The discovery of high-temperature superconductivity in hydrides provides a promising route to achieve the goal of room-temperature superconductivity, but the ultrahigh pressure required to be synthesized severely limits their application. The next challenge is to find novel hydrides with high $T_c$ at low pressure, even ambient pressure. Here, we propose a strategy that elements with little electronegativity, large atomic volume, and suitable valence electron number can be regarded as a candidate for reducing the stable pressure by summarizing the superconducting rules of the clathrate hexahydrides, and find that thorium is a good “precompressor.” Based on the above strategy, we doped thorium into clathrate hexahydrides with a ${\mathrm{H}}_{24}$ cage, and designed a series of hydrides. They could be dynamically stable at moderate pressure, which is much lower than that of the well-known hexahydrides $\mathrm{Ca}{\mathrm{H}}_6$. Remarkably, $\mathrm{LaT}{\mathrm{h}}_3{\mathrm{H}}_{24}, \mathrm{AcT}{\mathrm{h}}_3{\mathrm{H}}_{24}$, and $\mathrm{YTh}{\mathrm{H}}_{12}$ exhibit excellent superconductivity with high $T_c$ of 198 K at 50 GPa, 201 K at 60 GPa, and 208 K at 60 GPa, respectively. This work suggests that thorium doping is an effective method for finding hydrides with high $T_c$ at moderate pressure, and successfully helps us design a series of interesting high-temperature superconducting hydrides.

ZrSiS has been identified as an exemplary Dirac nodal-line semimetal, in which the Dirac band crossings extend along a closed loop in momentum space. Recently, the topology of the Fermi surface of ZrSiS was uncovered in great detail by quantum oscillation studies. For a magnetic field along the tetragonal $c$ axis, a rich frequency spectrum was observed stemming from the principal electron and hole pockets and multiple magnetic breakdown orbits. In this work we use uniaxial strain as a tuning parameter for the Fermi surface and the low-energy excitations. We measure the magnetoresistance of a single crystal under tensile (up to $0.34\%$) and compressive (up to $-0.28\%$) strain exerted along the $a$ axis and in magnetic fields up to 30 T. We observe a systematic weakening of the peak structure in the Shubnikov-de Haas frequency spectrum upon changing from compressive to tensile strain. This effect may be explained by a decrease in the effective quantum mobility upon decreasing the $c/a$ ratio, which is corroborated by a concurrent increase in the Dingle temperature.

Magnetic topological insulators have been attracting great interest in two dimensions for both fundamental physics and applications in spintronics. Here, we put forward that the topological phase transition between a second-order topological insulator and quantum anomalous Hall insulator with a strikingly different bulk-boundary correspondence is possible in two-dimensional ferromagnets. We employ the intrinsic ferromagnetic ${\mathrm{VSi}}_2{\mathrm{P}}_4$ monolayer with giant valley polarization as a material candidate and elucidate that the second-order topological insulator emerges, distinguished by the topological indices ${\chi }^{\left(3\right)}=(-3,2)$ and well-localized corner states. Remarkably, under strain engineering, a topological phase transition takes place under a 0.67% tensile strain accompanied by obtaining the quantum anomalous Hall effect with a Chern number $\mathcal{C}=-1$ and one chiral edge state. As the tensile strain further increases, another topological phase transition is realized as the ${\mathrm{VSi}}_2{\mathrm{P}}_4$ monolayer changes into a normal insulator. Our work considerably bridges the higher-order topology and quantum anomalous Hall effect with a high possibility of innovative applications in topotronic devices.

Non-Hermitian systems exhibit richer topological properties compared to their Hermitian counterparts. It is well known that non-Hermitian systems have been classified based on either the symmetry relations for non-Hermitian Hamiltonians or the symmetry relations for nonunitary time-evolution operators in the context of Floquet topological phases. In this work, we propose that non-Hermitian systems can always be classified in two ways; a non-Hermitian system can be classified using the symmetry relations for non-Hermitian Hamiltonians or time-evolution operator regardless of the Floquet topological phases or not. We refer to this as dual symmetry classification. To demonstrate this, we successfully introduce a nonunitary quantum walk that exhibits point gaps with a ${\mathbb{Z}}_2$ point-gap topological phase applying the dual symmetry classification and treating the time-evolution operator of this quantum walk as the non-Hermitian Hamiltonian.

We report the observation of second harmonic generation with high conversion efficiency $\sim 0.005\%$ in the terahertz regime from thin films of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ that exhibits the linear photogalvanic effect, measured via time-domain terahertz nonlinear spectroscopy and terahertz emission, respectively. As neither phenomena is observable from topologically trivial In-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, and since no enhancement is observed when subject to band bending, the efficient thickness-independent nonliear responses are attributable to the Dirac fermions of topological surface states of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. This observation of intrinsic terahertz second harmonic generation in an equilibrium system unlocks the full suite of both even and odd harmonic orders in the terahertz regime and opens new pathways to probing quantum geometry via intraband second-order nonlinear processes. We hope our work will motivate the theoretical development of a full treatment of second harmonic generation for probing the quantum geometry in various inversion-breaking topological and twisted materials.

We present a minimally entangled typical thermal state quantum impurity solver for general multiorbital systems at finite temperatures. We introduce an improved estimator for the single-particle Green's function that strongly reduces the large fluctuations at long imaginary time and low temperature, which were a severe limitation of the original algorithm. In combination with the fork tensor product states Ansatz, we obtain a dynamical mean field theory (DMFT) quantum impurity solver, which we benchmark for single and three-band models down to low temperatures, including the effect of spin-orbit coupling in a realistic DMFT computation for the Hund's metal ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ down to low temperatures.

The realization of topological insulators induced by correlation effects is one of the main issues of modern condensed matter physics. An intriguing example of correlated topological insulators is a magnetic Chern insulator induced by a noncoplanar multiple-$Q$ magnetic order. Although the realization of the magnetic Chern insulator has been studied in the classical limit of the Kondo lattice model, research on the magnetic Chern insulator in the original Kondo lattice model is limited. Here, we investigate the possibility of the many-body Chern insulator with the noncoplanar triple-$Q$ magnetic order in the Kondo lattice model on a triangular lattice. Using the many-variable variational Monte Carlo method, we reveal that the triple-$Q$ magnetic order becomes a ground state at quarter filling in an intermediate Kondo coupling region. We also show that the many-body Chern number is quantized to one in the triple-$Q$ magnetic ordered phase utilizing the polarization operators. Our results provide a pathway for the realization of the many-body Chern insulator in correlated electron systems.

Realizing flat bands via strong pseudomagnetic fields has recently inspired new opportunities for studying strong correlations in monolayer graphene, where a feasible idea is to utilize substrate topography to create strained graphene superlattices. Nevertheless, the effect from topography-induced strain relaxation on these isolated flat bands in monolayer graphene is unknown, posing challenges for rational design on flat bands and pseudomagnetic fields by strain engineering. In this work, we reveal analytical displacement fields due to strain relaxation in monolayer graphene conforming to substrate topography, allowing quantum mechanical design on flat band structures and strong pseudomagnetic fields in monolayer graphene with strain energy minimized. We find that the strong pseudomagnetic fields as well as flat band structures in monolayer graphene are highly sensitive to strain relaxation and substrate topography. The tunability of flat bands in monolayer graphene coupled to several topography designs is demonstrated. Our results contribute to the understanding of flat bands and strong pseudomagnetic fields in monolayer graphene enabled by topography strain engineering.

Using our rational methodology, we reveal a defect in wurtzite AlN that can serve as an optically controllable spin qubit. It combines an Al vacancy and a S atom substituting the neighboring N atom $\left(V_{\mathrm{Al}}{\mathrm{S}}_{\mathrm{N}}\right)$. Linear response GW and Bethe-Salpeter equation calculations guide us to find suitable ground and excited triplet and singlet states of $V_{\mathrm{Al}}{\mathrm{S}}_{\mathrm{N}}$. The obtained optical spin-polarization cycle is similar to that observed in the negative nitrogen-vacancy $\left(N{V}^{-}\right)$ center in diamond. The calculated optical oscillator strengths for $V_{\mathrm{Al}}{\mathrm{S}}_{\mathrm{N}}$ suggest that, in contrast to the $N{V}^{-}$ center, the optical emission in the singlet and triplet states have comparable rates, which is favorable for the qubit functionality.

Quantum loop and dimer models are archetypal examples of correlated systems with local constraints. Obtaining generic solutions for these models is difficult due to the lack of controlled methods to solve them in the thermodynamic limit. Nevertheless, these solutions are of immediate relevance to both statistical and quantum field theories, as well as the rapidly growing experiments in Rydberg atom arrays and quantum moiré materials, where the interplay between correlation and local constraints gives rise to a plethora of novel phenomena. In a recent work [X. Ran et al., arXiv:2205.04472], it was found through sweeping cluster quantum Monte Carlo (QMC) simulations and field theory analysis that the triangular lattice quantum loop model (QLM) hosts a rich ground-state phase diagram with lattice nematic, vison plaquette (VP) crystals, and the ${\mathbb{Z}}_2$ quantum spin liquid (QSL) close to the Rokhsar-Kivelson point. Here, we focus on the continuous quantum critical point separating the VP and QSL phases and demonstrate via both static and dynamic probes in QMC simulations that this transition is of the ($2+1$)D cubic* universality. In this transition, the fractionalized visons in QSL condense to give rise to the crystalline VP phase, while leaving their trace in the anomalously large anomalous dimension exponent and pronounced continua in the dimer and vison spectra compared with those at the conventional cubic or O(3) quantum critical points.

Frequency instabilities are a major source of errors in quantum devices. This Letter investigates frequency fluctuations in a surface acoustic wave (SAW) resonator, where reflection coefficients of 14 SAW modes are measured simultaneously for more than 7 h. We report two distinct noise characteristics. Multimode frequency noise caused by interactions with two-level system (TLS) defects shows significant degrees of correlation that diminish with increased detuning. This finding agrees with the current understanding of the parasitic TLS behavior as one of the dominant noise sources in quantum devices. In addition to the TLS-induced noise, we observe strong anomalous frequency fluctuations with slow, anticorrelated dynamics. These noise bursts resemble signatures of cosmic radiation observed in superconducting quantum systems.

Quantum entanglement at critical points is often marked by universal characteristics. Here, the entanglement entropy is calculated at the quantum multicritical point of the random transverse-field Ising model (RTIM). We use an efficient implementation of the strong disorder renormalization group method in two and three dimensions for two types of disorder. For cubic subsystems we find a universal logarithmic corner contribution to the area law $bln\left(\ell \right)$ that is independent of the form of disorder. Our results agree qualitatively with those at the quantum critical points of the RTIM, but with new $b$ prefactors due to having both geometric and quantum fluctuations at play. By studying the vicinity of the multicritical point, we demonstrate that the corner contribution serves as an “entanglement susceptibility,” a useful tool to locate the phase transition and to measure the correlation length critical exponents.

We consider a system exhibiting the bipolar non-Hermitian skin effect (NHSE), where bipolar skin states are localized at both edges, and discuss how the topological funneling effect can occur with a single focal point in this system. We explore a topological phase transition from bipolar NHSE to NHSE, which alters the localization characteristics of the eigenstates. After the transition, both the delocalized and bipolar skin states transform into skin states. The non-Hermitian edge burst is a novel and unexpected feature of non-Hermitian quantum dynamics, characterized by substantial particle loss at the boundary of a system. Using the model displaying bipolar NHSE, we show the emergence of the edge-burst effect with a large amount of loss observed only at one boundary. To enhance this effect, we introduce slightly asymmetric long-range couplings into a system that already exhibits the edge burst. This leads to a notable enhancement of the edge burst. We discuss the significance of asymmetrical local power generation in contributing to this enhanced edge-burst effect.

We present a comprehensive investigation of the electronic structure, magnetization, specific heat, and crystallography of TbSi (FeB structure type) and ${\mathrm{TbSi}}_{0.6}{\mathrm{Ge}}_{0.4}$ (CrB structure type) compounds. Both TbSi and ${\mathrm{TbSi}}_{0.6}{\mathrm{Ge}}_{0.4}$ exhibit two antiferromagnetic (AFM) transitions at $T_{\mathrm{N}1}\approx$ 58 and 57 K, and $T_{\mathrm{N}2}\approx$ 36 and 44 K, respectively, along with an onset of weak metamagneticlike transition around 6 T between $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2}$. High-resolution specific heat ($C_P$) measurements show the second- and first-order nature of the magnetic transition at $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2}$, respectively, for both samples. However, in the case of TbSi, the low-temperature (LT) AFM to high-temperature (HT) AFM transition takes place via an additional AFM phase at the intermediate temperature (IT), where both LT to IT AFM and IT to HT AFM phase transitions exhibit a first-order nature. Both TbSi and ${\mathrm{TbSi}}_{0.6}{\mathrm{Ge}}_{0.4}$ manifest significant magnetic entropy changes ($\mathrm{\Delta }{S}_M$) of 9.6 and 11.6 J/kg-K, respectively, for $\mathrm{\Delta }{\mu }_{0}H$=7 T, at $T_{\mathrm{N}2}$. The HT AFM phase of ${\mathrm{TbSi}}_{0.6}{\mathrm{Ge}}_{0.4}$ is found to be more susceptible to the external magnetic field, causing a significant broadening in the peaks of $\mathrm{\Delta }{S}_M$ curves at higher magnetic fields. Temperature- and field-dependent specific-heat data have been utilized to construct the complex HT phase diagram of these compounds. Furthermore, temperature-dependent x-ray diffraction measurements demonstrate substantial magnetostriction and anisotropic thermal expansion of the unit cell in both samples.

We provide a theoretical framework to describe the quantum many-body dynamics of Andreev states in Josephson junctions with spin-orbit coupling and a magnetic Zeeman field. In such cases, employing a doubled Nambu spinor description is technically advantageous but one then has to be careful to avoid double-counting problems. By deriving the Lindblad master equation in the so-called excitation picture, we show that a physically consistent many-body theory free from double-counting problems follows. We apply our formalism to a study of dynamical parity stabilization of the Andreev sector at intermediate times after an initial microwave pulse, in particular addressing the combined effects of spin-orbit coupling and Zeeman field.

Determining the fundamental mechanisms that are responsible for the observed anomalies in electrical transport is a key objective in condensed matter physics. Among these, quantum interference effects such as electron-electron (e-e) interaction and weak localization (WL) effects play an important role in determining the low-temperature ($T$) electrical transport behavior in disordered systems. Here, we provide experimental evidence for the observation of quantum interference effects driven low-temperature resistivity (ρ) minima in HoScSi and ErScSi. Temperature dependent magnetic and neutron diffraction studies clearly establish that HoScSi undergoes a transition from a paramagnetic to a commensurate antiferromagnetic (AFM) state near $T_{1\mathrm{Ho}}\sim 57$ K, which decreases to 28 K ($T_{1\mathrm{Er}}$) in ErScSi. Furthermore, in HoScSi, an incommensurate AFM state is observed below ∼35 K. Additionally, cluster glass phases are also noted in both intermetallics at lower $T$, which coexist with the antiferromagnetic state. These observed features have been attributed to the presence of ferromagnetic (FM) and AFM exchange interactions. In addition to this, ρ minima are noted near 20 K ($T_{\mathrm{minHo}}$) and 18 K ($T_{\mathrm{minEr}}$) in HoScSi and ErScSi, respectively. The systematic analysis of the magnetic field and $T$ dependence of ρ indicates that the observed anomaly originates from a disorder-enhanced e-e interaction and the WL effect.

This work merges two different types of many-electron ensembles, namely, the Theophilou-Gross-Oliveira-Kohn ensembles of ground and neutrally excited states, and the more recent $N$-centered ensembles of neutral and charged ground states. On that basis, an in-principle exact and general, so-called extended $N$-centered, ensemble density functional theory of charged and neutral electronic excitations is derived. We revisit in this context the concept of density functional derivative discontinuity for neutral excitations, without ever invoking nor using the asymptotic behavior of the ensemble electronic density. The present mathematical construction fully relies on the weight dependence of the ensemble Hartree-exchange-correlation density functional energy, which makes the theory applicable to lattice models and opens new perspectives for the description of gaps in mesoscopic systems.

Due to quantum confinement, excitons in finite-sized crystals behave rather differently than in bulk materials. We investigate the dependence of energies of Rydberg excitons on the strengths of parabolic as well as rectangular confinement potentials in finite-sized crystals. The evolution of the energy levels of hydrogenlike excitons in the crossover region from weak to strong parabolic confinement is analyzed for different quantum numbers by numerical solution of the two-dimensional Schrödinger equation. The energy spectrum of hydrogenlike excitons in ${\mathrm{Cu}}_2\mathrm{O}$-based rectangular quantum wells is, in turn, obtained numerically from the solution of the three-dimensional Schrödinger equation as a function of the quantum well width. Various crossings and avoided crossings of Rydberg energy levels are observed and categorized based on the symmetry properties of the exciton wave function. Particular attention is paid to the two limiting cases of narrow and wide quantum wells attributed to strong and weak confinement, respectively. The energies obtained with the pure Coulomb interaction are compared with the results originating from the Rytova-Keldysh potential, i.e., by taking into account the dielectric contrast in the quantum well and in the barrier.

Well-known Mott's formula links the thermoelectric power characterized by the Seebeck coefficient to conductivity. We calculate analytically the thermoelectric current and Seebeck coefficient in one-dimensional systems and show that, while the prediction of Mott's formula is valid for Dirac fermions, it is misleading for the carriers having a parabolic dispersion. We apply the developed formalism to metallic single wall carbon nanotubes and obtain a nontrivial nonmonotonic dependence of the Seebeck coefficient on the chemical potential. We emphasize that, in contrast to Mott's formula, the classical Kelvin's formula that links thermoelectric power to the temperature derivative of the chemical potential is perfectly valid in carbon nanotubes in the ballistic regime. Interestingly, however, the Kelvin's formula fails in two- and three-dimensional systems in the ballistic regime.