Quantum confinement has remarkable effects on the band structures and optoelectronic performance of semiconducting materials. The confinement of electronic states developed along van der Waals (vdW) gaps in transition metal dichalcogenides (TMDs) has unique advantages compared with those of artificial quantum wells. Here, we detected the quantized electronic states of few-layered ${\mathrm{MoS}}_2$ in real space using scanning tunneling microscope/spectroscopy. Combined with density-functional theory calculations, the quantized states were attributed to quantum-well states (QWSs), and the number of the states was strictly determined by the ${\mathrm{MoS}}_2$ layer thickness. We further regulated the QWSs of few-layered ${\mathrm{MoS}}_2$ by tuning the strength of interlayer hybridization through directly adjusting the interlayer distance. More importantly, substitutional defects in few-layered ${\mathrm{MoS}}_2$ were introduced to control the energy eigenvalues of the QWSs. Our work proves the existence of the interlayer electronic hybridization in conventional weakly coupled vdW interfaces, and provides a way to manipulate the electronic states of few-layered TMD through controlling interlayer hybridization. It also suggests potential applications of quantum-well materials in subband transitions, spin splitting, photoexcitation, and electronic devices.

$1T\text{−}{\mathrm{TiSe}}_2$ has been found to host a chiral charge density wave (CDW). Some studies suggest the microscopic origin of this phase is due to electron-phonon coupling while other studies suggest it is due to an excitonic insulator phase transition based on nonthermal melting of the charge density wave. First, we propose these interpretations can be reconciled if one analyzes the available experimental and theoretical data within a formal definition of what constitutes an excitonic insulator as initially proposed by Keldysh and Kopaev. Next, we present pump-probe measurements of circularly polarized optical transitions and first-principles calculations to highlight the role of elevated electronic temperatures on structural distortions to understand the nonthermal melting of the CDW phase. We also uncover a noncentrosymmetric CDW structure that explains the finite chirality of the optical transitions observed in the CDW phase of ${\mathrm{TiSe}}_2$.

We study a one-dimensional lattice model subject to non-Hermitian quasiperiodic potentials. First, we strictly demonstrate that there exists an interesting dual mapping relation between $\left|a\right|<1$ and $\left|a\right|>1$ with regard to the potential tuning parameter $a$. The localization property of $\left|a\right|<1$ can be directly mapped to that of $\left|a\right|>1$, the analytical expression of the mobility edge of $\left|a\right|>1$ is therefore obtained through the spectral properties of $\left|a\right|<1$. More impressive, we prove rigorously that, even if the phase $\theta \ne 0$ in quasiperiodic potentials, the model becomes non-$\mathcal{PT}$ symmetric, however, there still exists a type of real-complex transition driven by non-Hermitian disorder, which is a class beyond the $\mathcal{PT}$-symmetric class.

The eigenspectrum of a non-normal matrix, which does not commute with its Hermitian conjugate, is a central issue of non-Hermitian physics that has been extensively studied in the past few years. There is, however, another characteristic of a non-normal matrix that has often been overlooked: the pseudospectrum, or the set of spectra under small perturbations. In this paper, we study the dynamics driven by the non-normal matrix (Hamiltonian) realized as a continuous quantum trajectory of the Lindblad master equation in open quantum systems and point out that the dynamics can reveal the nature of unconventional pseudospectrum of the non-normal Hamiltonian. In particular, we focus on the transient dynamics of the norm of an unnormalized quantum state evolved with the non-normal Hamiltonian, which is related to the probability for observing the trajectory with no quantum jump. We formulate the transient suppression of the decay rate of the norm due to the pseudospectral behavior and derive a non-Hermitian/non-normal analog of the time-energy uncertainty relation. We also consider two methods to experimentally realize the non-normal dynamics and observe our theoretical findings on quantum computers: one uses a technique to realize nonunitary operations on quantum circuits and the other leverages a quantum-classical hybrid algorithm called variational quantum simulation. Our demonstrations using cloud-based quantum computers provided by IBM Quantum exhibit the frozen dynamics of the norm in transient time, which can be regarded as a non-normal analog of the quantum Zeno effect.

The host-guest structure compounds are potential candidates for high-efficiency thermoelectric (TE) materials because of their good electronic transport combined with ultralow thermal conductivity. In this paper, we theoretically investigate TE transport properties of the hexagonal $A_2{\mathrm{Se}}_2(A$ = K, Rb) with strongly quartic anharmonicity based on an incorporation of first-principles calculations with self-consistent phonon theory and Boltzmann transport equations. The calculation results reveal that $A_2{\mathrm{Se}}_2(A$ = K, Rb) have ultralow lattice thermal conductivities ${\kappa }_L$, e.g., 0.30–0.$34{\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at room temperature, which are only one third of quartz glass. The calculated results indicate that the coexistence of small phonon lifetime ${\tau }_{\mathrm{ph}}$ and phonon group velocity ${\upsilon }_{\mathrm{ph}}$ is the reason for the ultralow ${\kappa }_L$ in these two host-guest structure materials. Additionally, due to the anisotropic electronic structure, that is, the coexistence of high dispersion and flat band edges, these two materials capture a high TE power factor along the $c$ axis. As a consequence, the anomalous high $\mathit{ZT}$ values of 2.95 and 2.17 at 500 K along the $c$-axis direction are captured in $n$-type ${\mathrm{K}}_2{\mathrm{Se}}_2$ and ${\mathrm{Rb}}_2{\mathrm{Se}}_2$, which break the long-term record of $\mathit{ZT}<2$ in most TE materials reported so far. These findings reveal that $A_2{\mathrm{Se}}_2(A$ = K, Rb) has broad prospects in TE applications.

Using the first-principles nonequilibrium Green's function technique with supercell disorder averaging, we study the influence of interfacial intermixing on the spin-orbit torque in Co/Pt bilayers. Intermixing is modeled by inserting one or more monolayers of a disordered Co-Pt alloy between Co and Pt. Dampinglike torque is moderately enhanced by interfacial intermixing, while the fieldlike torque, which is small for abrupt interfaces, is strongly enhanced and becomes comparable to the dampinglike torque. The enhancement of the fieldlike torque is attributed to the interface between Co and the intermixed region. The planar-Hall-like torque increases with intermixing but remains relatively small. The behavior of the torques is similar for bilayers with (111)- and (001)-oriented interfaces. Strong dependence of the fieldlike torque on intermixing could provide a way to tune the fieldlike-to-dampinglike torque ratio by interface engineering.

We investigate the effects of magnetic and crystalline anisotropies on the topological superconducting state of planar Josephson junctions (JJs). In junctions where only Rashba spin-orbit coupling (SOC) is present, the topological phase diagram is insensitive to the supercurrent direction, but exhibits a strong dependence on the magnetic field orientation. However, when both Rashba and Dresselhaus SOCs coexist, the topological phase diagram strongly depends on both the magnetic field and junction crystallographic orientations. We examine the impact of the magnetic and crystalline anisotropy on the current-phase relation (CPR), energy spectrum, and topological gap of phase-biased JJs, where the junction is connected in a loop and the superconducting phase difference is fixed by a loop-threading magnetic flux. The anisotropic CPR can be used to extract the ground-sate phase (i.e., the superconducting phase difference that minimizes the system free energy) behavior in phase-unbiased JJs with no magnetic flux. Under appropriate conditions, phase-unbiased JJs can self-tune into or out of the topological superconducting state by rotating the in-plane magnetic field. The magnetic field orientations at which topological transitions occur strongly depend on both the junction crystallographic orientation and the relative strength between Rashba and Dresselhaus SOCs. We find that for an optimal practical application, in which the junction exhibits topological superconductivity with a sizable topological gap, a careful balancing of the magnetic field direction, the junction crystallographic orientation, and the relative strengths of the Rashba and Dresselhaus SOCs is required. We discuss the considerations that must be undertaken to achieve this balancing for various junction types and parameters.

In a superfluid system, off-diagonal long-range order is expected to be exhibited in the appropriate reduced density matrices when the relevant particles (either bosons or fermion pairs) are considered to recede sufficiently far apart from each other. This concept is usually exploited to identify the value of the condensate density, without explicit concern, however, as to the spatial range over which this asymptotic condition can effectively be achieved. Here, based on a diagrammatic approach that includes beyond-mean-field pairing fluctuations in the broken-symmetry phase at the level of the $t$-matrix also with the inclusion of the Gorkov-Melik-Barkhudarov (GMB) correction, we present a systematic study of the two-particle reduced density matrix for a superfluid fermionic system undergoing the BCS-BEC crossover, when the entities to recede far apart from each other evolve with continuity from largely overlapping Cooper pairs in the BCS limit to dilute composite bosons in the BEC limit. By this approach, we not only provide the coupling and temperature dependence of the condensate density at the level of our diagrammatic approach, which includes the GMB correction, but we also obtain the evolution of the spatial dependence of the two-particle reduced density matrix, from a power law at low temperature to an exponential dependence at high temperature in the superfluid phase, when the interparticle coupling spans the BCS-BEC crossover. Our results put limitations on the minimum spatial extent of a finite-size system for which superfluid correlations can effectively be established.

Van der Waals heterostructures have risen as a tunable platform to combine different electronic orders, due to the flexibility in stacking different materials with competing symmetry broken states. Among them, van der Waals ferromagnets such as ${\mathrm{CrI}}_3$, ${\mathrm{CrBr}}_3$, or ${\mathrm{CrCl}}_3$ and superconductors as ${\mathrm{NbSe}}_2$ provide a natural platform to engineer novel phenomena at ferromagnet-superconductor interfaces. In particular, ${\mathrm{NbSe}}_2$ is well known for hosting strong spin-orbit coupling effects that influence the properties of the superconducting state. Here we put forward a ferromagnet/${\mathrm{NbSe}}_2$/ferromagnet heterostructure where the interplay between Ising superconductivity in ${\mathrm{NbSe}}_2$ and magnetism controls the magnetic alignment of the heterostructure. In particular, we show that the interplay between spin-orbit coupling and superconductivity provides a new mechanism to control magnetic ordering in van der Waals materials. We show that this coupling allows creating heterostructures featuring a magnetic phase transition from in-plane to out-of-plane associated with the onset of superconductivity. Our results show how a hybrid van der Waals ferromagnet/superconductor heterostructure can be used as a tunable materials platform for superconducting spin-orbitronics.

We analyze the band topology of acoustic phonons in 2D materials by considering the interplay between spatial/internal symmetries and additional constraints that arise from the physical context. These supplemental constraints trace back to the Nambu-Goldstone theorem and the requirements of structural stability. We show that this interplay can give rise to previously unaddressed nontrivial nodal charges that are associated with the crossing of the acoustic phonon branches at the center ($\mathrm{\Gamma }$ point) of the phononic Brillouin zone. We moreover apply our perspective to the concrete context of graphene, where we demonstrate that the phonon spectrum harbors these kinds of nontrivial nodal charges. Apart from its fundamental appeal, this analysis is physically consequential and dictates how the phonon dispersion is affected when graphene is grown on a substrate. Given the generality of our framework, we anticipate that our strategy, which thrives on combining physical context with insights from topology, should be widely applicable in characterizing systems beyond electronic band theory.

The influence of the curvature on the dynamical properties of transversal domain walls in a thin antiferromagnetic wire is studied theoretically. Equations of motion for an antiferromagnetic domain wall are obtained within the collective variable approach ($q\text{−}\mathrm{\Phi }$ model). It is shown that (i) for the case of a localized bend, curvature results in a pinning potential for the domain wall. (ii) The gradient of the curvature results in a driving force on the domain wall and it effectively moves without any external stimuli. Although we showcase our approach on the specific parabola and Euler spiral geometries, the approach is general and valid for a wide class of geometries. All analytical predictions are confirmed by numerical simulations.

It has been recently shown that the presence of topological frustration, induced by periodic boundary conditions in an antiferromagnetic $XY$ chain made of an odd number of spins, prevents the realization of a perfectly staggered local order. Starting from this result and exploiting a recently introduced approach that enables the direct calculation of the expectation value of any operator with support over a finite range of lattice sites, in this work, we investigate the possible fates of local orders. We show that, regardless of the variety of possible situations, they can be all arranged in two different cases. A system admits a finite local order only if the ground state is degenerate, with at least two elements whose momenta differ, in the thermodynamic limit, by $\pi$, and this order breaks translational symmetry. In all other cases, any local order decays to zero, algebraically (or faster) in the chain length. Moreover, we show that, in some cases, which of the two possibilities is realized, may depend on the sequence of chain lengths with which the thermodynamic limit is reached. These results are established both analytically and by exact diagonalization and illustrated through examples.

In many strongly correlated insulators, antiferromagnetic order competes with exotic and technologically relevant phases, like superconductivity. While control of spin order is critical to stabilize different functional states, elucidating the mechanism of laser-induced demagnetization in complex oxides remains a challenge. It is unknown if the optical pulse can quench magnetization nonthermally or if it instead only acts as a heat source. Here, we use ultrafast, broadband, optical spectroscopy to track the responses of the electronic, lattice, and spin degrees of freedom and their relation to antiferromagnetism in the strongly spin-orbit coupled insulator ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$. We find that magnetization can be rapidly and strongly suppressed on a sub-150 fs timescale. At low excitation fluences, the magnetic recovery is fast; however, the recovery time increases dramatically with the magnitude of demagnetization. At the same time, we show that the lattice, evidenced through the $A_g$ phonon frequencies, appears to remain below $T_{\mathrm{N}}$, suggesting that the system remains nonthermal during the optical modulation of spin order. We suggest that photogenerated spin defects are responsible for the long-lived demagnetized state and discuss its implications for optical control of solids.

Ferrimagnets (FIMs) can function as high-frequency antiferromagnets while being easy to detect as ferromagnets, offering unique opportunities for ultrafast device applications. While the physical behavior of FIMs near the compensation point has been widely studied, there lacks a generic understanding of FIMs where the ratio of sublattice spins can vary freely between the ferromagnetic and antiferromagnetic limits. Here we investigate the physical properties of a two-sublattice FIM manipulated by static magnetic fields and current-induced torques. By continuously varying the ratio of sublattice spins, we clarify how the dynamical chiral modes in a FIM are intrinsically connected to their ferro- and antiferromagnetic counterparts, which reveals unique features not visible near the compensation point. In particular, we find that current-induced torques can trigger spontaneous oscillation of the terahertz exchange mode. Compared with its realization in antiferromagnets, a spin-torque oscillator using FIMs not only has a reduced threshold current density but also can be self-stabilized, obviating the need for dynamic feedback.

Interest in the spin-$\frac{1}{2}$ hexagonal perovskites, typified by $\mathrm{Cs}\mathrm{Co}{\mathrm{Cl}}_3$ and including $\mathrm{Rb}\mathrm{Co}{\mathrm{Cl}}_3$, has persisted over several decades due to their unusual Ising-chain-like antiferromagnetic ordering. Despite considerable theoretical and experimental work, evidence of the origin of the magnetic excitations of these materials has been limited by an incomplete knowledge of their magnetic ordering and exchange interactions. Now, with the availability of detailed measurements of such information obtained by neutron scattering [M. Mena et al., Phys. Rev. Lett. 124, 257201 (2020)], we are motivated to report a study for $\mathrm{Rb}\mathrm{Co}{\mathrm{Cl}}_3$. We employ the concept of a Zeeman ladder of magnetic excitations below the antiferromagnetic transition temperature at 28 K, as well as showing the existence of a bound magnetic state, to accurately predict the energies of five magnetic excitations observed by Raman scattering. Such a remarkable match of theory to experiment with just one adjustable parameter augers well for theoretical applications in other such perovskites.

Bilayer moiré structures are a highly tunable laboratory to investigate the physics of strongly correlated electron systems. Moiré transition metal dichalcogenides (TMDs) at low energies, in particular, are believed to be described by a single narrow band Hubbard model on a triangular lattice. Motivated by recent experimental evidence for superconductivity in twisted bilayer materials, we investigate the possible superconducting pairings for TMDs deposited on a substrate by studying a two-dimensional single-band Rashba-Hubbard model. Using a random-phase approximation in the presence of nearest- and next-nearest-neighbor hopping, we analyze the structure of spin fluctuations and the symmetry of the superconducting gap function. We show that Rashba spin-orbit coupling favors ferromagnetic fluctuations which strengthen triplet superconductivity. If parity is violated due to the absence of spatial inversion symmetry, singlet ($d$-wave) and triplet ($p$-wave) channels of superconductivity will be mixed. Moreover, we show that time-reversal symmetry can be spontaneously broken, leading to a chiral superconducting state. Finally, we consider quasiparticle interference as a possible experimental technique to observe the superconducting gap symmetry.

$1T^{\prime }\text{−}\mathrm{Ta}{\mathrm{Te}}_2$ exhibits an intriguing first-order structural phase transition at around 170 K. Understanding the electronic structural properties is a crucial way to comprehend the origin of this structural phase transition. We performed a combined optical and ultrafast pump-probe study on the compound across the transition temperature. The phase transition leads to abrupt changes in both optical spectra and ultrafast electronic relaxation dynamics. The measurements revealed a sudden reconstruction of the band structure. We elaborate that the phase transition is of first order and cannot be attributed to a conventional density-wave type instability. Our work is illuminating for understanding the origin of the structural phase transition.

It has been well established experimentally that the interplay of electronic correlations and spin-orbit interactions in ${\mathrm{Ir}}^{4+}$ and ${\mathrm{Ir}}^{5+}$ oxides results in insulating $J_{\mathrm{eff}}=1/2$ and $J_{\mathrm{eff}}=0$ ground states, respectively. However, in compounds where the structural dimerization of iridium ions is favorable, the direct Ir $d\text{−}d$ hybridization can be significant and takes a key role. Here, we investigate the effects of direct Ir $d\text{−}d$ hybridization in comparison with electronic correlations and spin-orbit coupling in ${\mathrm{Ba}}_5{\mathrm{AlIr}}_2{\mathrm{O}}_{11}$, a compound with Ir dimers. Using a combination of ab initio many-body wave-function quantum chemistry calculations and resonant inelastic x-ray scattering experiments, we elucidate the electronic structure of ${\mathrm{Ba}}_5{\mathrm{AlIr}}_2{\mathrm{O}}_{11}$. We find excellent agreement between the calculated and the measured spin-orbit excitations. Contrary to expectations, the analysis of the many-body wave function shows that the two Ir (${\mathrm{Ir}}^{4+}$ and ${\mathrm{Ir}}^{5+}$) ions in the ${\mathrm{Ir}}_2{\mathrm{O}}_9$ dimer unit in this compound preserve their local $J_{\mathrm{eff}}$ character close to 1/2 and 0, respectively. The local point group symmetry at each of the Ir ions plays an important role, significantly limiting the direct $d\text{−}d$ hybridization. Our results emphasize that minute details in the local crystal field environment can lead to dramatic differences in the electronic states in iridates and $5d$ oxides in general.

The exciton Bose liquid (EBL) is a hypothesized phase of bosons in $2+1\text{D}$ which possesses a dispersion that is gapless along the coordinate axes in momentum space. The low energy theory of the EBL involves modes on all length scales, extending all the way down to the lattice spacing. In this paper, we discuss a renormalization group scheme that can be used to address the stability of this and related phases of matter. We find that, in the absence of an extensively large symmetry group, realizing the simplest formulation of the EBL always requires fine-tuning. However, we also argue that the addition of certain marginal interactions can be used to realize a stable phase, without the need for fine-tuning. A simple generalization to $3+1\text{D}$ is also discussed.

The Anderson single impurity model, including its Kondo limit, can be seen as the hydrogen atom for correlated systems. Its most striking feature is the universal scaling of its low-energy properties governed by the appearance of a single emergent scale, the Kondo temperature $T_K$. In this work, we demonstrate the emergence of a second independent energy scale in a quantum impurity model, the interacting resonant level model, which is equivalent to the anisotropic Kondo model. We study the local density of states at the impurity site in the ground state (zero temperature limit) of the model. We see collapse of data onto universal curves at low energies, however this is only achieved by defining a second low-energy scale with a different power law dependence on model parameters than the Kondo temperature. We provide an exact expression for this second critical exponent. We also report on a splitting of the central resonance as the interaction strength is increased in the absence of any external magnetic field in the model.