Josephson currents are carried by sharp Andreev states within the superconducting energy gap. We theoretically study the electronic transport of a magnetically tunable nanoscale junction consisting of a quantum dot connected to two superconducting leads and coupled to the spin of a molecular magnet. The exchange interaction between the molecular magnet and the quantum dot modifies the Andreev states due to a spin-dependent renormalization of the quantum dot's energy level and the induction of spin flips. A magnetic field applied to the central region of the quantum dot and the molecular magnet further tunes the Josephson current and starts a precession of the molecular magnet's spin. We use a nonequilibrium Green's function approach to evaluate the transport properties of the junction. Our calculations reveal that the energy level of the dot, the magnetic field, and the exchange interaction between the molecular magnet and the electrons occupying the energy level of the quantum dot can trigger transitions from a 0 to a $\pi$ state of the Josephson junction. The redistribution of the occupied states induced by the magnetic field strongly modifies the current-phase relation. The critical current exhibits a sharp increase as a function of either the energy level of the dot, the magnetic field, or the exchange interaction.

Following an earlier construction of exactly soluble lattice models for Abelian fractional topological insulators in two and three dimensions, we construct here an exactly soluble lattice model for a non-Abelian $\nu =1$ quantum Hall state and a non-Abelian topological insulator in two dimensions. We show that both models are topologically ordered, exhibiting fractionalized charge, ground-state degeneracy on the torus, and protected edge modes. The models feature non-Abelian vortices which carry fractional electric charge in the quantum Hall case and spin in the topological insulator case. We analyze the statistical properties of the excitations in detail and discuss the possibility of extending this construction to three-dimensional non-Abelian topological insulators.

A theoretical study is reported of electron transport at finite temperature in a double quantum dot (DQD) capacitively coupled to a quantum point contact (QPC) for the measurement of the DQD charge state. Starting from a Hamiltonian model, a master equation is obtained for the stochastic process taking place in the DQD while the QPC is at or away from equilibrium, allowing us to study the measurement back-action of the QPC onto the DQD. The QPC is treated nonperturbatively in our analysis. Effective fluctuation theorems are established for the full counting statistics of the DQD current under different limiting conditions. These fluctuation theorems hold with respect to an effective affinity characterizing the nonequilibrium environment of the DQD and differing from the applied voltage if the QPC is out of equilibrium. The effective affinity may even change its sign if the Coulomb drag of the QPC reverses the DQD current. The thermodynamic implications of the effective fluctuation theorems are discussed.

We propose an experimental approach to extract separately the surface and interface phase shift of standing waves in metallic quantum well (QW) structures composed of isostructural perovskite oxides. The “asymmetric” vacuum/SrVO${}_3$/SrTiO${}_3$ and “symmetric” SrTiO${}_3$/SrVO${}_3$/SrTiO${}_3$ QW structures are fabricated in an epitaxial multilayer form. Using these metallic QW structures, the phase shifts at the surface (vacuum/SrVO${}_3$) and interface (SrTiO${}_3$/SrVO${}_3$) are successfully obtained by analyzing a thickness series of angle-resolved photoemission spectra. The difference of the phase shift between the two boundaries reveals that nearly ideal quantum confinement is achieved at the interface, indicating that a SrTiO${}_3$ layer acts as a useful potential barrier.

The interaction of a single manganese impurity with silicon is analyzed in a combined experimental and theoretical study of the electronic, magnetic, and structural properties of manganese-doped silicon clusters. The structural transition from exohedral to endohedral doping coincides with 3$d$ electron delocalization and a quenching of high-spin states. For all geometric structures investigated, we find a correlation of the magnetic moment with the manganese coordination number and nearest-neighbor distance. This observation can be generalized to manganese point defects in bulk silicon, whose magnetic moments fall within the observed magnetic-to-nonmagnetic transition, and therefore react very sensitively to changes in the local geometry. The results indicate that high-spin states in manganese-doped silicon could be stabilized by an appropriate lattice expansion.

We present an ab initio $GW$ calculation to study dynamical effects on an organic compound (TMTSF)${}_2$PF${}_6$. Calculated polarized reflectivities reproduce experimental plasma edges at around 0.2 eV for $E\parallel b^{\prime }$ and 1.0 eV for $E\parallel a$. The low-energy plasmons come out from the low-energy narrow bands energetically isolated from other higher-energy bands, and affect the low-energy electronic structure via the $GW$-type self-energy. Because of the quasi-one-dimensional band structure, a distinct plasmaron state is observed along the $Y$-$\Gamma$ line and a large plasmon-induced electron scattering is found in the low-energy occupied states along the $X$-$M$ line.

Typical persistent current ($I_{\mathrm{typ}}$) in a mesoscopic normal metal ring with disorder due to rough edges and random grain boundaries is calculated by use of a scattering matrix method. In addition, resistance of a corresponding metallic wire is obtained from the Landauer formula and the electron mean free path ($l$) is determined. If disorder is due to the rough edges, a ballistic persistent current $I_{\mathrm{typ}}\simeq e{v}_F/L$ is found to coexist with the diffusive resistance ($\propto L/l$), where $v_F$ is the Fermi velocity and $L\gg l$ is the ring length. This ballistic current is due to a single electron that moves almost in parallel with the rough edges and thus hits them rarely (it is shown that this parallel motion exists in the ring geometry due to the Hartree-Fock interaction). Our finding agrees with a puzzling experimental result $I_{\mathrm{typ}}\simeq e{v}_F/L$, reported by Chandrasekhar et al. [Phys. Rev. Lett. 67, 3578 (1991)] for metallic rings of length $L\simeq 100l$. If disorder is due to the grain boundaries, our data reproduce the theoretical result $I_{\mathrm{typ}}\simeq (e{v}_F/L)(l/L)$ that holds for the white-noise-like disorder and has been observed in recent experiments. Thus, result $I_{\mathrm{typ}}\simeq e{v}_F/L$ in a disordered metallic ring of length $L\gg l$ is as normal as result $I_{\mathrm{typ}}\simeq (e{v}_F/L)(l/L)$. Which result is observed depends on the nature of disorder. Experiments that would determine $I_{\mathrm{typ}}$ and $l$ in correlation with the nature of disorder can be instructive.

We probe the structure and electronic band structure of graphene grown on a Pt(100) substrate using scanning tunneling microscopy, low energy electron diffraction, and angle-resolved photoemission spectroscopy. It is found that the graphene layer lacks a well-defined azimuthal orientation with respect to the substrate, causing a circular smearing of the $\pi$ band instead of a well-defined Dirac cone near the Fermi level. The graphene is found to be electron doped placing the Dirac point $\sim$0.45 eV below the Fermi level, and a gap of $0.15\pm 0.03$ eV is found at the Dirac point. We dose atomic hydrogen and monitor the coverage on the graphene by analyzing the impurity-induced broadening of the $\pi$-band width. Saturation of graphene on Pt(100) with hydrogen does not expand the band gap, but instead hydrogen-mediated broadening and rehybridization of the graphene $s{p}^2$ bonds into $s{p}^3$ leads to a complete disruption of the graphene $\pi$ band, induces a lifting of the Pt(100) reconstruction, and introduces a dispersing Pt state near the Fermi level. Deposition of rubidium on graphene on Pt(100) leads to further electron doping, pushing the Dirac point to a binding energy of $\sim$1.35 eV, and increasing the band gap to $0.65\pm 0.04$ eV.

We used low-temperature synchrotron x-ray diffraction to investigate the structural phase transitions of Fe${}_{1+y}$Te in the vicinity of a tricitical point in the phase diagram. A detailed analysis of the powder diffraction patterns and temperature dependence of the peak widths in Fe${}_{1+y}$Te showed that two-step structural and magnetic phase transitions occur within the compositional range $0.11\le y\le 0.13$. The phase transitions are sluggish, indicating a strong competition between the orthorhombic and the monoclinic phases. We combine high-resolution diffraction experiments with specific heat, resistivity, and magnetization measurements and present a revised temperature-composition phase diagram for Fe${}_{1+y}$Te.

Spin wave transmission experiments are performed on a one-dimensional magnonic crystal (MC) where an injection pad for domain walls reverses the magnetization $M$ of selected nanostripes independently from the otherwise saturated MC. The MC consists of a periodic array of 255-nm-wide permalloy nanostripes with an edge-to-edge separation of 45 nm. In the experiment and simulations, we find that a single nanostripe with antiparallel $M$ performing opposite spin precession reduces significantly the transmission of long-wavelength spin waves. Our findings allow for the implementation and current-controlled operation of magnonic devices such as spin-wave-based logic on the nanoscale.

Epitaxial Y${}_3$Fe${}_5$O${}_{12}$ thin films deposited by off-axis sputtering exhibit excellent crystalline quality enabling observation of large spin pumping signals in Pt/Y${}_3$Fe${}_5$O${}_{12}$ and W/Y${}_3$Fe${}_5$O${}_{12}$ bilayers driven by cavity ferromagnetic resonance. The inverse spin Hall voltages reach 2.10 and −5.26 mV in 5-mm-long Pt/Y${}_3$Fe${}_5$O${}_{12}$ and W/Y${}_3$Fe${}_5$O${}_{12}$ bilayers, respectively, excited by a radio-frequency magnetic field of 0.3 Oe. From the ferromagnetic resonance linewidth broadening, we obtain high interfacial spin mixing conductances of 4.56×10${}^{14}$ and 2.30×10${}^{14}$ ${\Omega }^{-1}$ m${}^{-2}$ for Pt/Y${}_3$Fe${}_5$O${}_{12}$ and W/Y${}_3$Fe${}_5$O${}_{12}$ bilayers, respectively.

We report magnetic domain imaging of a terbium cobalt (TbCo) alloy thin film with a high perpendicular magnetic anisotropy in one- and two-photon photoemission electron microscopy (PEEM). Both photoemission processes deliver a clear magnetic circular dichroism (MCD) whose strength is strongly energy dependent. Comparing the energy dependence of the MCD signal in one- and two-photon photoemission, we conclude that the magnetic contrast is mainly an initial state effect. Our results ultimately show that MCD contrast can be obtained in PEEM in valence band photoemission from a material supporting all-optical magnetization switching. This opens the way for the investigation of the all-optical switching process with simultaneous ultrahigh temporal and spatial resolution.

Magnetic order consistent with multiferroism has been observed in the acentric LiNbO${}_3$-type, high pressure form II of MnTiO${}_3$ using neutron diffraction and magnetization measurements. Spin order below the 28 K magnetic transition has propagation vector (0 0 0), and spins lie in the $ab$ plane. Representation symmetry analysis shows that the antiferromagnetic Mn${}^{2+}$ spin component observed by neutron scattering, of magnitude 3.9(1) μ${}_B$ at 2 K, coexists with a weak ferromagnetic component of magnitude 0.0014 μ${}_B$. This magnetization is perpendicular to the electrical polarization resulting from cation displacements in this acentric structure, permitting coupled switches of the two ferroic orders. The spin order is stable to fields of at least 5 T; however, facile domain reorientation within the $ab$ plane enhances the antiferromagnetic susceptibility and a constant magnetization/field ($M$/$H$) is observed in field strengths greater than ∼1.5 T. Suppression of the {101} neutron intensity with field follows the same Brillouin-dependence as $M$/$H$ and enables the antiferromagnetic easy axes to be identified as parallel to the $ab$ plane hexagonal axes.

We have used soft x-ray magnetic diffraction at the Fe${}^{3+}$ $L_{2,3}$ edges to examine to what extent the Dzyaloshinsky-Moriya interaction in Ba${}_3$NbFe${}_3$Si${}_2$O${}_{14}$ influences its low-temperature magnetic structure. A modulated component of the moments along the $c$ axis is present, adding to the previously proposed helical magnetic configuration of coplanar moments in the $a,b$ plane. This leads to a “helical-butterfly” structure and suggests that both the multiaxial in-plane and the uniform out-of-plane Dzyaloshinsky-Moriya vectors are relevant. A nonzero orbital magnetic signal is also observed at the oxygen $K$ edge, which reflects the surprisingly strong hybridization between iron 3$d$ and oxygen 2$p$ states, given the nominal spherical symmetry of the Fe${}^{3+}$ half-filled shell.

We assess the performance of a recently proposed renormalized adiabatic local density approximation (rALDA) for ab initio calculations of electronic correlation energies in solids and molecules. The method is an extension of the random phase approximation (RPA) derived from time-dependent density functional theory and the adiabatic connection fluctuation-dissipation theorem and contains no fitted parameters. The new kernel is shown to preserve the accurate description of dispersive interactions from RPA while significantly improving the description of short-range correlation in molecules, insulators, and metals. For molecular atomization energies, the rALDA is a factor of 7 better than RPA and a factor of 4 better than the Perdew-Burke-Ernzerhof (PBE) functional when compared to experiments, and a factor of 3 (1.5) better than RPA (PBE) for cohesive energies of solids. For transition metals, the inclusion of full shell semicore states is found to be crucial for both RPA and rALDA calculations and can improve the cohesive energies by up to 0.4 eV. Finally, we discuss straightforward generalizations of the method, which might improve results even further.

We present the exact spectral function for a single oxygen hole with spin opposite to ferromagnetic order within a one-dimensional CuO${}_3$-like spin chain. We find that local Kondo-like exchange interaction generates five different states in the strong-coupling regime. It stabilizes a spin polaron which is a bound state of a moving charge dressed by magnon excitations, with essentially the same dispersion as predicted by mean-field theory. We then examine in detail the evolution of the spectral function for increasing strength of the hole-magnon interaction. We also demonstrate that the $s$ and $p$ symmetries of orbital states in the conduction band are essentially equivalent to each other and find that the simplified models do not suffice to reproduce subtle aspects of hole-magnon coupling in the charge-transfer model.

Quantum dots with light-hole ground states could find numerous applications including faster quantum bit operations or coherent conversion of photons into electron spins. Typically, however, holes confined in epitaxial quantum dots are of heavy-hole character. I show, by use of atomistic tight-binding theory, that the hole ground state undergoes a transition from heavy holelike to light holelike with increasing height of a nanowire InAs/InP quantum dot. The fine structure of the light-hole exciton consists of a dark ground state and three bright states. Two of the bright states are quasidegenerate and are in-plane polarized, whereas, the third energetically higher bright state is polarized in the perpendicular out-of-plane direction. The light-hole exciton fine structure is robust against alloying.

We study finite-temperature properties of strongly interacting fermions in the honeycomb lattice using numerical linked-cluster expansions and determinantal quantum Monte Carlo simulations. We analyze a number of thermodynamic quantities, including the entropy, the specific heat, uniform and staggered spin susceptibilities, short-range spin correlations, and the double occupancy at and away from half filling. We examine the viability of adiabatic cooling by increasing the interaction strength for homogeneous as well as for trapped systems. For the homogeneous case, this process is found to be more efficient at finite doping than at half filling. That, in turn, leads to an efficient adiabatic cooling in the presence of a trap, which, starting with even relatively high entropies, can drive the system to have a Mott insulating phase with substantial antiferromagnetic correlations.

A finite-element method is presented for calculating the quantum conductance of mesoscopic two-dimensional electron devices of complex geometry attached to semi-infinite leads. For computational purposes, the leads must be cut off at some finite length. To avoid spurious, unphysical reflections, this is modeled by transparent boundary conditions. We introduce the Hardy space infinite-element technique from acoustic scattering as a way of setting up transparent boundary conditions for transport computations spanning the range from the quantum mechanical to the quasiclassical regime. These boundary conditions are exact even for wave packets and thus are especially useful in the limit of high energies with many excited modes. Yet, they possess a memory-friendly sparse matrix representation. In addition to unbounded domains, Hardy space elements allow us to truncate those parts of the computational domain which are irrelevant for the calculation of the transport properties. Thus, the computation can be done only on the region that is essential for a physically meaningful simulation of the scattering states. The benefits of the method are demonstrated by three examples. The convergence properties are tested on the transport through a quasi-one-dimensional quantum wire. It is shown that higher-order finite elements considerably improve current conservation and establish the correct phase shift between the real and the imaginary parts of the electron wave function. The Aharonov-Bohm effect demonstrates that characteristic features of quantum interference can be assessed. A simulation of electron magnetic focusing exemplifies the capability of the computational framework to study the crossover from quantum to quasiclassical behavior.

The valley pseudospin is an inherent electron degree of freedom in graphene. This work establishes a theory for manipulation of valley pseudospins at the quantum level, in bilayer graphene. Two key mechanisms of valley manipulation are proposed and valley-based quantum devices—qubits and field-effect transistors—are implemented based on the mechanisms. This work provides a crucial step in paving the way for the experimental realization (expansion) of valley-based quantum (classical) information processing.