We analytically calculate the intrinsic longitudinal and transverse optical conductivities of electronic systems which govern by a modified Dirac fermion model Hamiltonian for materials beyond graphene such as monolayer ${\text{MoS}}_2$ and ultrathin film of the topological insulator. We analyze the effect of a topological term in the Hamiltonian on the optical conductivity and transmittance. We show that the optical response enhances in the nontrivial phase of the ultrathin film of the topological insulator and the optical Hall conductivity changes sign at transition from trivial to nontrivial phases which has significant consequences on the circular dichroism property and optical absorption of the system.

The numerical renormalization group is used to solve quantum impurity problems, which describe magnetic impurities in metals, nanodevices, and correlated materials within dynamical mean field theory. Here we present a simple generalization of the Wilson chain, which improves the scaling of computational cost with the number of conduction bands, bringing more complex problems within reach. The method is applied to calculate the $t$ matrix of the three-channel Kondo model at $T=0$, which shows universal crossovers near non-Fermi-liquid critical points. A nonintegrable three-impurity problem with three bands is also studied, revealing a rich phase diagram and novel screening and overscreening mechanisms.

The correlation between the $4f$-conduction electron ($c$-$f$) hybridization and the transition temperature $T_0$ into the unconventional antiferromagnetically (AFM) ordered state in $\text{Ce}({\text{Ru}}_{1-x}$${\text{Fe}}_x{)}_2{\text{Al}}_{10}$ has been investigated. The impact of pressure on the crystal structure and the electronic structure has been measured using x-ray diffraction and resonant x-ray emission spectroscopy at the Ce $L_3$ absorption edge in a diamond anvil cell. Our results show that the lattice spacings imply some correlation with the disappearance of $T_0$. Analyses of the measured valence states, however, show only a weak correlation with the corresponding critical disappearance of the AFM order within experimental errors. Our results may be explained by a scenario based on the Kondo-Heisenberg model beyond the conventional Doniach phase diagram.

We investigate transmission properties of electrons at the interfaces of cobalt-diamond hybrid systems. To this end, we employ a Green's function method based on spin-polarized density-functional theory calculations using a Gaussian orbital representation of the wave functions. The considered open systems are magnetic tunnel junctions consisting of semi-infinite hcp cobalt leads and $n$ layers of carbon atoms stacked in (111) direction. These systems are characterized by a very small lattice mismatch of their constituents. We have identified three types of states that give rise to an almost complete transmission. The transmission for minority electrons is considerably suppressed at the Fermi level in most parts of the interface Brillouin zone.

We performed in situ conductivity measurements by multiple microscopic probes of Ag nanofilms grown on a periodic array of indium atomic wires on Si(111). Within the investigated thickness range the transport properties differ markedly from the bulk case. The ratio between the in-plane conductivity along and perpendicular to the In wires is 1.4 for the 3 monolayer (0.7 nm) thick Ag film and approaches unity with increasing film thickness. The observed anisotropy at the initial stage of the film growth is far larger than expected from the quasi-one-dimensional quantum well electronic structure of the films and ascribed to the carrier scattering at the film/vacuum and film/substrate interfaces. Our findings show that an ordered monolayer inserted at the interface enables drastic changes of the transport behavior of the whole metal nanofilm.

High-resolution spin- and angle-resolved photoemission spectroscopy measurements were performed on the three-dimensional topological insulator Bi${}_2$Te${}_{2.4}$Se${}_{0.6}$, which is characterized by enhanced thermoelectric properties. The Fermi level position is found to be located in the bulk energy gap independent of temperature and it is stable over a long time. Spin textures in the Dirac-cone state at energies above and below the Dirac point as well as in the Rashba-type valence band surface state are observed in agreement with theoretical prediction. The calculations of the surface electronic structure demonstrate that the fractional stoichiometry induced disorder within the Te/Se sublattice does not influence the Dirac-cone state dispersion. In spite of relatively high resistivity, temperature dependence of conductivity shows a weak metallic behavior that could explain the effective thermoelectric properties of the Bi${}_2$Te${}_{2.4}$Se${}_{0.6}$ compound with the in-plane Seebeck coefficient reaching $-330$ $\mu$V/K at room temperature.

The electronic and magnetic bulk properties of half-metallic Heusler alloys such as Co${}_2$FeSi, Co${}_2$FeAl, Co${}_2$MnSi, and Co${}_2$MnAl are investigated by means of ab initio calculations in combination with Monte Carlo simulations. The electronic structure is analyzed using the plane-wave code quantum espresso and the magnetic exchange interactions are determined using the Korringa-Kohn-Rostoker (KKR) method. From the magnetic exchange interactions, the Curie temperature is obtained via Monte Carlo simulations. In addition, electronic transport properties of trilayer systems consisting of two semi-infinite platinum leads and a Heusler layer in-between are obtained from the fully relativistic screened KKR method by employing the Kubo-Greenwood formalism. The focus is on thermoelectric properties, namely, the Seebeck effect and its spin dependence. It turns out that already thin Heusler layers provide highly spin-polarized currents. This is attributed to the recovery of half-metallicity with increasing layer thickness. The absence of electronic states of spin-down electrons around the Fermi level suppresses the contribution of this spin channel to the total conductance, which strongly influences the thermoelectric properties and results in a spin polarization of thermoelectric currents.

We investigated the effects of phosphorus substitution on the shape of the $T_c\left(x\right)$ dome in 1111-type ${\mathrm{SmFeAs}}_{1-y}{\mathrm{P}}_y{\mathrm{O}}_{1-x}{\mathrm{H}}_x$ $(0<x<0.5)$. Hydride ion substitution of oxide sites $({\mathrm{O}}^{2-}\to {\mathrm{H}}^{-})$ exerts a chemical pressure effect, i.e., a structural reduction of the $\mathit{Pn}-\text{Fe}-\mathit{Pn}$ angle α $(\mathit{Pn}=\text{P},\text{As})$ and also dopes electrons into the $\text{Fe}\mathit{Pn}$ layer to induce superconductivity. Isovalent phosphorus substitution $({\mathrm{P}}^{3-}\to {\mathrm{As}}^{3-})$ can induce only a chemical pressure effect, i.e., an increase of α for La substitution of Sm sites. As $y$ increases from 0.0 to 0.5, the single $T_c$ dome gradually splits into two domes, similar to those of ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{H}}_x$ with a $T_c$ valley at $x\approx 0.16$. We found that the $T_c$ valley is located around $(x,\alpha )\approx (0.16,113°)$ for both of the ${\mathrm{SmFeAs}}_{1-y}{\mathrm{P}}_y{\mathrm{O}}_{1-x}{\mathrm{H}}_x$ and ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{H}}_x$ series, irrespective of changes in the Pn anion and Ln cation species. This result suggests that suppression of $T_c$ leads to the emergence of a $T_c$ valley when both the shape of $\mathrm{Fe}P{n}_4$ tetrahedra represented by α and electron-doping level of $x$ meet the above criterion in 1111-type iron oxypnictide superconductors.

We performed neutron single-crystal and synchrotron x-ray powder diffraction experiments in order to investigate the magnetic and crystal structures of the conductive layered triangular-lattice antiferromagnet PdCrO${}_2$ with a putative spin chirality, which contributes to an unconventional anomalous Hall effect. We revealed that the ground-state magnetic structure is a commensurate and nearly coplanar 120${}^{\circ }$ spin structure. The 120${}^{\circ }$ planes in different Cr layers seem to tilt with one another, leading to a small noncoplanarity. Such a small but finite noncoplanar stacking of the 120${}^{\circ }$ planes gives rise to a finite scalar spin chirality, which may be responsible for the unconventional nature of the Hall effect of PdCrO${}_2$.

Using Monte Carlo simulations, we explore the phase diagram and the phase transitions in $\mathrm{U}\left(1\right)\times {\mathbb{Z}}_2$ $n$-band superconductors with spontaneously broken time-reversal symmetry (also termed $s+is$ superconductors), focusing on the three-band case. In the limit of infinite penetration length, the system under consideration can, for a certain parameter regime, have a single first-order phase transition from a $\mathrm{U}\left(1\right)\times {\mathbb{Z}}_2$ broken state to a normal state due to a nontrivial interplay between $\mathrm{U}\left(1\right)$ vortices and ${\mathbb{Z}}_2$ domain walls. This regime may also apply to multicomponent superfluids. For other parameters, when the free energy of the domain walls is low, the system undergoes a restoration of broken ${\mathbb{Z}}_2$ time-reversal symmetry at temperatures lower than the temperature of the superconducting phase transition. We show that inclusion of fluctuations can strongly suppress the temperature of the ${\mathbb{Z}}_2$ transition when frustration is weak. The main result of our paper is that for relatively short magnetic field penetration lengths, the system has a superconducting phase transition at a temperature lower than the temperature of the restoration of the broken ${\mathbb{Z}}_2$ symmetry. Thus, there appears a new phase that is $\mathrm{U}\left(1\right)$ symmetric, but breaks ${\mathbb{Z}}_2$ time-reversal symmetry, an anomalous dissipative (metallic) state.

The first-principles density functional theory calculations have been performed to understand LaTiO${}_3$/LaAlO${}_3$ superlattice. By taking into account of the structural distortions, $U$ dependence, and the exchange correlation functional dependence, we show that the ferromagnetic spin and antiferro-orbital ordering is stabilized in the wide range of strains, which is notably different from the previous reports on the titanate systems. The ground-state spin and orbital configurations critically depend on the structural properties. Our results suggest a possible strain control of the magnetic property in transition-metal oxide heterostructures.

Motivated by recent proposals to realize Majorana bound states in chains and arrays of magnetic atoms deposited on top of a superconductor, we study the topological properties of various chain structures, ladders, and two-dimensional arrangements exhibiting magnetic helices. We show that magnetic domain walls where the chirality of a magnetic helix is inverted support two protected Majorana states giving rise to a tunneling conductance peak twice the height of a single Majorana state. The topological properties of coupled chains exhibit nontrivial behavior as a function of the number of chains beyond the even-odd dichotomy expected from the simple ${\mathbb{Z}}_2$ nature of coupled Majorana states. In addition, it is possible that a ladder of two or more coupled chains exhibit Majorana edge states even when decoupled chains are trivial. We formulate a general criterion for the number of Majorana edge states in multichain ladders and discuss some experimental consequences of our findings.

We analyze the transport properties of a double quantum dot device in the side-coupled configuration. A small quantum dot (QD), having a single relevant electronic level, is coupled to source and drain electrodes. A larger QD, whose multilevel nature is considered, is tunnel-coupled to the small QD. A Fermi-liquid analysis shows that the low-temperature conductance of the device is determined by the total electronic occupation of the double QD. When the small dot is in the Kondo regime, an even number of electrons in the large dot leads to a conductance that reaches the unitary limit, while for an odd number of electrons a two-stage Kondo effect is observed and the conductance is strongly suppressed. The Kondo temperature of the second-stage Kondo effect is strongly affected by the multilevel structure of the large QD. For increasing level spacing, a crossover from a large Kondo temperature regime to a small Kondo temperature regime is obtained when the level spacing becomes of the order of the large Kondo temperature.

In the search for topological phases in correlated electron systems, materials with $5d$ transition-metal ions, in particular the iridium-based pyrochlores $A_2$Ir${}_2$O${}_7$, provide fertile grounds. Several topological states have been predicted but the actual realization of such states is believed to critically depend on the strength of local potentials arising from distortions of the IrO${}_6$ cages. We test this hypothesis by measuring with resonant inelastic x-ray scattering the electronic level splittings in the $A=\text{Y}$, Eu systems, which we show to agree very well with ab initio quantum chemistry electronic-structure calculations for the series of materials with $A=\text{Sm}$, Eu, Lu, and Y. We find, however, that the primary source for quenching the spin-orbit interaction is not a distortion of the IrO${}_6$ octahedra but longer-range lattice anisotropies which inevitably break the local cubic symmetry.

To gain insight into the unconventional superconductivity of Fe pnictides with no electron pockets, we measure the thermal conductivity $\kappa$ and penetration depth $\lambda$ in the heavily hole-doped regime of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$. The residual thermal conductivity ${(\kappa /T)}_{T\to 0\mathrm{K}}$ and $T$ dependence of $\lambda$ consistently indicate the fully gapped superconductivity at $x=0.76$ and the (line) nodal superconductivity at higher hole concentrations. The magnitude of $\frac{\kappa }{T}{T}_c{|}_{T\to 0\mathrm{K}}$ and the slope of $\lambda \left(T\right)$ at low temperatures, both of which are determined by the properties of the low-energy excitations, exhibit a highly unusual nonmonotonic $x$ dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range. We suggest that the observed $x$ dependence is naturally explained by a doping crossover of the gap function between the $s$-wave states with and without sign reversal between $\Gamma$-centered hole pockets.

Monod and Beuneu [P. Monod and F. Beuneu, Phys. Rev. B 19, 911 (1979)] established the validity of the Elliott-Yafet theory for elemental metals through correlating the experimental electron spin resonance linewidth with the so-called spin-orbit admixture coefficients and the momentum-relaxation theory. The spin-orbit admixture coefficients data were based on atomic spin-orbit splitting. We highlight two shortcomings of the previous description: (i) the momentum-relaxation involves the Debye temperature and the electron-phonon coupling whose variation among the elemental metals was neglected, (ii) the Elliott-Yafet theory involves matrix elements of the spin-orbit coupling (SOC), which are however not identical to the SOC induced energy splitting of the atomic levels, even though the two have similar magnitudes. We obtain the empirical spin-orbit admixture parameters for the alkali metals by considering the proper description of the momentum relaxation theory. In addition we present a model calculation, which highlights the difference between the SOC matrix element and energy splitting.

We obtained strain and electric field gradient (EFG) in an $n$-GaAs/Al${}_{0.3}$Ga${}_{0.7}$As (110) quantum well (QW) by optically detected nuclear magnetic resonance (NMR). The dependence of the quadrupolar splitting on an angle between the QW plane and a static magnetic field provided the crystalline-orientation-dependent EFG and strain in the GaAs QW. We also explored the dependence of the NMR line widths on the direction of the external magnetic field with respect to the QW plane. It is likely that the nonuniform EFG as well as the hyperfine interaction governs the inhomogeneous broadening of NMR spectra.

We present a theoretical study of the spin dynamics of a single electron confined in a quantum dot. Spin dynamics is induced by the interplay of electrical driving and the presence of a spatially disordered magnetic field, the latter being transverse to a homogeneous magnetic field. We focus on the case of strong driving, i.e., when the oscillation amplitude $A$ of the electron's wave packet is comparable to the quantum dot length $L$. We show that electrically driven spin resonance can be induced in this system by subharmonic driving, i.e., if the excitation frequency is an integer fraction ($\frac{1}{2},\frac{1}{3}$, etc.) of the Larmor frequency. At strong driving we find that (i) the Rabi frequencies at the subharmonic resonances are comparable to the Rabi frequency at the fundamental resonance, and (ii) at each subharmonic resonance, the Rabi frequency can be maximized by setting the drive strength to an optimal, finite value. In the context of practical quantum information processing, these findings highlight the availability of subharmonic resonances for qubit control with effectivity close to that of the fundamental resonance, and the possibility that increasing the drive strength might lead to a decreasing qubit-flip speed. Our simple model is applied to describe electrical control of a spin-valley qubit in a weakly disordered carbon nanotube.

Through reactive molecular-beam epitaxy or Eu postoxidation and postannealing, large EuO(111) bilayer islands of high quality and exceptional stability are grown on Ir(111). We use scanning tunneling microscopy, low-energy electron diffraction, magneto-optical Kerr effect measurements, and density functional theory to characterize the properties of this ultrathin polar rare-earth metal oxide in atomistic detail. We analyze the crystallographic properties and their growth dependence, the film morphology including the atomic structure of defects, the mechanisms for reduction in the electrostatic potential related to the film polarity, as well as the work function, thermal stability, and magnetic properties, resulting in a comprehensive picture of this new two-dimensional material.