We review the fact that $U\left(1\right)$ gauge symmetry enables the mapping of one-dimensional Hubbard chains with Rashba-type spin-orbit coupling to renormalized Hubbard Hamiltonians. The existence of the mapping has important consequences for the interpretation of ARPES experiments on one-dimensional chains subject to Rashba spin-orbit coupling and can be exploited to check for the applicability of the mapping. We show numerical applications of the mapping and consider the implications for bosonization as well as for the Heisenberg limit of the Hubbard model. In addition we point out the consequences for various generalized Hubbard models.

Linear-response quantum excitation is proportional to the amplitude of the field, with the energy of the excitation given by the driving frequency. As the amplitude is increased, there is a crossover, where the excitation energy is governed by the amplitude of the driving field, not its frequency. As the amplitude is increased even further, then complex quantum oscillations develop. We illustrate this phenomena with the exact solution of the simplest model of a charge-density-wave insulator driven by a spatially uniform time-dependent electric field. The driving by the field can be mapped onto a series of Landau-Zener problems, but with a complex, nonmonochromatic drive that varies for each momentum point in the reduced Brillouin zone.

Realizing topological insulators in commonly used III-V semiconductors is of great importance for their potential application in spintronics and quantum computing. Here we propose a general strategy to realize topological insulators in conventional III-V semiconductors by bismuth substitution and external strain. Based on first-principles calculations, we identify that AlBi (GaBi and InBi) become topological insulators (semimetals) under proper external strain by directly calculating ${\mathbb{Z}}_2$ invariants and surface states. Furthermore, we demonstrate that a topological phase transition can be induced by Bi substitution in common III-V semiconductors like GaAs. These proposed topological insulators can be easily integrated into various semiconductor electronic devices and modulated by well-developed modern semiconductor technologies.

We have studied EuPtP, which undergoes two successive valence transitions at $T_{\mathrm{A}}\sim 240$ K and $T_{\mathrm{B}}\sim 200$ K by ${}^{31}\mathrm{P}$-nuclear magnetic resonance (NMR) measurements. From the analysis of NMR spectra, we obtained plausible ordered structures and Eu valence states in three phases divided by $T_{\mathrm{A}}$ and $T_{\mathrm{B}}$. These ordered structures well explain observed inequivalent P sites and the intensity ratio of the NMR spectra arising from these P sites. The results are also in good accordance with mean Eu valence measured by the x-ray absorption spectroscopy. We also discuss Eu $4f$ states and the origin of the transitions from the measurements of nuclear spin lattice relaxation rate and hyperfine coupling constant.

We derive the effective charge- and current-density operators for the strong-coupling limit of a single-band Mott insulator in the presence of spin-orbit coupling and show that the spin-orbit contribution to the effective charge density leads to novel mechanisms for multiferroic behavior. In some sense, these mechanisms are the electronic counterpart of the ionic-based mechanisms, which have been proposed for explaining the electric polarization induced by spiral spin orderings. The new electronic mechanisms are illustrated by considering cycloidal and proper-screw magnetic orderings on sawtooth and kagome lattices. As for the isotropic case, geometric frustration is crucial for achieving this purely electronic coupling between spin and charge degrees of freedom.

The electron Landé $g$ factor $\left(g^{*}\right)$ is investigated both experimentally and theoretically in a series of ${\mathrm{GaBi}}_x{\mathrm{As}}_{1-x}/\mathrm{GaAs}$ strained epitaxial layers, for bismuth compositions up to $x=3.8\%$. We measure $g^{*}$ via time-resolved photoluminescence spectroscopy, which we use to analyze the spin quantum beats in the polarization of the photoluminescence in the presence of an externally applied magnetic field. The experimental measurements are compared directly to atomistic tight-binding calculations on large supercells, which allows us to explicitly account for alloy disorder effects. We demonstrate that the magnitude of $g^{*}$ increases strongly with increasing Bi composition $x$ and, based on the agreement between the theoretical calculations and experimental measurements, elucidate the underlying causes of the observed variation of $g^{*}$. By performing measurements in which the orientation of the applied magnetic field is changed, we further demonstrate that $g^{*}$ is strongly anisotropic. We quantify the observed variation of $g^{*}$ with $x$, and its anisotropy, in terms of a combination of epitaxial strain and Bi-induced hybridization of valence states due to alloy disorder, which strongly perturbs the electronic structure.

Photoluminescence (PL) fluctuations of single semiconductor nanocrystals (NCs), such as PL blinking and spectral diffusion (SD), reflect the quantum nature of charges in the NCs. Through simultaneous measurements of PL spectra and lifetimes on single CdSe/ZnS NCs, PL of neutral excitons is found to exhibit a unique behavior of SD, which is accompanied by changes of radiative recombination lifetime. We find that the SD of neutral excitons originated from the quantum-confined Stark effect, which also affects the SD of charged excitons observed during PL blinking.

We study time-resolved transport properties of a Y junction composed of interacting one-dimensional quantum wires using a bosonization approach. In particular, we investigate the ac conductivity of the Y junction formed from finite-length Tomonaga-Luttinger liquid wires based on a plasmon scattering approach for injected charge pulses of arbitrary shapes. In addition, we calculate the tunneling current and quantum noise of the Y junction arising from pointlike tunneling impurities at the junction, including finite-temperature effects. Our results will be useful for designing nanoelectronic quantum circuits and for interpreting time-resolved experiments [Kamata et al., Nat. Nanotechnol. 9, 177 (2014)] in interacting wires and their junctions.

Quantum spin Hall insulators are characterized by topologically protected counterpropagating edge states. Here we study the dynamical response of these helical edge states under a time-dependent flux biasing, in the presence of a heat bath. It is shown that the relaxation time of the edge carriers can be determined from a measurement of the dissipative response of topological insulator disks. The effects of various perturbations, including Zeeman coupling and disorder, are also discussed.

The Korringa-Kohn-Rostoker (KKR) Green's function, multiple-scattering theory is an efficient site-centered, electronic-structure technique for addressing an assembly of $N$ scatterers. Wave functions are expanded in a spherical-wave basis on each scattering center and indexed up to a maximum orbital and azimuthal number $L_{max}={(l,m)}_{max}$, while scattering matrices, which determine spectral properties, are truncated at $L_{tr}={(l,m)}_{tr}$ where phase shifts ${\delta }_{l>l_{tr}}$ are negligible. Historically, $L_{max}$ is set equal to $L_{tr}$, which is correct for large enough $L_{max}$ but not computationally expedient; a better procedure retains higher-order (free-electron and single-site) contributions for $L_{max}>L_{tr}$ with ${\delta }_{l>l_{tr}}$ set to zero [X.-G. Zhang and W. H. Butler, Phys. Rev. B 46, 7433 (1992)]. We present a numerically efficient and accurate augmented-KKR Green's function formalism that solves the KKR equations by exact matrix inversion $[{\mathcal{R}}^3$ process with rank $N{(l_{tr}+1)}^2]$ and includes higher-$L$ contributions via linear algebra $[{\mathcal{R}}^2$ process with rank $N{(l_{max}+1)}^2]$. The augmented-KKR approach yields properly normalized wave functions, numerically cheaper basis-set convergence, and a total charge density and electron count that agrees with Lloyd's formula. We apply our formalism to fcc Cu, bcc Fe, and $\mathrm{L}{1}_0$ CoPt and present the numerical results for accuracy and for the convergence of the total energies, Fermi energies, and magnetic moments versus $L_{max}$ for a given $L_{tr}$.

We report ${}^{35}\mathrm{Cl}$ NMR, ESR, $\mu \mathrm{SR}$, and specific-heat measurements on the $S=\frac{1}{2}$ frustrated kagome magnet kapellasite $\alpha \text{−}{\mathrm{Cu}}_3{\text{Zn(OH)}}_6{\mathrm{Cl}}_2$, where a gapless spin-liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbor exchange interaction $J_1$ and give an energy scale for the competing interactions $\left|J\right|\sim 10$ K. The study of the temperature-dependent ESR line shift reveals a moderate symmetric exchange anisotropy term $D$, with $|D/J|\sim 3$%. These findings validate a posteriori the use of the $J_1\text{−}{J}_2\text{−}{J}_d$ Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagome lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by ${}^{35}\mathrm{Cl}$ NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and $\mu \mathrm{SR}$ in the low-$T$, correlated, spin-liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low-energy modes.

We recently studied a doped two-dimensional bosonic Hubbard model with two hard-core species, with different masses, using quantum Monte Carlo simulations [Hettiarachchilage, Phys. Rev. B 88, 161101(R) (2013)]. Upon doping away from half filling, we find several distinct phases, including a phase-separated ferromagnet with Mott behavior for the heavy species and both Mott-insulating and superfluid behaviors for the light species. Introducing polarization, i.e., an imbalance in the population between species, we find a fully phase-separated ferromagnet. This phase exists for a broad range of temperatures and polarizations. By using finite-size scaling of the susceptibility, we find a critical exponent which is consistent with the two-dimensional Ising universality class. Significantly, since the global entropy of this phase is higher than that of the ferromagnetic phase with single species, its experimental observation in cold atoms may be feasible.

The evolution of the electronic density of states at the Fermi level of intermetallic ${\mathrm{EuFe}}_{2-x}{\mathrm{Ru}}_x{\mathrm{As}}_2$ in the whole concentration range $(0\le x\le 2)$ is reported. Systematic electron spin resonance investigations of the Korringa-type Eu spin relaxation in combination with the results of band-structure calculations and magnetic susceptibility measurements can provide a clear picture of how the density of states is reduced by substituting Fe with Ru in ${\mathrm{EuFe}}_2{\mathrm{As}}_2$. Moreover, the strength of the exchange coupling between the local Eu and the itinerant spin systems can be estimated consistently with the weak coupling expected for this class of layered compounds.

In sliding charge-density wave (CDW) conductors such as ${\text{NbSe}}_3$, voltage oscillations at a frequency proportional to the local CDW velocity accompany CDW motion and can furnish many insights into the dynamics of these systems. We have used high-performance cryogenic differential amplifiers to make position-dependent measurements of voltage oscillations in the quasisteady state. These measurements yield voltage-voltage and velocity-velocity correlations and the temperature dependence of the CDW's longitudinal and transverse velocity coherence.

Carrier density $\left(n_{\mathrm{s}}\right)$ is a crucial parameter that governs the properties of correlated oxides, so the field-effect approach is an ideal tool to investigate the novel physics of the system. Here, the carrier-mediated transport of slightly doped anatase ${\mathrm{TiO}}_2$ epitaxial films were studied by electric double layer transistor (EDLT) gating. The $n_{\mathrm{s}}$ has been increased hugely, and concomitantly, the channels of anatase ${\mathrm{TiO}}_2$ films undergo an insulator-metal transition with a decrease in resistivity by almost three orders of magnitude. More fascinating, the Kondo effect depends very strongly on $n_{\mathrm{s}}$, and the Hall mobility could be enhanced about one order of magnitude with increasing $n_{\mathrm{s}}$. This study shows that EDLT gating is a powerful method to stimulate and mediate novel phenomena of correlated oxides.

We present an experimental study of x-ray absorption near edge structure (XANES) at $L_{2,3},M_{2,3}$, and $K$ edges of, respectively, Fe, Zr, and O in iron-doped zirconia $({\mathrm{ZrO}}_2:\mathrm{Fe})$ for different Fe dopant concentrations $x$ (from $x\sim 6\%$ to $x\sim 25\%$ at.) and make the comparison with ab initio simulations at the O $K$-edge. The x-ray magnetic circular dichroism (XMCD) measurements show no evidence of ferromagnetic (FM) order for all the analyzed samples in agreement with our ab initio simulations, which show an antiferromagnetic (AFM) order. We found that substituting Zr with Fe atoms leads to a radical change in the O $K$-edge XANES spectrum, especially in the pre-edge region where a pre-edge peak appears. This pre-edge peak is ascribed to dipole transitions from O $1s$ to O $2p$ states that are hybridized with the unoccupied Fe $3d$ states. Both theoretical and experimental results reveal that the intensity of the pre-edge peak increases with Fe concentration, suggesting the increase of unoccupied Fe $3d$ states. The increase of Fe concentration increases oxygen vacancies as required for charge neutrality and consequently improves AFM ordering. According to our first-principles calculations, the effect of one Fe atom is mostly localized in the first oxygen shell and vanishes as one moves far from it. Thus the increase of the O $K$-pre-edge peak with increasing Fe concentration is due to the increase of percentage of oxygen atoms that are near neighbors to Fe atoms.

Using density functional theory (DFT)-based calculations, we explore the extent to which achievable modes of atomic layering can tune the near-gap electronic structure of tin- and lead-halide perovskites with applications in dye-sensitized solar cells. We show that regardless of how atomic layering is achieved—whether by the growth of layered inorganic phases such as the Ruddlesden-Popper series, hybrid perovskites connected by organic linker molecules, or layered perovskite heterostructures—their band gaps can similarly be widened by several tenths of an eV or more. Because these classes of compounds are known to have band gaps spanning much of the visible region of the solar spectrum, the ability to controllably tune their near-gap electronic structure could further optimize their performance in solar energy conversion applications. Throughout this work, trends in band gap are explained based on the effects of atomic layering and quantum confinement on the character and energy of band-edge crystal orbitals.

Resistance between two ferromagnetic electrodes coupled to a normal channel depends on their relative magnetizations. The spin-dependent component, $R$, of the resistance changes with magnetic field, $B$, normal to the directions of magnetizations. In the field of spin transport, this change, $R\left(B\right)$, originating from the Larmour spin precession, is called the Hanle curve. We demonstrate that the shape of the Hanle curve evolves upon application of an ac drive and study this evolution theoretically as a function of the amplitude, $B_1$, and frequency, $\omega$, of the drive. If the distance between the electrodes, $L$, is smaller than the spin-diffusion length, ${\lambda }_s$, the prime effect of a weak circular-polarized drive is the shift of the center of the curve to the value of $B$ for which the Larmour frequency, ${\omega }_L$, is $\sim$$B_1^2/\omega$. Magnetic resonance at ${\omega }_L\sim \omega$ manifests itself in the derivative, $\frac{dR}{dB}$. For large $L\gg {\lambda }_s$ the ac drive affects the Hanle curve if the drive amplitude exceeds the spin-relaxation rate, ${\tau }_s^{-1}$, i.e., at $B_1{\tau }_s\gtrsim 1$. The prime effect of the drive is the elimination of a minimum in $R\left(B\right)$. A linearly polarized drive has a fundamentally different effect on the Hanle curve, affecting not its shape but rather its width.