We report measurements of London penetration depth $\lambda \left(T\right)$ for the noncentrosymmetric superconductor BiPd by using a tunnel diode oscillator. Pronounced anisotropic behavior is observed in the low-temperature penetration depth; the in-plane penetration depth ${\lambda }_{ac}\left(T\right)$ follows an exponential decrease, but the interplane penetration depth ${\lambda }_b\left(T\right)$ shows power-law-type behavior. The superfluid density ${\rho }_s\left(T\right)$, converted from the penetration depth $\lambda \left(T\right)$, is best fitted by an anisotropic two-band BCS model. We argue that such a complex order parameter is attributed to the admixture of spin-singlet and spin-triplet pairing states as a result of antisymmetric spin-orbit coupling in BiPd.

We calculate the anisotropic thermal expansion of wurtzite AlN within the quasiharmonic approximation by sampling the volume as a function of two unit cell axes. From the derived Gibbs energy for the low pressure wurtzite phase and the high pressure rocksalt phase, we calculate the phase diagram. By applying ab initio molecular dynamics, we can confirm recent experimental findings regarding a temperature-driven rocksalt to wurtzite backward phase transition. We propose a detailed mechanism for the transition and predict the existence of a $\beta$-BeO type high-temperature modification of AlN. Furthermore, we find the $h$-MgO type intermediate structure previously reported for AlN and other compounds which show the wurtzite to rocksalt forward pressure-induced phase transition.

We demonstrate the general principle which states that a dissipationless spin current flows between two coupled ferromagnets if their magnetic orders are misaligned. This principle applies regardless of whether the two ferromagnets are metallic or insulating, and also generally applies to bulk magnetic insulators. On a phenomenological level, this principle is analogous to the Josephson effect, and yields a dissipationless spin current that is independent of scattering. The microscopic mechanisms for the dissipationless spin current depend on the systems, which are elaborated in detail. A uniform, static magnetic field is further proposed to be an efficient handle to create the misaligned configuration and stabilize the dissipationless spin current.

Modern theory of the orbital magnetization is applied to the series of prototype insulating perovskite transition metal oxides (orthorhombic YTiO${}_3$, LaMnO${}_3$, and YVO${}_3$, as well as monoclinic YVO${}_3$), carrying a net ferromagnetic (FM) moment in the ground state. For these purposes, we use an effective Hubbard-type model, derived from the first-principles electronic structure calculations and describing the behavior of magnetically active states near the Fermi level. The solution of this model in the mean-field Hartree-Fock approximation with the relativistic spin-orbit coupling typically gives us a distribution of the local orbital magnetic moments, which are related to the site-diagonal part of the density matrix $\widehat{\mathcal{D}}$ by the “classical” expression ${\mu }^0=-{\mu }_{\mathrm{B}}\mathrm{Tr}\left\{\widehat{\mathbf{L}}\widehat{\mathcal{D}}\right\}$. These moments are usually well quenched by the crystal field. In this work, we evaluate “itinerant” corrections $\Delta \mathcal{M}$ to the net FM moment, suggested by the modern theory. We show that these corrections are small and in most cases can be neglected. Nevertheless, the most interesting aspect of our analysis is that, even for these compounds, which are frequently regarded as prototype Mott insulators, the “itinerant” corrections reveal a strong $\mathbf{k}$ dependence in the reciprocal space, following the behavior of Chern invariants. Therefore, the small value of $\Delta \mathcal{M}$ is the result of strong cancellation of relatively large contributions, coming from different parts of the Brillouin zone. We discuss details as well as possible implications of this cancellation, which depends on the crystal structure as well as the type of the magnetic ground state.

We study the spin and energy dynamics in one-dimensional spin-$\frac{1}{2}$ systems induced by local quantum quenches at finite temperatures using a time-dependent density matrix renormalization group method. System sizes are chosen large enough to ensure that the time-dependent data for the accessible time scales represent the behavior in the thermodynamic limit. As a main result, we observe a ballistic spreading of perturbations of the energy density in the integrable spin-$\frac{1}{2}$ XXZ chain for all temperatures and exchange anisotropies, related to the divergent thermal conductivity in this model and the exact conservation of the energy current. In contrast, the spin dynamics is ballistic in the massless phase, but shows a diffusive behavior at high temperatures in the easy-axis phase in the case of a vanishing background spin density. We extract a quantitative estimate for the spin-diffusion constant from the time dependence of the spatial variance of the spin density, which agrees well with values obtained from current-current correlation functions using an Einstein relation. Interestingly, the diffusion constant approaches a constant value deep in the easy-axis regime. As an example for nonintegrable models, we consider two-leg ladders, for which we observe indications of diffusive energy and spin dynamics. The relevance of our results for recent experiments with quantum magnets and bosons in optical lattices is discussed.

Compared to electrons, holes in InAs quantum dots have a significantly weaker hyperfine interaction that leads to less dephasing from nuclear spins. Thus many recent studies have suggested that nuclear spins are unimportant for hole-spin dynamics compared to electric-field fluctuations. We show that the hole hyperfine interaction can have a strong effect on hole-spin coherence measurements through a nuclear feedback effect. The nuclear polarization is generated through a unique process that is dependent on the anisotropy of the hole hyperfine interaction and the coherent precession of nuclear spins, giving rise to strong modulation at the nuclear precession frequency.

We present angle-resolved photoemission spectroscopy measurements of the surface states on in-situ grown (111) oriented films of Pb${}_{1-x}$Sn${}_x$Se, a three-dimensional topological crystalline insulator. We observe surface states with Dirac-like dispersion at $\overline{\Gamma }$ and $\overline{M}$ in the surface Brillouin zone, supporting recent theoretical predictions for this family of materials. We study the parallel dispersion isotropy and Dirac-point binding energy of the surface states, and perform tight-binding calculations to support our findings. The relative simplicity of the growth technique is encouraging, and suggests a clear path for future investigations into the role of strain, vicinality, and alternative surface orientations in (Pb,Sn)Se solid solutions.

We present a Keldysh nonlinear sigma-model approach to the renormalization group analysis of the disordered electron liquid. We include both the Coulomb interaction and Fermi-liquid type interactions in the singlet and triplet channels into the formalism. Based on this model, we reproduce the coupled renormalization group equations for the diffusion coefficient, the frequency, and interaction constants previously derived with the replica model in the imaginary time technique. With the help of source fields coupling to the particle-number and spin densities, we study the density-density and spin density-spin density correlation functions in the diffusive regime. This allows us to obtain results for the electric conductivity and the spin susceptibility and thereby to rederive the main results of the one-loop renormalization group analysis of the disordered electron liquid in the Keldysh formalism.

The intensity of surface spin wave excitations in inelastic electron scattering is measured as function of electron energy for fcc and hexagonal close packed (hcp) cobalt layers. Intensities are converted into scattering probabilities with the help of a recently established calibration of our spectrometer. The scattering probability as function of energy exhibits a peak around 7 and 3 eV for fcc and hcp cobalt, respectively, and decays to immeasurably small values at energies above 10 to 15 eV in stark contrast to theoretical predictions. By comparison to phonon scattering the peaks in the scattering probabilities at low energies are tentatively attributed to image potential induced resonances.

Electronic states within the band gap of a semiconductor have a significant influence on the electronic properties and the operational stability of electronic devices, especially in the case of organic semiconductors. Previous work has shown that the gap-state density in organic thin-film transistors (TFTs) is often so large that the Fermi level is pinned deep within the gap. For $p$-channel organic TFTs, we have recently reported that the density of gap states and hence the pinning of the Fermi level can be greatly reduced by optimizing the fabrication process. For $n$-channel organic TFTs, very little is known about the gap density of states and its impact on the performance and stability of the devices. Here, we report measurements of the density and distribution of gap states in $n$-channel TFTs based on two different organic semiconductors. We have found that the gap-state density in both semiconductors is relatively small (∼10${}^{17}$ cm${}^{-3}$) and no greater than in $p$-channel organic TFTs fabricated with the same technology and the same gate dielectric. This is a significant finding, since both the performance and the stability of $n$-channel organic TFTs are often greatly inferior to those of $p$-channel organic TFTs. We have also found that the gap-state density increases over time during air exposure and that this increase in gap-state density correlates with the time-dependent, air-induced decrease in electron mobility.

The canalization of terahertz surface plasmon polaritons using a modulated graphene monolayer is investigated for subwavelength imaging. An anisotropic surface conductivity formed by a set of parallel nanoribbons with alternating positive and negative imaginary conductivities is used to realize the canalization regime required for hyperlensing. The ribbons are narrow compared to the wavelength and are created electronically by gating a graphene layer over a corrugated ground plane. Good quality canalization of surface plasmon polaritons is shown in the terahertz even in the presence of realistic loss in graphene, with relevant implications for subwavelength imaging applications.

We present composite pulse sequences that perform fault-tolerant two-qubit gate operations on exchange-only quantum-dot spin qubits in various experimentally relevant geometries. We show how to perform dynamically corrected two-qubit gates in exchange-only systems with the leading hyperfine error term canceled. These pulse sequences are constructed to conform to the realistic experimental constraint of strictly non-negative couplings. We establish that our proposed pulse sequences lead to several orders of magnitude improvement in the gate fidelity compared with their uncorrected counterparts. Together with single-qubit dynamically corrected gates, our results enable noise-resistant universal quantum operations with exchange-only qubits.

The electronic transport properties in multiterminal graphene oxide framework (GOF) materials are investigated using a combination of theoretical and computational methods. GOFs make up four-terminal $[origin=c]90H$-shaped GNR-$L$-GNR junctions where sandwiched boronic acid molecules (L) are covalently linked to two graphene nanoribbons (GNRs) of different edge chiralities. The transport properties are governed by both tunneling and quasiresonant regimes. We determine how the presence of linker molecules affects the transport properties and establish that the through-molecule transport properties can be tuned by varying the chemical composition of the pillar molecules but are not significantly modified when changing the type of electrodes from zigzag GNRs to armchair GNRs. In addition, we find that in multilinker systems containing two parallel molecules in the device area, the coupling between the molecules can lead to both constructive and destructive quantum interferences. We also examine the inability of the classical Kirchhoff's superposition law to account for electron flow in multilinker GOF nanonetworks.

C${}_{60}$ monolayers grown on Ag(111) at room temperature form incommensurate lattices that convert into a commensurate (2$\sqrt{3}$ × 2$\sqrt{3}$)R30° phase upon annealing. The C${}_{60}$ molecules in the commensurate phase have been observed to exist in three different states on Ag(111), namely bright, dim, and superbright (SB). All three species are in dynamical equilibrium at 280 < $T$ < 440 K. The bright and dim species were the subject of an earlier low-energy electron diffraction study that determined their geometries on the surface and the dynamics of the switching between those two states. The study presented here takes a closer look at the SB molecules, which appear and disappear at temperature-dependent rates with a measured activation barrier of 1.5 eV. The SB molecules in the commensurate phase comprise about 0.5% of the molecules and have a spatially random distribution. The evidence suggests that the formation of the three different states of C${}_{60}$ on Ag(111) is a result of stress imposed by the substrate as the C${}_{60}$ adopts the commensurate spacing that is slightly smaller than its natural spacing. In the incommensurate phases, there is no bright-dim contrast, but SB C${}_{60}$ molecules form and organize into ordered arrays that appear to correspond to the moiré patterns that are produced by the mutually incommensurate lattices. This suggests that the substrate responds to the nonuniform forces imposed by the C${}_{60}$ molecules by producing raised islands of Ag atoms at the vertices of the moiré structure. A similar island structure may account for the SB molecules in the commensurate phase.

Silicene is a two-dimensional honeycomb lattice made of silicon atoms, which is considered to be a new Dirac fermion system. Based on first-principles calculations, we examine the possibility of the formation of solitonlike topological domain walls (DWs) in silicene. We show that the DWs between regions of distinct ground states of the buckled geometry should bind electrons when a uniform electric field is applied in the perpendicular direction to the sheet. The topological origin of the electron confinement is demonstrated based on numerical calculations of the valley-specific Hall conductivities, and possible experimental signatures of the quantum valley Hall effects are discussed using simulated scanning tunneling microscopy images. Our results strongly suggest that silicene could be an ideal host for the quantum valley Hall effect, thus providing a pathway to the valleytronics in silicon-based technology.

Graphene with Rashba spin-orbit coupling (RSOC) has attracted much attention so far. However, no one has noticed the topologically nontrivial changes of Berry phase for RSOC from the absence to the presence. We demonstrate that the Berry phase of electronic wave functions changes from $\pi$ to $2\pi$ in graphene monolayer (GML) and from $2\pi$ to $\pi$ in graphene bilayer (GBL), driven by RSOC. These reversals of Berry phase result in anomalous electron-hole conversions at normal conductor-superconductor junctions. The specular Andreev reflection can be significantly reduced in GML, but obviously enhanced in GBL. Another unusual point caused by RSOC is that the spin-flipped electron reflection happens due to the spin helical structures on equal-energy surfaces. An electrically observable effect induced by RSOC is proposed such that the differential conductance at voltages below the superconducting gap decreases strongly for GML while it increases remarkably for GBL, attributed to the Berry-phase-dictated interference between incident and reflected states.