The pursuit of quantum spin liquid (QSL) in the Kitaev honeycomb magnets has drawn intensive attention recently. In particular, $\alpha \text{−}{\mathrm{RuCl}}_3$ has been widely recognized as a promising candidate for the Kitaev QSL. Although the compound exhibits an antiferromagnetic order under zero field, it is believed to be endowed with fractionalized excitations and can be driven to the QSL phase by magnetic fields. Here, based on a realistic $K\text{−}J\text{−}\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}^{\prime }$ model for $\alpha \text{−}{\mathrm{RuCl}}_3$, we exploit the exponential tensor renormalization group approach to explore the phase diagram of the compound under magnetic fields. We calculate the thermodynamic quantities, including the specific heat, Grüneisen parameter, magnetic torque, magnetotropic susceptibility, and so on, under a magnetic field with a tilting angle $\theta$ to the $c^{*}$-axis perpendicular to the honeycomb plane. We find an extended QSL in the angle-field phase diagram determined with thermodynamic responses. The gapless nature of such field-induced QSL is identified from the specific heat and entropy data computed down to very low temperatures. The present study provides guidance for future high-field experiments for the QSL in $\alpha \text{−}{\mathrm{RuCl}}_3$ and other candidate Kitaev magnets.

Diamond like ternary chalcopyrites $MB{X}_2$ ($M=\text{Cu},\text{Ag}$; $B=\text{Ga},\text{In},\text{Tl}$; $X=\text{S},\text{Se},\text{Te}$) have attracted tremendous research interest in thermoelectric society due to the competitive performance and a great variety of transport properties. Interestingly, the lattice thermal conductivity of $\text{Ag}B{X}_2$ is systematically two to five times lower that of $\text{Cu}B{X}_2$, in spite of their similar crystal and band structures. Based on the careful theoretical analysis, we show that the ultralow thermal conductivity of $\text{Ag}B{X}_2$ originates in the mismatched atomic bonds between Ag-$X$ and B-$X$ pairs. Owing to the very expanded $4d$ orbital of Ag, the bonding strength of Ag-$X$ is much weaker than that of $B\text{−}X$. As a result, the vibrations of Ag-$X$ and B-$X$ are well separated within the low-frequency modes. The phonon density of states of $\text{Ag}B{X}_2$ exhibits four divided peaks, compared with the three peaks in $\text{Cu}B{X}_2$. The calculated joint phonon density of states reveals that such characteristic in $\text{Ag}B{X}_2$ would increase the scattering rate of low-frequency phonons significantly. This study provides essential understandings on the ultralow thermal conductivity of Ag-based ternary chalcopyrites and indicates a general strategy to suppress the thermal conductivity in ternary compounds.

Motivated by the discovery of superconductivity in the infinite-layer nickelate family, we report an experimental endeavor to clean the surface of nickelate superconductor ${\mathrm{Pr}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ films by ${\mathrm{Ar}}^{+}$ ion sputtering and subsequent annealing, and we study their electronic structures by cryogenic scanning tunneling microscopy and spectroscopy. The annealed surfaces are characterized by nanosized clusters and Coulomb staircases with periodicity inversely proportional to the projected area of the nanoclusters, consistent with a double-barrier tunneling junction model. Moreover, the dynamical Coulomb blockade effects are observed and result in well-defined energy gaps around the Fermi level, which correlate closely with the specific configuration of the junctions. These Coulomb blockade-related phenomena provide an alternative plausible cause of the observed gap structure that should be considered in the spectroscopic understanding of nickelate superconductors with the nanoclustered surface.

The transport and optical properties of Nb-doped $\mathrm{Cs}({\mathrm{V}}_{1-x}{\mathrm{Nb}}_x$)${}_3{\mathrm{Sb}}_5$ with $x=0.03$ and 0.07 have been investigated and compared with those of undoped ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. Upon Nb doping, the charge density wave (CDW) transition temperature $T_{\text{CDW}}$ is suppressed, and the superconducting temperature $T_c$ rises. The residual resistivity ratio decreases with Nb doping, suggesting an increase of disorder. For all compounds, the optical conductivity in the pristine phase reveals two Drude components (D1 and D2). The substitution of Nb causes an increase of D1 alongside a reduction of D2 in weight, which implies a change of the Fermi surface. The total Drude weight is reduced with increasing Nb content, signifying an enhancement of electronic correlations. Below $T_{\text{CDW}}$, while the optical conductivity clearly manifests the CDW gap in all materials, the gapped portion of the Fermi surface shrinks as the Nb content grows. A comprehensive analysis indicates that the change of the Fermi surface, the enhancement of electronic correlations, the shrinkage of the removed Fermi surface by the CDW gap, and the increase of disorder may all have a considerable impact on the interplay between the CDW and superconductivity in $\mathrm{Cs}({\mathrm{V}}_{1-x}{\mathrm{Nb}}_x$)${}_3{\mathrm{Sb}}_5$.

We present a temperature-dependent Raman scattering study of CdO in the temperature range between 80 and 500 K. Distinctive features present in the second-order Raman spectra of CdO are identified as overtones and combination modes with the help of density functional perturbation theory calculations. A wide, broad band in the region between 250 and $470 {\mathrm{cm}}^{-1}$ is attributed to Raman scattering by charge-density fluctuations in the high-density overdamped electron gas present in the sample. Very good agreement is found between the Raman spectra and the expected scattering arising from the overtone density of states and plasmon excitations. The temperature dependence of the modes yields information about the anharmonic phonon interactions. Four-phonon processes arising from quartic anharmonicity play a significant role in the observed temperature dependence.

The axion insulator is believed to host half-quantized chiral currents running antiparallelly on its top and bottom surfaces. However, the experimental detection of the half quantization in axion insulators remains elusive. In this paper, we propose a mechanism to explain why the half quantization is hard to be probed by showing that the half-quantized counterpropagating currents in axion insulator thin films are strongly suppressed due to the hybridization mediated by the massless side-surface states. This side-surface-mediated hybridization leads to a different type of finite-size effect, which features a power-law decay with the increasing film thickness, different from the exponential decay in topological insulators. Moreover, we show that the half quantization can be extracted in the axion insulator phase by adopting the nonlocal transport measurement.

We show that tailoring the dissipative environment allows us to change the features of continuous quantum phase transitions and even induce first-order transitions in ferromagnetic spin chains. In particular, using a numerically exact quantum Monte Carlo method for the paradigmatic Ising chain of one-half spins in a transverse magnetic field, we find that spin couplings to local quantum boson baths (in the Ohmic regime) can drive the transition from the second to the first order even for a low dissipation strength. Moreover, using a variational mean-field approach for the treatment of spin-spin and spin-boson interactions, we point out that phase discontinuities are ascribable to a dissipation-induced effective magnetic field which is intrinsically related to the bath quantum fluctuations and vanishes for classical baths. The effective field is able to switch the sign of the magnetization along the direction of spin-spin interactions. The results can be potentially tested in recent quantum simulators and are relevant for quantum sensing since the spin system could not only detect the properties of nonclassical baths, but also the effects of weak magnetic fields.

Located at crystal voids, interstitial anion electrons (IAEs) have diverse topologies, which may be tuned to achieve different properties. Elucidating the role of IAEs in electron-phonon coupling (EPC), and using it to design electride superconductors, leads to the current prediction of superconducting ${\mathrm{Li}}_8\mathrm{Au}$ at high pressure. We suggest that the occurence of high-temperature superconductivity in electrides requires high-symmetry structures with hydrogenlike cages, an electron acceptor element to balance charges, and isolated IAEs coupled with medium-frequency vibrations. The uniquely designed ${\mathrm{Li}}_8\mathrm{Au}$ electride has a NaCl-type ($B1$) lattice, with atomic Au and cubic ${\mathrm{Li}}_8$ cages as bases. Isolated IAEs are formed at the cage centers, with extra charges taken up by Au. These octahedrally coordinated IAEs have a $p$-orbital-like attribute and are strongly coupled with atomic vibrations in the ${\mathrm{Li}}_8$ cages. The strong EPC in ${\mathrm{Li}}_8\mathrm{Au}$ results in a calculated $T_c$ of 73.1 K at 250 GPa, which is the highest $T_c$ reported to date for all the electrides. A slight substitutional Pt doping can enhance the $T_c$ of ${\mathrm{Li}}_8\mathrm{Au}$ to exceed liquid nitrogen temperature.

We generalize the modern theory of electric polarization to the case of one-dimensional (1D) non-Hermitian systems with a line-gapped spectrum. In these systems, the electronic position operator is non-Hermitian even when projected into the subspace of states below the energy gap. However, in the framework of biorthogonal quantum mechanics, the associated Wilson-loop operator is unitary in the thermodynamic limit, thereby leading to real-valued electronic positions that allow for a clear definition of polarization. Non-Hermitian polarization can be quantized in the presence of certain symmetries, as for Hermitian insulators. Differently from the latter case, however, in this regime polarization quantization depends also on the type of energy gap, which can be either real or imaginary, leading to a richer variety of topological phases. The most counterintuitive example is the 1D non-Hermitian chain with time-reversal symmetry only, where non-Hermitian polarization is quantized in the presence of an imaginary-line gap. We propose two specific models to provide numerical evidence supporting our findings.

We investigate the number probability density function that characterizes subportions of a quantum many-body system with globally conserved number of particles. We put forward a linear fitting protocol capable of mapping out the ground-state phase diagram of the rich one-dimensional extended Bose-Hubbard model: The results are quantitatively comparable with more sophisticated traditional and machine learning techniques. We argue that the studied quantity should be considered among the most informative bipartite properties, being moreover readily accessible in atomic gases experiments.

In time-resolved pump-probe vibrational spectroscopy the internal stretch mode of polar molecules is utilized as a key observable to characterize the ultrafast dynamics of adsorbates on surfaces. The adsorbate nonadiabatic intermode couplings are the commonly accepted mechanisms behind the observed transient frequency shifts. Here, we study the CO/Pd(111) system with a robust theoretical framework that includes electron-hole pair excitations and electron-mediated coupling between the vibrational modes. A mechanism is revealed that screens the electron-phonon interaction and originates a blueshift under ultrafast nonequilibrium conditions. The results are explained in terms of the abrupt change in the density of states around the Fermi level, and are instrumental for understanding the dynamics at multicomponent surfaces involving localized and standard $s$ or $p$ states.

The creation of well-understood structures using spectral hole burning is an important task in the use of technologies based on rare-earth ion-doped crystals. We apply a series of different techniques to model and improve the frequency dependent population change in the atomic level structure of thulium yttrium gallium garnet (Tm:YGG). In particular we demonstrate that, at zero applied magnetic field, numerical solutions to frequency-dependent three-level rate equations show good agreement with spectral hole-burning results. This allows us to predict spectral structures given a specific hole-burning sequence, the underpinning spectroscopic material properties, and the relevant laser parameters. This enables us to largely eliminate power-dependent hole broadening through the use of adiabatic hole-burning pulses. Although this system of rate equations shows good agreement at zero field, the addition of a magnetic field results in unexpected spectral diffusion proportional to the induced Tm ion magnetic-dipole moment and average magnetic-field strength, which, through the quadratic Zeeman effect, dominates the optical spectrum over long timescales. Our results allow optimization of the preparation process for spectral structures in a large variety of rare-earth ion-doped materials for quantum memories and other applications.

The gap functions for a single-band model for unconventional superconductivity are distinguished by their unitary or nonunitary forms. Here we generalize this classification to a two-band superconductor with two nearly degenerate orbitals. We focus on spin-singlet pairings and investigate the effects of the atomic spin-orbit coupling (SOC) on superconductivity, which is a driving force behind the discovery of a new spin-orbit coupled nonunitary superconductor. Multiorbital effects such as orbital hybridization and strain-induced anisotropy will also be considered. The spin-orbit coupled nonunitary superconductor has three main features. First, the atomic SOC locks the electron spins to be out-of-plane, leading to a new Type II Ising superconductor with a large in-plane upper critical field beyond the conventional Pauli limit. Second, it provides a promising platform to realize the topological chiral or helical Majorana edge state even without external magnetic fields or Zeeman fields. More surprisingly, a spin-polarized superconducting state could be generated by spin-singlet nonunitary pairings when time-reversal symmetry is spontaneously broken, which serves as a smoking gun to detect this exotic state by measuring the spin-resolved density of states. Our work indicates the essential roles of orbital-triplet pairings in both unconventional and topological superconductivity.

We use nonequilibrium steady states to study the effect of dissipation-assisted operator evolution (DAOE) on the scaling behavior of transport in one-dimensional spin chains. We consider three models in the XXZ family: the XXZ model with staggered anisotropy, which is chaotic; XXZ model with no external field and tunable interaction, which is Bethe-ansatz integrable and (in the zero interaction limit) free-fermion integrable; and the disordered XY model, which is free-fermion integrable and Anderson localized. We find evidence that DAOE's effect on transport is controlled by its effect on the system's conserved quantities. To the extent that DAOE preserves those symmetries, it preserves the scaling of the system's transport properties; to the extent it breaks those conserved quantities, it pushes the system towards diffusive scaling of transport.

The quantum spin liquid as a natural ground state of the Kitaev model has led to a quest for new materials candidates hosting Kitaev physics. Yet, there are very few material candidates in this category. Using a combination of ab initio and model Hamiltonian methods, we propose that the Ruddlesden-Popper compound ${\mathrm{Sr}}_4{\mathrm{RhO}}_6$ belongs to this category. With a tight-binding model and exact-diagonalization approach, we show that despite substantial trigonal-like distortion, the electronic and magnetic properties of ${\mathrm{Sr}}_4{\mathrm{RhO}}_6$ can be well described in terms of pseudospin-1/2 states. Magnetic interactions among pseudospins, estimated using the second-order perturbation method, are highly bond-dependent anisotropic in nature with two particularly noticeable features, antiferromagnetic Kitaev and Dzyaloshinskii-Moriya interactions. The gapped spin-wave spectrum of ${\mathrm{Sr}}_4{\mathrm{RhO}}_6$ obtained with linear spin-wave theory is consistent with the underlying magnetic frustration. Additional analysis of the role of individual or a particular combination of magnetic interactions reveals that the spin-wave spectrum of ${\mathrm{Sr}}_4{\mathrm{RhO}}_6$ is a combined effect of the highly anisotropic interactions, and a relatively simpler minimal model may not be plausible in the current case. The crucial insights about coupling between the local structural features and magnetic properties of ${\mathrm{Sr}}_4{\mathrm{RhO}}_6$ obtained in this study may be helpful for future studies belonging to this class.

We construct a general theory of ${\mathbb{Z}}_2$ topological phase transitions in two-dimensional systems with time-reversal symmetry. We investigate the possibilities of ${\mathbb{Z}}_2$ topological phase transitions at band inversions at all high-symmetry points in $k$ space in all 80 layer groups. We exclude the layer groups with inversion symmetry because the ${\mathbb{Z}}_2$ topological phase transition is known to be associated with band inversions with an exchange of parities. Among the other layer groups, we find 21 layer groups with insulator-to-insulator transitions with band inversion, and this problem is finally reduced to five point groups $C_3,C_4,C_6,S_4$, and $C_{3h}$. We show how the change of the ${\mathbb{Z}}_2$ topological invariant at a band inversion is entirely determined by the irreducible representations of occupied and unoccupied bands at the high-symmetry point. For example, in the case of $C_3$, we show that the ${\mathbb{Z}}_2$ topological invariants change whenever the band inversion occurs between two Kramers pairs whose $C_3$ eigenvalues are $\{e^{\pi i/3},e^{-\pi i/3}\}$ and $\{-1,-1\}$. These results are not included in the theory of symmetry-based indicators or topological quantum chemistry.