm{N}{\mathrm{d}}_{n+1}\mathrm{N}{\mathrm{i}}_n{\mathrm{O}}_{2n+2}$. Importantly, the charge-transfer energy between O $2p$ and Ni $3d$, one of the key factors determining the superconducting temperature in cuprates, is largely and continuously tunable by $n$ and $x$ in nickelates. Further, from the perspective of orbital hybridization and orbital polarization, the quasi-two-dimensional electronic properties are more pronounced for nickelates with smaller $n$, potentially facilitating superconductivity. These findings shed light on the general roles of H in controlling electronic properties in nickelates and provide valuable guidance for the experimental preparation of superconducting $R_{n+1}\mathrm{N}{\mathrm{i}}_n{\mathrm{O}}_{2n+2}$ materials.

${\mathrm{Ta}}_2{\mathrm{NiS}}_5$, a semiconductor at ambient conditions, does not exhibit an excitonic insulating state like its selenium counterpart ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, owing to its large band gap. Using a combination of Raman spectroscopy and analysis with first-principles effective Hamiltonian, we explore its instability toward an excitonic insulating state as a function of pressure, and affirm that excitonic insulating state does not get stabilized in ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ with pressure. We observe pressure-induced structural phase transition from its orthorhombic $Cmcm$ structure to another orthorhombic $Pmnm$ structure, with onset at $\sim$ 4.2 GPa and this transition gets completed at $\sim$ 6 GPa. We observe Raman signatures of an additional phase transition at $\sim$ 10.8 GPa, which is suggested to be associated with a semiconductor to metal transition.

Band-structure engineering is an important strategy that can improve the properties of functional materials or even bring new features to existing systems. Band gap (or energy gap, Eg) plays a crucial role in deciding the electronic or optical properties of one material. Isoelectronic and isostructural alloys usually exhibit similar electronic band structures, but the related effect of Eg variation was found to be distinct and sometimes controversial. Herein, we provided a deep understanding of the origin of band tuning in isoelectronic alloys based on experimental characterizations and theoretical calculations. The prerequisites of an isoelectronic alloy system with an Eg that is sensitive to composition are thoroughly disclosed by studying ${\left(\mathrm{Sb},\mathrm{Bi}\right)}_2{\mathrm{Te}}_3$ and Pb(Se,Te). In the promising ${\mathrm{Mg}}_3{\left(\mathrm{Sb},\mathrm{Bi}\right)}_2$ thermoelectric materials, it is found that Bi isoelectronic substitution can significantly decrease the Eg, due to the combination of the higher energy of Bi $6p$ orbit than that of Sb $5p$ orbit and the varied bond strength induced by lattice expanding. A high room-temperature power factor (PF) value of $38.5\mu \mathrm{W}\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{K}}^{-2}$ is obtained in nearly zero-gap ${\mathrm{Mg}}_{3.2}{\mathrm{Sb}}_{0.3}{\mathrm{Bi}}_{1.7}$ samples, which rationally guides the design of thermoelectric materials in the aspect of electronic transport.

At the core of the success of semiconducting spin qubits is the ability to manipulate them electrically, enabled by the spin-orbit interactions. However, most implementations require external magnetic fields to define the spin qubit, which in turn activate various charge-noise mechanisms. Here we study spin qubits confined in quantum dots at zero magnetic fields that are driven periodically by electrical fields and are coupled to a microwave resonator. Using Floquet theory, we identify a well-defined Floquet spin-qubit originating from the lowest degenerate spin states in the absence of driving. We find both transverse and longitudinal couplings between the Floquet spin qubit and the resonator, which can be selectively activated by modifying the driving frequency. We show how these couplings can facilitate fast qubit readout and the implementation of a two-qubit CPHASE gate. Finally, we use adiabatic perturbation theory to demonstrate that the spin-photon couplings originate from the non-Abelian geometry of states endowed by the spin-orbit interactions, rendering these findings general and applicable to a wide range of solid-state spin qubits.

We present an experimental protocol using cathodoluminescence measurements as a function of the electron incident energy to study both exciton diffusion in a directional way and surface exciton recombination. Our approach overcomes the challenges of anisotropic diffusion and the limited applicability of existing methods to the bulk counterparts of two-dimensional (2D) materials. The protocol is then applied at room and at cryogenic temperatures to four bulk hexagonal boron nitride crystals grown by different synthesis routes. The exciton diffusivity depends on the sample quality but not on the temperature, indicating it is limited by defect scattering even in the best quality crystals. The lower limit for the diffusivity by phonon scattering is 0.2 ${\mathrm{cm}}^2{\mathrm{s}}^{-1}$. Diffusion lengths were as much as 570 nm. Finally, the surface recombination velocity exceeds ${10}^5{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, at a level similar to silicon or diamond. This result reveals that surface recombination could strongly limit light-emitting devices based on 2D materials.

The observation temperatures of quantum anomalous Hall effect in semimagnetic and magnetic topological insulator (MTI) heterostructures are always much lower than Curie temperature. Here, we theoretically demonstrate that floating of topological surface states (TSSs) into magnetic insulators is not only able to produce ideal semi-MTI heterostructures, but also provides insight into the origin of extremely low observation temperature of the quantum anomalous Hall effect in different MTIs. We show that the emergence of diving and floating of TSSs can be observed in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ family heterostructures. Within the TSSs floating systems, ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/{\mathrm{Sb}}_2{\mathrm{Te}}_3$ and ${\mathrm{NiBi}}_2{\mathrm{Te}}_4/{\mathrm{Sb}}_2{\mathrm{Te}}_3$ exhibit characteristics of ideal semi-MTIs, featuring a tunable Fermi level, and ${\mathrm{MnSb}}_2{\mathrm{Te}}_4/{\mathrm{Bi}}_2{\mathrm{Te}}_3/{\mathrm{NiBi}}_2{\mathrm{Te}}_4$ $({\mathrm{NiBi}}_2{\mathrm{Te}}_4/{\mathrm{Sb}}_2{\mathrm{Te}}_3/{\mathrm{VBi}}_2{\mathrm{Te}}_4)$ is found to be MTI with weak (strong) long-range ferromagnetic coupling and large (small) surface gap. Our findings reveal that the observation temperature of quantum anomalous Hall effect is governed by the smaller of the two surface gaps, rather than being directly linked to the Curie temperature of MTIs.

We propose a two-mode, two-photon microlaser using a single semiconductor quantum dot (QD) grown inside a two-mode microcavity. We explore both incoherent and coherent pumping at low temperatures to achieve suitable conditions for the lasing. The two-mode, two-photon-stimulated emission is strongly suppressed when exciton-phonon interactions occur, but the single-photon-stimulated emission is enhanced in this case. In coherently pumped QDs, one can achieve large two-mode, two-photon lasing, while the single-photon lasing is almost absent here. We further extend our study for the generation of a two-mode entangled state using two-photon resonant pumping.

We theoretically investigate the Josephson phase transition in a superconductor/quantum-dot/superconductor junction with side-coupled Majorana bound states (MBSs), by means of the zero-bandwidth approximation and numerical renormalization group (NRG) methods. It is found that the bistable phase of the Josephson current is allowed to arise in this system, when the dot level is of electron-hole symmetry. The NRG calculation shows that such a phase is induced by the appearance of the spin-dependent states in the quantum dot, due to the side coupling of MBSs. Through further research, we observe that the MBS leads to the formation of two distinct spin-resolved Yu-Shiba-Rusinov (YSR) states. The spin screening of the superconductors to the YSR states causes the occurrence of spin-dependent Kondo effect, and then the two-stage Kondo effect can be observed with the special and nontrivial contributions of the MBS. It also shows that the properties of the first-stage Kondo effect can be modulated in detail by adjusting the dot-MBS and inter-MBS couplings, which leads to the emergence of the new bistable phase of the Josephson current. All these results are helpful for understanding the Kondo physics and phase transition in a Josephson junction with side-coupled MBSs.

Cavity embedding is an emerging paradigm for the control of quantum matter, offering avenues to manipulate electronic states and potentially drive topological phase transitions. In this work, we address the stability of a one-dimensional topological superconducting phase to the vacuum quantum fluctuations brought by a global cavity mode. By employing a quasiadiabatic analytical approach completed by density matrix renormalization group calculations, we show that the Majorana end modes evolve into composite polaritonic modes while maintaining the topological order intact and robust to disorder. These Majorana polaritons keep their non-Abelian exchange properties and protect a twofold exponentially degenerate ground state for an open chain. They become, however, weak edge modes in the sense that they no longer commute with the full Hamiltonian and protect the exponential degeneracy only in the ground-state manifold.

A growing body of theoretical and experimental evidence suggests that inert gases (He, Ne, Ar, Kr, Xe, Rn) become less and less inert under increasing pressure. Here we use crystal structure prediction software based on the ab initio evolutionary algorithm, named uspex, to predict stable compounds of Xe and Na at pressures below 100 GPa, and we find three stable compounds, NaXe, ${\mathrm{NaXe}}_3$, and ${\mathrm{NaXe}}_4$. The NaXe belongs to a well-known cubic CsCl structure type. ${\mathrm{NaXe}}_4$'s structure is common in amphiboles, whereas ${\mathrm{NaXe}}_3$ has a unique structure, analogous to the “post-perovskite” orthorhombic ${\mathrm{CaIrO}}_3$-type structure with Ir atoms removed. NaXe, ${\mathrm{NaXe}}_3$, and ${\mathrm{NaXe}}_4$ are found to be metallic.

Understanding the relationship between quantum geometry and topological invariants is a central problem in the study of topological states. In this work, we establish the relationship between the quantum metric and the Euler curvature in two-dimensional systems with space-time inversion $I_{ST}$ symmetry satisfying $I_{ST}^2=+1$. As $I_{ST}$ symmetry imposes the reality of the wave function with vanishing Berry curvature, the well-known inequality between the quantum metric and the Berry curvature is not meaningful in this class of systems. We find that the non-Abelian quantum geometric tensor of two real bands exhibits an intriguing inequality between the off-diagonal Berry curvature and the quantum metric, which in turn gives the inequality between the quantum volume and the Euler invariant. Moreover, we show that the saturation condition of the inequality is deeply related to the ideal condition for Euler bands, which provides a criterion for the stability of fractional topological phases in interacting Euler bands. Our findings demonstrate the potential of the quantum geometry as a powerful tool for characterizing symmetry-protected topological states and their interaction effect.

Artificial lattices have been used as a platform to extend the application of topological band theory beyond electronic systems. Here, using the two-dimensional Lieb lattice as a prototypical example, we show that an array of disks which each support localized plasmon modes gives rise to an analog of the quantum spin-Hall state enforced by a synthetic time-reversal symmetry. We find that the plasmonic modes naturally possess a synthetic spin degree of freedom which leads to a spin-dependent second-neighbor coupling mechanism mediated by interorbital coupling. This interaction introduces a nontrivial $Z_2$ topological order and gaps out the Bloch spectrum. A faithful mapping of the plasmonic system onto a tight-binding model is developed and shown to capture its essential topological signatures. Full wave numerical simulations of graphene disks arranged in a Lieb lattice confirm the existence of propagating helical boundary modes in the nontrivial band gap.

We construct two-band effective models describing Majorana edge states in a short Kitaev chain. We derive an analytical formula for the effective Hamiltonian as a function of model parameters. Then, we discuss the robustness of Majorana edge states as a function of model parameters. We have found an even-odd effect on the robustness of Majorana states, which is characteristic of short Kitaev chains. It is experimentally observable as a differential conductance in quantum dot systems. We also study effects of coupling to an environment based on non-Hermitian Hamiltonians derived from the Lindblad equation. It is found that the Majorana zero-energy edge states acquire nonzero energy such as $E\propto \pm {\left(i\mathrm{\Gamma }\right)}^L$ for the local dissipation, where $\mathrm{\Gamma }$ is the magnitude of the dissipation and $L$ is the length of the chain. The even-odd effect is manifest for small $L$ in this formula. On the other hand, the Majorana zero-energy edge states acquire nonzero energy such as $E\propto \pm i\mathrm{\Gamma }$ for small $\mathrm{\Gamma }$ irrespective of the length $L$ for the global dissipation.

The phase diagram of the one-dimensional hexagonal Harper (HH) model reveals the presence of two metallic phases and one insulating phase, separated by critical lines and a bicritical point. In our work, we investigate transport in the different phases by considering both the isolated and open system scenario. For the case of the isolated system, we study the single-particle dynamics at the bicritical point and along the critical lines. We find that the single-particle wave packet dynamics is superdiffusive in the critical regions with the transport at the bicritical point faster than that along the critical lines. In addition, we study domain wall (DW) dynamics via unconventional multiparticle states in the two metallic phases. The DW state is constructed by partially filling half of the chain while the other half is kept empty; the dynamics of this state reveals distinct behavior in the deep metallic regime of the two metallic phases. Interestingly, the distinct behavior is absent if one instead considers a fully filled half chain. For the open system scenario, we study transport in the nonequilibrium steady state (NESS). We observe that in the critical regions, transport is subdiffusive in nature. Moreover, we find that the transport scaling exponents for the open system scenario are the same at the bicritical point and along the critical lines, unlike the closed system case. In addition, we observe an even-odd size effect on the NESS density and current and on the optimal system-bath coupling parameter (corresponding to the maximum current) in the deep metallic regime of the HH model.

We investigate the characteristic modes of polarization waves in ferroelectric films. This is motivated by the recent surge of interest in the excitations of the ferroelectric order inspired by the duality between electric dipoles in ferroelectrics and magnetic dipoles in ferromagnets that has led to the introduction of the area of ferronics by analogy to magnonics. We report that a ferroelectric film supports surface and volume modes of polarization waves, analogous to the surface and volume magnetostatic spin-wave modes in a ferromagnetic film. However, while in ferromagnets each type of mode has only one (positive) frequency band, in a ferroelectric film the surface and volume modes have two frequency bands each. We present the dependence of the frequencies on the wave vector for both modes for the parameters of the classic ferroelectric $\mathrm{LiNb}{\mathrm{O}}_3$, with polarization either parallel or perpendicular to the film. The frequencies lie in the low terahertz band that is experimentally accessible.

We report a detailed study of the physical properties of single crystals of ${\mathrm{DyCoGa}}_5$ using magnetic susceptibility, specific heat, and resistivity measurements. ${\mathrm{DyCoGa}}_5$ crystallizes in the layered tetragonal ${\mathrm{HoCoGa}}_5$-type structure and undergoes two successive antiferromagnetic transitions at $T_{N1}$ = 24.7 K and $T_{N2}=22.9$ K, which are associated with ordering of the ${\mathrm{Dy}}^{3+}$ moments. We characterize the temperature-field phase diagrams of ${\mathrm{DyCoGa}}_5$ for fields both along the $c$ axis and within the $ab$ plane, where highly anisotropic magnetic behaviors are observed. When fields are applied along the easy $c$ axis, both $T_{N1}$ and $T_{N2}$ are suppressed with increasing field, and multiple metamagnetic transitions are observed, while these transitions exhibit only a weak field dependence for $H\parallel ab$. From our analysis we propose a crystalline-electric-field scheme that gives rise to the observed Ising anisotropy, and a simple model of the magnetic exchange interactions can account for the low-temperature antiferromagnetic phase, as well as the field-induced phase with a magnetization half the saturated value.

${\mathrm{Cr}}_2{\mathrm{Te}}_3$ is a ferromagnetic, quasi-two-dimensional layered material with perpendicular magnetic anisotropy, strong spin-orbit coupling, and nontrivial band topology. The nontrivial topology results in an intrinsic anomalous Hall conductivity (AHC) that switches sign under filling and biaxial strain. Thin films can exhibit half-metallicity. Using density-functional theory combined with maximally localized Wannier functions, we reveal the physical origins of the sensitivity of the sign of the AHC to strain and filling, and we determine the effect of surface termination on the half-metallicity. We find that thin films terminated on the Te layers are the most energetically stable, but only the thin films terminated on both sides with the partially occupied Cr layers are half metals. In bulk ${\text{Cr}}_2{\text{Te}}_3$, the sensitivity of the sign of the AHC to strain and filling results from the complex Fermi surface comprised of three bands. Filling of local minima and bands near anticrossings alters the local Berry curvature, consistent with the negative-to-positive switching of the AHC. Similarly, strain depopulates a local minimum, shifts a degenerate point closer to the Fermi energy, and causes two spin-orbit split bands to reverse their order. These findings provide a physical understanding of the evolution of the Berry phase, AHC, and half-metallicity in ${\text{Cr}}_2{\text{Te}}_3$.