Laser-pulse-induced ultrafast electronic excitation energy transfer in molecular systems is studied theoretically. As a particular excitation channel, we consider the simultaneous transition of different molecules into their first excited state. Accordingly, a restricted picture of the total nonlinear response of the molecular system on laser pulse excitation is obtained, but complete insight is gained on the contribution of the first excited states. To describe the temporal evolution of this multiexciton state, a direct computation of the related density matrix is circumvented. Instead, we derive equations of motion for expectation values of interest in using the quantum master equation governing the respective density operator. The subsequently generated hierarchy of kinetic equations is truncated following a general scheme. The whole approach is applied to a chain and a disk of para-sexiphenyl molecules. Different signatures of multiexciton effects are discussed.

We study Shubnikov–de Haas oscillations in single crystals of TaAs and find a previously undetected two-dimensional quantum oscillation that does not belong to the bulk Fermi surface. We cannot find an impurity phase consistent with our observations, and extensive diffraction measurements have not shown the presence of known impurity phases. We conjecture that the frequency originates from surface states that are sensitive to surface disorder. One candidate is the interference of coherent quasiparticles traversing two distinct Fermi arcs on the [001] crystallographic surface. The frequency and effective mass quantitatively agree with predictions of density functional theory and previous angle-resolved photoemission spectroscopy measurements of the Fermi arcs.

We investigate the bulk topological proximity effect in multilayer hexagonal lattice systems by which one can introduce topological properties into a system composed of multiple trivial layers by tunnel coupling to a single nontrivial layer described by the Haldane model. In a multilayer system arrangement of different layers can significantly affect its properties. This mechanism is relevant for the bulk topological proximity effect leading to complex behavior like the emergence of dark states. The response of a trivial system to the proximity of a topological insulator appears to be highly nonlocal, in contrast to the proximity effect observed in context of superconductivity. Furthermore, for a wide range of parameters our system is semimetallic with multiple Dirac points emerging in the Brillouin zone.

We study the single-impurity problem in the non-Hermitian lattice described by the nonreciprocal Su-Schrieffer-Heeger model and obtain the phase diagram of localized bound states induced by the impurity. The existence of analytical results permits us to determine the phase boundaries exactly, which separate regions with different numbers of bound states. Particularly, in the limit of the strong impurity potential, we find that the phase boundaries of midgap bound states are identical to the boundaries of topological phase diagrams of the bulk system in the absence of impurities. The existence of a correspondence between midgap impurity modes and bulk phases indicates that we are able to diagnose the bulk phase diagram of the nonreciprocal topological model by its impurity modes.

The local current flow through three small aromatic carbon molecules, namely, benzene, naphthalene, and anthracene, is studied. Applying density functional theory and the nonequilibrium Green's function method for transport, we demonstrate that pronounced current vortices exist at certain electron energies for these molecules. The intensity of these circular currents, which appear not only at the antiresonances of the transmission but also in the vicinity of its maxima, can exceed the total current flowing through the molecular junction and generate considerable magnetic fields. The $\pi$ electron system of the molecular junctions is emulated experimentally by a network of macroscopic microwave resonators. The local current flows in these experiments confirm the existence of current vortices as a robust property of ring structures. The circular currents can be understood in terms of a simple nearest-neighbor tight-binding Hückel model. Current vortices are caused by the interplay of the complex eigenstates of the open system which have energies close to the considered electron energy. Degeneracies, as observed in benzene and anthracene, can thus generate strong circular currents, but also nondegenerate systems like naphthalene exhibit current vortices. Small imperfections and perturbations can couple otherwise uncoupled states and induce circular currents.

We address two long-standing questions regarding the hidden order in ${\mathrm{URu}}_2{\mathrm{Si}}_2$: Is it associated with the hybridization process, and what are the distinct roles played by the localized and itinerant electrons? Our quasiparticle scattering spectroscopy reveals a hybridization gap ubiquitous in the entire phase space spanned by P and Fe substitutions in ${\mathrm{URu}}_2{\mathrm{Si}}_2$, including the no-order and antiferromagnetic regions, with minimal change upon crossing the phase boundary. This indicates its opening is not associated with the ordering, and thus localized electrons must be the major player. Towards a consistent understanding of all the other gaplike behaviors observed only below transition temperatures, we analyze the electrical resistivity using a model in which gapped bosonic excitations are the dominant scattering source. With their stiffness set to follow an unusual temperature dependence (decreasing with decreasing temperature), this model fits all of our resistivity data well including the jump at the transition. Remarkably, the extracted gap increases slowly with increasing Fe content, similarly to the gap detected by inelastic neutron scattering at $Q_1=(1.4,0,0)$, suggesting a common origin. Such a model can also naturally explain the Hall effect temperature dependence without invoking Fermi surface gapping.

The origin of intrinsic quantum criticality in the heavy-fermion superconductor $\beta -{\mathrm{YbAlB}}_4$ has been attributed to strong Yb valence fluctuations and its peculiar crystal structure. Here, we assess these contributions individually by studying the isostructural but fixed-valence compound $\beta -{\mathrm{LuAlB}}_4$. Quantum oscillation measurements and density functional theory calculations reveal a Fermi surface in $\beta -{\mathrm{LuAlB}}_4$ markedly different from that of $\beta -{\mathrm{YbAlB}}_4$, consistent with a “large” Fermi surface in the latter. We also find an unexpected in-plane anisotropy of the electronic structure, in contrast to the isotropic Kondo hybridization in $\beta -{\mathrm{YbAlB}}_4$.

Although hybrid halide perovskites $(\text{MAPb}{X}_3,$ $\mathrm{MA}={\mathrm{CH}}_3{\mathrm{NH}}_3 \mathrm{and} X=\mathrm{I}, \mathrm{Br}, \mathrm{Cl})$ have been ubiquitously explored from the photovoltaic perspective, there are still a few unanswered questions which require a more fundamental understanding. One such unsettled issue is the puzzling behavior of the band gap. Unlike conventional semiconductors, $\text{MAPb}{X}_3$ $(X=\mathrm{I}, \mathrm{Br})$ is found to show a blueshift (increase) in the band gap $\left(E_g\right)$ with increasing temperature $\left(T\right)$, while ${\mathrm{MAPbCl}}_3$ shows an unusual redshift (decrease). In order to understand this, we performed a detailed $T$-dependent study of electronic, optical, and structural properties of $\text{MAPb}{X}_3$ combined with the state-of-the-art first-principles calculations. With increasing $T$, two dominant mechanisms which come into play are lattice dilation and electron-phonon coupling (EPC). The former (latter) is responsible for an increase (decrease) in $E_g$. We found that lattice dilation effect dominates in $\text{MAPb}{X}_3$ $(X=\mathrm{I}, \mathrm{Br})$, causing an enhancement in $E_g$. EPC involves various contributions, of which the interaction of charge carriers with the longitudinal optical phonon mode via Fröhlich interaction is the most dominant one at room temperature. We quantify this contribution using Fröhlich's theory of large polarons and show that the $E_g$ correction due to this effect in ${\mathrm{MAPbCl}}_3$ is almost double as compared to ${\mathrm{MAPbBr}}_3$ and thus explain the reduction in $E_g$ for the former.

We present results of a theoretical study of a prototypical weak ferromagnet ${\mathrm{ZrZn}}_2$. We use the density-functional theory $\left(\mathrm{DFT}\right)+\mathrm{dynamical}$ mean-field theory (DMFT) method to study the electronic and local magnetic properties. The obtained $\mathrm{DFT}+\mathrm{DMFT}$ electronic self-energies are Fermi-liquid-like, indicating a small effective mass enhancement of the Zr $4d$ states $m^{*}/m\sim 1.1–1.3$ accompanied by partly formed local moments within the electronic states of $t_{2g}$ symmetry. The effect of electronic interaction is shown to be essential for determining the correct topology of some of the Fermi surface sheets. To study in detail the pressure dependence of the Curie temperature $T_{\mathrm{C}}$ and corresponding pressure-induced quantum phase transition, we consider an effective single-band model, constructed using the Zr $4d$ contribution to the total density of states. The model is studied within static and dynamic mean-field theory, as well as spin-fermion approach. We show that the spin-fermion approach yields the temperature dependence of susceptibility at ambient pressure and the pressure dependence $T_{\mathrm{C}}\left(p\right)$, including the first-order quantum phase transition at $p\approx 1.7$ GPa, comparable well with the experimental data.

The Hund's coupling in multiorbital Hubbard systems induces spin freezing and associated Hund metal behavior. Using dynamical mean-field theory, we explore the effect of local moment formation, spin, and charge excitations on the entropy and specific heat of the three-orbital model. For fillings $2\lesssim n<3$ and low temperature, we demonstrate a substantial enhancement of the entropy in the spin-frozen metal phase to values comparable to the half-filled Mott insulator. We also discuss the appearance of entropy plateaus and peaks in the specific heat associated with the activation of spin and charge fluctuations at high temperature. The temperature scale for charge excitations is almost independent of filling and given by $\approx 0.2U$, with $U$ the intraorbital repulsion. Local spin excitations become relevant for filling $n>1$ and their characteristic temperature is proportional to the Hund coupling $J$, with a filling-dependent prefactor. The analysis of the specific heat in the atomic limit yields accurate predictions for these features in the strong-coupling regime.

We examine applicability of the valence bond basis correlator product state ansatz, equivalent to the restricted Boltzmann machine quantum artificial neural-network ansatz, and variational Monte Carlo method for direct optimization of excited energy states to study properties of strongly correlated and frustrated quantum systems. The energy eigenstates are found by stochastic minimization of the variational function for the energy eigenstates, which allows direct optimization of particular energy state without knowledge of the lower energy states. This approach combined with numerous tensor network or artificial neural-network ansatz wave functions then allows further insight into quantum phases and phase transitions in various strongly correlated models by considering properties of these systems beyond the ground-state properties. Also, the method is in general applicable to any dimension and has no sign instability. An example that we consider is the square lattice $J_1-J_2$ antiferromagnetic Heisenberg model. The model is one of the most studied models in frustrated quantum magnetism since it is closely related to the disappearance of the antiferromagnetic order in the high-Tc superconducting materials and there is still no agreement about the properties of the system in the highly frustrated regime near $J_2/J_1=0.5$. For the $J_1-J_2$ model, we write the variational ansatz in terms of the two site correlators and in the valence bond basis and calculate the lowest energy eigenstates in the highly frustrated regime near $J_2/J_1=0.5$ where the system has a paramagnetic phase. We find that our results are in good agreement with previously obtained results, which confirms applicability of the method to study frustrated spin systems.

The accurate ab initio simulation of periodic solids with strong correlations is one of the grand challenges of condensed matter. While mature methods exist for weakly correlated solids, the ab initio description of strongly correlated systems is an active field of research. In this work, we show results for the single-particle spectral function of the two correlated $d$-electron solids NiO and MnO from self-energy embedding theory. Unlike earlier work, the theory does not use any adjustable parameters and is fully ab initio, while being able to treat both the strong correlation and the nonlocal screening physics of these materials. We derive the method, discuss aspects of the embedding and choices of physically important orbitals, and compare our results to x-ray and angle-resolved photoemission spectroscopy as well as bremsstrahlung-isochromat spectroscopy.

Vertex functions are a crucial ingredient of several forefront many-body algorithms in condensed matter physics. However, the full treatment of their frequency and momentum dependence severely restricts numerical calculations. A significant advancement requires an efficient treatment of the high-frequency asymptotic behavior of the vertex functions. In this work, we first provide a detailed diagrammatic analysis of the high-frequency structures and their physical interpretation. Based on these insights, we propose a parametrization scheme, which captures the whole high-frequency domain for arbitrary values of the Coulomb interaction and electronic density, and we discuss the details of its algorithmic implementation in many-body solvers based on parquet equations as well as functional renormalization group schemes. Finally, we assess its validity by comparing our results for a single impurity Anderson model with exact diagonalization calculations. The proposed parametrization is pivotal for the algorithmic development of all quantum many-body methods based on vertex functions arising from both local and nonlocal static microscopic interactions as well as effective dynamic interactions which uniformly approach a static value for large frequencies. In this way, our present technique can substantially improve vertex-based diagrammatic approaches including spatial correlations beyond dynamical mean-field theory.

We report an algorithm designed to perform computer-automated tuning of a single quantum dot with a charge sensor. The algorithm performs an adaptive measurement sequence of subsized stability diagrams until the single-electron regime is identified and reached. For each measurement, the signal processing module removes the physical background of the charge sensor to generate a binary image of charge transitions. Then, the image analysis module identifies the position and number of lines using two line detection schemes that are robust to noise and missing data.

We study the constraints on reciprocal and non-reciprocal many-body radiative heat transfer imposed by symmetry and the second law of thermodynamics. We show that the symmetry of such a many-body system in general forms a magnetic group, and the constraints of the magnetic group on the heat transfer can be derived using a generalized reciprocity theorem. We also show that the second law of thermodynamics provides additional constraints in the form of a nodal conservation law of heat flow at equilibrium. As an application, we provide a systematic approach to determine the existence of persistent heat current in arbitrary many-body systems.

In this paper we propose a new perspective to analyze the many-body localization (MBL) transition when recast in terms of a single-particle tight-binding model in the space of many-body configurations. We compute the distribution of tunneling rates between many-body states separated by an extensive number of spin flips at the leading order in perturbation theory starting from the insulator and determine the scaling of their typical amplitude with the number of accessible states in the Hilbert space. By using an analogy with the Rosenzweig-Porter random matrix ensemble, we propose an ergodicity breaking criterion for the MBL transition based on the Fermi golden rule. According to this criterion, in the MBL phase many resonances are formed at large distance from an infinite temperature initial state, but they are not enough for the quantum dynamics to decorrelate from it in a finite time. This implies that, differently from Anderson localized states, in the insulating phase many-body eigenstates are multifractal in the Hilbert space, as they occupy a large but subexponential part of the total volume, in agreement with recent numerical results, perturbative calculations, and intuitive arguments. Possible limitations and implications of our interpretation are discussed in the conclusions.

The recent development in the field of two-dimensional magnetic materials urges reliable theoretical methodology for determination of magnetic properties. Among the available methods, ab initio four-state energy mapping based on density functional theory stands out as a powerful technique to calculate the magnetic exchange interaction in the Heisenberg spin model. Although the required formulas were explained in earlier works, the considered Hamiltonian in those studies always corresponded to the specific case that the off-diagonal part of $J$ matrix is antisymmetric, which may be misleading in other cases. Therefore, using the most general form of the Heisenberg spin Hamiltonian, we here derive the generic formulas. With a proper choice of four different magnetic states, a single formula governs all elements of the exchange interaction matrix for any considered pair of spin sites.

We present a theoretical study on nucleation and propagation of antiferromagnetic skyrmionium induced by spin current injection. A skyrmionium, also known as $2\pi$ skyrmion, is a vortexlike magnetic structure characterized by a topological charge, the so-called skyrmion number, being zero. We find that an antiferromagnetic skyrmionium can be generated via a local injection of spin current with toroidal distribution. We examine the threshold current density for successful skyrmionium nucleation and the robustness of the proposed mechanism against thermal fluctuation. A spatially uniform spin current is then demonstrated to induce propagation of the skyrmionium. We derive an expression for the skyrmionium velocity based on a collective-coordinate model, which agrees well with our numerical results.

A single-phase sample of $\mathrm{Ta}{\mathrm{H}}_{2.2\left(1\right)}$ with a hexagonal close-packed metal lattice (hcp; space group $P{6}_3/mmc$) was synthesized under a hydrogen pressure of 9 GPa and a temperature of 100 °C; quenched to the liquid nitrogen temperature; recovered to ambient pressure and studied by neutron diffraction (ND) and inelastic neutron scattering (INS). The ND study showed that hydrogen atoms occupied one half of the tetrahedral (T) and all octahedral (O) interstitial sites in the hcp lattice of Ta atoms. The arrangement of the H atoms over the T sites was proven to be ordered, which lowered the symmetry of the full crystal structure of the dihydride to $P{6}_{3}mc$. Due to the resulting asymmetry in the local environment of the O sites, the H atoms were considerably displaced from the centers of these sites along the $z$ axis, away from the H atoms occupying the neighboring T sites. The INS study demonstrated that the potential wells for H atoms at both the T and O sites are highly anharmonic and anisotropic. The potential wells at the O sites are softer along the $z$ axis than in the $x, y$ plane, while the T sites show opposite anisotropy.