Pulsed laser deposition (PLD) is widely used to grow epitaxial thin films of quantum materials. Here, we use in situ x-ray scattering to study homoepitaxy of ${\mathrm{SrTiO}}_3$ by energetic deposition (e-PLD) versus PLD thermalized by a He background gas (th-PLD). Energetic PLD suppresses the lateral growth of two-dimensional islands, which suggests that particles with kinetic energies of $\sim 100$ eV break up smaller islands. Fast interlayer transport occurs for th-PLD as well as e-PLD, implying a process operating on submicrosecond time scales for incident particles with kinetic energies below 10 eV.

The quantum anomalous Hall effect (QAHE) has been experimentally observed in magnetically doped topological insulators at ultralow temperatures. Inhomogeneous ferromagnetism is considered to be one of the main factors that lead to the unexpected low QAHE observation temperature. Dopant-induced disorder is usually the origin of the inhomogeneous ferromagnetism. Here, our systematic first-principles calculations demonstrate that inhomogeneous mixing of Bi and Sb in a ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ system is the intrinsic origin of inhomogeneous ferromagnetism. Different from diluted magnetic semiconductors, the mixing energy of Cr and V in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, and ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ topological insulators clearly show that magnetic dopants are homogeneously distributed even in the presence of naturally formed crystalline defects. Surprisingly, our study shows that the mixing energies of Sb and Bi in a ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ system are all positive in the whole range of doping concentration, indicating that Bi elements in a ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ system are inhomogeneously distributed. Moreover, the formation energies of Cr and V suggest that they are relatively easy to substitute Bi sites in Bi inhomogeneously distributed ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ systems, which leads to inhomogeneous ferromagnetism of the experimental QAHE system. The influence of the inhomogeneous distribution of Bi on the electronic structures of bulk ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ systems is also analyzed. We believe that our finding of the intrinsic origin of magnetic inhomogeneity should be beneficial for the experimental enhancement of the QAHE observation temperature in magnetically doped topological insulators.

The time evolution in quantum many-body systems after external excitations is attracting high interest in many fields, including dense plasmas, correlated solids, laser-excited materials, or fermionic and bosonic atoms in optical lattices. The theoretical modeling of these processes is challenging, and the only rigorous quantum-dynamics approach that can treat correlated fermions in two and three dimensions is nonequilibrium Green functions (NEGF). However, NEGF simulations are computationally expensive due to their $T^3$ scaling with the simulation duration $T$. Recently, $T^2$ scaling was achieved with the generalized Kadanoff-Baym ansatz (GKBA), for the second-order Born (SOA) self energy, which has substantially extended the scope of NEGF simulations. In a recent Letter [Schlünzen et al., Phys. Rev. Lett. 124, 076601 (2020).] we demonstrated that GKBA-NEGF simulations can be efficiently mapped onto coupled time-local equations for the single-particle and two-particle Green functions on the time diagonal, hence the method has been called the G1-G2 scheme. This allows one to perform the same simulations with order $T^1$ scaling, both for SOA and $GW$ self energies giving rise to a dramatic speedup. Here we present more details on the G1-G2 scheme, including derivations of the basic equations including results for a general basis, for Hubbard systems, and for jellium. Also, we demonstrate how to incorporate initial correlations into the G1-G2 scheme. Further, the derivations are extended to a broader class of self energies, including the $T$ matrix in the particle-particle and particle-hole channels and the dynamically-screened-ladder approximation. Finally, we demonstrate that, for all self energies, the CPU-time scaling of the G1-G2 scheme with the basis dimension $N_b$ can be improved compared to our first report: The overhead compared to the original GKBA is not more than an additional factor $N_b$, even for Hubbard systems.

Because of their unique electronic structure and peculiar physical properties, topological materials have attracted considerable attention in condensed matter physics. In both physics and material science, exploring materials with a topological state or Dirac fermions near the Fermi level is important. Here, we report the transport evidence of the topological state for ${\mathrm{Ag}}_3\mathrm{Sn}$. Under a high magnetic field, we observed clear Shubnikov–de Haas (SdH) oscillations with three major frequencies and relatively small effective masses. Our SdH quantum oscillation analyses reveal that the Berry phase of the $\gamma$ band is very close to the nontrivial value $\pi$; the nonzero Berry phase for the $\alpha$ band indicates the ${\mathrm{Ag}}_3\mathrm{Sn}$ alloy to be a possible topological material. Moreover, we performed angle-resolved magnetoresistance (MR) measurements. Note that MR displays an unusual angular dependence, which can be tuned from twofold to fourfold symmetry by temperature and field. This feature makes ${\mathrm{Ag}}_3\mathrm{Sn}$ a candidate for valleytronics.

A polarized electronic Raman scattering study reveals the emergence of symmetry dependence in the electronic Raman response of single-crystalline ${\mathrm{URu}}_2{\mathrm{Si}}_2$ below the Kondo crossover scale $T_K\sim$ 100 K. In particular, the development of a coherent Kondo pseudogap predominantly in the $E_g$ channel highlights strong anisotropy in the Kondo physics in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ that was previously neglected in theoretical models of this system and more generally has been sparsely treated for Kondo systems. A calculation of the Raman vertices demonstrates that the strongest Raman vertex does, indeed, develop within the $E_g$ channel for interband transitions and reaches a maximum along the diagonals of the Brillouin zone, implying a $d$-wave-like geometry for the Kondo pseudogap. This generally overlooked property may have important consequences on the formation of the subsequent phases of this system, namely, the hidden order and the superconductivity.

We present the topology of spin-split Fermi surface of CaAgAs as determined by de Haas-van Alphen (dHvA) effect measurements combined with ab initio calculations. We have determined the torus-shaped nodal-line Fermi surface from the dHvA oscillations of $\beta$ and $\gamma$ orbits. The former orbit encircles the nodal line, while the latter does not. Nevertheless, a nontrivial Berry phase is found for both orbits. The nontrivial phase of $\beta$ arises from the orbital characters, which can be expressed as a pseudospin rotating around the nodal line. On the other hand, the phase of $\gamma$ is attributed to the vortex of real spin texture induced by an antisymmetric spin-orbit interaction. Our result demonstrates that both the real spin and pseudospin textures are indispensable in interpreting the electronic topology in noncentrosymmetric nodal-line semimetals.

Electron–core hole interactions are critical for proper interpretation of core-level spectroscopies commonly used as analytical tools in materials science. Here we utilize resonant Auger-electron spectroscopy to uniquely identify exciton, shake, and charge-transfer processes that result from the sudden creation of the core hole in both x-ray-absorption and photoemission spectra. These effects are captured for the transition-metal compounds $\mathrm{SrTi}{\mathrm{O}}_3$ and $\mathrm{Mo}{\mathrm{S}}_2$ by fully ab initio, combined real-time cumulant, and Bethe-Salpeter equation approaches to account for core hole dynamics and screening. Atomic charges and excited-state electron-density fluctuations reflect materials’ solid-state electronic structure, loss of translational symmetry around the core hole, and breakdown of the sudden approximation. They also demonstrate competition between long- and short-range screening in a solid.

V $3d$ charge and orbital states in ${\mathrm{V}}_2{\mathrm{OPO}}_4$ have been investigated by means of x-ray absorption spectroscopy (XAS). The electronic structure of ${\mathrm{V}}_2{\mathrm{OPO}}_4$ is very unique in that the charge transfer between ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ in face-sharing ${\mathrm{VO}}_6$ chains provides negative thermal expansion as reported by E. Pachoud et al. [J. Am. Chem. Soc. 140, 636 (2018)]. The near-edge region of O $1s$ XAS exhibits the three features which can be assigned to transitions to O $2p$ mixed into the unoccupied V $3d{t}_{2g}$ and $e_g$ orbitals of ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$. The V $2p$ XAS line shape can be reproduced by multiplet calculations for a mixed-valence state with ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$. The polarization dependence of the O $1s$ and V $2p$ XAS spectra indicates V $3d$ orbital order in which $xy$ and $yz$ (or $zx$) orbitals are occupied at the ${\mathrm{V}}^{3+}$ site in the face-sharing chains. The occupied $xy$ orbital is essential for the antiferromagnetic coupling between the ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ sites along the chains, while the occupied $yz$ (or $zx$) orbital provides the antiferromagnetic coupling between the ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ sites between the chains.

The presence of quantum scars, athermal eigenstates of a many-body Hamiltonian with finite-energy density, leads to an absence of ergodicity and long-time coherent dynamics starting from initial states that have a high overlap with scars as experimentally observed in a chain of ultracold Rydberg atoms. We show, via study of a periodically driven Rydberg chain, that the drive frequency acts as a tuning parameter for several reentrant transitions between ergodic and weak ergodicity breaking regimes. The former regime shows rapid thermalization of correlation functions and absence of scars in the Floquet spectrum of the system. The latter regime, in contrast, has scars; they lead to long-time coherent dynamics of correlation functions. We provide an analytical explanation of the existence of these regimes by going beyond a high-frequency Magnus expansion and using a novel perturbative approach valid at large drive amplitudes to derive a Floquet Hamiltonian which qualitatively explains the behavior of the driven system at arbitrary frequencies. We also discuss experiments involving finite Rydberg chains which can validate our theory. Our results demonstrate the possibility of drive-frequency-induced tuning between ergodic and weak ergodicity breaking dynamics in a disorder-free quantum many-body system.

Despite decades of efforts, the origin of metal-insulator transitions (MITs) in strongly correlated materials remains one of the main long-standing problems in condensed-matter physics. An archetypal example is ${\mathrm{V}}_2{\mathrm{O}}_3$, which undergoes simultaneous electronic, structural, and magnetic phase transitions. This remarkable feature highlights the many degrees of freedom at play in this material. In this work, acting solely on the magnetic degree of freedom, we reveal an anomalous feature in the electronic transport of ${\mathrm{V}}_2{\mathrm{O}}_3$: On cooling, the magnetoresistance changes from positive to negative values well above the MIT temperature, and shows divergent behavior at the transition. The effects are attributed to the magnetic field quenching antiferromagnetic fluctuations above the Néel temperature $T_N$, and preventing long-range antiferromagnetic ordering below $T_N$. In both cases, suppressing the antiferromagnetic ordering prevents the opening of the incipient electronic gap. This interpretation is supported by Hubbard model calculations which fully reproduce the experimental behavior. Our study sheds light on this classic problem providing a clear and physical interpretation of the nature of the metal-insulator transition.

We predict that a family of antiperovskite materials realize a higher-order topological insulator phase, characterized by a previously introduced ${\mathbb{Z}}_4$ index. A tight-binding model and a $k·p$ model are used to capture the physics of the bulk, surface, and hinge states of these materials. A phase diagram of the higher-order and weak topological invariants is obtained for the tight-binding model. The mirror Chern number is also discussed. In order to reveal the gapless hinge states in the presence of mirror Chern surface states, several ways of opening the surface gap are proposed and confirmed by calculation, including cleaving the crystal to reveal a low-symmetry surface, building a heterostructure, and applying strain. Upon opening the surface gap, we are able to study the hinge states by computing the momentum-space band structure and real-space distribution of midgap states.

The ability to actively control heat flows in the nanoscale can open up a plethora of opportunities for applications that require thermal management and regulation. We show that it is possible to build a three-terminal quantum thermal gating device which regulates the thermal conductivity between two of its terminals in response to light incident upon the remaining terminal. We model our device as three mutually coupled two-level systems which interact thermally and optically with their environment. To incorporate the thermal interactions of our device, we adopt an open quantum systems framework under the Born-Markov approximation. We subsequently use detailed quantum mechanical state analysis to illustrate its operating principle. Through numerical simulations, we further explore the nonlinear relationship between the optical field amplitude and the thermal conductivity of the device in the steady-state regime. Based on our investigations, we find that the energy-gating behavior of our device is highly efficient in that it can control a significantly larger thermal energy flow compared to the amount of energy it absorbs from the optical field in the process. The approach we have taken to analyze the system, in particular the graphical representations we developed to intuitively and concisely represent the quantum states, energy flows, and the relationships between them, could be of value in analyzing other similar thermo-optical systems. Thus, we envision that both the device concept and its analysis technique would be useful to researchers working in this area.

Due to the exponential increase in the numerical effort with the number of degrees of freedom, moving basis functions have a long history in quantum dynamics. In addition, spawning of new basis functions is routinely applied. Here, we advocate the opposite process: the programed removal of motional freedom of selected basis functions. This is a necessity for converged numerical results with respect to the size of a nonorthogonal basis because generically two or more states approach each other too closely early on, rendering unstable the matrix inversion, required to make the equations of motion explicit. Applications to the sub-Ohmic spin-boson model as well as to polaron dynamics in a Holstein molecular-crystal model demonstrate the power of the proposed methodology.

Floquet engineering of closed quantum systems can lead to the formation of long-lived prethermal states that, in general, eventually thermalize to infinite temperature. Coupling these driven systems to dissipative baths can stabilize such states, establishing a true nonequilibrium steady state. We demonstrate that, in a certain strongly interacting lattice model coupled to a bath and driven by an electric field, such steady states can have the remarkable property that the cycle-averaged rate of energy transfer between the lattice and the baths vanishes. Despite this, we show that these states retain a clear nonequilibrium nature.

In the search for experimental realizations of bond-anisotropic Kitaev interactions and resulting spin-liquid phases, the layered magnet $\alpha \text{−}{\mathrm{RuCl}}_3$ is a prime candidate. Its modeling typically involves Heisenberg, Kitaev, and symmetric off-diagonal $\mathrm{\Gamma }$ interactions on the two-dimensional honeycomb lattice. However, recent neutron-scattering experiments point towards a sizable magnetic interlayer coupling. Here we study three-dimensional exchange models for $\alpha \text{−}{\mathrm{RuCl}}_3$, for both possible $R\overline{3}$ and $C2/m$ crystal structures. We discuss the symmetry constraints on the interlayer couplings, construct minimal models, and use them to compute the magnetic mode dispersion and the dynamical spin structure factor, in both the zero-field zigzag phase and the paramagnetic high-field phase. Our predictions for the interlayer mode dispersion shall guide future experiments; they also call for a reevaluation of the quantitative model parameters relevant for $\alpha \text{−}{\mathrm{RuCl}}_3$.

We have elucidated the spin, lattice, charge, and orbital coupling mechanism underlying the multiferroic character in tensile-strained $\mathrm{EuTi}{\mathrm{O}}_3$ films. Symmetry determined by oxygen octahedral tilting shapes the hybridization between the Eu $4f$ and the Ti $3d$ orbitals, and this inhibits predicted Ti displacement proper ferroelectricity. Instead, phonon softening emerges at low temperatures within the pseudocube (110) plane, orthogonal to the anticipated ferroelectric polarization symmetry. Additionally, the magnetic anisotropy is determined by orbital distortion through hybridization between the Ti $3d$ and the typically isotropic $\mathrm{E}{\mathrm{u}}^{2+}4f$ states. This unique scenario demonstrates the critical role symmetry plays in the coupling of order parameters defining multiferroic behavior.

The topological material of $\mathrm{C}{\mathrm{o}}_2\mathrm{MnGa}$ with potentially high spin polarization has attracted more and more attention, in the fields of fundamental science and spintronic applications. Here we report the electrical- and magnetotransport behaviors in epitaxial $\mathrm{C}{\mathrm{o}}_2\mathrm{MnGa}$ thin films. An angle-independent negative linear magnetoresistance and a negative anisotropy magnetoresistance are observed in the whole temperature range from 3 to 300 K, indicating the existence of weak spin disorder and a high spin polarization. Intriguingly, with decreasing temperature an anomalous resistivity upturn that exhibits $-T^{1/2}$ dependence is observed, and this phenomenon is robust under magnetic fields. Further analysis demonstrates that the three-dimensional enhanced electron-electron interaction effect is responsible for these phenomena rather than the orbital two-channel Kondo effect or other quantum interference effects, which provides deeper insight into the transport behavior of the $\mathrm{C}{\mathrm{o}}_2\mathrm{MnGa}$ materials.

We study the quasiparticle current in clean ferromagnetic Josephson structures of the form $S_1/F_1/N/F_2/S_2$, where $S,F$, and $N$ denote superconducting, ferromagnetic, or normal layers, respectively. Our focus is on the structure of the conductance $G$ as a function of bias $V$, emphasizing the subgap region. We use a fully self-consistent numerical method, coupled to a transfer matrix procedure to extract $G\left(V\right)$. We choose material parameters appropriate to experimentally realized Co Cu Nb structures. We find a resonance peak structure as a function of the intermediate layer thickness and of the misalignment angle $\varphi$ between $F$ layers. To understand this resonance structure, we develop an approximate analytic method. For experimentally relevant thicknesses, the conductance has multiple subgap peaks, which oscillate in position between low and critical bias positions. These oscillations occur in both $\varphi$ and the layer thicknesses. We compare our results with those obtained for the spin valve structures $(F_1/N/F_2/S_2)$ and discuss the implications of our results for the fabrication of spin Josephson devices.

Non-Hermitian Hamiltonians are generally sensitive to boundary conditions, and their spectra and wave functions under open boundary conditions are not necessarily predicted by the Bloch band theory for periodic boundary conditions. To elucidate such a non-Bloch feature, recent works have developed a non-Bloch band theory that works even under arbitrary boundary conditions. Here, it is demonstrated that the standard non-Bloch band theory breaks down in the symplectic class, in which non-Hermitian Hamiltonians exhibit Kramers degeneracy because of reciprocity. Instead, a modified non-Bloch band theory for the symplectic class is developed in a general manner, as well as illustrative examples. This nonstandard non-Bloch band theory underlies the ${\mathbb{Z}}_2$ non-Hermitian skin effect protected by reciprocity.