We formulate the problem of electron transport through an interacting quantum dot system in the framework of the self-consistent perturbation theory of the nonequilibrium Green's function and show that the current conservation through the central region is automatically guaranteed owing to the gauge-invariant properties of the resulting Green's functions and the generalized Ward identity. By using a generating functional for the statistics of the nonequilibrium system, we obtain general formulas for calculating the current and the current fluctuations in the presence of arbitrary time-dependent potentials. As an illustration of application, we study the interaction effects on the finite-frequency noise for electron resonant tunneling through an Anderson impurity and obtain an analytical expression for the finite-frequency current noise within the Hartree approximation, which is an extension of the results obtained by Hershfield on zero-frequency shot noise.

We consider the tension-induced nonlinearities of mechanical resonators and derive the Hamiltonian of the flexural modes up to the fourth order in the position operators. This tension can be controlled by a nearby gate voltage. We focus on systems which allow large deformations, $u\left(x\right)\gg h$, compared to the thickness $h$ of the resonator and show that in this case, the third-order coupling can become nonzero due to the induced dc deformation and offers the possibility to realize radiation-pressure-type equations of motion encountered in optomechanics. The fourth-order coupling is relevant especially for relatively low voltages. It can be detected by accessing the Duffing regime and by measuring frequency shifts due to mode-mode coupling.

We investigate the Fano-Kondo interplay in an Aharonov-Bohm ring with an embedded noninteracting quantum dot and a Coulomb interacting quantum dot. Using a slave-boson mean-field approximation we diagonalize the Hamiltonian via scattering matrix theory and derive the conductance in the form of a Fano expression, which depends on the mean-field parameters. We predict that in the Kondo regime the magnetic field leads to a gapped energy level spectrum due to hybridization of the noninteracting QD state and the Kondo state, and can quantum-mechanically alter the electron's path preference. We demonstrate that an abrupt symmetry change in the Fano resonance, as seen experimentally, could be a consequence of an underlying Kondo channel.

We present transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact. Zero-bias features in tunneling spectroscopy appear at finite magnetic fields and oscillate in amplitude and split away from zero bias as a function of magnetic field and gate voltage. A crossover in magnetoconductance is observed: Magnetic fields above $\sim$0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance in the high-conductance (multichannel) regime. We consider these results in the context of Majorana zero modes as well as alternatives, including the Kondo effect and analogs of 0.7 structure in a disordered nanowire.

We present detailed results from a recent microscopic theory of extremely correlated Fermi liquids, applied to the $t$-$J$ model in two dimensions, developed recently by Shastry [Phys. Rev. Lett. 107, 056403 (2011); Phys. Rev. B 87, 125124 (2013)]. The second-order theory in the parameter $\lambda$, related to the density, is argued to be quantitatively valid in the overdoped regime for $0\le n\lesssim 0.75$, with $n$ denoting the particle density. The calculation involves the self-consistent solution of equations for an auxiliary Fermi liquid Green's function and an adaptive spectral weight. We present numerical results at low as well as high $T$, at various low to intermediate densities in the normal phase, using a minimal set of band parameters relevant to the cuprate superconductors. We display the momentum space occupation function $m_k$, energy dispersion curves locating the peaks of spectral functions, the optical conductivity, relaxation rates for quasiparticles, and the electronic spectral functions on an absolute scale. The line shapes have an asymmetric shape and a broad background that is also seen in experiments, and our calculations validate approximate recent versions of the theory. The results also display the experimentally noted high-energy kink and provide an in-depth understanding of its origin and dependence on band parameters.

We explore the anisotropy of the magnetoresistance of individual GaAs/(Ga,Mn)As core-shell nanowires which feature a strong negative magnetoresistance (NMR) and a very large magnetic anisotropy field. Our analysis of the magnetoresistance shows that the resistance anisotropy is dominated by the effective magnetic field and that the origin of the NMR is related to spin scattering rather than to weak localization in (Ga,Mn)As core-shell nanowires.

We demonstrate that a quantum spin Hall current, spontaneously generated at the edge of a two-dimensional topological insulator, acts as a source of spin pumping for a magnetic impurity with uniaxial anisotropy. One can then manipulate the impurity spin direction by means of an electrical current without using either magnetic electrodes or an external magnetic field. Furthermore we show that the unique properties of the quantum spin Hall topological state have profound effects on the inelastic electron tunneling spectrum of the impurity. For low current densities inelastic spin-flip events do not contribute to the conductance. As a consequence the conductance steps, normally appearing at voltages corresponding to the spin excitations, are completely suppressed. In contrast an intense current leads to spin pumping and generates a transverse component of the impurity spin. This breaks the topological phase yielding to the conductance steps.

Motivated by the transmission of topological surface states through atomic scale steps, we study the transport of gapless Dirac fermions on hyperbolic surfaces. We show that the density of states of the electrons shows a dip at concave step edges which can be measured by a scanning tunneling microscope tip. We also show that the tunneling conductance measured by a polarized scanning tunneling probe exhibits an unconventional dependence on the polar and azimuthal angles of the magnetization of the tip as a function of the curvature of the surface and the sharpness of the edge. However, no backward scattering takes place and transmission of the topological surface states remains completely independent of the geometrical shape (within the hyperbolic model) of the surface.

Water is common and plays a crucial role in biological, chemical, and physical processes, but its crystalline or ice state has a complicated structure. In this work, we study the lattice mismatch challenge for ice nucleation on silver iodide, the sublimation energy for different ice phases, and the structural phase-transition pressures of ice, with various density functionals. Our calculations show that the recently developed meta-generalized gradient approximation made simple (MGGA_MS) yields a lattice mismatch (3$\%$) of hexagonal ice (ice Ih) with β-AgI in good agreement with experiment (2$\%$), significantly better than the Perdew-Burke-Ernzerhof (PBE) GGA mismatch (6$\%$). MGGA_MS is a computationally efficient semilocal functional that incorporates intermediate-range van der Waals (vdW) interaction, which, overall, performs well for ice and may be expected to improve upon PBE for liquid water. While MGGA_MS predicts the most realistic volumes and volume changes in the phase transitions of ice Ih to trigonal ice (ice II) and tetragonal ice (ice VIII), a more accurate description of some other properties of the higher-pressure phases (ice II and ice VIII) is provided by some functionals that include long-range vdW corrections (e.g., revised Tao-Perdew-Staroverov-Scuseria+vdW for sublimation energy and optB88-vdW for transition pressure).

The generation, relaxation, and diffusion dynamics of optically induced lattice distortion in the relaxed excited state of SrTiO${}_3$ are studied by using polarization spectroscopy with the pump-probe technique. The relaxed excited state is generated with a rise time on the order of 100 ps. Three kinds of thermal activation processes of the localized lattice distortion are found, and these processes are considered to be caused by photogenerated carriers in trapped states, which play important roles in photoluminescence or photoconductivity. We observed the lattice distortion induced by a separated pump beam from the probe beam to investigate its itineracy. The lattice-distortion signal appears later as the separation along the [100] axis becomes larger. The temperature dependence of the mobility suggests that the observed diffusive lattice distortion is caused by photogenerated electrons accompanied by lattice distortion, or electron polarons. Thus, the spatial and temporal dynamics of polaron diffusion were observed directly in our experiment.

We present a study of the single spin flip stochastic dynamics of the two-dimensional 16-vertex model. We single out several dynamic regimes controlled by different processes that we describe. We analyze the emergence of very long-lived metastable states and their dependence on the system size, boundary conditions, and working parameters. We investigate the coarsening process after quenches into the ordered ferromagnetic and antiferromagnetic phases. We discuss our results in the context of artificial spin ice on square lattices.

Motivated by recent numerical and experimental studies of the spin-$1/2$ Heisenberg antiferromagnet on the kagome lattice, we formulate a many-body model for fermionic spinons introduced by us earlier [Phys. Rev. Lett. 103, 187203 (2009)]. The spinons interact with an emergent $U\left(1\right)$ gauge field and experience strong short-range attraction in the $S=0$ channel. The ground state of the model is generically a $Z_2$ liquid. We calculate the edge of the two-spinon continuum and compare the theory to the slave-fermion approach to the Heisenberg model.

Magnetism in SmPd${}_2$Al${}_3$ was investigated on a single crystal by magnetometry and neutron diffraction. SmPd${}_2$Al${}_3$ represents a distinctive example of a Sm magnetism exhibiting complex magnetic behavior at low temperatures with four consecutive magnetic phase transitions at 3.4, 3.9, 4.4, and 12.5 K. The rich magnetic phase diagram of this compound reflects the specific features of the Sm${}^{3+}$ ion, namely, the energy nearness of the ground-state multiplet $J$ = 5/2 and the first excited multiplet $J$ = 7/2 in conjunction with strong crystal field influence. Consequently, a significantly reduced Sm magnetic moment in comparison with the theoretical Sm${}^{3+}$ free-ion value is observed. Despite the strong neutron absorption by natural samarium and the small Sm magnetic moment (∼0.2 ${\mu }_{\mathrm{B}}$), we have successfully determined the magnetic k vector (1/3, 1/3, 0) of the phase existing in the temperature interval 12.5–4.4 K. This observation classifies the SmPd${}_2$Al${}_3$ compound as a magnetically frustrated system. The complex magnetic behavior of this material is further illustrated by kinetic effects of the magnetization, inducing a rather complicated magnetic structure with various metastable states.

By discussing field-induced quantum interference effects due to monopole moments and those due to dipole moments on equal footing, their similarities and differences are clarified. First, we demonstrate the general principle for flux quantization. For particles carrying a monopole moment, the interference causes the monopole current to oscillate periodically, with the flux defined as the inner product of the field and area, whereas for particles carrying a fixed dipole moment the dipole current oscillates periodically, with the flux vector defined as the cross product of the field and trajectory. Our analysis unifies the oscillation of monopole or dipole currents in various devices, such as the superconducting quantum interference device and spin field-effect transistor, into the same physical picture. Second, we show that interference effects can also happen in open trajectory devices that transport dipole currents, such as the spin Josephson effect, based on the non-gauge-field nature of the interference effects of dipole moments. In addition, we propose that the interference effect of electric dipoles, known as the He-McKellar-Wilkens effect, can be realized by the bilayer exciton condensates observed in semiconductor heterostructures and bilayer graphene.

The maximum current (critical current) a type-II superconductor can transmit without energy loss is limited by the motion of the quantized magnetic flux penetrating into a superconductor. Introducing nanoscale holes into a superconducting film has been long pursued as a promising way to increase the critical current. So far the critical current enhancement was found to be mostly limited to low magnetic fields. Here we experimentally investigate the critical currents of superconducting films with a conformal array of nanoscale holes that have nonuniform density while preserving the local ordering. We find that the conformal array of nanoscale holes provides a more significant critical current enhancement at high magnetic fields. The better performance can be attributed to its arching effect that not only gives rise to the gradient in hole density for pinning vortices with a wide range of densities but also prevents vortex channeling occurring in samples with a regular lattice of holes.

The influence of the valence electron concentration of $X$ in fcc Fe-Mn-$X$ ($X$=Cr, Co, Ni, Cu) alloys on the elastic and magnetic properties has been studied by means of ab initio calculations for alloy element concentrations of up to 8 at. $\%$ $X$. We observe that Cu increases the bulk-to-shear modulus ($B/G$) ratio by 19.2$\%$. Simultaneously magnetic moments of Fe and Mn increase strongly. The other alloying elements induce less significant changes in $B/G$. The trends in $B/G$ may be understood by considering the changes in $G$ induced by the variation in valence electron concentration (VEC). As the VEC is increased, more pronounced metallic bonds are formed, giving rise to smaller shear modulus values. The changes in magnetic moments may be explained by the magnetovolume effect. Alloys with smaller VEC as Fe-Mn exhibit a decrease in local magnetic moments and equilibrium lattice parameters, while alloys with larger VEC as Fe-Mn demonstrate an increase in local magnetic moments and equilibrium lattice parameters. These data provide the basis for the design of Mn-rich steels with enhanced elastic properties.

We experimentally investigate and quantitatively analyze the spin Hall magnetoresistance effect in ferromagnetic insulator/platinum and ferromagnetic insulator/nonferromagnetic metal/platinum hybrid structures. For the ferromagnetic insulator, we use either yttrium iron garnet, nickel ferrite, or magnetite and for the nonferromagnet, copper or gold. The spin Hall magnetoresistance effect is theoretically ascribed to the combined action of spin Hall and inverse spin Hall effect in the platinum metal top layer. It therefore should characteristically depend upon the orientation of the magnetization in the adjacent ferromagnet and prevail even if an additional, nonferromagnetic metal layer is inserted between Pt and the ferromagnet. Our experimental data corroborate these theoretical conjectures. Using the spin Hall magnetoresistance theory to analyze our data, we extract the spin Hall angle and the spin diffusion length in platinum. For a spin-mixing conductance of $4\times {10}^{14}{\Omega }^{-1}{\mathrm{m}}^{-2}$, we obtain a spin Hall angle of $0.11\pm 0.08$ and a spin diffusion length of $(1.5\pm 0.5)\mathrm{nm}$ for Pt in our thin-film samples.

The magnetization behavior of long nanowires with square cross section under the influence of strong perpendicular spin-polarized current is studied theoretically. The study is based on Landau-Lifshitz-Slonczewski phenomenology. Without the current the wire is magnetized uniformly along its axis. For small currents the wire magnetization remains uniform, but it inclines with respect to the wire axis within the plane perpendicular to the current direction. With increasing current the inclination angle increases up to the maximum value $\pi /4$. Further current increase leads either to saturation or to a stable periodic multidomain structure depending on the wire thickness. For thick wires a hysteresis is observed in the saturation process under the action of the current. All critical parameters of the current induced magnetization behavior are determined theoretically. The study is carried out both analytically and numerically using micromagnetic simulations.

We consider an infinitely long 0-$\pi$ Josephson junction consisting of 0 and $\pi$ regions having different critical current densities $j_{c,0}$ and $j_{c,\pi }$. The ground state of such a junction corresponds to a spontaneously formed asymmetric semifluxon with tails decaying on different length scales. We calculate the depinning current of such a fractional vortex and show that it is different for positive and negative bias polarity. We also show that upon application of a bias current, the fractional flux (topological charge) associated with the vortex changes. We calculate the range of fractional flux associated with the vortex when the bias changes from negative to positive critical (depinning) values.

The intermetallic compound Y${}_3$Pt${}_4$Ge${}_{13}$ has been synthesized by high-pressure high-temperature technique. It crystalizes with its own noncentrosymmetric type of crystal structure in the monoclinic space group $Cc$ and is superconducting below $T_c=4.5$ K. Thermodynamic properties as well as transport measurements were performed down to 0.4 K. The upper critical field is ${\mu }_0{H}_{c2}=3.8$ T, the lower one only ${\mu }_0{H}_{c1}=5$ mT. All observations are consistent with an $s$-wave symmetry single-energy gap for the superconducting phase.