The nonlocal conductance and the current cross-correlations are investigated within scattering theory for three-terminal normal metal–superconductor–normal metal (NSN) hybrid structures. The positive cross-correlations at high transparency found by Mélin, Benjamin, and Martin [Phys. Rev. B 77, 094512 (2008)] are not due to crossed Andreev reflection. On the other hand, local processes can be enhanced by reflectionless tunneling but this mechanism has little influence on nonlocal processes and on current cross-correlations. Therefore Cooper pair splitting cannot be enhanced by reflectionless tunneling. Overall, this shows that NSN structures with highly transparent or effectively highly transparent interfaces are not suited to experimentally producing entangled split pairs of electrons.

Magnetization, dielectric, and calorimetric studies on Bi${}_{0.8}$ Pb${}_{0.2}$ Fe${}_{0.9}$ Nb${}_{0.1}$O${}_3$ (BF-0.2PFN) reveal very weak ferromagnetism but strong dielectric anomaly at the antiferromagnetic transition temperature ($T_N$) characteristic of magnetoelectric coupling. We correlate these results with nuclear and magnetic structure studies using x-ray and neutron powder diffraction techniques, respectively. Rietveld refinements using x-ray powder diffraction data in the temperature range 300 to 673 K reveal pronounced anomalies in the unit cell parameters at $T_N$${}_{,}$ indicating strong magnetoelastic coupling. The nuclear and magnetic structures of BF-0.2PFN were determined from neutron powder diffraction data using a representation theory approach. They show the occurrence of a first-order isostructural phase transition (IPT) accompanying the magnetic ordering below $T_N\sim 566$ K, leading to significant discontinuous change in the ionic polarization (Δ$P_z\sim 1.6\left(3\right)$ μC/cm${}^2$) and octahedral tilt angle (∼0.3°) at $T_N$. The ionic polarization obtained from refined positional coordinates of the nuclear structure and Born effective charges is shown to scale linearly with sublattice magnetization, confirming the presence of linear magnetoelectric coupling in BF-0.2PFN at the atomic level, despite the very low value of remanent magnetization ($M_r$).

We report on ab initio time-dependent spin-dynamics simulations for a two-center magnetic molecular complex within the framework of the time-dependent noncollinear spin-density functional theory. In particular, we discuss how the dynamical behavior of the ab initio spin-density in the time domain can be mapped onto a model Hamiltonian based on the classical Heisenberg spin-spin interaction $J{\mathrm{S}}_1·{\mathrm{S}}_2$. By analyzing individual localized-spin trajectories, extracted from the spin-density evolution, we demonstrate a practical method for evaluating the effective Heisenberg exchange coupling constant $J$ from first-principles simulations. We find that $J$, extracted in such a dynamical way, agrees quantitatively with that calculated by the standard density functional theory broken-symmetry scheme.

We study the interplay between the effects of surface anisotropy and dipolar interactions in monodisperse assemblies of nanomagnets with oriented anisotropy. We derive asymptotic formulas for the assembly magnetization, taking into account temperature, applied field, core and surface anisotropy, and dipolar interparticle interactions. We find that the interplay between surface anisotropy and dipolar interactions is well described by the analytical expression of the assembly magnetization derived here: the overall sign of the product of the two parameters governing the surface and the dipolar contributions determines whether intrinsic and collective terms compete or have synergistic effects on the magnetization. This is illustrated by the magnetization curves of $\gamma$-Fe${}_2$O${}_3$ nanoparticle assemblies in the low concentration limit.

The longitudinal thermopower of a Cu/Co/Cu trilayer system exhibits an oscillatory dependence on the thickness of the Co layer, a behavior related to the formation of quantum well states in the minority spin channel. In addition, it is found to be very sensitive to a switching between an in-plane and out-of-plane magnetization. The resulting magnetothermopower (MTP) is therefore much larger than anticipated from the conventional anisotropic magnetoresistance (AMR). Our calculations establish a direct connection between the magnitude of the MTP signal and the asymmetry of the AMR around the Fermi energy. An enhancement of MTP based on this understanding may offer the possibility of implementing an efficient spin read-out thermoelectric device based on a single ferromagnetic layer.

We study the valley-dependent magnetic and transport properties of massive Dirac fermions in multivalley systems such as transition metal dichalcogenides. The asymmetry of the zeroth Landau level between valleys and the enhanced magnetic susceptibility can be attributed to the different orbital magnetic moment tied with each valley. This allows the valley polarization to be controlled by tuning the external magnetic field and the doping level. As a result of this magnetic-field-induced valley polarization, there exists an extra contribution to the ordinary Hall effect. All these effects can be captured by a low-energy effective theory with a valley-orbit coupling term.

Dielectric properties of α-MoO${}_3$ are investigated by a combination of valence electron-energy-loss spectroscopy and ab initio calculation at the random-phase approximation level with the inclusion of local-field effects (LFE). A meticulous comparison between experimental and calculated spectra is performed in order to interpret calculated dielectric properties. The dielectric function of MoO${}_3$ has been obtained along the three axes and the importance of LFE has been shown. In particular, taking into account LFE is shown to be essential to describe properly the intensity and position of the Mo-$N_{2,3}$ edges as well as the low-energy part of the spectrum. A detailed study of the energy-loss function in connection with the dielectric response function also shows that the strong anisotropy of the energy-loss function of α-MoO${}_3$ is driven by an anisotropic influence of LFE. These LFE significantly dampen a large peak in ε${}_2$, but only along the [010] direction. Thanks to a detailed analysis at specific $k$ points of the orbitals involved in this transition, the origin of this peak has not only been evidenced but a connection between the inhomogeneity of the electron density and the anisotropic influence of local-field effects has also been established. More specifically, this anisotropy is governed by a strongly inhomogeneous spatial distribution of the empty states. This depletion of the empty states is localized around the terminal oxygens and accentuates the electron inhomogeneity.

The electronic structure of recently synthesized square superlattices with atomic coherence composed of PbSe, CdSe, or CdTe nanocrystals (NCs) attached along $\left\{100\right\}$ facets is investigated using tight-binding calculations. In experimental realizations of these systems [W. H. Evers et al., Nano Lett. 13, 2317 (2013)], NC facets are atomically bonded, resulting in single-crystalline sheets, which, due to their nanogeometry, have an effective dimensionality below two. We predict electronic structures composed of successive bands formed by strong coupling between the wave functions of nearest-neighbor NCs. This coupling is mainly determined by the number of atoms at the NC bonding plane. The band structures deviate markedly from that of the corresponding two-dimensional (2D) quantum well; the 2D case can be recovered, however, if the effects of the nanogeometry are gradually reduced. The width of the bands can reach hundreds of meV, ascribing highly promising transport properties to square superlattices. The band edges are located at $k=0$ except for PbSe superlattices, where their position in $k$ space surprisingly depends on the parity of the number of $\left\{100\right\}$ atomic planes in the NCs. Our calculations demonstrate that semiconductors with dimensionality below two have a strong potential for (opto-)electronic, photovoltaic, and spintronic applications.

Silicene is a honeycomb structure of silicon atoms which shares many intriguing properties with graphene. We investigate the electronic properties of a silicene nanodisk, which is silicene with a closed edge. In the case of a graphene nanodisk, the number of the zero-energy modes is given by the lower bound of the inequality in a bipartite system, ${\xi }_0\ge 2|N_A-N_B|$, where ${\xi }_0$ is the number of zero-energy modes and $N_{A/B}$ is the number of sites in a $A/B$ sublattice. They are standing wave states. In the case of a silicene nanodisk, they propagate around a nanodisk as a helical current, because silicene is a topological insulator due to the spin-orbit interaction. We have found the counting rule of the zero-energy states and constructed the low-energy theory of zigzag triangular silicene. We also show the validity of the bulk-edge correspondence in a nanodisk with a rough edge by calculating the local probability amplitude and current. Finally, the signatures of the topological phase transition induced by an external electric field are discussed. Our results will be observable by means of scanning tunneling microscopy experiments.

The topological entanglement entropy (TEE) of Gutzwiller projected RVB state is studied with Monte Carlo simulation. With tricks to speed up the convergence of TEE, we are able to show that the spin liquid state studied in Zhang et al. [Phys. Rev. B 84, 075128 (2011)] actually does not support $Z_2$ topological order. We find that both an approximate Marshall sign structure and a long-ranged RVB amplitude are responsible for the suppression of the vison gap in this spin liquid state. On the other hand, a TEE of $\ln 2$ is clearly demonstrated for a short range RVB state on triangular lattice proposed in Yunoki et al. [Phys. Rev. B 74, 014408 (2006)]. We find the TEE of the RVB wave function is mainly determined by its phase rather than its amplitude. Our results indicate that the effective field theory misses some key information for the topological property of a RVB state.

The dispersion relations for surface plasmon polaritons propagating in the Voigt geometry in a metal-insulator-metal waveguide with a magneto-optically active dielectric medium are derived. The symmetry between the upper and lower interfaces is broken by the introduction of the magnetic field; the balance between the field distributions on the two interfaces can be controlled by the applied field. This control is illustrated by finite-element method numerical simulations of the field distributions around a point dipole placed in the center of the short waveguide; it is shown that both the total emission of radiation from the cavity and the distribution of the far-field radiation can be strongly modified by tuning the magnetization of the waveguide. This raises the possibility of using magnetic fields to control light propagation in nanostructures.

Polarization fields associated with one-monolayer-thick InN/GaN multiple quantum wells (MQWs) cause shifts of the photoluminescence peak that depend on the GaN barrier layer thickness. Diffraction contrast and aberration-corrected scanning transmission electron microscopy show that the InN QWs are well defined and coherently strained. Mapping of electrostatic potential using off-axis electron holography shows that the electric fields inside the GaN barriers decrease from ∼0.7 to ∼0.2 MV/cm as the barrier layer thickness increases from 5 to 20 nm. Atomistic tight-binding calculations agree closely with experiment, and confirm that changes in optical emission of these III-nitride quantum wells result from changes in the spontaneous and piezoelectric polarization fields in the InN quantum wells and the GaN barrier layers. Overall, this QW system provides the basis for InN-based light-emitting devices operating across a useful band of wavelengths at room temperature.

The spin polarization of chromium at the interface next to a ferromagnetic layer is of general interest because of the competing ferromagnetic and antiferromagnetic exchange coupling. While the Fe/Cr interface has been well studied, information on the Co/Cr interface still remains scarce. Here we show for epitaxially grown Cr/Co/Cr(100) trilayers with smooth interfaces x-ray resonant magnetic scattering (XRMS) results in a saturation field of $\pm$270 mT, recorded at the Co and Cr $L_3$ edges, respectively. The XRMS results at the Co edge show the expected asymmetry and a ferromagnetic hysteresis for different incident angles $\theta$. Furthermore, XRMS measurements with the energy tuned to the Cr $L_3$ edge also exhibit an asymmetry, albeit much smaller than the one at the Co $L_3$ edge. Moreover, the magnetic hysteresis of Cr taken at the $L_3$ edge has a sign opposite to that of Co at the $L_3$ edge over a broad range of incident angles. From these results we infer first that at the Co/Cr interface chromium is ferromagnetically polarized, and second that its spin structure is oriented opposite to the magnetization of Co.

The magnetization reversal processes in ferromagnetic nanowires with sinusoidally modulated edges were investigated as a function of modulation amplitude and wavelength. The reversal processes were studied in two regimes: nucleation controlled reversal and magnetization reversal mediated by domain-wall propagation. In the latter case, domain walls were introduced using both nucleation-pad structures and local pulsed-field injection techniques. The reversal behavior shows that competing effects govern the switching fields in these structures, giving a minimum as a function of modulation wavelength, showing promising results for improved control of domain-wall propagation behavior. The experimental results were interpreted with detailed micromagnetic simulations and an analytical model, based on the demagnetization effects of the modulation upon the spin structure of the wire. The analysis highlights consistent trends in the reversal behavior resulting from modulation, and, significantly, the switching behavior is found to be scalable in relation to the amplitude and wavelength.

We theoretically investigate the behavior of Andreev interferometers with three superconducting electrodes in the current-biased regime. Our analysis allows us to predict a number of interesting features of such devices, such as both hysteretic and nonhysteretic behavior, negative magnetoresistance, and two different sets of singularities of the differential resistance at subgap voltages. In the nonhysteretic regime we find a pronounced voltage modulation with the magnetic flux which can be used for improving the sensitivity of Andreev interferometers.

We suggest a mechanism causing large positive magnetoresistance (MR) in dilute magnetic semiconductors when hopping via nonmagnetic donor impurities dominates the conductivity. The effect is due to the increase in the characteristic width $\sigma$ of the donor energy distribution with increasing magnetic field $B$, caused by exchange interactions between magnetic Mn atoms and the electrons localized on nonmagnetic Cl donor impurities. Using general scaling arguments based solely on the dependencies of hopping rates on temperature and on the energies of hopping sites we show that this mechanism accounts quantitatively for our experimental data on MR in $n$-type Zn${}_{1-x}$Mn${}_x$Se:Cl. We suggest a method for extracting the dependence of $\sigma$ on magnetic field from the MR data. The mechanism explains the experimentally observed universal dependence of the MR effect on the ratio $B/T$ at different temperatures $T$ under the premise that transport is due to the nearest-neighbor or Mott hopping mechanism.

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm${}^{-1}$ and its second-order overtone at twice this wavenumber move up in energy with decreasing nanocrystal diameter. This anomalous behavior is interpreted in terms of quantum confinement of the longitudinal optical (LO) phonon whose frequency displays a minimum at Γ in the dispersion of bulk PbSe. We perform ab initio calculations of the phonons of PbSe slabs with up to 15 layers. The LO mode perpendicular to the slab shifts indeed upwards with decreasing layer thickness, thus validating the interpretation of the anomalous radius dependence of the Raman spectra in terms of quantum confinement.

In the framework of density functional theory combined with the Lippmann-Schwinger formalism in scattering approaches, we calculate the Seebeck coefficients for carbon atom chains sandwiched between aluminum electrodes from first principles. The most striking feature is that the Seebeck coefficients are length dependent, and can be negative ($n$ type) or positive ($p$ type) for odd- or even-numbered carbon atom chains. Alternating $n$-type and $p$-type material properties of carbon atom chains are due to the full- and half-filled ${\pi }^{*}$-orbital states near the chemical potential.

We have investigated the Fermi surface nesting-derived magnetic phases in the graphenelike BC${}_3$ via density functional theory. Two nesting vectors emerge when the conduction $\pi$ band is half filled through chemical doping. The density of states at the Fermi level exhibits the van Hove singularities. This fermiology induces competing spin density waves and ferromagnetism. We show that the fermiology and competitions among magnetic orders are tunable under biaxial strain which induces rich magnetic quantum phase transitions. Systematic calculations are performed to elucidate the strain-induced quantum phase transitions and map out the phase diagram. Our results suggest BC${}_3$ as an experimentally feasible nanosystem for exploring the nesting-driven quantum phases.