We examine the appearance of a spontaneous bulk spin current in a triplet superconductor in contact with a metallic ferromagnet. The spin current results from the spin flip of Cooper pairs upon reflection from the interface with the ferromagnet, and is shown to display strong similarities to the spontaneous charge current in a Josephson junction. We express the spin current in terms of the Andreev reflection coefficients, which are derived by the construction of the quasiclassical scattering wave functions. The dependence of the spin current upon a number of parameters is investigated, in particular the orientation of the magnetic moment of the ferromagnet, the exchange splitting, the temperature, and the orbital pairing state of the triplet superconductor.

We describe two different nuclear quadrupole resonance (NQR) based techniques, designed to measure the local asymmetry of the internal electric field gradient $\eta$ and the tilt angle $\alpha$ of the main NQR principal axis $\hat{\mathbf{z}}$ from the crystallographic axis $\hat{\mathbf{c}}$. These techniques use the dependence of the NQR signal on the duration of the radio frequency (rf) pulse and on the direction of the rf field $H_1$ with respect to the crystal axis. The techniques are applied to oriented powder of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_y$ fully enriched with ${}^{63}\text{C}\text{u}$. Measurements were performed at different frequencies, corresponding to different in-plane copper sites with respect to the dopant. Combining the results from both techniques, we conclude that oxygen deficiency in the chain layer lead to a rotation of the NQR main principal axis at the nearby Cu on the ${\text{CuO}}_2$ planes by $\alpha \simeq 20°\pm 5°$. This occurs with no change to $\eta$. The axis rotation associated with oxygen deficiency means that there must be electric field inhomogeneities in the ${\text{CuO}}_2$ planes only in the vicinity of the missing oxygen.

We report inelastic x-ray scattering experiments on the lattice dynamics in SmFeAsO and superconducting ${\text{SmFeAsO}}_{0.60}{\text{F}}_{0.35}$ single crystals. Particular attention was paid to the dispersions along the [100] direction of three optical modes close to 23 meV, polarized out of the FeAs planes. Remarkably, two of these modes are strongly renormalized upon fluorine doping. These results provide significant insight into the energy and momentum dependence of the coupling of the lattice to the electron system and underline the importance of spin-phonon coupling in the superconducting iron pnictides.

In this paper we provide a comprehensive analysis of different properties of pnictides both in the normal and superconducting state, with a particular focus on the optimally doped ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ system. We show that, by using the band dispersions experimentally measured by angle-resolved photoemission spectroscopy, a four-band Eliashberg model in the intermediate-coupling regime can account for both the measured hierarchy of the gaps and for several spectroscopic and thermodynamic signatures of low-energy renormalization. These include the kinks in the band dispersion and the effective masses determined via specific-heat and superfluid-density measurements. We also show that, although an intermediate-coupling Eliashberg approach is needed to account for the magnitude of the gaps, the temperature behavior of the thermodynamic quantities does not show in this regime a significant deviation with respect to weak-coupling BCS calculations. This can explain the apparent success of two-band BCS fits of experimental data reported often in the literature.

We calculate the transmission of electrons and holes between two normal-metal (N) electrodes, separated over a distance $L$ by an impurity-free superconductor (S) with $d$-wave symmetry of the order parameter. Nodal lines of vanishing excitation gap form ballistic conduction channels for coupled electron-hole excitations, described by an anisotropic two-dimensional Dirac equation. We find that the transmitted electrical and thermal currents both have the pseudodiffusive $1/L$ scaling characteristic of massless Dirac fermions—regardless of the presence of tunnel barriers at the NS interfaces. Tunnel barriers reduce the slope of the $1/L$ scaling in the case of the electrical current while leaving the thermal current unaffected.

We derive boundary conditions for the electrically induced spin accumulation in a finite, disordered two-dimensional semiconductor channel. While for dc electric fields these boundary conditions select spatially constant spin profiles equivalent to a vanishing spin-Hall effect, we show that an in-plane ac electric field results in a nonzero ac spin-Hall effect, i.e., it generates a spatially nonuniform out-of-plane polarization even for linear intrinsic spin-orbit interactions. Analyzing different geometries in [001]- and [110]-grown quantum wells, we find that although this out-of-plane polarization is typically confined to within a few spin-orbit lengths from the channel edges, it is also possible to generate spatially oscillating spin profiles which extend over the whole channel. The latter is due to the excitation of a driven spin-helix mode in the transverse direction of the channel. We show that while finite frequencies suppress this mode, it can be amplified by a magnetic field tuned to resonance with the frequency of the electric field. In this case, finite-size effects at equal strengths of Rashba and Dresselhaus SOI lead to an enhancement of the magnitude of this helix mode. We comment on the relation between spin currents and boundary conditions.

We establish the phase diagram of the disordered three-dimensional Bose-Hubbard model at unity filling which has been controversial for many years. The theorem of inclusions, proven by Pollet et al. [Phys. Rev. Lett. 103, 140402 (2009)] states that the Bose-glass phase always intervenes between the Mott insulating and superfluid phases. Here, we note that assumptions on which the theorem is based exclude phase transitions between gapped (Mott insulator) and gapless phases (Bose glass). The apparent paradox is resolved through a unique mechanism: such transitions have to be of the Griffiths type when the vanishing of the gap at the critical point is due to a zero concentration of rare regions where extreme fluctuations of disorder mimic a regular gapless system. An exactly solvable random transverse field Ising model in one dimension is used to illustrate the point. A highly nontrivial overall shape of the phase diagram is revealed with the worm algorithm. The phase diagram features a long superfluid finger at strong disorder and on-site interaction. Moreover, bosonic superfluidity is extremely robust against disorder in a broad range of interaction parameters; it persists in random potentials nearly 50 (!) times larger than the particle half-bandwidth. Finally, we comment on the feasibility of obtaining this phase diagram in cold-atom experiments, which work with trapped systems at finite temperature.

Photoexcited dynamics of a spin-density-wave (SDW) state in an organic linear-chain compound bis-(tetramethyl-tetraselenafulvalene)hexafluorophosphate, ${\left(\text{TMTSF}\right)}_2{\text{PF}}_6$, is investigated by optical pump and terahertz probe experiments. After the ultrashort laser-pulse excitation, a sudden closing followed by a recovery of the SDW gap is clearly observed in the far-infrared spectrum. The recovery time is found to diverge toward the phase-transition temperature, $T_{SDW}$, while the gap remains open at $T_{SDW}$. The observed critical slowing down behavior is interpreted in terms of three-dimensional fluctuation which enlarges near the phase-transition temperature.

Photoluminescence studies of a wide asymmetric quantum well hosting two strongly coupled electron layers demonstrate unambiguously how such a system deforms itself due to interlayer charge transfer in a quantizing magnetic field thus manifesting its overall single-layer appearance. The charge distribution inside the quantum well is balanced for total filling factor $\nu =4N$ ($N$ integer), while it is imbalanced at most for $\nu =4N+2$. The intersubband energy oscillates enormously with the applied magnetic field and reaches values as large as the electron cyclotron energy for filling factors $4N$.

The magnetic susceptibilities and specific heats of the crystalline garnet and glass forms of ${\text{Mn}}_3{\text{Al}}_2{\text{Si}}_3{\text{O}}_{12}$ are reported. This allows a direct comparison of the degree of magnetic frustration of the triangle-based garnet lattice and the structurally disordered solid at the same composition for isotropic spin $5/2{\text{Mn}}^{2+}\left(3d^5\right)$. The results show that the glass phase shows more pronounced signs of magnetic frustration than the crystalline phase. Through comparison of the specific heats of ${\text{Ca}}_3{\text{Al}}_2{\text{Si}}_3{\text{O}}_{12}$ (grossular) and ${\text{Mn}}_3{\text{Al}}_2{\text{Si}}_3{\text{O}}_{12}$ (spessartine) garnets, information is provided concerning the anomalous extra specific heat in the latter material.

Edge channels play a crucial role for electron transport in two-dimensional electron gas under magnetic field. It is usually thought that ballistic transport along edges occurs only in the quantum regime with low filling factors. We show that a microwave field can stabilize edge trajectories even in the semiclassical regime leading to a vanishing longitudinal resistance. This mechanism gives a clear physical interpretation for observed zero-resistance states.

Low-energy electron microscopy reveals a mode of graphene growth on Ru(0001) in which Ru atoms are etched from a step edge and injected under a growing graphene sheet. Based on density-functional calculations, we propose a model wherein injected Ru atoms form metastable islands under the graphene. Scanning tunneling microscopy reveals that dislocation networks exist near step edges, consistent with some of the injected atoms being incorporated into the topmost Ru layer, thereby increasing its density

We have studied the ${\text{Tb}}_2{\text{Sn}}_2{\text{O}}_7$ ordered spin ice by neutron diffraction under an isotropic pressure of 4.6 GPa, combined with a stress of 0.3(1) GPa. Measurements down to a temperature of 0.06 K and up to 100 K probe the effect of pressure both on ground state and spin fluctuation in the paramagnetic region. In the pressure-induced ground state, the ordered spin ice structure with a ${\mathbf{k}}_0=0$ propagation vector persists, but it coexists with a structure with ${\mathbf{k}}_1=\left(0,0,1\right)$. The ordered moment at 0.06 K is reduced, suggesting that pressure also enhances the spin liquid fluctuations at $T\sim 0$. In the paramagnetic region, applying pressure changes the short-range spin correlations and suppresses the ferromagnetic correlations. The influence of pressure is discussed considering both isotropic pressure and stress effects.

The transport properties of the $\text{Si}\left(111\right)\sqrt{3}\times \sqrt{3}\text{-Sn}$ surface are investigated by micro-four-point-probe conductivity measurements. The temperature dependence of the surface-state conductivity showed an insulating behavior from room temperature down to 15 K although the surface was believed to be metallic. Furthermore, with changing the band filling by partially replacing Sn atoms with In or Na deposition, we found that the conductivity showed a metallic behavior down to 260 K and upon further cooling, the carriers became strongly localized possibly due to the dopants themselves. Our result suggests that the ground state of this surface is insulating with a very small energy gap, which is consistent with a recent theoretical study [G. Profeta and E. Tosatti, Phys. Rev. Lett. 98, 086401 (2007)] predicting this surface to be a Mott insulator.

Iron telluride (FeTe), a relative of the iron-based high-temperature superconductors, displays unusual magnetic order and structural transitions. Here, we explore the idea that strong correlations may play an important role in these materials. We argue that the unusual orders observed in FeTe can be understood from a picture of correlated local moments with orbital degeneracy, coupled to a small density of itinerant electrons. A component of the structural transition is attributed to orbital, rather than magnetic ordering, introducing a strongly anisotropic character to the system along the diagonal directions of the iron lattice. Double exchange interactions couple the diagonal chains leading to the observed ordering wave vector. The incommensurate order in samples with excess iron arises from electron doping in this scenario. The strong anisotropy of physical properties in the ordered phase should be detectable by transport in single domains. Predictions for ARPES, inelastic neutron scattering and hole/electron doping studies are also made.

Recent experiments have demonstrated that neutral graphene sheets have an insulating ground state in the presence of an external magnetic field. We report on a $\pi$-band tight-binding-model Hartree-Fock calculation which examines the competition between distinct candidate insulating ground states. We conclude that for graphene sheets on substrates the ground state is most likely a field-induced spin-density wave and that a charge-density-wave state is possible for suspended samples. Neither of these density-wave states support gapless edge excitations.

Electronic and magnetic dynamics at a single interface of antiferromagnetic manganites, ${\text{LaMnO}}_3/{\text{SrMnO}}_3$, were studied with time-resolved second-harmonic generation and time-resolved nonlinear magneto-optical Kerr effect. The temporal evolution of the respective signals shows clear differences; an optical pump makes an instantaneous response of the electrons and, in contrast, induces slow demagnetization, which points to a possibility of two-dimensional half-metallic ferromagnetism for this heterointerface. Detailed analysis with various pump and probe polarizations reveals possible pathways of the optical charge-transfer excitation between $\text{Mn}d$ orbitals, leading to a subsequent perturbation to the interface spin system.

As devices to control spin currents using the spin-orbit interaction are proposed and implemented, it is important to understand the fluctuations that spin-orbit coupling can impose on transmission through a quantum dot. Using random matrix theory, we estimate the typical scale of transmitted charge and spin currents when a spin current is injected into a chaotic quantum dot with strong spin-orbit coupling. These results have implications for the functioning of the spin transistor proposed by Schliemann, Egues, and Loss. We use a density matrix formalism appropriate for treating arbitrary input currents and indicate its connections to the widely used spin-conductance picture. We further consider the case of currents entangled between two leads, finding larger fluctuations.

We examine the growth of entanglement under a quantum quench at point contacts of simple fractional quantum Hall fluids and its relation with the measurement of local observables. Recently Klich and Levitov proposed that the noise generated from a local quantum quench provides a measure of the entanglement entropy. Their methods were specific to noninteracting electrons and the generalization to interacting systems was left as an open question. In this work, we generalize their result to the Laughlin states. We investigate the noise generated in the current along the edge of a fractional quantum Hall state at filling factors $\nu =1/m$, when a quantum point contact, initially closed, is fully opened at some initial time $t_0=0$. We find that local quenching in these systems gives time dependent correlation functions that have universal behavior on sufficiently long time and length scales. We calculate the noise and full counting statistics for $\nu =1/m$ and find that in general, the entanglement entropy and noise generated are unrelated quantities. We also discuss a generalization of this problem to the critical quantum Ising spin chain.

The discovery of superplasticity in ceramics polycrystals led to debates about whether or not earlier models developed for metallic polycrystals can apply to these systems. In particular, all existing models require some mobility of lattice or grain-boundary dislocations whereas such activity is not observed in most ceramic systems. A model is presented that accounts for the occurrence of superplasticity in the absence of dislocation motion. It is based on a mechanism of grain-boundary sliding by pure-shear motion under stationary conditions, which is accommodated by lattice or grain-boundary diffusion. The prediction of this model regarding the temperature dependences of the stress exponent and of the effective activation energy are found in agreement with experimental results and literature data on five ceramic systems where dislocation activity could not be recorded: $\beta$-SiAlON polycrystals, Al-doped SiC polycrystals, nanocrystalline MgO, yttria-tetragonal zirconia polycrystals, and alumina ceramics polycrystals.