In the absence of any symmetry constraints we address universal properties of the boundary charge $Q_B$ for a wide class of nearest-neighbor tight-binding models in one dimension with one orbital per site but generic modulations of on-site potentials and hoppings. We provide a precise formulation of the bulk-boundary correspondence relating the boundary charge of a single band uniquely to the Zak phase evaluated in a particular gauge. We reveal the topological nature of $Q_B$ by proving the quantization of a topological index $eI=\mathrm{\Delta }{Q}_B-\overline{\rho }$, where $\mathrm{\Delta }{Q}_B$ is the change of $Q_B$ when shifting the lattice by one site towards a boundary and $\overline{\rho }$ is the average charge per site. For a single band we find this index to be given by the winding number of the fundamental phase difference of the Bloch wave function between the two lattice sites defining the boundary of a half-infinite system. For a given chemical potential we establish a central topological constraint $I\in \{-1,0\}$ related only to charge conservation of particles and holes. Our results are shown to be stable against disorder and we propose generalizations to multichannel and interacting systems.

We investigate the strong-field dynamics of a paramagnetic two-band Mott insulator using real-time dynamical mean-field theory. We demonstrate that strong electric fields can lead to a transient localization of electrons. This nonequilibrium quantum effect allows us to reveal specific signatures of local correlations in the time-resolved photoemission spectrum. In particular, we demonstrate that the localization can be strong enough to produce atomiclike spin multiplets determined by the Hund's coupling $J$, and thus provide a way of measuring $J$ inside the solid. Our simulation also fully incorporates nonlinear field-induced tunneling processes, which would lead to a dielectric breakdown in the steady state limit. A careful analysis of these processes, however, shows that they remain weak enough and do not prevent the measurement of the transiently localized spectra.

${\mathrm{CeRhIn}}_5$ is a Kondo-lattice prototype in which a magnetic field $B^{*}\simeq 31\mathrm{T}$ induces an abrupt Fermi-surface (FS) reconstruction and pronounced in-plane electrical transport anisotropy all within its antiferromagnetic state. Though the antiferromagnetic order at zero field is well understood, the origin of an emergent state at $B^{*}$ remains unknown due to challenges inherent to probing states microscopically at high fields. Here we report low-temperature nuclear magnetic resonance (NMR) measurements revealing a pronounced decrease in the ${}^{115}\mathrm{In}$ formal Knight shift, without changes in crystal or magnetic structures, of ${\mathrm{CeRhIn}}_5$ at fields ($\parallel c$) spanning $B^{*}$. We discuss the emergent state above $B^{*}$ in terms of a change in Ce's $4f$ orbitals that arises from field-induced evolution of crystal-electric field (CEF) energy levels. This change in orbital character enhances hybridization between the $4f$ and the conduction electrons that leads ultimately to an itinerant quantum-critical point at $B_{c0}\simeq 50\mathrm{T}$.

For a wide class of noninteracting tight-binding models in one dimension, we present an analytical solution for all scattering and edge states on a half-infinite system. Without assuming any symmetry constraints, we consider models with nearest-neighbor hoppings and one orbital per site but the arbitrary size of the unit cell and generic modulations of on-site potentials and hoppings. The solutions are parametrized by determinants that can be straightforwardly calculated from recursion relations. We show that this representation allows for an elegant analytic continuation to complex quasimomentum consistent with previous treatments for continuum models. Two important analytical results are obtained based on the explicit knowledge of all eigenstates. (1) An explicit proof of the surface charge theorem is presented including a unique relationship between the boundary charge $Q_B^{\left(\alpha \right)}$ of a single band $\alpha$ and the bulk polarization in terms of the Zak-Berry phase. In particular, the Zak-Berry phase is determined within a special gauge of the Bloch states such that no unknown integer is left. This establishes a precise form of a bulk-boundary correspondence relating the boundary charge of a single band to bulk properties. (2) We derive a topological constraint for the phase dependence of the edge state energies, where the phase variable describes a continuous shift of the lattice towards the boundary. The topological constraint is shown to be equivalent to the quantization of a topological index $I=\mathrm{\Delta }{Q}_B-\overline{\rho }\in \{-1,0\}$ introduced in an accompanying paper [M. Pletyukhov et al., Phys. Rev. B 101, 161106 (2020)]. Here, $\mathrm{\Delta }{Q}_B$ is the change of the boundary charge $Q_B$ for a given chemical potential in the insulating regime when the lattice is shifted by one site towards the boundary, and $\overline{\rho }$ is the average charge per site (both in units of the elementary charge $e=1$). This establishes an interesting link between universal properties of the boundary charge and edge state physics discussed within the field of topological insulators. In accordance with previous results for continuum systems, we also establish the localization of the boundary charge and determine the explicit form of the density given by an exponential decay and a pre-exponential function following a power law with generic exponent $-1/2$ at large distances.

The MOF $\left[{\text{Fe(tvp)}}_2{\left(\text{NCS}\right)}_2\right]·2\text{BzCHO}$ is demonstrated to undergo a complex sequence of phase transitions and spin-crossover behavior of its constitutive ${\text{Fe}}^{\text{II}}$ ions upon adsorption of benzaldehyde guest molecules. Our study, combining Raman and synchrotron x-ray diffraction measurements on a single crystal, reveals that the conversion from the pure high-spin to the pure low-spin phases implies a rich sequence of intermediate phases, with symmetry breaking forming at least three different space groups. These different symmetries involve spin-state ordering, ligand ordering, and guest ordering, interpreted by combining magnetic susceptibility, Raman measurements, density functional theory, and force field molecular dynamics calculations.

While exploring potential superconductors in the C-S-H ternary system using first-principles crystal structure prediction methods, we uncovered a class of hydride perovskites based on the intercalation of methane into an ${\mathrm{H}}_3\mathrm{S}$ framework. These intriguing ${\mathrm{H}}_3\mathrm{S}\text{−}{\mathrm{CH}}_4$ structures emerge as metastable at $\sim 100$ GPa. Electron-phonon coupling calculations indicate that phases with ${\mathrm{CSH}}_7$ stoichiometry are potential superconductors with $T_c$ values ranging from 100 K to 190 K at megabar pressures. The results are expected to guide the experimental search for new high-$T_c$ superconductors, including those stable at lower pressures than previously documented superconducting hydrides such as ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$.

We show how an accurate first-principles treatment of the canted-anti-ferromagnetic ground state of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, a prototypical $5d$ correlated spin-orbit coupled material, can be obtained without invoking any free parameters, such as the Hubbard $U$ or tuning the spin-orbit coupling strength. Our theoretically predicted iridium magnetic moment of $0.250{\mu }_B$, canted by $12.6^{\circ }$ off the $a$ axis, is in accord with experimental results. By resolving the magnetic moments into their spin and orbital components, we show that our theoretically obtained variation of the magnetic scattering amplitude $\langle M_m\rangle$ as a function of the polarization angle is consistent with recent nonresonant magnetic x-ray scattering measurements. The computed value of the band gap (55 meV) is also in line with the corresponding experimental values. A comparison of the band structure to that of the cuprates suggests the presence of incommensurate charge-density wave phases in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

The spin splitting driven by spin-orbit coupling in the monolayer (ML) transition metal dichalcogenide (TMDC) family has been widely studied only for the $1H$-phase structure, while its $1T$-phase structure has not been as widely researched due to the centrosymmetry of the crystal. Based on first-principles calculations, we show that significant spin splitting can be induced in the ML $1T$-TMDCs by introducing the line defect. Taking the ML ${\mathrm{PtSe}}_2$ as a representative example, we considered the stablest form of the line defects, namely, the Se vacancy line defect (Se-VLD). We find that large spin splitting is observed in the defect states of the Se-VLD, exhibiting a highly unidirectional spin configuration in the momentum space. This peculiar spin configuration may yield the so-called persistent spin textures (PSTs), a specific spin structure resulting in protection against spin decoherence and supporting an extraordinarily long spin lifetime. Moreover, by using $\vec{k}·\vec{p}$ perturbation theory supplemented with symmetry analysis, we clarified that the emerging of the spin splitting maintaining the PSTs in the defect states originates from the inversion symmetry breaking together with the one-dimensional nature of the Se-VLD engineered ML ${\mathrm{PtSe}}_2$. Our findings pave a possible way to induce significant spin splitting in the ML $1T$-TMDCs, which could be highly important for designing spintronic devices.

One obstacle that has slowed the development of electrically gated metal-oxide-semiconductor (MOS) singlet-triplet qubits is the frequent lack of observed spin blockade, even in samples with large singlet-triplet energy splittings. We present theoretical and experimental evidence that this problem in MOS double quantum dots can be caused by stray positive charges in the oxide, inducing accidental localized levels near the device's active region that lead to the lifting of the spin blockade. We also present evidence that these effects can be mitigated by device design modifications, such as overlapping gates.

Liu et al. [Phys. Rev. B 98, 241109 (2018)] used Monte Carlo sampling of the physical degrees of freedom of a projected entangled pair state type wave function for the $S=1/2$ frustrated $J_1-J_2$ Heisenberg model on the square lattice and found a nonmagnetic state argued to be a gapless spin liquid when the coupling ratio $g=J_2/J_1$ is in the range $g\in [0.42,0.6]$. Here we show that their definition of the order parameter for another candidate ground state within this coupling window—a spontaneously dimerized state—is problematic. The order parameter as defined will not detect dimer order when lattice symmeties are broken due to open boundaries or asymmetries originating from the calculation itself. Thus, a dimerized phase for some range of $g$ cannot be excluded (and is likely based on several other recent works).

The origin of the $3\times 1$ reconstruction observed in epitaxial ${\mathrm{LaCoO}}_3$ films on ${\mathrm{SrTiO}}_3\left(001\right)$ is assessed by first-principles calculations including a Coulomb repulsion term. We compile a phase diagram as a function of the oxygen pressure, which shows that ($3\times 1$)-ordered oxygen vacancies (${\mathrm{LaCoO}}_{2.67}$) are favored under commonly used growth conditions, while stoichiometric films emerge under oxygen-rich conditions. Growth of further reduced ${\mathrm{LaCoO}}_{2.5}$ brownmillerite films is impeded by phase separation. We report two competing ground-state candidates for stoichiometric films: a semimetallic phase with $3\times 1$ low-spin/intermediate-spin/intermediate-spin (LS/IS/IS) magnetic order and a semiconducting phase with IS/IS/IS magnetic order. This demonstrates that tensile strain induces ferromagnetism even in the absence of oxygen vacancies. Both phases exhibit an intriguing ($3\times 1$)-reconstructed octahedral rotation pattern and accordingly modulated La-La distances. In particular, charge and bond disproportionation and concomitant orbital order of the $t_{2g}$ hole emerge at the Co sites that are also observed for unstrained bulk ${\mathrm{LaCoO}}_3$ in the IS state and explain structural data obtained by x-ray diffraction at elevated temperature. Site disproportionation drives a metal-to-semiconductor transition that reconciles the IS state with the experimentally observed low conductivity during spin-state crossover without the presence of Jahn-Teller distortions.

In itinerant systems, electron-electron interactions may lead to the formation of local magnetic moments and their effective exchange coupling, which in turn gives rise to long-range magnetic order. Therefore, when moment formation is weakened, such as in the single-band Hubbard model on a square lattice with the on-site repulsion being randomly switched off on a fraction $x$ of sites, magnetic order is suppressed beyond some critical $x_c$, which was found to lie below the classical percolation threshold $x_c^{\text{(perc,sq)}}$. Here we study dilute magnetism in flat band systems, namely, in the Hubbard model on a “Lieb” lattice. Interestingly, we show that magnetic order persists to $x$ almost twice as large as the classical percolation threshold for the lattice, thus emphasizing the central role of electron itinerancy to the magnetic response. The analysis of the orbital-resolved order parameters reveals that the contribution of the fourfold coordinated “$d$” sites to magnetism is dramatically affected by dilution, while the localized “$p$” states of the flat band provide the dominant contribution to long-range correlations. We also examine the transport properties, which suggest the existence of an insulator-to-metal transition in the same range of the critical magnetic dilution.

The effective interaction between electrons in two-dimensional materials can be modified by their environment, enabling control of electronic correlations and phases. Here, we study the dependence of electronic correlations in twisted bilayer graphene (tBLG) on the separation to the metallic gate(s) in two device configurations. Using an atomistic tight-binding model, we determine the Hubbard parameters of the flat bands as a function of gate separation, taking into account the screening from the metallic gate(s), the dielectric spacer layers, and the tBLG itself. We determine the critical gate separation at which the Hubbard parameters become smaller than the critical value required for a transition from a correlated insulator state to a (semi)metallic phase. We show how this critical gate separation depends on twist angle, doping, and the device configuration. These calculations may help rationalize the reported differences between recent measurements of tBLG's phase diagram and suggest that correlated insulator states can be screened out in devices with thin dielectric layers.

We detected and analyzed a spin-dependent intensity asymmetry for spin-degenerate surface states at the $\overline{\mathrm{M}}$ point in the unoccupied electronic structure of Tl/Ge(111). Approximating the initial state by a plane wave, we calculated the inverse-photoemission process and obtained good agreement with the experimental data. Our model reveals that this spin asymmetry at a point of time-reversal invariant momentum (TRIM) is of different origin than other effects discussed in the literature, which are based on the light detection geometry, photon energy, and experimental probing depth of the electrons. Instead, the spin asymmetry in this nonmagnetic material with spin-orbit interaction is caused by breaking the initial-state time-reversal symmetry in the experiment. The effect is of general nature and can occur in spin- and angle-resolved inverse photoemission as well as photoemission at TRIM points.

By using density functional theory (DFT) calculations, we predict a puckered, dynamically stable Janus single-layer black arsenic-phosphorus (b-AsP), which is composed of two different atomic sublayers, arsenic and phosphorus atoms. The calculated phonon spectrum reveals that Janus single-layer b-AsP is dynamically stable with either pure or coupled optical phonon branches arising from As and P atoms. The calculated Raman spectrum indicates that due to the relatively strong P-P bonds, As atoms have no contribution to the high-frequency optical vibrations. In addition, the orientation-dependent isovolume heat capacity reveals anisotropic contributions of LA and TA phonon branches to the low-temperature thermal properties. Unlike pristine single layers of b-As and b-P, Janus single-layer b-AsP exhibits additional out-of-plane asymmetry which leads to important consequences for its electronic, optical, and elastic properties. In contrast to single-layer b-As, Janus single-layer b-AsP is found to possess a direct band gap dominated by the P atoms. Moreover, real and imaginary parts of the dynamical dielectric function, including excitonic effects, reveal the highly anisotropic optical feature of the Janus single-layer. A tight-binding (TB) model is also presented for Janus single-layer b-AsP, and it is shown that, with up to seven nearest hoppings, the TB model reproduces well the DFT band structure in the low-energy region around the band gap. This TB model can be used in combination with the Green's function approach to study, e.g., quantum transport in finite systems based on Janus single-layer b-AsP. Furthermore, the linear-elastic properties of Janus single-layer b-AsP are investigated, and the orientation-dependent in-plane stiffness and Poisson ratio are calculated. It is found that the Janus single layer exhibits strong in-plane anisotropy in its Poisson ratio much larger than that of single-layer b-P. This Janus single layer is relevant for promising applications in optical dichroism and anisotropic nanoelasticity.

Ab initio calculations of the spin-wave stiffness constant $D$ for elemental Fe and Ni performed by different groups in the past have led to values with a considerable spread of 50–100%. We present results for the stiffness constant $D$ of Fe, Ni, and permalloy ${\mathrm{Fe}}_{0.19}{\mathrm{Ni}}_{0.81}$ obtained by three different approaches: (i) by finding the quadratic term coefficient of the power expansion of the spin-wave energy dispersion, (ii) by a damped real-space summation of weighted exchange coupling constants, and (iii) by integrating the appropriate expression in reciprocal space. All approaches are implemented by means of the same Korringa-Kohn-Rostoker (KKR) Green's function formalism. We demonstrate that if properly converged, all procedures yield comparable values, with uncertainties of 5–10% remaining. By a careful analysis of the influence of various technical parameters, we estimate the margin of errors for the stiffness constants evaluated by different approaches and suggest procedures to minimize the risk of getting incorrect results.

We present an ab initio theory of the spin-wave stiffness tensor for ordered and disordered itinerant ferromagnets with pair exchange interactions derived from a method of infinitesimal spin rotations. The resulting formula bears an explicit form of a linear-response coefficient which involves one-particle Green's functions and effective velocity operators encountered in a recent theory of electron transport. Application of this approach to ideal metal crystals yields more reliable values of the spin stiffness than traditional ill-converging real-space lattice summations. The formalism can also be combined with the coherent potential approximation for an effective-medium treatment of random alloys, which leads naturally to an inclusion of disorder-induced vertex corrections to the spin stiffness. The calculated concentration dependence of the spin-wave stiffness of random fcc Ni-Fe alloys can be ascribed to a variation of the reciprocal value of alloy magnetization. Calculations for random iron-rich bcc Fe-Al alloys reveal that their spin-wave stiffness is strongly reduced owing to the atomic ordering; this effect takes place due to weakly coupled local magnetic moments of Fe atoms surrounded by a reduced number of Fe nearest neighbors.

Tip-induced superconductivity at point contacts on the nonsuperconducting Weyl semimetal TaAs has been reported recently. Meanwhile, both the main tetragonal phase and the intergrown hexagonal phase of TaAs were identified to be topologically nontrivial according to theoretical calculations. Here, we report the observation of two independent local superconducting states induced at the point contacts formed on TaAs, which exhibit distinct temperature dependencies of the critical magnetic field $H_{c2}\left(T\right)$. A universal relationship between $H_{c2}\left(0\right)$, $T_c$, and the Fermi velocities of bulk bands near Weyl points of the two TaAs phases was uncovered, demonstrating the dominant role of the bulk bands in the emergence of superconductivity, rather than the surface states. These results provide a direction to study the mechanism of the tip-induced superconductivity in topological semimetals.

We study the interplay of two interacting discrete time quantum walks in the presence of disorder. Each walk is described by a Floquet unitary map defined on a chain of two-level systems. Strong disorder induces a novel Anderson localization phase with a gapless Floquet spectrum and one unique localization length ${\xi }_1$ for all eigenstates for noninteracting walks. We add a local contact interaction which is parametrized by a phase shift $\gamma$. A wave packet is spreading subdiffusively beyond the bounds set by ${\xi }_1$ and saturates at a new length scale ${\xi }_2\gg {\xi }_1$. In particular we find ${\xi }_2\sim {\xi }_1^{1.2}$ for $\gamma =\pi$. We observe a nontrivial dependence of ${\xi }_2$ on $\gamma$, with a maximum value observed for $\gamma$ values which are shifted away from the expected strongest interaction case $\gamma =\pi$. The novel Anderson localization regime indicates violation of single parameter scaling for both interacting and noninteracting walks.