The description of the dynamics of strongly correlated quantum matter is a challenge, particularly in physical situations where a quasiparticle description is absent. In such situations, however, the many-body Kubo formula from linear response theory, involving matrix elements of the current operator computed with many-body wave functions, remains valid. Working directly in the many-body Hilbert space and not making any reference to quasiparticles (or lack thereof), we address the puzzle of linear in temperature ($T$-linear) resistivity seen in non-Fermi-liquid phases that occur in several strongly correlated condensed matter systems. We derive a simple criterion for the occurrence of $T$-linear resistivity based on an analysis of the contributions to the many-body Kubo formula, determined by an energy invariant “$f$ function” involving current matrix elements and energy eigenvalues that describes the dc conductivity of the system in the microcanonical ensemble. Using full diagonalization, we test this criterion for the $f$ function in the spinless nearest-neighbor Hubbard model and in a system of Sachdev-Ye-Kitaev dots coupled by weak single-particle hopping. We also study the $f$ function for the spin conductivity in the two-dimensional Heisenberg model and arrive at similar conclusions. Our work suggests that a general principle, formulated in terms of many-body Hilbert space concepts, is at the core of the occurrence of $T$-linear resistivity in a wide range of systems, and precisely translates $T$-linear resistivity into a notion of energy scale invariance far beyond what is typically associated with quantum critical points.

We have observed short-lived optical birefringence in a ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ single crystal induced by a powerful nearly single-cycle terahertz pulse. Apart from the instantaneous contribution that follows the profile of the squared pump electric field, the recorded anisotropy signal contains an exponential component decaying in $\sim 350\mathrm{fs}$, underdamped oscillations at the frequency of $\sim 0.16\mathrm{THz}$ and an intermediate picosecond relaxation process with a Gaussian tail. We associate these three nontrivial features with, respectively, Kerr effect in the inorganic lattice, terahertz-induced transient alignment of ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$ cations, and their coherent rotation excited by the terahertz pulse in a Raman process.

A ferromagnetic insulator (FI) in contact with a superconductor (S) is known to induce a spin splitting of the BCS density of states at the FI/S interface. This spin splitting causes the Cooper pairs to reduce their singlet-state correlations and acquire odd-in-frequency triplet correlations. We consider a diffusive FI/S bilayer with a sharp magnetic domain wall in the FI, and we study the local quasiparticle density of states and triplet superconducting correlations. In the case of collinear alignment of the domains, we obtain analytical results by solving the Usadel equation. For a small enough exchange field or weak superconductivity, we also find an analytical expressions for arbitrary magnetic textures, which reveals how the triplet component vector depends on the local magnetization of the FI. For an arbitrary angle between the magnetizations and the strength of the exchange field, we numerically solve the problem of a sharp domain wall. We finally propose two different setups based on FI/S/F stacks, where F is a ferromagnetic layer, to filter out singlet pairs and detect the presence of triplet correlations via tunneling differential conductance measurements.

In this article we employ density functional theory in the generalized gradient approximation to investigate the structural and electronic properties of the solid solution alloy $\text{Zn}{\text{Se}}_x{\text{S}}_{1-x}$ in the wurtzite structure. We analyzed the character of the bond lengths and angles at the atomic scale, using a supercell approach that does not impose any constraint on the crystal potential. We show that the bond lengths of pristine ZnS and ZnSe compounds are almost preserved between nearest neighbors, which is different from what would be anticipated if Vegard's law were valid at the atomic level. We also show that bond lengths start behaving in accordance with Vegard's law from the third shell of nearest neighbors onward, which in turn determines the average lattice parameters of the alloys determined by diffraction experiments. Fundamental building blocks around the anions are identified and are shown to be nonrigid but still volume preserving. Finally, the geometrical analysis is connected to the trend exhibited by the electronic structure, and in particular by the band gap. The latter is found to exhibit a small deviation from linearity with respect to the Se concentration, in accordance with available experimental data. By assuming a quadratic dependence, we can extract a bowing parameter and analyze various contributions to it with various calculations under selected constraints. The structural deformation in response to the doping process is shown to be the driving force behind the deviation from linearity. The difference in stiffness between ZnS and ZnSe is shown to play a key role in the asymmetric behavior of the bowing parameter observed in the S-rich and Se-rich regions.

We present the influences of electronic and magnetic correlations and doping on the ground-state properties of the recently discovered superconductor ${\mathrm{Ba}}_2{\mathrm{CuO}}_{4-\delta }$ by utilizing the rotationally invariant slave-boson method. Starting with an effective two-orbital Hubbard model [Scalapino et al., Phys. Rev. B 99, 224515 (2019)], we demonstrate that with increasing doping concentration, the paramagnetic (PM) system evolves from a two-band character to single band around the electron filling $n=2.5$, with the band nature of the $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ orbitals to the $d_{3z^2-r^2}$ orbital, slightly affected when the electronic correlation $U$ varies from 2 to 4 eV. Considering the magnetic correlations, the system displays one antiferromagnetic (AFM) metallic phase in $2<n<2.14$ and a PM phase in $n>2.14$ at $U=2\mathrm{eV}$, or two AFM metallic phases in $2<n<2.52$ and $2.74<n<3$, and a PM phase in $2.52<n<2.74$, respectively, at $U=4\mathrm{eV}$. Our results show that near the realistic superconducting state around $n=2.6$ the intermediate correlated ${\mathrm{Ba}}_2{\mathrm{CuO}}_{3,2}$ should be of single-band character. The $s$-wave superconducting pairing strength becomes significant when $U>2\mathrm{eV}$, and crosses over to $d$ wave when $U>2.2\mathrm{eV}$.

We extend finite temperature tensor network methods to compute Matsubara imaginary time correlation functions, building on the minimally entangled typical thermal states (METTS) and purification algorithms. While imaginary time correlation functions are straightforward to formulate with these methods, care is needed to avoid convergence issues that would result from naive estimators. As a benchmark, we study the single-band Anderson impurity model, even though the algorithm is quite general and applies to lattice models. The special structure of the impurity model benchmark system and our choice of basis enable techniques such as the reuse of high-probability METTS for increasing algorithm efficiency. The results are competitive with state-of-the-art continuous-time Monte Carlo. We discuss the behavior of computation time and error as a function of the number of purified sites in the Hamiltonian.

Kondo lattice models have established themselves as an ideal platform for studying the interplay between topology and strong correlations such as in topological Kondo insulators or Weyl-Kondo semimetals. The nature of these systems requires the use of nonperturbative techniques, which are few in number, especially in high dimensions. Motivated by this we study a model of Dirac fermions in $(3+1)$ dimensions coupled to an arbitrary array of spins via a generalization of functional non-Abelian bosonization. We show that there exists an exact transformation of the fermions, which allows us to write the system as decoupled free fermions and interacting spins. This decoupling transformation consists of a local chiral, Weyl, and Lorentz transformation parameterized by solutions to a set of nonlinear differential equations, which order by order takes the form of Maxwell's equations with the spins acting as sources. Owing to its chiral and Weyl components this transformation is anomalous and generates a contribution to the action. From this we obtain the effective action for the spins and expressions for the anomalous transport in the system. In the former we find that the coupling to the fermions generates kinetic terms for the spins, a long-ranged interaction, and a Wess-Zumino-like term. In the latter we find generalizations of the chiral magnetic and Hall effects.

Combined measurements including Raman scattering and optical pump-probe spectroscopy (OPPS) are carried out to investigate the lattice and electron dynamics of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ under hydrostatic pressures. The pressure evolutions of nonequilibrium photocarrier dynamics, containing the hot electron relaxations and coherent acoustic phonons, have been accessed up to 30 GPa. With increasing pressure, the Raman vibrations and electron relaxations manifest anomalies around 3 GPa and 5 GPa, indicating the sudden changes of electron-phonon couplings across the electronic topological transition (ETT) and semiconductor-semimetal transition (SST). Especially, the OPPS reveals that the hot phonon bottleneck is effectively suppressed along with the onset of ETT, which is likely due to the abrupt increase in the density of state and the number of energy valley in conduction band, basing on the calculated electronic and lattice structures. Furthermore, a comprehensive phase diagram of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ composed with ETT, SST, as well as the additional three high-pressure structures and a mixture phase has been distinguished according to the pressure behavior of OPPS. Our paper not only develop understandings for the interactions between electron and lattice in ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, but also may provide a strategy to probe the electronic topological changes under pressure.

Spin-to-charge interconversion has been extensively investigated to improve its efficiency of charge-to-spin conversion, realizing magnetic switching by the technique of low-power spin-orbit torque (SOT). The quasi-two-dimensional electron gas (q2DEG) formed at the interface between insulating ${\mathrm{SrTiO}}_3$ (STO) and ${\mathrm{LaTiO}}_{3+\delta }$ (LTO) is supposed to possess strong Rashba spin-orbit coupling. Here, we show that a charge current flowing along the LTO/STO interface, which hosts a q2DEG, can efficiently generate a spin current, producing a strong SOT on adjacent NiFe layer as confirmed by spin-torque ferromagnetic resonance. The maximal charge-to-spin conversion efficiency is as large as $\sim 2.4$ at room temperature, appearing in the NiFe/LTO/STO structure with a LTO thickness of 4 nm. This efficiency is much larger than that of heavy meals. With the decrease of temperature, the conversion efficiency only exhibits a slight decrease, remaining $\sim 1.5$ at 20 K. The present work reveals the great potential of conducting oxide interface for oxide-based spintronics.

Angular momentum conversion between mechanical rotation and the valley degree of freedom in 2D Dirac materials is investigated theoretically. Coupling between the valley and vorticity of dynamic lattice distortions is derived by applying the $k·p$ method to 2D Dirac materials with an inertial effect. Lattice strain effects are also incorporated. Valley transfer and valley-dependent carrier localization are predicted using the dynamic lattice distortions. The transport properties are found to be controllable, allowing the system to be insulating and to generate pulsed charge current. Our formalism offers a route toward mechanical manipulation of valley dynamics in 2D Dirac materials.

Controlling magnetism and electronic properties of two-dimensional (2D) materials by purely electrical means is crucial and highly sought for high-efficiency spintronics devices since electric field can be easily applied locally compared with magnetic field. The recently discovered 2D Janus crystals provide a platform for nanoscale electronics and spintronics due to their broken inversion symmetry. Here, through comprehensive density functional theory calculations and Monte Carlo simulations, we demonstrate that both the electronic and magnetic properties of Janus monolayer MnSSe, a 2D ferromagnetic half-metal with high Curie temperature, are electrically tunable. The exchange coupling can be significantly enhanced or quenched by hole and electron doping, respectively. In particular, with a small amount of hole doping, MnSSe can tune its magnetization easy axis in between out-of-plane and in-plane directions, which is conducive to designing a 2D spin field effect transistor for spin-dependent transport. We also find a reversible longitudinal intralayer charge transfer between S and Se layers that is highly sensitive to the external electric field. Interestingly, the directions of charge flow and the applied field are the same. This behavior originates from the coexistence and/or the competition of external and built-in electric fields. These findings, together with the excellent stability and large in-plane stiffness, can greatly facilitate the development of nanoscale electronics and spintronics devices based on 2D MnSSe crystal.

We demonstrated on Ag(111) and Ag(100) surfaces that the reciprocal of the field emission resonance (FER) linewidth, which is proportional to the mean lifetime of resonant electrons in FER, may vary with the electric field. The variation on Ag(111) was nearly smooth, whereas that on Ag(100) was sporadic and fluctuated remarkably. This drastic difference can be explained through their dissimilar projected bulk band structures and the ensemble interpretation of quantum mechanics, according to which all resonant electrons are governed by a single wave function (WF). Ag(100) has an energy gap above its vacuum level, whereas Ag(111) does not. Consequently, the dissipation rate of the WF, which is relevant to the FER linewidth, on Ag(111) was almost stable, whereas that on Ag(100) fluctuated. The fluctuation originated from the quantum trapping effect and surface dipole layer (SDL) on Ag(100) surface. Through FER linewidth, we also demonstrated that SDLs of Cu(100) and Ag(100) are different.

Undistorted monolayer graphene has energy bands which cross at protected Dirac points. It elastically deforms, and much research has assumed the Dirac description persists, now in a curved space and coupled to a gauge field related to lattice strain. We show this is incorrect by using a real space gradient expansion to study how the Dirac equation derives from the tight-binding model. Generic spatially varying hopping functions give rise to large magnetic fields which spoil the truncation in derivatives. In the perturbative regime, the only consistent truncation to Dirac is one with nontrivial gauge field but in flat space. One can instead fine-tune the magnetic field to be small, and we derive the resulting differential condition that the hopping functions must satisfy to yield a consistent truncation to Dirac in curved space. We consider whether mechanical effects might impose this fine tuning but find this is not the case for a simple elastic membrane model.

We studied the band structure of $\mathrm{Nd}\mathrm{Fe}{\mathrm{As}}_{1-x}{\mathrm{P}}_x{\mathrm{O}}_{0.9}{\mathrm{F}}_{0.1}$ ($x=0$, 0.2, 0.4, and 0.6) using angle-resolved photoemission spectroscopy (ARPES) measurements. Two of the hole bands, ${\alpha }_1$ $(d_{xz})$ and ${\alpha }_3$ $(d_{z^2})$, were observed at the Brillouin zone center in the $P$-polarized light configuration, while the other two hole bands, ${\alpha }_2$ $\left(d_{yz}\right)$ and γ $(d_{xy})$, were observed in the $S$-polarized alternative. The observed γ band shifts downward as $x$ increases, which is consistent with the theoretical prediction for the change in bond angle of As/P-Fe-As/P. Furthermore, a small amount of the $d_{xy}$ orbital component was observed at the same binding energy as that of the top of the ${\alpha }_1$ band, thus indicating the band reconstruction of the originally degenerate ${\alpha }_1$ and ${\alpha }_2$ $(d_{xz}/d_{yz})$ bands by the unoccupied $d_{xy}$ band. The change in the energy level of the ${\alpha }_1$ band top with $d_{xy}$ orbital character is accompanied by a $T_{\mathrm{c}}$ upturn at $0.2<x<0.4$. The $T_{\mathrm{c}}$ continues to increase as the ${\alpha }_1$ band shifts downward, crossing the Fermi level. The incipient band with the $d_{xy}$ orbital character on its top could be an important ingredient for high $T_{\mathrm{c}}$ 1111-type iron-based superconductors.

The record-breaking thermoelectric performance of tin selenide (SnSe) has motivated the investigation of analog compounds with the same structure. A promising candidate that emerged recently is germanium selenide (GeSe). Here, using extensive first-principles calculations of the hole-phonon and hole-impurity scattering, we investigate the thermoelectric transport properties of the orthorhombic phase of $p$-doped GeSe. We predict outstanding thermoelectric performance for GeSe over a broad range of temperatures due to its high Seebeck coefficients, extremely low Lorenz numbers, ultralow total thermal conductivity, and relatively large band gap. In particular, the out-of-plane direction in GeSe presents equivalent or even higher performance than SnSe for temperatures above 500 K. By extending the analysis to 900 K, we obtained an ultrahigh value for the thermoelectric figure of merit ($zT=$ 3.2) at the optimal hole density of $4\times {10}^{19}{\mathrm{cm}}^{-3}$. Our work provides strong motivation for continued experimental work focusing on improving the GeSe doping efficiency in order to achieve this optimal hole density.

We investigate the interplay of two highly localized, nearly degenerate electronic states, namely, a zero-energy edge mode in a graphene nanoribbon on the one hand and an Abrikosov-Suhl resonance located at a Kondo impurity on the other. On-surface synthesis of the ribbon structures in combination with intercalation of single-atom Kondo impurities by atomic manipulation in a scanning tunneling microscope junction offer full control of the atomic geometry of the system. Density functional theory provides the microscopic description to scrutinize the electronic features observed in experiment. We find the interaction of the two localized states and the resulting signatures of Kondo physics to be very sensitive to the placing of the atom, suggesting its use as a laboratory to study the interplay of the Kondo effect with other zero-bias anomalies as well as to tailor these states by controlling the atomic-scale coupling.

Calculations of the diffusion of a Au adatom on the dimer reconstructed Si(100)-$2\times 1$ surface reveal an interesting mechanism that differs significantly from a direct path between optimal binding sites, which are located in between dimer rows. Instead, the active diffusion mechanism involves promotion of the adatom to higher-energy sites on top of a dimer row and then fast migration along the row, visiting ca. a hundred sites at room temperature, before falling back down into an optimal binding site. This top-of-row mechanism becomes more important the lower is the temperature. The calculations are carried out by finding minimum energy paths on the energy surface obtained from density functional theory within the PBEsol functional approximation followed by kinetic Monte Carlo simulations of the diffusion over a range of temperature from 200 to 900 K. While the activation energy for the direct diffusion mechanism, both parallel and perpendicular to the dimer rows, is calculated to be 0.84 eV, the effective activation energy for the indirect mechanism parallel to the rows is on average 0.56 eV.

We present a unified theory of quantum phase transitions for half-filled quantum dots (QDs) coupled to gapped host bands. We augment the bands by an additional weakly coupled metallic lead, which allows us to analyze the system by using standard numerical renormalization-group techniques. The ground-state properties of the systems without the additional metallic lead are then extrapolated in a controlled way from the broadened subgap spectral functions. We show that a broad class of narrow-gap-semiconductor tunneling densities of states (TDOSs) support the existence of two distinct phases known from their superconducting counterpart: the 0 phase, which is marked by the singlet ground state, and the $\pi$ phase regime with the doublet ground state. To keep a close analogy with the superconducting case, we focus on the influence of particle-hole asymmetry of the TDOS on the subgap spectral features. Nevertheless, we also discuss the possibility of inducing singlet-doublet quantum phase transitions in experimental setups by varying the filling of the QD. In addition, for gapped TDOS functions with smoothed gap edges, we demonstrate that all subgap peaks may leak out of the gap into the continuous part of the spectrum, an effect that has no counterpart in the superconducting Anderson model.

We consider the effects of strong dissipation in quantum systems with a notion of locality, which induces a hierarchy of many-body relaxation timescales as shown by K. Wang, F. Piazza, and D. J. Luitz [Phys. Rev. Lett. 124, 100604 (2020)]. If the strength of the dissipation varies strongly in the system, additional separations of timescales can emerge, inducing a manifold of metastable states, to which observables relax first, before relaxing to the steady state. Our simple model, involving one or two “good” qubits with dissipation reduced by a factor $\alpha <1$ compared to the other “bad” qubits, confirms this picture and admits a perturbative treatment.