We report high-pressure Raman-scattering studies on single-crystal $\mathrm{Re}{\mathrm{O}}_3$ up to 26.9 GPa at room temperature, complemented by first-principles density functional calculations to assign the modes and to develop understanding of the subtle features of the low-pressure phase transition. The pressure $\left(P\right)$ dependence of phonon frequencies $\left(\omega \right)$ reveals three phase transitions at 0.6, 3, and 12.5 GPa with characteristic splitting and changes in the slope of $\omega \left(P\right)$. Our first-principles theoretical analysis confirms the role of the rotational modes of $\mathrm{Re}{\mathrm{O}}_6,M_3$, to the lowest pressure structural transition, and shows that the transition from the $Pm3m$ to the $Im3$ structure is a weak first-order transition, originating from the strong anharmonic coupling of the $M_3$ modes with the acoustic modes (strain).

${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$, crystallizing in the noncentrosymmetric but nonpolar $P\overline{4}{2}_{1}m$ structure, belongs to a special class of multiferroic materials, whose properties are featured by the presence of rotoinversion symmetry. Unlike inversion, the rotoinversion symmetry can be easily destroyed by magnetization. Moreover, due to the specific structural pattern, in which magnetic ${\mathrm{Co}}^{2+}$ ions are separated by nonmagnetic ${\mathrm{GeO}}_4$ tetrahedra, the magnetic structure of ${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$ is relatively soft. Altogether, this leads to a rich variety of multiferroic properties, where the magnetic structure of ${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$ can be easily deformed by the magnetic field, inducing the net electric polarization in the direction depending on the magnetic symmetry of the system, which itself depends on the direction of the magnetic field. In this paper, we show that all these properties can be successfully explained on the basis of a realistic low-energy model, derived from first-principles electronic structure calculations for the magnetically active Co $3d$ bands, and the Berry-phase theory of electronic polarization. Particularly, we argue that the magnetization-induced electric polarization in ${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$ is essentially local and can be expressed via the expectation values $\langle \widehat{\mathit{p}}\rangle =\mathrm{Tr}\left[\widehat{\mathit{p}}\widehat{\mathcal{D}}\right]$ of some dipole matrices $\widehat{\mathit{p}}$ and the site-diagonal density matrices $\widehat{\mathcal{D}}$ of the magnetic Co atoms. Thus, the basic aspects of multiferroic properties of ${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$ can be understood already in the atomic limit, where both magnetic anisotropy and magnetoelectric coupling are specified by $\widehat{\mathcal{D}}$. Then, the observable polarization is the macroscopic average over the microscopic electric dipoles $\langle \widehat{\mathit{p}}\rangle$. We discuss the behavior of interatomic magnetic interactions, the main contributions to the magnetocrystalline anisotropy, and the spin canting in the $xy$ plane, as well as the similarities and differences between the proposed picture and the phenomenological model of spin-dependent $p-d$ hybridization.

We perform time-resolved photoelectron spectroscopy measurements of optimally doped $B{i}_{2}S{r}_{2}CaC{u}_2{O}_{8+\delta }$ (Bi-2212) and $B{i}_{2}S{r}_{2-x}L{a}_{x}Cu{O}_{6+\delta }$ (Bi-2201). The electron dynamics shows that inelastic scattering by nodal quasiparticles decreases when the temperature is lowered below the critical value of the superconducting phase transition. This drop in electronic dissipation is astonishingly robust and survives to photoexcitation densities much larger than the value sustained by long-range superconductivity. The unconventional behavior of quasiparticle scattering is ascribed to superconducting correlations extending on a length scale comparable to the inelastic path. Our measurements indicate that strongly driven superconductors enter in a regime without phase coherence but finite pairing amplitude. The latter vanishes near to the critical temperature and has no evident link to the pseudogap observed by angle-resolved photoelectron spectroscopy.

Properly modulated flat-band lattices have a divergent density of states at the flat-band energy. Quasiperiodic modulations are known to host a metal-insulator transition already in one space dimension. Their embedding into flat-band geometries consequently allows for a precise engineering and fine tuning of mobility edges. We obtain analytic expressions for singular mobility edges for two flat-band lattice examples. In particular, we engineer cases with arbitrarily small energy separations of mobility edge, zeroes, and divergencies.

The ability to tune exchange (magnetic) interactions between $3d$ transition metals in perovskite structures has proven to be a powerful route to discovery of novel properties. Here we demonstrate that the introduction of $3d\text{−}5d$ exchange pathways in double perovskites enables additional tunability, a result of the large spatial extent of $5d$ wave functions. Using x-ray probes of magnetism and structure at high pressure, we show that compression of ${\mathrm{Sr}}_2{\mathrm{FeOsO}}_6$ drives an unexpected continuous change in the sign of Fe-Os exchange interactions and a transition from antiferromagnetic to ferrimagnetic order. We analyze the relevant electron-electron interactions, shedding light into fundamental differences with the more thoroughly studied $3d\text{−}3d$ systems.

We introduce and theoretically study the plasmon-photogalvanic effect in a planar noncentrosymmetric plasmonic crystal containing a homogeneous two-dimensional electron system gated by a periodic metal grating with an asymmetric unit cell. The plasmon-photogalvanic dc current arises due to the two-dimensional electron drag by the noncentrosymmetric plasmon modes excited under normal incidence of terahertz radiation. We show that the collective plasmon modes of the planar plasmonic crystal become strongly noncentrosymmetric in the weak-coupling regime of their anticrossing. A large plasmon wave vector (which is typically by two-three orders of magnitude greater than the terahertz photon wave vector) along with strong near-field enhancement at the plasmon resonance make the plasmonic drag a much stronger effect compared to the photon drag observed in conventional two-dimensional electron systems.

Intensive studies of the interplay between spin-orbit coupling (SOC) and electronic correlations in transition-metal compounds have recently been undertaken. In particular, $j_{\mathrm{eff}}=1/2$ bands on a honeycomb lattice provide a pathway to realize Kitaev's exactly solvable spin model. However, since current wisdom requires strong atomic SOC to make $j_{\mathrm{eff}}=1/2$ bands, studies have been limited to iridium oxides. Contrary to this expectation, we demonstrate how Kitaev interactions arise in $4d$-orbital honeycomb $\alpha \text{−}{\mathrm{RuCl}}_3$, despite having significantly weaker SOC than the iridium oxides, via assistance from electron correlations. A strong-coupling spin model for these correlation-assisted $j_{\mathrm{eff}}=1/2$ bands is derived, in which large antiferromagnetic Kitaev interactions emerge along with ferromagnetic Heisenberg interactions. Our analyses suggest that the ground state is a zigzag-ordered phase lying close to the antiferromagnetic Kitaev spin liquid. Experimental implications for angle-resolved photoemission spectroscopy, neutron scattering, and optical conductivities are discussed.

We study the spin-crossover molecule $\text{Fe}{\left(\mathrm{phen}\right)}_2{\left(\mathrm{NCS}\right)}_2$ using density functional theory (DFT) plus dynamical mean-field theory, which allows access to observables not attainable with traditional quantum chemical or electronic structure methods. The temperature dependent magnetic susceptibility, electron addition and removal spectra, and total energies are calculated and compared to experiment. We demonstrate that the proper quantitative energy difference between the high-spin and low-spin state, as well as reasonably accurate values of the magnetic susceptibility can be obtained when using reasonable interaction parameters. Comparisons to DFT and $\text{DFT}+U$ calculations demonstrate that dynamical correlations are critical to the energetics of the low-spin state. Additionally, we elucidate the differences between $\text{DFT}+U$ and spin density functional theory (SDFT) plus $U$ methodologies, demonstrating that $\text{DFT}+U$ can recover $\text{SDFT}+U$ results for an appropriately chosen on-site exchange interaction.

At low temperatures, electrons in semiconductors are bound to shallow donor impurity ions, neutralizing their charge in equilibrium. Inelastic scattering of other externally injected conduction electrons accelerated by electric fields can excite transitions within the manifold of these localized states. Promotion of the bound electron into highly spin-orbit-mixed excited states drives a strong spin relaxation of the conduction electrons via exchange interactions, reminiscent of the Bir-Aronov-Pikus process where exchange occurs with valence band hole states. Through low-temperature experiments with silicon spin transport devices and complementary theory, we reveal the consequences of this spin depolarization mechanism both below and above the impact ionization threshold.

We show that topological transitions in electronic spin transport are feasible by a controlled manipulation of spin-guiding fields. The transitions are determined by the topology of the fields texture through an effective Berry phase (related to the winding parity of spin modes around poles in the Bloch sphere), irrespective of the actual complexity of the nonadiabatic spin dynamics. This manifests as a distinct dislocation of the interference pattern in the quantum conductance of mesoscopic loops. The phenomenon is robust against disorder, and can be experimentally exploited to determine the magnitude of inner spin-orbit fields.

An exotic type of topological spin-helical surface state was found in layered van der Waals bonded ${\left(\mathrm{SnTe}\right)}_{n=2,3}{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_{m=1}$ compounds which comprise two covalently bonded band inverted sublattices, SnTe and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, within a building block. This topological state demonstrates unusual dispersion within the band gap. The dispersion of the surface state has two linear sections of different slope with a shoulder feature between them. Such a dispersion of the topological surface state enables an effective switch of the velocity of topological carriers by means of applying an external electric field.

The thermodynamics of the antiferromagnetic ordering transition in NiO and the photoelectron spectra in the antiferromagnetic phase are studied by the variational cluster approximation. Using realistic Racah parameters to describe the Coulomb interaction in the Ni $3d$ shell and a Slater-Koster parameter $\left(pd\sigma \right)$ which is slightly ($10\%$) increased over the band-structure estimate, the calculated Néel temperature is 481 K (experimental value: 523 K). The magnetic susceptibility above $T_N$ has Curie-Weiss form. A significant contribution to the stabilization of the antiferromagnetic phase comes from electron hopping between oxygen which would be missed in theories that consider superexchange along a single bond only. The single-particle spectral function in the ordered phase is in good agreement with experiment, in particular, a number of dispersionless bands which are not reproduced by most calculations are obtained correctly. These flat bands are shown to be direct experimental evidence for a dispersionless electronic self-energy with several poles in the energy range of the valence band which originate from the multiplets of the ${\mathrm{Ni}}^{3+}$ ion. Small but possibly experimentally detectable changes of the photoelectron spectra with temperature are discussed, in particular, a widening of the insulating gap in the paramagnetic phase by approximately $10\%$ is predicted.

A lattice-dynamics study of the cubic ${\mathrm{Li}}_3\mathrm{OCl}$ antiperovskite, a candidate solid electrolyte in lithium-ion batteries, reveals the presence of dynamical instabilities with respect to rotations of the ${\mathrm{Li}}_6\mathrm{O}$ octahedra. Calculated energy landscapes in the subspace of unstable octahedral rotational modes are very shallow with at most a 1 meV per formula unit reduction in energy upon breaking the cubic symmetry. While ${\mathrm{Li}}_3\mathrm{OCl}$ is not stable relative to decomposition into ${\mathrm{Li}}_2\mathrm{O}$ and LiCl at 0 K, estimates of the vibrational free energy suggest that ${\mathrm{Li}}_3\mathrm{OCl}$ antiperovskite should become entropically stabilized above approximately 480 K.

We present a pressure study of the electrical resistivity, ac magnetic susceptibility, and powder x-ray diffraction (XRD) of the recently discovered ${\text{BiS}}_2$-based superconductor ${\text{EuBiS}}_2\text{F}$. At ambient pressure, ${\text{EuBiS}}_2\text{F}$ shows an anomaly in the resistivity at around $T_0\approx 280$ K and a superconducting transition at $T_c\approx 0.3$ K. Upon applying hydrostatic pressure, there is little change in $T_0$ but the amplitude of the resistive anomaly is suppressed, whereas there is a dramatic enhancement of $T_c$ from 0.3 K to about 8.6 K at a critical pressure of $p_c\approx 1.4$ GPa. XRD measurements confirm that this enhancement of $T_c$ coincides with a structural phase transition from a tetragonal phase ($P4/nmm$) to a monoclinic phase ($P{2}_1/m$), which is similar to that observed in isostructural ${\text{LaO}}_{0.5}{\text{F}}_{0.5}{\text{BiS}}_2$. Our results suggest the presence of two different superconducting phases with distinct crystal structures in ${\text{EuBiS}}_2\text{F}$, which may be a general property of this family of ${\text{BiS}}_2$-based superconductors.

We study the zero-temperature transport properties of one-dimensional normal metal-superconductor (NS) junctions with topological superconductors across their topological transitions. Working within the Blonder-Tinkham-Klapwijk (BTK) formalism generalized for topological NS junctions, we analytically calculate the differential conductance for tunneling into two models of a topological superconductor: a spinless intrinsic $p$-wave superconductor and a spin-orbit-coupled $s$-wave superconductor in a Zeeman field. In both cases we verify that the zero-bias conductance is robustly quantized at $2e^2/h$ in the topological regime, while it takes nonuniversal values in the nontopological phase. The conductance spectra in the topological state develops a peak at zero bias for certain parameter regimes, with the peak width controlled by the strength of spin-orbit coupling and barrier transparency.

We study the long-range Coulomb interaction effects on the double-Weyl fermion system, which is possibly realized in the three-dimensional semimetal ${\mathrm{HgCr}}_2{\mathrm{Se}}_4$ in the ferromagnetic phase. Within the one-loop renormalization-group analysis, we find that there exists a stable fixed point at which the Coulomb interaction is screened anisotropically. At the stable fixed point, the renormalized Coulomb interaction induces nontrivial logarithmic corrections to the physical quantities, such as specific heat, compressibility, the electrical conductivity, and the diamagnetic susceptibility that are obtained utilizing the renormalization-group equations.

We study nonequilibrium dynamics in the one-dimensional Bose-Hubbard model starting from an initial product state with one boson per site and a single-hole defect. We find that for parameters close to the quantum critical point, the hole splits into a core showing a very slow diffusive dynamics, and a fast mode which propagates ballistically. Using an effective fermionic model at large Hubbard interactions $U$, we show that the ballistic mode is a consequence of an interference between slow-holon and fast-doublon dynamics, which occurs once the hole defect starts propagating into the bosonic background at unit filling. Within this model, the signal velocity of the fast ballistic mode is given by the maximum slope of the dispersion of the doublon quasiparticle in good agreement with the numerical data. Furthermore, we contrast this global quench with the dynamics of a single-hole defect in the ground state of the Bose-Hubbard model and show that the dynamics in the latter case is very different even for large values of $U$. This can also be seen in the entanglement entropy of the time-evolved states which grows much more rapidly in time in the global quench case.