We propose and analyze theoretically a one-dimensional solid-state electronic setup that operates as a topological charge pump in the complete absence of superimposed oscillating local voltages. The system consists of a semiconducting narrow channel with a strong Rashba spin-orbit interaction patterned in a mesoscale serpentine shape. A rotating planar magnetic field serves as the external ac perturbation, and cooperates with the Rashba spin-orbit interaction, which is modulated by the geometric curvature of the electronic channel to realize the topological pumping protocol, originally introduced by Thouless, in a different fashion. We expect the precise pumping of electric charges in our mesoscopic quantum device to be relevant for quantum metrology purposes.

We devise a generic recipe for constructing $D$-dimensional lattice models whose $d$-dimensional boundary states, located on surfaces, hinges, corners, and so forth, can be obtained exactly. The solvability is rooted in the underlying lattice structure and as such does not depend on fine tuning, allowing us to track their evolution throughout various phases and across phase transitions. Most saliently, our models provide “boundary solvable” examples of the recently introduced higher-order topological phases. We apply our general approach to breathing and anisotropic kagome and pyrochlore lattices for which we obtain exact corner eigenstates, and to periodically driven two-dimensional models as well as to three-dimensional lattices where we present exact solutions corresponding to one-dimensional chiral states at the hinges of the lattice. We relate the higher-order topological nature of these models to reflection symmetries in combination with their provenance from lower-dimensional conventional topological phases.

We consider the strongly correlated limit of variants of the Hubbard model in which on parts of the system it is energetically favorable to project out doublons from the low-energy Hilbert space while on other sites of the system it is favorable to project out holes while still allowing for doublons. As an effect the low-energy Hilbert space itself varies with sites of the system. Though the formalism is well developed for the case of doublon projection in the literature, the case of hole projection has not been explored in detail so far. We derive the basic framework by defining creation and annihilation operators for electrons in a restricted Hilbert space where holes are projected out but which still allows for doublons. We generalize the idea of the Gutzwiller approximation for the case of hole projection, which has been done in the literature for the case of doublon projection. To be specific, we provide a detailed analysis of the strongly correlated limit of the ionic Hubbard model which has a staggered potential $\mathrm{\Delta }$ on two sublattices of a bipartite lattice and the correlated binary alloys which have binary disorder $\pm V/2$ randomly distributed on sites of the lattice. In both cases, for $\mathrm{\Delta }\sim U\gg t$ and for $V\sim U\gg t$, where $U$ is the Hubbard energy cost for having a doublon at a site, there are sites on which doublons are allowed while holes are the maximum energy states. We do a systematic generalization of similarity transformation for both these cases and obtain the effective low-energy Hamiltonian. We further derive Gutzwiller approximation factors which provide renormalization of various terms in the effective low-energy Hamiltonian due to the Gutzwiller projection operators, excluding holes on some sites and doublons on the remaining sites.

The dynamics induced by femtosecond-laser excitation of the commensurate phase of the charge-density wave (CDW) in $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ have been studied using both time-resolved electron diffraction and the time-resolved spectroscopy of coherent-phonon dynamics. Electron diffraction results show that the commensurate CDW phase is transformed into a new phase with CDW order that is similar to the nearly commensurate phase with threshold-type transition rates; the threshold excitation density of 0.2 per 13 Ta atoms is evaluated. Coherent-phonon spectroscopy results show that, together with the amplitude mode of CDW with a frequency of 2.41 THz, two other modes with frequencies of 2.34 and 2.07 THz are excited in the photoexcited commensurate CDW phase over a timescale of several tens of picoseconds after excitation. Spectroscopic, temporal, and excitation-intensity dependent characteristics of the three coherent phonons reveal that a photoinduced decomposition of the commensurate CDW order into an ensemble of domains with different CDW orders is induced before the CDW-phase transition occurs. The physics underlying the photoinduced decomposition and evolution into discommensurations responsible for the CDW-order transformation are discussed.

In the context of an isolated three-dimensional noninteracting fermionic lattice system, we study the effects of a sudden quantum quench between a disorder-free situation and one in which disorder results in a mobility edge and associated Anderson localization. Salient post-quench features hinge upon the overlap between momentum states and post-quench eigenstates and whether these latter states are extended or localized. We find that the post-quench momentum distribution directly reflects these overlaps. For the local density, we show that disorder generically prevents the equilibration of quantum expectation values to a steady state and that the persistent fluctuations have a nonmonotonic dependence on the strength of disorder. We identify two distinct types of fluctuations, namely, temporal fluctuations describing the time-dependent fluctuations of the local density around its time average and sample-to-sample fluctuations characterizing the variations of these time averages from one realization of disorder to another. We demonstrate that both of these fluctuations vanish for extremely extended as well as extremely localized states, peaking at some intermediate value.

There are reasons to believe that the ground state of the magnetic rare-earth pyrochlore ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is on the boundary between competing ground states. We have carried out ab initio density functional calculations to determine the most stable chemical formula as a function of the oxygen chemical potential and the likely location of the oxygen atoms in the unit cell of the “stuffed” system. We find that it is energetically favorable in the stuffed crystal (with an Yb replacement on a Ti site) to contain oxygen vacancies which dope the Yb $4f$ orbitals and qualitatively change the electronic properties of the system. In addition, with the inclusion of the contribution of spin-orbit coupling (SOC) on top of the GGA + $U$ approach, we investigated the electronic structure and the magnetic moments of the most stable stuffed system. In our determined stuffed structure the valence bands as compared to those of the pure system are pushed down and a change in hybridization between the O $2p$ orbitals and the metal ion states is found. Our first-principle findings should form a foundation for effective models describing the low-temperature properties of this material whose true ground state remains controversial.

We present electronic structure and quantum transport calculations that predict conducting channels induced in graphene quantum dots by lines of adsorbed hydrogen atoms to function as highly efficient, experimentally realizable valley filters, accumulators, and switches. The underlying physics is an interesting property of graphene Dirac point resonances (DPRs) that is revealed here, namely, that an electric current passing through a DPR-mediated conducting channel in a given direction is carried by electrons of only one of the two graphene valleys. Our predictions apply to lines of hydrogen atoms adsorbed on graphene quantum dots that are either free standing or supported on a hexagonal boron nitride substrate.

In this paper, we analyze the effects of nonlocality on the optical properties of a system consisting of a thin metallic film separated from a graphene sheet by a hexagonal boron nitride (hBN) layer. We show that nonlocal effects in the metal have a strong impact on the spectrum of the surface plasmon-polaritons on graphene. If the graphene sheet is nanostructured into a periodic grating, we show that the resulting extinction curves can be used to shed light on the importance of nonlocal effects in metals. Therefore graphene surface plasmons emerge as a tool for probing nonlocal effects in metallic nanostructures, including thin metallic films. As a byproduct of our study, we show that nonlocal effects may lead to smaller losses for the graphene plasmons than what is predicted by a local calculation. Finally, we demonstrate that such nonlocal effects can be very well mimicked using a local theory with an effective spacer thickness larger than its actual value.

We study optically the coherent evolution of trions and excitons in a $\delta$-doped 3.5-nm-thick ZnO/${\mathrm{Zn}}_{0.91}{\mathrm{Mg}}_{0.09}\mathrm{O}$ multiple quantum well by means of time-resolved four-wave mixing at a temperature of 1.5 K. Employing spectrally narrow picosecond laser pulses in the ${\chi }^{\left(3\right)}$ regime allows us to address differently localized trion and exciton states, thereby avoiding many-body interactions and excitation-induced dephasing. The signal in the form of photon echoes from the negatively charged $A$ excitons ($T_{\mathrm{A}}$, trions) decays with coherence times varying from 8 up to 60 ps, depending on the trion energy: more strongly localized trions reveal longer coherence dynamics. The localized neutral excitons decay on the picosecond time scale with coherence times up to $T_2=4.5$ ps. The coherent dynamics of the $X_{\text{B}}$ exciton and $T_{\text{B}}$ trion are very short ($T_2<1$ ps), which is attributed to the fast energy relaxation from the trion and exciton $B$ states to the respective $A$ states. The trion population dynamics is characterized by the decay time $T_1$, rising from 30 to 100 ps with decreasing trion energy.

Possibility of in-plane and out-of-plane magnetization generated by synchrotron radiation (SR) in magnetically doped and pristine topological insulators (TIs) is demonstrated and studied by angle-resolved photoemission spectroscopy. We show experimentally and by ab initio calculations how nonequal depopulation of the Dirac cone (DC) states with opposite momenta in V-doped and pristine TIs generated by linearly polarized SR leads to the hole-generated uncompensated spin accumulation followed by the SR-induced magnetization via spin-torque effect. Moreover, the photoexcitation of the DC is asymmetric, and it varies with the photon energy. We find a relation between the photoexcitation asymmetry, the generated spin accumulation, and the induced in-plane and out-of-plane magnetic field. Experimentally the SR-generated in-plane and out-of-plane magnetization is confirmed by the $k_{\parallel }$ shift of the DC position and by the gap opening at the Dirac point even above the Curie temperature. Theoretical predictions and estimations of the measurable physical quantities substantiate the experimental results.

We study the structural and vibrational properties of the high-temperature superconducting sulfur trihydride and trideuteride in the high-pressure $Im\overline{3}m$ and $R3m$ phases by first-principles density-functional-theory calculations. On lowering pressure, the rhombohedral transition $Im\overline{3}m\to R3m$ is expected, with hydrogen-bond desymmetrization and occurrence of trigonal lattice distortion. With both Perdew-Burke-Ernzerhof (PBE) and Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional, in hydrostatic conditions we find that, contrary to what is suggested in some recent experiments, if the rhombohedral distortion exists it affects mainly the hydrogen bonds, whereas the resulting cell distortion is minimal. We estimate that the occurrence of a stress anisotropy of approximately $10\%$ could explain this discrepancy. Assuming hydrostatic conditions, we calculate the critical pressure at which the rhombohedral transition occurs. Quantum and anharmonic effects, which are relevant in this system, are included at nonperturbative level with the stochastic self-consistent harmonic approximation. Within this approach, we determine the transition pressure by calculating the free-energy Hessian, a method that allows to estimate the critical pressure with much higher precision (and much lower computational cost) compared with the free-energy “finite-difference” approach previously used. Using PBE and BLYP, we find that quantum anharmonic effects are responsible for a strong reduction of the critical pressure with respect to the one obtained with the classical harmonic approach. Interestingly, for the two functionals, even if the transition pressures at classical harmonic level differ by 83 GPa, the transition pressures including quantum anharmonic effects differ only by 23 GPa. Moreover, we observe a prominent isotope effect, as we estimate higher transition pressure for ${\mathrm{D}}_3\mathrm{S}$ than for ${\mathrm{H}}_3\mathrm{S}$. Finally, within the stochastic self-consistent harmonic approximation, with PBE we calculate the anharmonic phonon spectral functions in the $Im\overline{3}m$ phase. The strong anharmonicity of the system is confirmed by the occurrence of very large anharmonic broadenings leading to complex non-Lorentzian line shapes. Generally, for the high-energy hydrogen bond-stretching modes, the anharmonic phonon broadening is of the same magnitude of the electron-phonon one. However, for the vibrational spectra at zone center, accessible, e.g., by infrared spectroscopy, the broadenings are very small (linewidth at most around 2 meV) and anharmonic phonon quasiparticles are well defined.

Electronic and magnetic properties of molecular nanomagnets are determined by competing energy scales due to the crystal field splitting, the exchange interactions between transition metal atoms, and relativistic effects. We present a comprehensive theory embracing all these phenomena based on first-principles calculations. In order to achieve this goal, we start from the ${\mathrm{FeNi}}_4$ cluster as a paradigm. The system can be accurately described on the ab initio level yielding all expected electronic states in a range of multiplicities from 1 to 9, with a ferromagnetic ground state. By adding the spin-orbit coupling between them we obtain the zero-field splitting. This allows to introduce a spin Hamiltonian of a giant spin model, which operates on a smaller energy scale. We compare the computed parameters of this Hamiltonian with the experimental and theoretical magnetic anisotropy energies of the monolayer Ni/Cu(001). In line with them, we find that the anisotropy almost entirely originates from the second-order spin-orbit coupling, the spin-spin coupling constitutes only a small fraction. Finally, we include the ligand atoms in our consideration. This component has a decisive role for the stabilization of molecules in experimental synthesis and characterization, and also substantially complicates the theory by bringing the superexchange mechanisms into play. Since they are higher-order effects involving two hopping matrix elements, not every theory can describe them. Our generalization of the corresponding perturbation theory substantiates the use of complete active space methods for the description of superexchange. At the same time, our numerical results for the $\left\{{\mathrm{CuFe}}_4\right\}$ system demonstrate that the Goodenough-Kanamori rules, which are often used to determine the sign of these exchange interactions, cannot deliver quantitative predictions due to the interplay of other mechanisms, e. g., involving multicenter Coulomb integrals. We conclude by comparing ab initio values of the exchange interaction constants for the $\left\{{\mathrm{CuCu}}_4\right\}$ and $\left\{{\mathrm{CuFe}}_4\right\}$ metallacrown magnetic molecules with experimental values determined by fitting of the magnetic susceptibility curves ${\chi }_{M}T\left(T\right)$, and attribute the remaining discrepancy between them to the role of virtual electron excitations into and out of the active space (dynamical correlations).

We employ a combination of spectrally resolved optical pump-probe spectroscopy and excited-state ab initio molecular dynamics (ESAIMD) simulations to study the ultrafast vibrational relaxation dynamics of the ${}^{3}E$ excited state of negatively charged nitrogen vacancy ($\mathrm{N}{\mathrm{V}}^{-}$) defects. The experimental results reveal vibrational relaxation in the phonon sideband with a time constant of approximately 50 fs, in excellent agreement with the $\sim 40$-fs structural equilibration timescale predicted by ESAIMD simulations. The observed ultrafast vibrational energy relaxation implies that dynamical processes triggered by photoexcitation into the phonon sideband of the $\mathrm{N}{\mathrm{V}}^{-}$ center occur primarily in the lowest vibronic level of the ${}^{3}E$ state.

We report experiments to determine the effect of radiation damage on the phonon spectra of the most common nuclear fuel, ${\mathrm{UO}}_2$. We irradiated thin ($\sim 300$ nm) epitaxial films of ${\mathrm{UO}}_2$ with 2.1 MeV ${\mathrm{He}}^{2+}$ ions to 0.15 displacements per atom and a lattice swelling of $\mathrm{\Delta }a/a\sim 0.6\%$ and then used grazing-incidence inelastic x-ray scattering to measure the phonon spectrum. We succeeded in observing the acoustic modes, both transverse and longitudinal, across the Brillouin zone. The phonon energies, in both the pristine and irradiated samples, are unchanged from those observed in bulk material. On the other hand, the phonon linewidths (inversely proportional to the phonon lifetimes) show a significant broadening when comparing the pristine and irradiated samples. This effect is shown to increase with phonon energy across the Brillouin zone. The decreases in the phonon lifetimes of the acoustic modes are roughly consistent with a 50% reduction in the thermal conductivity.

We compare calculations based on density functional theory (DFT) with available experimental data and analyze the origin of magnetic anisotropies in ${\mathrm{MnF}}_2, {\mathrm{FeF}}_2, {\mathrm{CoF}}_2$, and ${\mathrm{NiF}}_2$. We confirm that the magnetic anisotropy of ${\mathrm{MnF}}_2$ stems almost completely from the dipolar interaction, while magnetocrystalline anisotropy energy (originating in spin-orbit interaction) plays a dominant role in the other three compounds, and discuss how it depends on the details of band structure. The latter is critically compared to available optical measurements. The case of ${\mathrm{CoF}}_2$, where magnetocrystalline anisotropy energy strongly depends on $U$ (the Hubbard parameter in $\mathrm{DFT}+\mathrm{U}$), is put into contrast with ${\mathrm{FeF}}_2$ where theoretical predictions of magnetic anisotropies are nearly quantitative.

Majority of the $A_2{B}_3$-type chalcogenide systems with strong spin-orbit coupling (SOC), such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3,{\mathrm{Bi}}_2{\mathrm{Te}}_3$, and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, etc., are topological insulators. One important exception is ${\mathrm{Sb}}_2{\mathrm{Se}}_3$ where a topological nontrivial phase was argued to be possible under ambient conditions, but such a phase could be detected to exist only under pressure. In this paper, we show that ${\mathrm{Sb}}_2{\mathrm{Se}}_3$ like ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ displays a generation of highly spin-polarized current under mesoscopic superconducting point contacts as measured by point-contact Andreev reflection spectroscopy. In addition, we observe a large negative and anisotropic magnetoresistance of the mesoscopic metallic point contacts formed on ${\mathrm{Sb}}_2{\mathrm{Se}}_3$. Our band-structure calculations confirm the trivial nature of ${\mathrm{Sb}}_2{\mathrm{Se}}_3$ crystals and reveal two trivial surface states one of which shows large spin splitting due to Rashba-type SOC. The observed high spin polarization and related phenomena in ${\mathrm{Sb}}_2{\mathrm{Se}}_3$ can be attributed to this spin splitting.

A uniaxial strain applied to graphenelike materials moves the Dirac nodes along the boundary of the Brillouin zone. An extreme case is the merging of the Dirac node positions to a single degenerate spectral node, which gives rise to a new topological phase. Then isotropic Dirac nodes are replaced by a node with a linear behavior in one and a parabolic behavior in the other direction. This anisotropy influences substantially the optical properties. We propose a method to determine characteristic spectral and transport properties in black phosphorus layers, which were recently studied by several groups with angle-resolved photoemission spectroscopy, and discuss how the transmittance, the reflectance, and the optical absorption of this material can be tuned. In particular, we demonstrate that the transmittance of linearly polarized incident light varies from nearly 0% to almost 100% in the microwave and far-infrared regime.

We report a combined theoretical and experimental study on $\mathrm{TaIrT}{\mathrm{e}}_4$, a potential candidate for a minimal model of type-II Weyl semimetals. Unexpectedly, an intriguing node structure with 12 Weyl points and a pair of nodal lines protected by mirror symmetry was found by first-principles calculations. Some signatures of the complex electronic structure, such as topologically nontrivial band crossings and topologically trivial Fermi arcs, are cross-validated by angle-resolved photoemission spectroscopy. Through external strain, the number of Weyl points can be reduced to a theoretical minimum of four, and the appearance of the nodal lines can be switched between different mirror planes in momentum space. The coexistence of switchable Weyl points and nodal lines establishes transition-metal chalcogenides as a unique test ground for topological state characterization and engineering.