In this paper, we demonstrate the capability to establish spin-polarized currents in doped $\mathrm{SrTi}{\mathrm{O}}_3$ (STO). The results are based on the study of charge and spin transport in STO layers doped by the reversible electromigration of oxygen atoms in resistive-switching ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3/\mathrm{STO}/\mathrm{Co}$ vertical stacks. The formation of oxygen vacancies inside STO results in a metallic conductivity at temperatures $<200–250$ K, above which a transition to an insulatinglike behavior is detected. A detailed theoretical analysis shows that the behavior of the metallic phase in our samples corresponds to the well-known state of the thermodynamically doped STO featuring the so-called bad metal behavior. Thus, our findings introduce this class of unconventional materials as valuable candidates for innovative spintronic devices.

We study the current-voltage characteristics of a superconducting junction with particle losses at the contacts. We adopt the Keldysh formalism to compute the steady-state current for varying transmission of the contact. In the low-transmission regime, the dissipation leads to an enhancement of the current at low bias, a nonmonotonic dependence of current on dissipation, and the emergence of new structures in the current-voltage curves. The effect of dissipation by particle loss is found to be qualitatively different from that of a finite temperature and a finite inelastic scattering rate in the reservoirs.

In one dimension, quantum fluctuations prevent the appearance of long-range order in a supersolid, and only quasi-long-range order can survive. We derive this quantum critical behavior and study its influence on the superfluid response and properties of the solid. The analysis is based on an effective low-energy description accounting for the two coupled Goldstone modes. We find that the quantum phase transition from the superfluid to the supersolid is shifted by quantum fluctuations from the position where the local formation of a solid structure takes place. For current experimental parameters with dipolar atomic gases, this shift is extremely small and cannot be resolved yet, i.e., current observations in experiments are expected to be in agreement with predictions from mean-field theory based on the extended Gross-Pitaevskii formalism.

Dilute superconductivity survives in bulk strontium titanate when the Fermi temperature falls well below the Debye temperature. Here, we show that the onset of the superconducting dome is dopant dependent. When mobile electrons are introduced by removing oxygen atoms, the superconducting transition survives down to $2\times {10}^{17}{\mathrm{cm}}^{-3}$, but when they are brought by substituting Ti with Nb, the threshold density for superconductivity is an order of magnitude higher. Our study of quantum oscillations reveals a difference in the band dispersion between the dilute metals made by these doping routes and our band calculations demonstrate that the rigid band approximation does not hold when mobile electrons are introduced by oxygen vacancies. We identify the band sculpted by these vacancies as the exclusive locus of superconducting instability in the ultradilute limit.

The Fermi sea topology is characterized by the Euler characteristic ${\chi }_F$. In this paper, we examine how ${\chi }_F$ of the metallic state is inherited by the topological invariant of the superconducting state. We establish a correspondence between the Euler characteristic and the Chern number $C$ of $p$-wave topological superconductors without time-reversal symmetry in two dimensions. By rewriting the pairing potential ${\mathrm{\Delta }}_{\mathbf{k}}={\mathrm{\Delta }}_1-i{\mathrm{\Delta }}_2$ as a vector field $\mathbf{u}=({\mathrm{\Delta }}_1,{\mathrm{\Delta }}_2)$, we found that ${\chi }_F=C$ when $\mathbf{u}$ and fermion velocity $\mathbf{v}$ can be smoothly deformed to be parallel or antiparallel on each Fermi surface. We also discuss a similar correspondence between the Euler characteristic and three-dimensional winding number of time-reversal-invariant $p$-wave topological superconductors in three dimensions.

We explore anomalous skin effects at non-Hermitian impurities by studying their interplay with potential disorder and by exactly solving a minimal lattice model. A striking feature of the solvable single-impurity model is that the presence of anisotropic hopping terms can induce a scale-free accumulation of all eigenstates opposite to the bulk hopping direction, although the nonmonotonic behavior is fine tuned and further increasing such hopping weakens and eventually reverses the effect. The interplay with bulk potential disorder, however, qualitatively enriches this phenomenology leading to a robust nonmonotonic localization behavior as directional hopping strengths are tuned. Nonmonotonicity persists even in the limit of an entirely Hermitian bulk with a single non-Hermitian impurity.

The $A\mathrm{Yb}C{h}_2$ ($A=\text{alkali}\text{metal}, Ch=\text{chalcogen}$) delafossites are a family of crystals which have a triangular lattice of effective $S=1/2$ moments on their Yb atoms. They hold great promise for the realization of the triangular quantum spin liquid state because of their defect-minimized growth and the ability to interchange chemical constituencies among the family. Here we use ab initio computations to evaluate the exchange couplings of four realized (and one theoretical) rhombohedral delafossite structures and examine the influence of chemical substitution on promoting the development of a quantum spin liquid state. We find good agreement with experiment regarding the antiferromagnetic nearest-neighbor exchange $J_1$, but our calculations underestimate $J_2$.

We present our results for simultaneous measurement of the refractive indices of gallium arsenide (GaAs) and aluminum gallium arsenide $\left({\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{As}\right)$ in the spectral region from $2.0\mu \mathrm{m}\mathrm{to}7.1\mu \mathrm{m}$ $\left(5000{\mathrm{cm}}^{-1}\mathrm{to}1400{\mathrm{cm}}^{-1}\right)$. We obtain these values from a monocrystalline superlattice Bragg mirror of excellent purity (background doping $\le 1\times {10}^{-14}/{\mathrm{cm}}^3$), grown via molecular beam epitaxy. To recover the refractive indices over such a broad wavelength range, we fit a dispersion model for each material. In a novel combination of well-established methods, we measure both a photometrically accurate transmittance spectrum of the Bragg mirror via Fourier-transform infrared spectrometry and the individual physical layer thicknesses of the structure via scanning electron microscopy. To infer the uncertainty of the refractive index values, we estimate relevant measurement uncertainties and propagate them via a Monte Carlo method. This highly-adaptable approach conclusively yields propagated relative uncertainties on the order of ${10}^{-4}$ over the measured spectral range for both GaAs and ${\mathrm{Al}}_{0.929}{\mathrm{Ga}}_{0.071}\mathrm{As}$. The fitted model can also approximate the refractive index for MBE-grown ${\mathrm{Al}}_x{\mathrm{Ga}}_{1\text{−}x}\mathrm{As}$ for $0\le x\le 1$. Both these updated values and the measurement approach will be essential in the design, fabrication, and characterization of next-generation active and passive optical devices in a spectral region that is of high interest in many fields, e.g., laser design and cavity-enhanced spectroscopy in the mid-infrared.

Employing large-scale quantum Monte Carlo simulations, we systematically compute the energy spectra of the two-dimensional (2D) spin-1/2 Heisenberg model with long-range interactions. With the $1/r^{\alpha }$ ferromagnetic and staggered antiferromagnetic interactions, we find the explicit range in $\alpha$ for the short-range Goldstone-type (gapless), anomalous Goldstone-type (gapless), and Higgs-type (gapped) spectra. Accompanied by the spin-wave analysis, our numerical results vividly reveal how the long-range interactions alter the usual linear and quadratic magnon dispersions in 2D quantum magnets and give rise to anomalous dynamical exponents. Moreover, we find the explicit case where the gapped excitation emerges at a noninteger decay exponent $\alpha$ for the antiferromagnetic Hamiltonian. This work provides the first set of unbiased dynamical data of long-range quantum many-body systems and suggests that many universally accepted low-energy customs for short-range systems need to be substantially modified for long-range ones, which are of immediate relevance to the ongoing experimental efforts from quantum simulators to 2D quantum moiré materials.

Using a nonperturbative classical model for ionic motion through one-dimensional (1D) solids, we explore how thermal lattice vibrations affect ionic transport properties. Based on analytic and numerical calculations, we find that the mean dissipation experienced by the mobile ion is similar to that of the nonthermal case, with thermal motion only contributing stochastic noise. A nonmonotonic dependence of drag on speed, predicted in earlier work, persists in the presence of thermal motion. The inverse relation between drag and speed at high speeds results in non-Fickian diffusion dominated by Lévy flights. This suppression of drag at high speeds, combined with enhanced activation frequency, improves the particle mobility at high temperatures, where typical particles move faster.

We develop a topological classification of non-Hermitian effective Hamiltonians that depend on momentum and frequency. Such effective Hamiltonians are in one-to-one correspondence to single-particle Green's functions of systems that satisfy translational invariance in space and time but may be interacting or open. We employ $K$ theory, which for the special case of noninteracting systems leads to the well-known 10-fold-way topological classification of insulators and fully gapped superconductors. Relevant theorems for $K$ groups are reformulated and proven in the more transparent language of Hamiltonians instead of vector bundles. We obtain 54 symmetry classes for frequency-dependent non-Hermitian Hamiltonians satisfying antiunitary symmetries. Employing dimensional reduction, the group structure for all these classes is calculated. This classification leads to a group structure with one component from the momentum dependence, which corresponds to the non-Hermitian generalization of topological insulators and superconductors, and two additional parts resulting from the frequency dependence. These parts describe winding of the effective Hamiltonian in the frequency direction and in combined momentum-frequency space.

Differentiable programming has emerged as a key programming paradigm empowering rapid developments of deep learning while its applications to important computational methods such as Monte Carlo remain largely unexplored. Here we present the general theory enabling infinite-order automatic differentiation on expectations computed by Monte Carlo with unnormalized probability distributions, which we call automatic differentiable Monte Carlo (ADMC). By implementing ADMC algorithms on computational graphs, one can also leverage state-of-the-art machine learning frameworks and techniques in traditional Monte Carlo applications in statistics and physics. We illustrate the versatility of ADMC by showing some applications: fast search of phase transitions and accurately finding ground states of interacting many-body models in two dimensions. ADMC paves a promising way to innovate Monte Carlo in various aspects to achieve higher accuracy and efficiency.

Stacking-dependent physics is emerging as a fascinating research topic for two-dimensional materials. A variety of novel properties can be achieved by layer stacking according to different modes. However, most of the studies focus on the impact of electronic, superconducting, optical, and magnetic properties. In this work, we have systematically studied the stacking-dependent topological quantum states in bilayer ${\mathrm{Mn}}_2{\mathrm{Cl}}_3{\mathrm{Br}}_3$ by the first-principles electronic structure calculations. Here, ferromagnetic layer coupling is always favored in different stacking modes. Many exotic topological quantum states, such as topological nodal-ring spin-gapless semimetal state, spin-valley polarized quantum valley Hall (SVP-QVH) effect, and high Chern number quantum anomalous Hall effect can be realized in this single system. Among these states, the SVP-QVH is a new topological phase discovered for the first time, in which there are two topologically protected gapless chiral edge states that carry the same spin, localize respectively on the two polarized valleys, and propagate in opposite directions along the edges. The spin-valley polarized anomalous valley Hall effect can be further realized with electron doping or gate tuning, resulting in edge states that integrate multiple degrees of freedom of valley, spin, and charge. Our research therefore provides an idea for discovery of new topological phases and design of two-dimensional multifunctional electronic, spintronic, and topological devices.