Although the challenge of GW calculations of the quasiparticle properties of $3d$ transition metal oxides (TMOs) is well recognized, TMOs involving $4d$ electrons may be considered borderline systems and have received less attention. Here we present the quasiparticle band structures of $\mathrm{SrZr}{\mathrm{O}}_3$ and $\mathrm{BaZr}{\mathrm{O}}_3$, two relatively simple wide band gap oxides for which accurate calculations of the electronic structure are scarce despite their technological importance. We show that fully converged GW calculations can accurately predict the quasiparticle properties of $4d$ TMO perovskites $\mathrm{SrZr}{\mathrm{O}}_3$ and $\mathrm{BaZr}{\mathrm{O}}_3$, regardless of the starting mean-field solution being calculated within straightforward density functional theory (DFT) or the $\mathrm{DFT}\text{+}U$ method. This is in contrast to the cases of $3d$ TMO perovskites $\mathrm{SrTi}{\mathrm{O}}_3$ and $\mathrm{BaTi}{\mathrm{O}}_3$, for which the $\mathrm{DFT}\text{+}U$ method is shown to provide a much better starting point for subsequent GW calculations. Compared with the fairly localized $3d$ states, the more extended $4d$ orbitals seem to be well described within DFT using local or semilocal functionals. Our results reaffirm the accuracy and robustness of the GW method provided that faithful zeroth-order mean-field solutions are available, and that the results are adequately converged.

After the discovery of graphene, there have been tremendous efforts in exploring various layered two-dimensional (2D) materials for their potential applications in electronics, optoelectronics, as well as energy conversion and storage. One of such 2D materials, $\mathrm{Sn}{\mathrm{S}}_2$, which is earth abundant, low in toxicity, and cost effective, has been reported to show a high on/off current ratio, fast photodetection, and high optical absorption, thus making this material promising for device applications. Further, a few recent theoretical reports predict high electrical conductivity and Seebeck coefficient in its bulk counterparts. However, the thermal properties of $\mathrm{Sn}{\mathrm{S}}_2$ have not yet been properly explored, which are important to materialize many of its potential applications. Here, we report the thermal properties of $\mathrm{Sn}{\mathrm{S}}_2$ measured using the optothermal method and supported by density functional theory (DFT) calculations. Our experiments suggest very low in-plane lattice thermal conductivity ($\kappa =3.20\pm 0.57\mathrm{W}{\mathrm{m}}^{–1}{\mathrm{K}}^{–1}$) and cross-plane interfacial thermal conductance per unit area $(g=0.53\pm 0.09\mathrm{MW}{\mathrm{m}}^{–2}{\mathrm{K}}^{–1})$ for monolayer $\mathrm{Sn}{\mathrm{S}}_2$ supported on a $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrate. The thermal properties show a dependence on the thickness of the $\mathrm{Sn}{\mathrm{S}}_2$ flake. Based on the findings of our DFT calculations, the very low value of the lattice thermal conductivity can be attributed to low group velocity, a shorter lifetime of the phonons, and strong anharmonicity in the crystal. Materials with low thermal conductivity are important for thermoelectric applications as the thermoelectric power coefficient goes inversely with the thermal conductivity.

We study the construction of programable integrated circuits with the help of disordered Chern insulators. Specifically, the schemes for low dissipation logic devices and connecting wires are proposed. We use the external-gate-induced step voltage to construct spatially adjustable channels, where these channels take the place of the conventional wires. Our numerical calculation manifests that the external gates can be adopted to program the arbitrary number of wires ($n$-to-$m$ connections). We find that their electron transport is dissipationless and robust against gate voltage fluctuation and disorder strength. Furthermore, seven basic logic gates distinct from the conventional structures are proposed. Our proposal has potential applications in low power-integrated circuits and enlightens the building of integrated circuits in topological materials.

Under a perfect periodic potential, the electric current density induced by a constant electric field may exhibit nontrivial oscillations, so-called Bloch oscillations, with an amplitude that remains nonzero in the large system size limit. Such oscillations have been well studied for nearly noninteracting particles and observed in experiments. In this work we revisit Bloch oscillations in strongly interacting systems. By analyzing the spin-$\frac{1}{2}$ XXZ chain, we demonstrate that the current density at special values of the anisotropy parameter $\mathrm{\Delta }=-cos(\pi /p)$ $(p=3,4,5,\cdots )$ in the ferromagnetic gapless regime behaves qualitatively the same as in the noninteracting case $(\mathrm{\Delta }=0)$ even in the weak electric field limit. When $\mathrm{\Delta }$ deviates from these values, the amplitude of the oscillation under a weak electric field is suppressed by a factor of the system size. We estimate the strength of the electric field required to observe such a behavior using the Landau-Zener formula.

We study nonlinear Drude weights (NLDWs) for the spin-$\frac{1}{2}$ XXZ chain in the critical regime at zero temperature. The NLDWs are generalizations of the linear Drude weight. Via the nonlinear extension of the Kohn formula, they can be read off from higher-order finite-size corrections to the ground-state energy in the presence of a $U\left(1\right)$ magnetic flux. The analysis of the ground-state energy based on the Bethe ansatz reveals that the NLDWs exhibit convergence, power-law, and logarithmic divergence, depending on the anisotropy parameter $\mathrm{\Delta }$. We determine the convergent and power-law divergent regions, which depend on the order of the response $n$. Then, we examine the behavior of the NLDWs at the boundary between the two regions and find that they converge for $n=0,1,2\left(\mathrm{mod}4\right)$, while they show logarithmic divergence for $n=3\left(\mathrm{mod}4\right)$. Furthermore, we identify particular anisotropies $\mathrm{\Delta }=cos(\pi r/(r+1))$ ($r=1,2,3,...$) at which the NLDW at any order $n$ converges to a finite value.

Second-order topological semimetals (SOTSMs) are featured with hinge Fermi arcs. How to generate them in different systems has attracted much attention. We propose a scheme to create exotic SOTSMs by periodic driving. Novel Dirac SOTSMs, with a widely tunable number of nodes and hinge Fermi arcs, the adjacent nodes having the same chirality, and the coexisting nodal points and loops, are generated at ease by the periodic driving. When the time-reversal symmetry is broken, our scheme permits us to realize hybrid-order Weyl semimetals with the coexisting hinge and surface Fermi arcs. Our Weyl semimetals possess a rich hybrid of 2D sliced zero- and $\pi /T$-mode topological phases, which may be any combination of the normal insulator, Chern insulator, and second-order topological insulator. Enriching the family of topological semimetals, our scheme supplies a convenient way to artificially synthesize exotic topological phases by periodic driving.

In the past decade, strong coupling of two-dimensional transition metal dichalcogenides (TMDs) with plasmonic and optical resonators has become an attractive field in cavity quantum electrodynamics. On one hand, how to enhance the Rabi splitting in such a system is important for applications. On the other hand, the underlying strong-coupling mechanism is not clear yet, especially for the complicated coupling case related to multiple photonic quasiparticles. Here we propose a plasmonic-optical hybrid resonator composed of a Au nanoantenna and photonic crystal cavity. When monolayer $\mathrm{Mo}{\mathrm{S}}_2$ is coupled with this hybrid resonator, we demonstrate a large Rabi splitting up to 460 meV, which is better than ever reported in similar systems. More importantly, we provide a deeper understanding of complicated coupling mechanisms in such a hybrid system, paving the way for studying strong light–matter interactions with multiple photonic quasiparticles in TMD materials coupled with different plasmonic-optical hybrid resonators.

The existence of a three-dimensional quantum Hall effect (3D QHE) due to spontaneous Fermi surface instabilities in strong magnetic fields was proposed decades ago, and has stimulated recent progress in experiments. The reports in recent experiments show that the Hall plateaus and vanishing transverse magnetoresistivities (TMRs) (which are two main signatures of 3D QHE) are not easily observed in natural materials. Two main explanations for the slowly varying slopelike Hall plateaus and nonvanishing TMRs [which can be referred to as the quasiquantized Hall effect (QQHE)] have been proposed. By studying the magnetotransport with a simple effective periodic 3D system, we show how 3D QHE can be achieved in certain parameter regimes. We find two mechanisms that may give rise to QQHE. One mechanism is the “low” Fermi energy effect, and the other is the “strong” impurity effect. Our studies also prove that an artificial superlattice is an ideal platform for realizing 3D QHE with a high layer barrier periodic potential.

Monolayer transition metal dichalcogenides (TMDs) with spin-valley coupling are a well-studied class of two-dimensional materials with potential for novel optoelectronics applications. Breaking time-reversal symmetry via an external magnetic field or supporting magnetic substrate can lift the degeneracy of the band gaps at the inequivalent $K$ and $K^{\prime }$ high symmetry points, or valleys, in the monolayer TMD Brillouin zone, a phenomenon known as valley splitting. However, reported valley splittings thus far are modest, and a detailed structural and chemical understanding of valley splitting via magnetic substrates is lacking. Here we probe the underlying physical mechanism with a series of density functional theory (DFT) calculations of magnetic atoms with varying coverage on the surface of prototypical monolayer ${\mathrm{WSe}}_2$ and ${\mathrm{MoS}}_2$ TMDs. Near-valence band edge energies for variable magnetic atom height, lateral registry, and magnetic moment are calculated with DFT, and trends are rationalized with a model Hamiltonian with second-order spin-dependent exchange coupling. From our analysis, we demonstrate how large valley splittings may be achieved and that the valley splitting can be understood with a superexchange mechanism, which strongly depends on overlaps of TMD Bloch states at the valley extrema with the localized $d$ states of the magnetic atom, as well as the out-of-plane component of the magnetic moment of the magnetic atom. Our calculations provide a basis for understanding prior measurements of valley splitting and suggest routes for enhancing valley splitting in future systems of interest.

A spark-plasma sintered ${\mathrm{GdCrO}}_3$ (SPS-GCO) is found to stabilize in its ferroelectric phase beyond room temperature. The intrinsic nature of this room-temperature ferroelectricity is established using ferroelectric positive-up–negative-down measurements and supported through piezoresponce force microscopy measurements. The SPS-GCO undergoes antiferromagnetic ordering at much lower temperatures, only below $\approx 170\mathrm{K}$. Thus, any role of magnetism to the observed room temperature ferroelectricity in SPS-GCO can be ruled out. This is contrast to the concomitant antiferromagnetic and ferroelectric ordering observed below $\approx 170\mathrm{K}$ in ${\mathrm{GdCrO}}_3$ (GCO) (prepared using standard solid-state synthesis technique). Using detailed Rietveld refinements of room-temperature x-ray diffraction patterns, SPS-GCO is found to stabilize in the noncentrosymmetric orthorhombic $Pna{2}_1$ space group (the reported low-temperature ferroelectric phase in GCO), while GCO stabilizes in the centrosymmetric $Pbnm$ space group at room temperature. Using first-principles calculations, we investigated the relative energies among various possible structures of ${\mathrm{GdCrO}}_3$ and found that the orthorhombic $Pna{2}_1$ and $Pbnm$ space groups are the most stable structures. The ferroelectric $Pna{2}_1$ phase of SPS-GCO (stabilized at room temperature using the high-pressure and high-temperature spark-plasma sintering process) undergoes transition to the paraelectric centrosymmetric phase upon heating beyond $\approx 450\mathrm{K}$ (as confirmed using dielectric and calorimetric measurements), which on subsequent cooling to room temperature does not undergo a transition back to the ferroelectric phase and remains in the centrosymmetric $Pbnm$ phase.

We study the effect of infinitesimal onsite disorder on $d$-dimensional all bands flat lattices. The lattices are generated from diagonal Hamiltonians by a sequence of $(d+1)$ local unitary transformations parametrized by angles ${\theta }_i$. Without loss of generality, we consider the case of two flat bands separated by a finite gap, $\mathrm{\Delta }$. The perturbed states originating from the flat bands are described by an effective tight-binding network with finite on- and off-diagonal disorder strength which depends on the manifold angles ${\theta }_i$. The original infinitesimal on-site disorder strength $W$ is only affecting the overall scale of the effective Hamiltonian. Upon variation of the manifold angles for $d=1$ and $d=2$ we find that localization persists for any choice of local unitaries, and the localization length can be maximized for specific values of ${\theta }_i$. Instead, in $d=3$ we identify a nonperturbative metal-insulator transition upon varying all bands' flat manifold angles.

We investigate the Josephson current through a suspended carbon nanotube double quantum dot which, at sufficiently low temperatures, is characterized by the ground state of the electronic subsystem. Depending on parameters such as a magnetic field or the interdot coupling, the ground state can either be a current-carrying singlet or doublet, or a blockaded triplet state. Since the electron-vibration interaction has been demonstrated to be electrostatically tunable, we study in particular its effect on the current-phase relation. We show that the coupling to the vibration mode can lift the current-suppressing triplet blockade by inducing a quantum phase transition to a ground state of a different total spin. Our key finding is the development of a triple point in the Josephson current parametrized by the resonator coupling and the Josephson phase. The quantum phase transitions around the triple point are directly accessible through the critical current and resilient to moderately finite temperatures. The proposed setup makes the mechanical degree of freedom part of a superconducting hybrid device which is interesting for ultrasensitive displacement detectors.

We study the emergence of non-Hermitian band topology in a two-dimensional metal with planar spiral magnetism due to a momentum-dependent relaxation rate. A sufficiently strong momentum dependence of the relaxation rate leads to exceptional points in the Brillouin zone, where the Hamiltonian is nondiagonalizable. The exceptional points appear in pairs with opposite topological charges and are connected by arc-shaped branch cuts. We show that exceptional points inside hole and electron pockets, which are generally present in a spiral magnetic state with a small magnetic gap, can cause a drastic change of the Fermi surface topology by merging those pockets at isolated points in the Brillouin zone. We derive simple rules for the evolution of the eigenstates under semiclassical motion through these crossing points, which yield geometric phases depending only on the Fermi surface topology. The spectral function observed in photoemission exhibits Fermi arcs. Its momentum dependence is smooth—despite of the nonanalyticities in the complex quasiparticle band structure.

We study the nature of the electronic states in the intermediate band formed by interstitial titanium in silicon. Our single-site description combines effects of electronic correlations, captured by dynamical mean-field theory, and disorder, modeled using the coherent potential approximation and the typical medium mean-field theory. For all studied concentrations an extended metallic state with a strongly depleted density of states at the Fermi level is obtained. The self-energy is characteristic of Fermi liquids and for certain temperatures reveals the existence of coherent quasiparticles.

Observations of electron-hole asymmetry in transport through graphene devices at high magnetic field challenge prevalent models of the graphene quantum Hall effect. Here we study this asymmetry both in conventional magnetotransport and in scanning gate microscopy maps measured in an encapsulated graphene constriction. We reveal that the presence of upstream modes and local doping in the vicinity of electrical contacts leads to a totally different picture of topological breakdown for electrons and holes, explaining the observed asymmetry.

The exchange interaction arising from particle indistinguishability is of central importance in the physics of many-particle quantum systems. Here we study analytically the dynamical generation of quantum entanglement induced by this interaction in an isolated system, namely, an ideal Fermi gas confined in a chaotic cavity, which is described by a non-Gaussian pure state and evolves unitarily from it. We find that the breakdown of the quantum-classical correspondence of particle motion, via the dramatic change in the spatial structure of the many-body wave function, leads to profound changes of the entanglement. Furthermore, for a class of initial states, the changes engender the approach to thermal equilibrium everywhere in the cavity, with the well-known Ehrenfest time in quantum chaos as the thermalization time. Specifically, the quantum expectation values of various correlation functions at different spatial scales are all determined by the Fermi-Dirac distribution. In addition, by using the reduced density matrix (RDM) and the entanglement entropy (EE) as local probes, we find that the gas inside a subsystem is in equilibrium with that outside, and that its thermal entropy is the EE, even though the whole system is in a pure state.

The pairing mechanism in different classes of correlated materials, including iron-based superconductors, is still under debate. For FeSe monolayers, uniform nematic fluctuations have been shown in a lattice Monte Carlo study to play a potentially important role. Here, using dynamical mean-field theory calculations for the same model system, we obtain a similar phase diagram and provide an alternative interpretation of the superconductivity in terms of local orbital fluctuations and phase rigidity. Our study clarifies the relation between the superconducting order parameter, superfluid stiffness, and orbital fluctuations, and provides a link between the spin/orbital freezing theory of unconventional superconductivity and theoretical works considering the role of nematic fluctuations.

We present the results of a study of the vortex lattice in the heavy fermion superconductor ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$, using small-angle neutron scattering (SANS). In this material at temperatures well below $T_{\mathrm{c}}\sim 0.6$ K, the value of the upper critical field $B_{\mathrm{c}2}\sim 2.2$ T is strongly limited by the Pauli paramagnetism of the heavy fermions. In this temperature region, our SANS data show an increase in the magnetization of the flux line cores with field, followed by a rapid fall near $B_{\mathrm{c}2}$. This behavior is the effect of Pauli paramagnetism and we present a theory-based model, which can be used to describe this effect in a range of materials. The pairing in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ appears to arise from the effect of magnetic fluctuations, but the evidence for a $d$-wave order parameter is rather weak. We find that the vortex lattice structure in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ is close to regular hexagonal. There are no phase transitions to square or rhombic structures; such transitions are expected for $d$-wave superconductors and observed in ${\mathrm{CeCoIn}}_5$; however, the temperature dependence of the SANS intensity indicates that both large and small gap values are present, most likely due to multiband $s$-wave superconductivity, rather than a nodal gap structure.

We theoretically analyze a state-of-the-art experimental method based on a combination of electron spin resonance and scanning tunneling microscopy (ESR-STM), to directly probe the spin fluctuations in the Kondo effect. The Kondo impurity is exchange coupled to the probe spin, and the ESR-STM setup detects the small level shifts in the probe spin induced by the spin fluctuations of the Kondo impurity. We use the open quantum system approach by regarding the probe spin as the “system” and the Kondo impurity spin as the fluctuating “bath” to evaluate the resonance line shifts in terms of the dynamic spin susceptibility of the Kondo impurity. We consider various common adatoms on surfaces as possible probe spins and estimate the corresponding level shifts. It is found that the sensitivity is most pronounced for the probe spins with transverse magnetic anisotropy.

By means of first-principles calculations, we study the electronic structures, lattice dynamics, electron-phonon coupling (EPC), and superconductivity of ${\mathrm{TiB}}_4$ monolayer (TBML) and ${\mathrm{TiB}}_4$ bilayer (TBBL). We find that both TBML and TBBL are nodal line semimetals, and the occurrences of their nodal lines are mainly due to the band inversions between B-$p_x+p_y$ and B-$p_z$ for TBML and between Ti-$d_{xz}+d_{yz}$ and Ti-$d_{z^2}$ for TBBL. The distortion of Ti atoms in the TBBL induces a horizontal glide mirror plane, which protects its nodal line against the spin-orbit coupling. The computed EPC constant $\lambda$ of TBML is 0.65, higher than that of the TBBL with $\lambda =0.35$. Both TBML and TBBL are identified to be phonon-mediated two-dimensional (2D) superconductors with the calculated $T_c=1.66\mathrm{K}$ and 0.82 K, respectively. The $T_c$ of the TBBL can be further enhanced to 6.43 K by applying a tensile strain of 11%. Moreover, they exhibit excellent thermal stability. The coexistence of the topological nodal-line states around the Fermi level and superconductivity in the square-lattice ${\mathrm{TiB}}_4$ monolayer may show more potential for realization of exotic physics.