Current-induced spin transfer torque and magnetization dynamics in the presence of spin diffusion in disordered magnetic textures is studied theoretically. We demonstrate using tight-binding calculations that weak, spin-conserving impurity scattering dramatically enhances the nonadiabaticity. To further explore this mechanism, a phenomenological drift-diffusion model for incoherent spin transport is investigated. We show that incoherent spin diffusion indeed produces an additional spatially dependent torque of the form $\sim {\nabla }^2[\mathbf{m}\times (\mathbf{u}·\nabla )\mathbf{m}]+\xi {\nabla }^2\left[(\mathbf{u}·\nabla )\mathbf{m}\right]$, where $\mathbf{m}$ is the local magnetization direction, $\mathbf{u}$ is the direction of injected current, and $\xi$ is a parameter characterizing the spin dynamics (precession, dephasing, and spin-flip). This torque, which scales as the inverse square of the domain wall width, only weakly enhances the longitudinal velocity of a transverse domain wall but significantly enhances the transverse velocity of vortex walls. The spatial-dependent spin transfer torque uncovered in this study is expected to have significant impact on the current-driven motion of abrupt two-dimensional textures such as vortices, skyrmions, and merons.

Motivated by a recent experiment in which zero-bias peaks have been observed in scanning tunneling microscopy (STM) experiments performed on chains of magnetic atoms on a superconductor, we show, by generalizing earlier work, that a multichannel ferromagnetic wire deposited on a spin-orbit coupled superconducting substrate can realize a nontrivial chiral topological superconducting state with Majorana bound states localized at the wire ends. The nontrivial topological state occurs for generic parameters requiring no fine tuning, at least for very large exchange spin splitting in the wire. We theoretically obtain the signatures which appear in the presence of an arbitrary number of Majorana modes in multiwire systems incorporating the role of finite temperature, finite potential barrier at the STM tip, and finite wire length. These signatures are presented in terms of spatial profiles of STM differential conductance, which clearly reveal zero-energy Majorana end modes and the prediction of a multiple Majorana based fractional Josephson effect. A substantial part of this work is devoted to a detailed critical comparison between our theory and the recent STM experiment claiming the observation of Majorana fermions in ferromagnetic atomic chains on a superconductor. The conclusion of this detailed comparison is that although the experimental observations are not manifestly inconsistent with our theoretical findings, the very small topological superconducting gap and the very high temperature of the experiment make it impossible to decisively verify the existence of a localized Majorana zero mode, as the spectral weight of the Majorana mode is necessarily spread over a very broad energy regime exceeding the size of the gap. Such an extremely broad (and extremely weak) conductance peak could easily arise from any subgap states existing in the rather complex system studied experimentally and may or may not have anything to do with a putative Majorana zero mode as discussed in the first half of our paper. Thus, although the experimental findings are indeed consistent with a highly broadened and weakened Majorana zero-bias peak, much lower experimental temperatures (and/or much larger experimental topological superconducting gaps) are necessary for any definitive conclusion.

We report elastic neutron scattering and transport measurements on the Ni and Cr equivalently doped iron pnictide ${\mathrm{BaFe}}_{2-2x}{\mathrm{Ni}}_x{\mathrm{Cr}}_x{\mathrm{As}}_2$. Compared with the electron-doped ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$, the long-range antiferromagnetic (AF) order in ${\mathrm{BaFe}}_{2-2x}{\mathrm{Ni}}_x{\mathrm{Cr}}_x{\mathrm{As}}_2$ is gradually suppressed with vanishing ordered moment and Néel temperature near $x=0.20$ without the appearance of superconductivity. A detailed analysis on the transport properties of ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x\mathrm{As}$ and ${\mathrm{BaFe}}_{2-2x}{\mathrm{Ni}}_x{\mathrm{Cr}}_x{\mathrm{As}}_2$ suggests that the non-Fermi-liquid behavior associated with the linear resistivity as a function of temperature may not correspond to the disappearance of the static AF order. From the temperature dependence of the resistivity in overdoped compounds without static AF order, we find that the transport properties are actually affected by Cr impurity scattering, which may induce a metal-to-insulator crossover in highly doped ${\mathrm{BaFe}}_{1.7-y}{\mathrm{Ni}}_{0.3}{\mathrm{Cr}}_y{\mathrm{As}}_2$.

The edges of time-reversal symmetry breaking topological superconductors support chiral Majorana bound states as well as spontaneous charge currents. The Majorana modes are a robust, topological property, but the charge currents are nontopological—and therefore sensitive to microscopic details—even if we neglect Meissner screening. We give insight into the nontopological nature of edge currents in chiral $p$-wave superconductors using a variety of theoretical techniques, including lattice Bogoliubov–de Gennes equations, the quasiclassical approximation, and the gradient expansion, and we describe those special cases in which edge currents do have a topological character. While edge currents are not quantized, they are generically large, but they can be substantially reduced for a sufficiently anisotropic gap function, a scenario of possible relevance for the putative chiral $p$-wave superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

Using all-atom molecular dynamics (MD) simulations, we have studied the mechanical properties of ZnS/CdS core/shell nanowires. Our results show that the coating of a few-atomic-layer CdS shell on the ZnS nanowire leads to a significant change in the stiffness of the core/shell nanowires compared to the stiffness of pure ZnS nanowires. The binding energy between the core and shell region decreases due to the lattice mismatch at the core-shell interface. This reduction in binding energy plays an important role in determining the stiffness of a core/shell nanowire. We have also investigated the effects of the shell on the thermal conductivity and melting behavior of the nanowires.

We investigate the possibility of exciting self-oscillation in a perpendicular ferromagnet by the spin Hall effect on the basis of a nonlinear analysis of the Landau-Lifshitz-Gilbert (LLG) equation. In the self-oscillation state, the energy supplied by the spin torque during a precession on a constant energy curve should equal the dissipation due to damping. Also, the current to balance the spin torque and the damping torque in the self-oscillation state should be larger than the critical current to destabilize the initial state. We find that these conditions in the spin Hall system are not satisfied by deriving analytical solutions of the energy supplied by the spin transfer effect and the dissipation due to the damping from the nonlinear LLG equation. This indicates that the self-oscillation of a perpendicular ferromagnet cannot be excited solely by the spin Hall torque.

The ultrafast laser-induced response of spins and charges in $\mathrm{CoFe}/{\mathrm{Al}}_2{\mathrm{O}}_3$ multilayers are studied using THz and optical pump-probe spectroscopies. We demonstrate the possibility of ultrafast manipulation of the transport and magnetic properties of the multilayers with femtosecond laser excitation. In particular, using time-resolved THz transmission experiments we found that such an excitation leads to a rapid increase of the THz transmission (i.e., electric resistivity). Our experiments also reveal that femtosecond laser excitation results in the emission of broadband THz radiation. To reveal the origin of the emitted THz radiation, we performed magnetic-dependent measurements of the THz emission. We also compared the observed electric field of the THz radiation to calculations performed using subpicosecond laser-induced demagnetization measurements. The good agreement between the experimentally obtained spectra and the calculations corroborates that the measured THz emission originates from the demagnetization process.

We study $s$-wave Cooper pairing in heavy-fermion systems. We analyze the periodic Anderson model by means of the variational cluster approach (VCA) focusing on the interorbital Cooper pairing between a conduction electron ($c$ electron) and an $f$ electron, called the “$c-f$ pairing.” It is shown that the $s$-wave superconductivity appears coexisting with long-range antiferromagnetic order when electrons or holes are doped into the system at half filling. The antiferromagnetic order vanishes when the doping concentration exceeds a certain critical value, leading to a pure $s$-wave superconducting state. Moreover, the comparative study with different reference systems used in the VCA shows that the interorbital $c-f$ pairing is essential for the appearance of the $s$-wave superconductivity.

Many cuprate superconductors possess an unusual charge-ordered phase that is characterized by an approximate $d_{x^2-y^2}$ intraunit cell form factor and a finite modulation wave vector ${\mathbf{q}}^{*}$. We study the effects of impurities on this charge-ordered phase via a single-band model in which bond order is the analog of charge order in the cuprates. Impurities are assumed to be pointlike and are treated within the self-consistent $t$-matrix approximation. We show that suppression of bond order by impurities occurs through the local disruption of the $d_{x^2-y^2}$ form factor near individual impurities. Unlike $d$-wave superconductors, where the sensitivity of $T_c$ to impurities can be traced to a vanishing average of the $d_{x^2-y^2}$ order parameter over the Fermi surface, the response of bond order to impurities is dictated by a few Fermi surface “hotspots.” The bond order transition temperature $T_{\mathrm{bo}}$ thus follows a different universal dependence on impurity concentration $n_i$ than does the superconducting $T_c$. In particular, $T_{\mathrm{bo}}$ decreases more rapidly than $T_c$ with increasing $n_i$ when there is a nonzero Fermi surface curvature at the hotspots. Based on experimental evidence that the pseudogap is insensitive to Zn doping, we conclude that a direct connection between charge order and the pseudogap is unlikely. Furthermore, the enhancement of stripe correlations in the La-based cuprates by Zn doping is evidence that this charge order is also distinct from stripes.

The effect of K, Co, and P dopings on the lattice dynamics in the ${{\mathrm{BaFe}}_2\mathrm{As}}_2$ system is studied by infrared spectroscopy. We focus on the phonon at $\sim 253{\mathrm{cm}}^{-1}$, the highest energy in-plane infrared-active Fe-As mode in ${{\mathrm{BaFe}}_2\mathrm{As}}_2$. Our studies show that Co and P dopings lead to a blueshift of this phonon in frequency, which can be simply interpreted by the change in lattice parameters induced by doping. In sharp contrast, an unusual redshift of the same mode was observed in the K-doped compound, at odds with the above explanation. This anomalous behavior in K-doped ${{\mathrm{BaFe}}_2\mathrm{As}}_2$ is more likely associated with the coupling between lattice vibrations and other channels, such as charge or spin. This coupling scenario is also supported by the asymmetric line shape and intensity growth of the phonon in the K-doped compound.

Here we report a further investigation on the thermodynamic and transport properties, and an assessment of theoretical calculations, for the $\mathrm{Bi}{\mathrm{S}}_2$-layered $\mathrm{B}{\mathrm{i}}_3{\mathrm{O}}_2{\mathrm{S}}_3$ superconductor. The polycrystalline sample is synthesized with a superconducting transition temperature of $T_{\mathrm{c}}^{\mathrm{onset}}=5.75\mathrm{K}$ and $T_{\mathrm{c}}^{\mathrm{zero}}=4.03\mathrm{K}$ ($\approx T_{\mathrm{c}}^{\mathrm{mag}}$) that drops to 3.3 K by applying a hydrostatic pressure of 6 kbar. Density-of-states (DOS) calculations give substantial hybridization between Bi, O, and S, with Bi the largest component of DOS, which supports the idea that the $\mathrm{Bi}{\mathrm{S}}_2$ layer is relevant for producing electron-phonon coupling. An analysis of previously published specific heat data for $\mathrm{B}{\mathrm{i}}_3{\mathrm{O}}_2{\mathrm{S}}_3$ is additionally suggestive of a strong electron-phonon interaction in the Bi-O-S system. The analysis of the Seebeck coefficient results strongly suggests that $\mathrm{B}{\mathrm{i}}_3{\mathrm{O}}_2{\mathrm{S}}_3$ is a semimetal. In fact, the semimetallic or narrow band gap behavior may occur in certain other materials in the $\mathrm{Bi}{\mathrm{S}}_2$-layered class of materials, such as $\mathrm{B}{\mathrm{i}}_4{\mathrm{O}}_4{\mathrm{S}}_3$.

Motivated by recent experiments on ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, we provide a theory of quantum oscillations in the Luttinger model with quadratic band touching, modeled for the spin-orbit-coupled conduction electrons in pyrochlore iridates. The magneto- and Hall resistivities are computed for electron- and hole-doped systems, and the corresponding Shubnikov–de Haas (SdH) signals are investigated. The SdH signals are characterized by aperiodic behaviors that originate from the unconventional Landau level structures of the Luttinger model near the neutrality point, such as inter-Landau-level crossing, nonuniform Landau level spacings, and nonparabolic dispersions along the applied magnetic-field direction. The aperiodic SdH signals observed in the paramagnetic state of ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ are shown to be consistent with such behaviors, justifying the use of the Luttinger model and the quadratic band touching spectrum as excellent starting points for physics of pyrochlore iridates. The implications of these results are discussed in light of recent theoretical and experimental developments in these systems.

We have calculated the transition energies of neutral and charged excitons in ${\mathrm{In}}_{0.5}\mathrm{Ga}{}_{0.5}\mathrm{As}$/GaAs quantum dots (QDs) under a lateral electric field up to 40 kV/cm. First, the single-particle electron and hole states under the lateral field are calculated using the 8-band $\mathbf{k}·\mathbf{p}$ theory. The linear and quadratic piezoelectricity is included. Next, the transition energies are calculated from the electron-hole, electron-electron, and hole-hole Coulomb energies. For a QD with 15-nm base length under a lateral electric field along the $\left[100\right]$ direction, the transition energy of the positively charged exciton exhibits a blueshift with increasing field up to 28 kV/cm, followed by a redshift under higher field. In contrast, those of the neutral and negatively charged excitons exhibit only redshifts accompanied by a crossing of the two exciton levels. The calculated result for the positively charged exciton reproduces the unconventional “M”-shaped exciton energy shift observed in our experiment by Nakaoka et al. [Appl. Phys. Lett. 99, 181109 (2011)]. The origin of the blueshift calculated for the positively charged exciton is the enhanced hole density in the QD base corner due to the modification of the piezoelectric potential by the lateral electric field, which causes an increase in the hole-hole Coulomb energy. We found that the amount of the blueshift increases with the QD size. In order to understand the effect of the lateral field direction, we calculate the transition energies under a lateral electric field of 20 kV/cm along the $\left[1\overline{1}0\right],\left[100\right]$, and $\left[110\right]$ directions. For the positively charged exciton, the transition energy exhibits a redshift for the $\left[1\overline{1}0\right]$ direction, and blueshifts of different amounts for the $\left[100\right]$ and $\left[110\right]$ directions, indicating that the effects of the lateral field are not equivalent for the $\left[1\overline{1}0\right]$ and $\left[110\right]$ directions. It is demonstrated that the direction dependence of the transition energy reflects the symmetry of the confinement potential in the QDs due to piezoelectricity.

We investigate the hydrodynamic theory of metals, offering systematic studies of the linear-response dynamics for an inhomogeneous electron gas. We include the quantum functional terms of the Thomas-Fermi kinetic energy, the von Weizsäcker kinetic energy, and the exchange-correlation Coulomb energies under the local density approximation. The advantages, limitations, and possible improvements of the hydrodynamic theory are transparently demonstrated. The roles of various parameters in the theory are identified. We anticipate that the hydrodynamic theory can be applied to investigate the linear response of complex metallic nanostructures, including quantum effects, by adjusting theory parameters appropriately.

We present a method, based on characterizing efficiency fluctuations, to assess the performance of nanoscale thermoelectric junctions. This method accounts for effects typically arising in small junctions, namely, stochasticity in the junction's performance, quantum effects, and nonequilibrium features preventing a linear response analysis. It is based on a nonequilibrium Green's function (NEGF) approach, which we use to derive the full counting statistics (FCS) for heat and work, and which in turn allows us to calculate the statistical properties of efficiency fluctuations. We simulate the latter for a variety of simple models where our method is exact. By analyzing the discrepancies with the semiclassical prediction of a quantum master equation (QME) approach, we emphasize the quantum nature of efficiency fluctuations for realistic junction parameters. We finally propose an approximate Gaussian method to express efficiency fluctuations in terms of nonequilibrium currents and noises which are experimentally measurable in molecular junctions.

Quantitative adsorption structure determinations on quasicrystals are scarce because most techniques for measuring surface structures are not well suited to the complex and infinite unit cells of quasicrystals. The normal incidence standing x-ray wave field technique presents a solution to these problems because it can be made inherently surface sensitive and does not involve extensive computational effort. We describe a method for applying this technique to adsorbates on quasicrystals, with specific application to a submonolayer of Si atoms on a decagonal Al-Co-Ni surface. We demonstrate the sensitivity of the technique to both adsorption site and geometry, leading to the conclusion that the Si atoms, which form six-atom pentagonal clusters, have an average height of $1.77\pm 0.05\AA$ above pentagonal hollow sites, with a significant height variation among the Si atoms in the cluster. In particular, the central Si atom sits more deeply than the five surrounding Si atoms, which are, on average, $2.7\AA$ away from the central Si atom. Although this study was performed on a decagonal quasicrystal that is periodic perpendicular to the surface, we describe how the technique can be applied to cases with no periodicity.

We report the observation of the de Haas–van Alphen effect in ${\mathrm{IrTe}}_2$ measured using torque magnetometry at low temperatures down to 0.4 K and in high magnetic fields up to 33 T. ${\mathrm{IrTe}}_2$ undergoes a major structural transition around $\sim 283\left(1\right)\mathrm{K}$ due to the formation of planes of Ir and Te dimers that cut diagonally through the lattice planes, with its electronic structure predicted to change significantly from a layered system with predominantly three-dimensional character to a tilted quasi-two-dimensional Fermi surface. Quantum oscillations provide direct confirmation of this unusual tilted Fermi surface and also reveal very light quasiparticle masses (less than $1m_e$), with no significant enhancement due to electronic correlations. We find good agreement between the angular dependence of the observed and calculated de Haas–van Alphen frequencies, taking into account the contribution of different structural domains that form while cooling ${\mathrm{IrTe}}_2$.

We report on the feedback-optimized extraordinary optical transmission of continuous-variable entangled states through a hexagonal metal-hole array. The continuous-variable entanglements from a nondegenerate optical parametric amplifier are first demonstrated to survive after a photon-plasmon-photon conversion process. By controlling the reflected light from the metal-hole array, a significant enhancement of quantum correlations has then been experimentally achieved, compared to the case without such coherent feedback control. This result presents a useful technique to efficiently recover the substantial reflective losses in plasmonic circuits for quantum information processing.

We investigate theoretically the nonlocal conductance through a superconducting wire in tunnel contact with normal and ferromagnetic leads. In the presence of an in-plane magnetic field, the superconducting density of states is spin split, and the current injected from the normal lead is spin polarized. A nonlocal conductance that is antisymmetric with the applied voltage can be measured with a ferromagnetic lead. It persists for a distance between the contacts that is larger than both the charge-imbalance relaxation length and the normal-state spin relaxation length. We determine its amplitude by considering two extreme models of weak and strong internal equilibration of the superconducting quasiparticles due to electron-electron interactions. We find that the nonlocal signal, which was measured in recent experiments and discussed as a spin-imbalance effect, can be interpreted alternatively as the signature of a thermoelectric effect.