Magnetic properties and the Mott transition are studied in the Hubbard model on the anisotropic triangular lattice described by two hopping parameters $t$ and $t^{\prime }$ in different spatial directions using the variational cluster approximation. Taking into account Néel (AF), 120${}^{\circ }$ Néel (spiral), and collinear (AFC) orderings, the magnetic phase diagram is analyzed at zero temperature and half-filling. We found six phases, AF metal, AF insulator, spiral, AFC, paramagnetic metal, and nonmagnetic insulator, which is a candidate for a spin liquid. Direct transitions from a paramagnetic metal to an AF insulator take place for $0.6\lesssim t^{\prime }/t\lesssim 0.8$, and a nonmagnetic insulator is realized between the paramagnetic metal and magnetic states for $0.8\lesssim t^{\prime }/t\lesssim 1.2$. Around $t^{\prime }/t\simeq 1.2$, the magnetic state (AFC or spiral) is realized above the paramagnetic metal, and as the on-site Coulomb repulsion $U$ increases, it changes to a nonmagnetic insulator. Implications for the $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ are discussed. As for the Mott transition, the structure of the self-energy in the spectral representation is studied in detail. As $U$ increases around the Mott transition point, a single dispersion evolves in the spectral weights of the self-energy, which yields the Mott gap.

The few determinant (FED) approximation introduced in our previous work [Phys. Rev. B 87, 235129 (2013)] is used to describe the ground state, characterized by well-defined spin and space group symmetry quantum numbers as well as doping fractions $N_e/N_{\mathrm{sites}}$, of one-dimensional Hubbard lattices with nearest-neighbor hopping and periodic boundary conditions. Within this multireference scheme, each ground state is expanded in a given number of nonorthogonal and variationally determined symmetry-projected configurations. The results obtained for the ground-state and correlation energies of half-filled and doped lattices with 30, 34, and 50 sites compare well with the exact Lieb-Wu solutions as well as with those obtained with other state-of-the-art approximations. The structure of the intrinsic symmetry-broken determinants resulting from the variational procedure is interpreted in terms of solitons whose translational and breathing motions can be regarded as basic units of quantum fluctuations. It is also shown that in the case of doped one-dimensional lattices, a part of such fluctuations can also be interpreted in terms of polarons. In addition to momentum distributions, both spin-spin and density-density correlation functions are studied as functions of doping. The spectral functions and density of states, computed with an ansatz whose quality can be well controlled by the number of symmetry-projected configurations used to approximate the $N_e\pm 1$ electron systems, display features beyond a simple quasiparticle distribution, as well as spin-charge separation trends.

We study the Kondo effect in a CNT(left lead)-CNT(QD)-CNT(right lead) structure. Here CNT is a single-wall metallic carbon nanotube, for which (1) the valence and conduction bands of electrons with zero orbital angular momentum ($m=0$) coalesce at the two valley points $\mathbf{K}$ and ${\mathbf{K}}^{\prime }$ of the first Brillouin zone and (2) the energy spectrum of electrons with $m\ne 0$ has a gap whose size is proportional to $\left|m\right|$. Following adsorption of hydrogen atoms and application of an appropriately designed gate potential, electron energy levels in the CNT(QD) are tunable to have (1) twofold spin degeneracy; (2) twofold isospin (valley) degeneracy; and (3) threefold orbital degeneracy $m=0,\pm 1$. As a result, an SU(12) Kondo effect is realized with remarkably high Kondo temperature. Unlike the SU(2) case, the low temperature conductance and magnetic susceptibility have a peak at finite temperature. Moreover, the magnetic susceptibilities for parallel and perpendicular magnetic fields (with respect to the tube axis) display anisotropy with a universal ratio ${\chi }_{\mathrm{imp}}^{\parallel }/{\chi }_{\mathrm{imp}}^{\perp }=\eta$ that depends only on the electron's orbital and spin $g$ factors.

We calculate the local current density in pristine armchair graphene nanoribbons (AGNRs) with varying width, $N_{\mathrm{C}}$, employing a density-functional-theory-based ab initio transport formalism. We observe very pronounced current patterns (“streamlines”) with threefold periodicity in $N_{\mathrm{C}}$. They arise as a consequence of quantum confinement in the transverse flow direction. Neighboring streamlines are separated by stripes of almost vanishing flow. As a consequence, the response of the current to functionalizing adsorbates is very sensitive to their placement: adsorbates located within the current filaments lead to strong backscattering, while adsorbates placed in other regions have almost no impact at all.

We investigate theoretically the Josephson junction of semiconductor nanowire with strong spin-orbit (SO) interaction in the presence of magnetic field. By using a tight-binding model, the energy levels $E_n$ of Andreev bound states are numerically calculated as a function of phase difference $\phi$ between two superconductors in the case of short junctions. The dc Josephson current is evaluated from the Andreev levels. In the absence of SO interaction, a 0-$\pi$ transition due to the magnetic field is clearly observed. In the presence of SO interaction, the coexistence of SO interaction and Zeeman effect results in $E_n(-\phi )\ne E_n\left(\phi \right)$, where the anomalous Josephson current flows even at $\phi =0$. In addition, the direction dependence of critical current is observed, in accordance with experimental results.

We report on a colossal negative magnetoresistance (MR) in a GaAs/AlGaAs quantum well which, at low temperatures, is manifested by a drop of the resistivity by more than an order of magnitude at a magnetic field $B\approx 1$ kG. In contrast to MR effects discussed earlier, the MR reported here is not parabolic, even at small $B$, and persists to much higher in-plane magnetic fields and temperatures. Remarkably, the temperature dependence of the resistivity at $B\approx 1$ kG is linear over the entire temperature range studied (from 1 to 30 K) and appears to coincide with the high-temperature limit of the zero-field resistivity, hinting on the important role of acoustic phonons.

We study the neutral exciton energy spectrum fine structure and its spin dephasing in transition metal dichalcogenides such as ${\mathrm{MoS}}_2$. The interaction of the mechanical exciton with its macroscopic longitudinal electric field is taken into account. The splitting between the longitudinal and transverse excitons is calculated by means of the both electrodynamical approach and $\mathit{k}·\mathit{p}$ perturbation theory. This long-range exciton exchange interaction can induce valley polarization decay. The estimated exciton spin dephasing time is in the picosecond range, in agreement with available experimental data.

We develop an approximate theory of phonon-induced topological insulation in Dirac materials. In the weak-coupling regime, long-wavelength phonons may favor topological phases in Dirac insulators with direct and narrow band gaps. This phenomenon originates from electron-phonon matrix elements, which change qualitatively under a band inversion. A similar mechanism applies to weak Coulomb interactions and spin-independent disorder; however, the influence of these on band topology is largely independent of temperature. As applications of the theory, we evaluate the temperature dependence of the critical thickness and the critical stoichiometric ratio for the topological transition in CdTe/HgTe quantum wells and in BiTl(S${}_{1-\delta }$Se${}_{\delta }{)}_2$, respectively.

Nodal angle-resolved photoemission spectra taken on overdoped La${}_{1.77}$Sr${}_{0.23}$CuO${}_4$ are presented and analyzed. It is proven that the low-energy excitations are true Landau Fermi-liquid quasiparticles. We show that momentum and energy distribution curves can be analyzed self-consistently without quantitative knowledge of the bare band dispersion. Finally, by imposing Kramers-Kronig consistency on the self-energy $\Sigma$, insight into the quasiparticle residue is gained. We conclude by comparing our results to quasiparticle properties extracted from thermodynamic, magnetoresistance, and high-field quantum oscillation experiments on overdoped Tl${}_2$Ba${}_2$CuO${}_{6+\delta }$.

A fascinating type of symmetry-protected topological states of matter are topological Kondo insulators, where insulating behavior arises from Kondo screening of localized moments via conduction electrons, and nontrivial topology emerges from the structure of the hybridization between the local-moment and conduction bands. Here we study the physics of Kondo holes, i.e., missing local moments, in three-dimensional topological Kondo insulators, using a self-consistent real-space mean-field theory. Such Kondo holes quite generically induce in-gap states which, for Kondo holes at or near the surface, hybridize with the topological surface states. In particular, we study the surface-state quasiparticle interference (QPI) induced by a dilute concentration of surface Kondo holes and compare this to QPI from conventional potential scatterers. We treat both strong and weak topological-insulator phases and, for the latter, specifically discuss the contributions to QPI from inter-Dirac-cone scattering.

A few organic conductors show a diversity of exciting properties like Mott insulating behavior, spin liquid, antiferromagnetism, bad metal, or unconventional superconductivity controlled by small changes in temperature, pressure, or chemical substitution. While such a behavior can be technologically relevant for functional switches, a full understanding of its microscopic origin is still lacking and poses a challenge in condensed matter physics since these phases may be a manifestation of electronic correlation. Here we determine from first principles the microscopic nature of the electronic phases in the family of organic systems $\kappa$-(ET)${}_2\mathrm{Cu}$[N(CN)${}_2$]Br${}_x$Cl${}_{1-x}$ by a combination of density functional theory calculations and the dynamical mean field theory approach in a form adapted for organic systems. By computing spectral and optical properties we are able to disentangle the origin of the various optical transitions in these materials and prove that correlations are responsible for relevant features. Remarkably, while some transitions are inherently affected by correlations, others are completely uncorrelated. We discuss the consequences of our findings for the phase diagram in these materials.

A recent experiment [E. A. Chekhovich et al., Phys. Rev. Lett. 104, 066804 (2010)] has demonstrated that high nuclear spin polarization can be achieved in self-assembled quantum dots by exploiting an optically forbidden transition between a heavy hole and a trion state. However, a fully polarized state is not achieved as expected from a classical rate equation. Here, we theoretically investigate this problem with the help of a quantum master equation and we demonstrate that a fully polarized state cannot be achieved due to formation of a nuclear dark state. Moreover, we show that the maximal degree of polarization depends on structural properties of the quantum dot.

We have reconstructed third-dimension specimen information from convergent-beam electron diffraction (CBED) patterns simulated using the stacked-Bloch-wave method. By reformulating the stacked-Bloch-wave formalism as an artificial neural network and optimizing with resilient back propagation, we demonstrate specimen orientation reconstructions with depth resolutions down to 5 nm. To show our algorithm's ability to analyze realistic data, we also discuss and demonstrate our algorithm reconstructing from noisy data and using a limited number of CBED disks. Applicability of this reconstruction algorithm to other specimen parameters is discussed.

Despite the fact that structures and properties of tilt grain boundaries of graphite surface and graphene have been extensively studied, their effect on the structures and electronic spectra of graphene layers has not been fully addressed. Here we study effects of one-dimensional tilt grain boundaries on structures and electronic spectra of graphene multilayers by scanning tunneling microscopy and spectroscopy. A tilt grain boundary of a top graphene sheet in graphene multilayers leads to a twist between consecutive layers and generates superstructures (Moiré patterns) on one side of the boundary. Our results demonstrate that the twisting changes the electronic spectra of Bernal graphene bilayer and graphene trilayers dramatically. We also study quantum-confined twisted graphene bilayer generated between two adjacent tilt grain boundaries and find that the band structure of such a system is still valid even when the number of superstructures is reduced to two in one direction. It implies that the electronic structure of this system is driven by the physics of a single Moiré spot.

In contrast to the well-studied multiferroic manganites with a spiral structure, the electric polarization in multiferroic borates is induced within a collinear antiferromagnetic structure and can easily be switched by small static fields. Because of specific symmetry conditions, the static and dynamic properties in borates are directly connected, which leads to giant magnetoelectric and magnetodielectric effects. Here we prove experimentally that the giant magnetodielectric effect in samarium ferroborate, ${\mathrm{SmFe}}_3$(${\mathrm{BO}}_3$)${}_4$, is of intrinsic origin and is caused by an unusually large electromagnon situated in the microwave range. This electromagnon reveals a strong optical activity exceeding 120 degrees of polarization rotation in a millimeter thick sample.

According to earlier first-principles calculations, the spin-disorder contribution to the resistivity of rare-earth metals in the paramagnetic state is strongly underestimated if Matthiessen's rule is assumed to hold. To understand this discrepancy, the resistivity of paramagnetic Fe and Gd is evaluated by taking into account both spin and phonon disorder. Calculations are performed using the supercell approach within the linear muffin-tin orbital method. Phonon disorder is modeled by introducing random displacements of the atomic nuclei, and the results are compared with the case of fictitious Anderson disorder. In both cases, the resistivity shows a nonlinear dependence on the square of the disorder potential, which is interpreted as a resistivity saturation effect. This effect is much stronger in Gd than in Fe. The nonlinearity makes the phonon and spin-disorder contributions to the resistivity nonadditive, and the standard procedure of extracting the spin-disorder resistivity by extrapolation from high temperatures becomes ambiguous. An “apparent” spin-disorder resistivity obtained through such extrapolation is in much better agreement with experiment compared to the results obtained by considering only spin disorder. By analyzing the spectral function of the paramagnetic Gd in the presence of Anderson disorder, the resistivity saturation is explained by the collapse of a large area of the Fermi surface due to the disorder-induced mixing between the electron and hole sheets.

The far-infrared optical transmission has been studied for two lanthanide-based single-chain magnets $\text{DyPhOPh}$ and $\text{TbPhOPh}$ in the frequency range between 3 and 80 ${\mathrm{cm}}^{-1}$. The spectra were acquired at temperatures between 2 and 80 K and magnetic fields up to 6 T. Based on their magnetic field dependence in $\text{DyPhOPh}$ two of the observed absorption lines are identified as transitions inside the crystal field split ${\mathrm{Dy}}^{3+}$ ground multiplet ${}^6{\mathrm{H}}_{15/2}$, coupled to the neighboring spins. In $\text{TbPhOPh}$ one transition was observed inside the crystal-field-split ${\mathrm{Tb}}^{3+}$ ground multiplet ${}^7{\mathrm{F}}_6$. The results allow a spectroscopic investigation of the role of single-ion anisotropy and exchange in Glauber dynamics.

The influence of Co and Cu doping on Ni-Mn-Ga Heusler alloy is investigated using the first-principles exact muffin-tin orbital method in combination with the coherent-potential approximation. Single-element doping and simultaneous doping by both elements are investigated in Ni${}_{50-x}$Co${}_x$Mn${}_{25-y}$Ga${}_{25-z}$Cu${}_{y+z}$ alloys, with dopant concentrations $x,y$, and $z$ up to 7.5 at. %. Doping with Co in the Ni sublattice decreases the ${(c/a)}_{\text{NM}}$ ratio of the nonmodulated (NM) martensite, but it simultaneously increases the cubic phase stability with respect to the NM phase. Doping with Cu in the Mn or in Ga sublattices does not change the ${(c/a)}_{\text{NM}}$ ratio significantly and it decreases the cubic phase stability. For simultaneous doping by Co in the Ni sublattice and Cu in the Mn or Ga sublattices, the effects of the individual dopants are independent and about the same as for the single-element doping. Thus, the ${(c/a)}_{\text{NM}}$ ratio can be adjusted by Co doping while the phase stability can be balanced by Cu doping, resulting in stable martensite with a reduced ${(c/a)}_{\text{NM}}$. The local stability of the cubic phase with respect to the tetragonal deformation can be understood on the basis of a density-of-states analysis.

The structural and magnetic properties of functional Ni-Mn-$Z$ ($Z=\text{Ga}$, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferro- and antiferromagnetic interactions with increasing chemical disorder. The resulting $d$-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition.

We study localized in-gap states and quantum spin Hall effect in Si-doped InAs/GaSb quantum wells. We propose a model with donor and/or acceptor impurities to describe Si dopants. This model shows in-gap bound states and wide conductance plateau with the quantized value $2e^2/h$ in light dopant concentration, consistent with recent experiments by Du et al. (Lingjie Du et al., arXiv:1306.1925). We predict a conductance dip structure due to backward scattering in the region where the localization length $\xi$ is comparable with the sample width $L_y$ but much smaller than the sample length $L_x$.