The photoinduced valence dynamics of ${\mathrm{EuNi}}_2{\left({\mathrm{Si}}_{0.21}{\mathrm{Ge}}_{0.79}\right)}_2$ were investigated using time-resolved x-ray absorption spectroscopy for Eu $M_5$ edge. Through the pump-probe technique with synchrotron x-ray and Ti:sapphire laser pulses, a photoinduced valence transition was observed from ${\mathrm{Eu}}^{3+}$ to ${\mathrm{Eu}}^{2+}$. Because the lifetime of a photoinduced state can be up to 3 ns, a metastable state is considered to be realized. By comparing the experimental results with the theoretical calculations, the photoinduced valence transition between Eu $4f$ and conduction electrons was quantitatively evaluated.

We analyze a diamond-lattice Hubbard model with the spatially modulated Hubbard interaction. Our dynamical mean-field analysis with special emphasis on non-Hermitian properties elucidates that the gapless nodal line changes into symmetry-protected exceptional torus (SPET) at the Fermi level enclosing the three-dimensional open Fermi surface, which is unique to non-Hermitian physics with chiral symmetry. Furthermore, we also elucidate the effects of the SPETs on the magnetic response; our results based on the random-phase approximation combined with the dynamical mean-field theory show that SPETs enhance the magnetic susceptibility at the weakly correlated sites, exemplifying effects of non-Hermitian degeneracies on responses to external fields.

Late-transition-metal oxides with small charge-transfer energy $\mathrm{\Delta }$ raise issues for state-of-the-art correlated electronic structure schemes, such as the combination of density functional theory (DFT) with dynamical mean-field theory (DMFT). The accentuated role of the oxygen valence orbitals in these compounds asks for an enhanced description of ligand-based correlations. Utilizing the rocksaltlike NiO as an example, we present an advancement of charge self-consistent DFT $+$ DMFT by including self-interaction correction (SIC) applied to oxygen. This introduces explicit on-site O correlations as well as an improved treatment of intersite $p\text{−}d$ correlations. Due to the efficient SIC incorporation in a pseudopotential form, the DFT $+$ sicDMFT framework is an advanced but still versatile method to address the interplay of charge-transfer and Mott-Hubbard physics. We revisit the spectral features of stoichiometric NiO and reveal the qualitative sufficiency of local DMFT self-energies in describing spectral peak structures usually associated with explicit nonlocal processes. For ${\mathrm{Li}}_x{\mathrm{Ni}}_{1-x}\mathrm{O}$, prominent in-gap states are verified by the present theoretical paper.

Topological quantum materials have stimulated growing attention because they reveal novel aspects of condensed matter physics and point to new opportunities in materials science, in particular for thermoelectrics. Here, we experimentally study thermoelectric effects in ${\mathrm{HfTe}}_5$, which was predicted to be at the boundary between strong and weak topological insulators. The magnetic field dependence of ${\mathrm{HfTe}}_5$ thermoelectric properties attests to the anomalous character of this material, supported by our angle-resolved photoemission spectroscopy (ARPES) measurements. At 36 K, the thermopower of $-277\mu \mathrm{V}/\mathrm{K}$ is reached when a field of 0.4 Tesla is applied, while it is $-157\mu \mathrm{V}/\mathrm{K}$ at zero field and a large Nernst coefficient up to $600\mu \mathrm{V}/\mathrm{K}$ is observed at 100 K with magnetic field of 4 T. A possible topologically nontrivial band structure is proposed to account for our observations. Our results constitute a highly constraining set of data for any model of transport based on ${\mathrm{HfTe}}_5$ band structure. Furthermore, the extraordinary thermoelectric properties suggest a new paradigm for the development of thermoelectric applications based on layered transition-metal chalcogenides.

Within density functional theory we study the bulk band structure and surface states of bismuth oxychalcogenides ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ and ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Te}$. We consider both polar and nonpolar surface terminations. On the basis of relativistic ab initio calculations, we show that both unreconstructed (polar) and reconstructed (nonpolar) surfaces possess the Rashba spin-split surface states. The metallic Rashba-split states on polar surfaces stem from huge potential bending, positive or negative, depending on surface polarity. On the nonpolar surfaces resulting from single-crystal cleavage the emerging Rashba-split states are nonmetallic.

We study the time evolution of the two-point correlation functions in the XXZ Heisenberg chain after a finite-time quantum quench in the anisotropy. We compare results from numerical simulations to ones obtained in the Luttinger model and find good agreement. We analyze the spreading of the correlations and the associated light-cone features. We observe a delay in the appearance of the light cone as compared to the sudden-quench setup, and link this delay to the properties of the quench protocol.

The shifts of core-level binding energies can provide powerful information about the electronic structure of a material. Understanding the physical origin of these shifts for catalytically relevant oxides may provide important insight into their properties. This requires reliable theoretical methods which are able to relate the binding energy shifts to the electronic structure. In order to establish such a methodology, the CaO(100) surface to bulk core-level binding energy shifts have been studied with Hartree-Fock and density-functional theory methods using both cluster and periodic slab models. The shifts obtained from the different theoretical methods are compared with each other and with data from synchrotron-based x-ray photoelectron spectroscopy (XPS) measurements. With a common approximation for the slab model treatment of XPS, the predicted binding energy shifts are seriously in error. The origin of the error is identified as arising from a flawed treatment of the surface atom binding energies, and a method for correcting the failure is presented.

We predict by ab initio calculations an $\mathrm{all}-s{p}^2$ nanostructured carbon phase of a topological hybrid nodal-loop semimetal with momentum-dependent crossings of linear bands near the Fermi level. This carbon phase is assembled from interconnected Riemann surfaces, naturally occurring as screw dislocations in graphitic carbons, which not only result in notably higher stability than most previously reported allotropes but also sustain a strain-robust nodal loop up to 75% elastic stretch due to mirror symmetry. The design strategy, based on the elegant topology of Riemann surfaces, can yield a family of topological carbon materials with fascinating mechanical and electronic properties.

Recent experiments on cuprates show that as a function of doping, the normal-state specific heat sharply peaks at the doping ${\delta }^{*}$, where the pseudogap ends at low temperature. This finding is taken as the thermodynamic signature of a quantum critical point, whose nature has not yet been identified. Here we present calculations for the two-dimensional Hubbard model in the doped Mott insulator regime, which indicate that the specific heat anomaly can arise from the finite-temperature critical end point of a first-order transition between a pseudogap phase with dominant singlet correlations and a metal. As a function of doping at the temperature of the end point, the specific heat diverges. Upon increasing temperature, the peak becomes broader. The diverging correlation length is associated with uniform density fluctuations. No broken symmetries are needed. These anomalies also occur at half-filling as a function of interaction strength, and are relevant for organic superconductors and ultracold atoms.

The amplitude decay of folded longitudinal acoustic phonon oscillations in GaAs/AlAs superlattices grown in the [100] and [111] directions is studied as a function of carrier excitation density. Time-resolved pump-probe measurements in reflection reveal decay times on the order of several tens of picoseconds, which are independent of the excitation density for all phonons in the [100] sample and for phonons at wave vectors $q\ne 0$ in the [111] sample. In contrast, the decay time increases with carrier density for the phonon at $q=0$ in the [111] sample. This phonon couples to the photoexcited electron-hole plasma via the piezoelectric interaction. Friction in the carrier plasma induces a damping of the phonon oscillations. The impact of the piezoelectric phonon-electron coupling on the amplitude decay is reduced by screening at high carrier density, resulting in longer phonon decay times.

We study corner states on a flat band in the square lattice. To this end, we introduce a two-dimensional model including Su-Schrieffer-Heeger type bond alternation responsible for corner states as well as next-nearest-neighbor hoppings yielding flat bands. The key symmetry of the model for corner states is space-time inversion ($\mathcal{PT}$) symmetry, which guarantees quantized Berry phases. This implies that edge states as well as corner states would show up if boundaries are introduced to the system. We also argue that an infinitesimal $\mathcal{PT}$ symmetry-breaking perturbation could drive flat bands into flat Chern bands.

We report our NMR study of the spin excitations in the quasi-one-dimensional $S=\frac{1}{2}$ quantum magnet ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{Cu}{\left(\mathrm{HCOO}\right)}_3$ lying in the one-dimensional–three-dimensional crossover regime with Dzyaloshinskii-Moriya (DM) interactions. Above $T_N$, the spinon excitation is observed from the constant $1/T_1$ at low temperatures contributed from the staggered spin susceptibility. At low temperatures well below $T_N$, the $1/T_1$ begins to flatten out under weak magnetic fields. With the increasing field intensity, $1/T_1$ tends to show a power-law temperature dependence gradually, with the index increasing from zero to $\sim 3$, and finally $\sim 5$, which are the respective typical characteristics for the dominating two-magnon Raman process and three-magnon scattering contributions to the nuclear relaxation in a conventional three-dimensional magnet. A possible physical mechanism for this magnetic field tuning of quantum spin excitations related with the enhanced effective staggered field created by the DM interactions under magnetic field is discussed.

Experimental observables around the metal-insulator (M-I) transition are key to unveiling the underneath mechanism that induces the transition. From the heuristic argument, two different characteristics of charge carriers are observed on approaching the M-I transition point from the metallic side: (i) the Mott insulators in which the carrier effective mass diverges, and (ii) charge-transfer insulators in which the carrier-number vanishes. The rare-earth nickelates exhibiting M-I transition have recently emerged as materials of enormous contemporary relevance; however, with no conclusive understanding of the origin of their M-I transition. Here we have investigated the charge-transfer phase of two prototypical systems ${\text{Pr}}_{1-x}{\text{Nd}}_x{\text{NiO}}_3$ ($x=0$, 0.5) to construct a unified picture of the metal-insulator transition in the presence of electron and hole bands. Contrary to the existing understanding, our analysis of two-band framework modeled to the terahertz (THz) optical conductivity of the two systems suggests the effective mass divergence upon approaching the transition from the metallic regime as the cause of electron correlations; and resultantly, an unambiguous manifestation of a negative charge-transfer insulating ground state is established in rare-earth nickelates.

We derive an exact matrix product state representation of the Haldane-Rezayi state on both the cylinder and torus geometry. Our derivation is based on the description of the Haldane-Rezayi state as a correlator in a nonunitary logarithmic conformal field theory. This construction faithfully captures the 10 degenerate ground states of this model state on the torus. Using the cylinder geometry, we probe the gapless nature of the phase by extracting the correlation length, which diverges in the thermodynamic limit. The numerically extracted topological entanglement entropies seem to only probe the Abelian part of the theory, which is reminiscent of the Gaffnian state, another model state deriving from a nonunitary conformal field theory.

We present a quantum Monte Carlo algorithm for computing the perturbative expansion in power of the coupling constant $U$ of the out-of-equilibrium Green's functions of interacting Hamiltonians of fermions. The algorithm extends the one presented in Profumo et al. [Phys. Rev. B 91, 245154 (2015)], and inherits its main property: it can reach the infinite time (steady state) limit since the computational cost to compute order $U^n$ is uniform versus time; the computing time increases as $2^n$. The algorithm is based on the Schwinger-Keldysh formalism and can be used for both equilibrium and out-of-equilibrium calculations. It is stable at both small and long real times including in the stationary regime, because of its automatic cancellation of the disconnected Feynman diagrams. We apply this technique to the Anderson quantum impurity model in the quantum dot geometry to obtain the Green's function and self-energy expansion up to order $U^{10}$ at very low temperature. We benchmark our results at weak and intermediate coupling with high precision numerical renormalization-group computations as well as analytical results.

The charge transport mechanism in amorphous oxide semiconductors (AOS) is a matter of controversial debates. Most theoretical studies so far neglected the percolation nature of the phenomenon. In this paper, a recipe for theoretical description of charge transport in AOSs is formulated using the percolation arguments. Comparison with the previous theoretical studies shows a superiority of the percolation approach. The results of the percolation theory are compared to experimental data obtained in various InGaZnO materials revealing parameters of the disorder potential in such AOS.

Applications of the multipole decomposition method for investigations of directional light scattering by a single nanoparticle and nanoparticle structures located in a finite spatial region are discussed. It is shown that, even in the case of relatively large scatterers, the multipole decomposition obtained in the long-wavelength approximation (LWA) may provide much better convergence than the multipole decomposition with the exact multipoles obtained from the spherical harmonics expansion. For an explanation of this seeming paradox, we derive in real space the exact multipole decomposition based on the spherical harmonics, presenting exact expressions for multipoles up to the electric 16-pole. Results obtained with the exact and approximate multipole expressions are discussed and compared. It is shown that for shape-anisotropic finite-size scatterers with different geometrical dimensions (like plates, rods, disks, rings, etc.), the required number of approximate multipoles providing accurate results may be much smaller than the required number of exact multipoles. For applicability of the LWA multipole decomposition, the only important parameter is the small ratio of the scatter size (its projection) in the scattering direction to the light wavelength. If this condition is fulfilled, the multipole decomposition with a small number of LWA multipoles is simpler than that based on the exact multipoles.

We study the electronic transport through a pair of distant nanosystems ($S_a$ and $S_b$) embedded in a single-mode cavity. Each system is connected to source and drain particle reservoirs and the electron-photon coupling is described by the Tavis-Cummings model. The generalized master equation approach provides the reduced density operator of the double system in the dressed-states basis. It is shown that the photon-mediated coupling between the two subsystems leaves a signature on their transient and steady-state currents. In particular, a suitable bias applied on subsystem $S_b$ induces a photon-assisted current in the other subsystem $S_a$ which is otherwise in the Coulomb blockade. We also predict that a transient current passing through one subsystem triggers a charge transfer between the optically active levels of the second subsystem even if the latter is not connected to the leads. As a result of backaction, the transient current through the open system develops Rabi oscillations (ROs) whose period depends on the initial state of the closed system.

Motivated by the recently developed duality [Pretko and Radzihovsky, Phys. Rev. Lett. 120, 195301 (2018)] between the elasticity of a crystal and a symmetric tensor gauge theory, we explore its classical analog, which is a dual theory of the dislocation-mediated melting of a two-dimensional crystal, formulated in terms of a higher derivative vector sine-Gordon model. It provides a transparent description of the continuous two-stage melting in terms of the renormalization-group relevance of two cosine operators that control the sequential unbinding of dislocations and disclinations, respectively, corresponding to the crystal-to-hexatic and hexatic-to-isotropic fluid transitions. This renormalization-group analysis reproduces the seminal results of the Coulomb gas description, such as the flows of the elastic couplings and of the dislocation and disclination fugacities, as well as the temperature dependence of the associated correlation lengths.