We investigate the transport properties of high-quality single-layer graphene, epitaxially grown on a 6H-SiC(0001) substrate. We have measured transport properties, in particular charge-carrier density, mobility, conductivity, and magnetoconductance of large samples as well as submicrometer-sized Hall bars which are entirely lying on atomically flat substrate terraces. The results display high mobilities, independent of sample size. The temperature dependence of the conductance indicates a rather strong coupling to the SiC substrate. An analysis of the Shubnikov-de Haas effect yields the Landau-level spectrum of single-layer graphene. When gated close to the Dirac point, the mobility increases substantially and the graphenelike quantum Hall effect occurs.

We consider excitations with the quantum numbers of a hole in the one-dimensional Hubbard model below half-filling. We calculate the finite-size corrections to the energy. The results are then used to determine threshold singularities in the single-particle Green’s function for commensurate fillings. We present the analogous results for the Yang-Gaudin model (electron gas with $\delta$-function interactions).

The influence of spin-orbit coupling (SOC) on the electron-phonon interaction and on the pairing strength is investigated for Pb and Tl within density-functional theory. For Pb we find that SOC increases the coupling constant $\lambda$ by 44%, solving the longstanding puzzle of too small $\lambda$ values obtained consistently in previous first-principles calculations. The origin of the SOC-induced enhancement of $\lambda$ lies both in a softening of the phonon spectrum and in an increase in the electron-phonon coupling matrix elements. In contrast, for the neighboring element Tl only weak effects of SOC on the phonon spectrum and the electron-phonon interaction is obtained, leading to an overall reduction in $\lambda$ by 12%. In both cases, the inclusion of SOC improves the agreement with experiment.

A nanoscale structure analysis method using convergent-beam electron diffraction (CBED) has been applied to an orbital-ordered state of ${\text{Fe}}^{2+}3d$ electrons in a spinel oxide ${\text{FeCr}}_2{\text{O}}_4$. The atom positions, atomic displacement parameters, electrostatic potential, and electron density have been determined using CBED intensity data alone. A ferro-type ordered state of the $3z^2-r^2$ orbitals at the tetrahedrally coordinated Fe atoms has been successfully observed through the electrostatic potential obtained in the present analysis.

Magnetoresistance effects in organic semiconductor spin valves have recently been reported, and have been variously interpreted as being due to tunneling magnetoresistance or giant magnetoresistance. We introduce a criterion for distinguishing between tunneling and injection conductivity necessary for properly analyzing organic spin-valve phenomena. We measure current-voltage $\left(I\text{−}V\right)$ characteristics in $\text{Co}/{\text{AlO}}_x/\text{rubrene}/\text{Fe}$ junctions with a rubrene layer thickness, $d$, ranging from 5 to 50 nm. For $d\le 10\text{nm}$, the $I\text{−}V$ traces are typical of tunnel junctions. At $d>15\text{nm}$, the tunneling current becomes negligibly small. At larger biases, however, a second type of conductivity sets in. In this regime, the $I\text{−}V$ curves are strongly nonlinear and temperature dependent. By comparing these to $I\text{−}V$ curves measured in organic light-emitting diodes, we assign the latter mode to injection into the organic layer followed by hopping transport. We observe a spin-valve effect only in the tunneling regime.

We have studied experimentally the formation of magnetic solitons in a periodic system constituted by a ferromagnetic film and a periodic metal grating. We show that due to the finite-size effects and the influence of dynamic magnetic losses, the wave dispersion within the stop bands caused by the periodicity allows observation of both bright and dark solitons formed via the induced and the coupled modulation instabilities. We find that the flexibility of the studied system enables a wide variation in the characteristics of the excited solitons by the proper choice of the experimental conditions.

We extend the recently developed converse NMR approach [Thonhauser et al., J. Chem. Phys. 131, 101101 (2009)] such that it can be used in conjunction with norm-conserving, nonlocal pseudopotentials. This extension permits the efficient ab initio calculation of NMR chemical shifts for elements other than hydrogen within the convenience of a plane-wave pseudopotential approach. We have tested our approach on several finite and periodic systems, finding very good agreement with established methods and experimental results.

We demonstrate a direct connection between the magnetic easy axis in Mn-doped GaP and epitaxial strain by a combined ferromagnetic resonance, x-ray diffraction and superconducting quantum interference device magnetometry study. The magnetic easy axis of ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{P}$ is gradually rotated from the in-plane $\left[0\overline{1}1\right]$ direction toward the film normal [100] through alloying with isovalent N which changes the strain state of the film from compressive to tensile. For a nearly lattice-matched film the strain-related component to the out-of-plane uniaxial anisotropy field is close to zero. Both in-plane and out-of-plane magnetization reversal processes are explored by a simple model that considers the combination of coherent spin rotation and noncoherent spin switching. We use our results to estimate domain-wall sizes and energetics, which have yet to be directly measured in this materials system. The band structure and electrical properties of ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{P}$ imply that holes localized within a Mn-derived impurity band are capable of mediating the same anisotropic exchange interactions as the itinerant carriers in the canonical ${\text{Ga}}_{1-x}{\text{Mn}}_x\text{As}$ system.

It is shown how the exchange interaction, the dipole-dipole interaction, and the Dzyaloshinsky-Moriya interaction between electronic spin-density fluctuations emerge naturally from a field-theoretic framework that couples electrons to the fluctuating electromagnetic potential. Semiquantitative estimates are given to determine when the dipole-dipole interaction, which is often neglected, needs to be considered and various applications are discussed, with an emphasis on weak ferromagnets and on helimagnets.

We study AKLT models on locally treelike lattices of fixed connectivity and find that they exhibit a variety of ground states depending upon the spin, coordination, and global (graph) topology. We find (a) quantum paramagnetic or valence-bond solid ground states, (b) critical and ordered Néel states on bipartite infinite Cayley trees, and (c) critical and ordered quantum vector spin glass states on random graphs of fixed connectivity. We argue, in consonance with a previous analysis [C. R. Laumann, S. A. Parameswaran, and S. L. Sondhi, Phys. Rev. B 80, 144415 (2009)], that all phases are characterized by gaps to local excitations. The spin glass states we report arise from random long-ranged loops which frustrate Néel ordering despite the lack of randomness in the coupling strengths.

Oscillatory surface-density profiles (layers) have previously been reported in several metallic liquids, one dielectric liquid, and in computer simulations of dielectric liquids. We have now seen surface layers in two other dielectric liquids, pentaphenyl trimethyl trisiloxane, and pentavinyl pentamethyl cyclopentasiloxane. These layers appear below $T\sim 285\text{K}$ and $T\sim 130\text{K}$, respectively; both thresholds correspond to $T/T_{\text{c}}\sim 0.2$ where $T_{\text{c}}$ is the liquid-gas critical temperature. All metallic and dielectric liquid surfaces previously studied are also consistent with the existence of this $T/T_{\text{c}}$ threshold, first indicated by the simulations of Chacón et al. [Phys. Rev. Lett. 87, 166101 (2001)]. The layer width parameters, determined using a distorted-crystal fitting model, follow common trends as functions of $T_{\text{c}}$ for both metallic and dielectric liquids.

Superconducting quantum interference device microscopy shows stripes of increased diamagnetic susceptibility in the superconducting state of twinned, orthorhombic, underdoped crystals of $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$, but not in tetragonal overdoped crystals. These stripes are consistent with enhanced superfluid density on twin boundaries.

We calculate the change in susceptibility resulting from a thin sheet with reduced penetration depth embedded perpendicular to the surface of an isotropic superconductor, in a geometry applicable to scanning superconducting quantum interference device microscopy, by numerically solving Maxwell’s and London’s equations using the finite-element method. The predicted stripes in susceptibility agree well in shape with the observations of Kalisky et al. [Phys. Rev. B 81, 184513 (2010)] of enhanced susceptibility above twin planes in the underdoped pnictide superconductor $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$. By comparing the predicted stripe amplitudes with experiment and using the London relation between penetration depth and superfluid density, we estimate the enhanced Cooper-pair density on the twin planes, and the barrier force for a vortex to cross a twin plane. Fits to the observed temperature dependence of the stripe amplitude suggest that the twin planes have a higher critical temperature than the bulk, although stripes are not observed above the bulk critical temperature.

We have achieved the few-electron regime in InAs nanowire double quantum dots. Spin blockade is observed for the first two half-filled orbitals, where the transport cycle is interrupted by forbidden transitions between triplet and singlet states. Partial lifting of spin blockade is explained by spin-orbit and hyperfine mechanisms that enable triplet to singlet transitions. The measurements over a wide range of interdot coupling and tunneling rates to the leads are well reproduced by a simple transport model. This allows us to separate and quantify the contributions of the spin-orbit and hyperfine interactions.

A bilayer system of two-dimensional electron gases in a perpendicular magnetic field exhibits rich phenomena. At total filling factor ${\nu }_{tot}=1$, as one increases the layer separation, the bilayer system goes from an interlayer-coherent exciton condensed state to an incoherent phase of, most likely, two decoupled composite-fermion Fermi liquids. Many questions still remain as to the nature of the transition between these two phases. Recent experiments have demonstrated that spin plays an important role in this transition. Assuming that there is a direct first-order transition between the spin-polarized interlayer-coherent quantum Hall state and spin partially polarized composite Fermi-liquid state, we calculate the phase boundary ${\left(d/l\right)}_c$ as a function of parallel magnetic field, NMR/heat pulse, temperature, and density imbalance, and compare with experimental results. Remarkably good agreement is found between theory and various experiments.

We have discussed the tensor-network representation of classical statistical or interacting quantum lattice models, and given a comprehensive introduction to the numerical methods we recently proposed for studying the tensor-network states/models in two dimensions. A second renormalization scheme is introduced to take into account the environment contribution in the calculation of the partition function of classical tensor-network models or the expectation values of quantum tensor-network states. It improves significantly the accuracy of the coarse-grained tensor renormalization-group method. In the study of the quantum tensor-network states, we point out that the renormalization effect of the environment can be efficiently and accurately described by the bond vector. This, combined with the imaginary-time evolution of the wave function, provides an accurate projection method to determine the tensor-network wave function. It reduces significantly the truncation error and enables a tensor-network state with a large bond dimension, which is difficult to be accessed by other methods, to be accurately determined.

The heavy-fermion antiferromagnet ${\text{CePt}}_2{\text{In}}_7$ is a new, structurally more two-dimensional member of the ${\text{Ce}}_m{M}_n{\text{In}}_{3m+2n}$ family. Applying pressure to ${\text{CePt}}_2{\text{In}}_7$ induces a broad dome of superconductivity that coexists with magnetic order for $1\le P\le 3\text{GPa}$. The maximum $T_c=2.1\text{K}$ appears near the critical pressure $P_c=3.5\text{GPa}$ where the Néel temperature extrapolates to zero temperature. An analysis of the initial slope of the upper critical field, the $T^2$ coefficient of the electrical resistivity, and specific heat indicates an enhancement of the effective mass $m^{\ast }$ as $P_c$ is approached, suggesting that critical fluctuations may mediate superconductivity. Electronic-structure calculations reveal a delicate balance between structural anisotropy and $f\text{−}d$ hybridization, which may account for comparable $T_c$’s in ${\text{CePt}}_2{\text{In}}_7$ and more three-dimensional ${\text{CeRhIn}}_5$.

We reiterate, in more details, our previous proposal of using quasiparticle interference to determine the pairing form factor in iron-based superconductors. We also present our functional renormalization group (FRG) results on LaFePO and Fe(Se,Te) superconductors. In particular, we found that the leading pairing channel in LaFePO is nodal $s_{\pm }$ with nodes on electron Fermi surfaces. For Fe(Se,Te) system we found fully gapped $s_{\pm }$ pairing with substantial gap anisotropy on electron Fermi surfaces and large gap is concentrated in regions with dominant $xy$ orbital character. We further fit the form factor obtained by FRG to real-space orbital basis pairing picture, which shows more clearly the differences between different iron-based superconductors.

The bilayer quantum Hall system at a total filling of ${\nu }_T=1$ has long resisted explanation in terms of a true counterflow superfluid, though many experimental features can be seen to be “almost” that of a superfluid. It is widely believed that quenched disorder is the root cause of this puzzle. Here we model the nonperturbative effects of disorder by investigating the $\nu =1$ bilayer in a strong periodic potential. Our model assumes that fermions are gapped and real spins are fully polarized, and concentrates on the pseudospin variable (the layer index), with the external potential coupling to the topological (Pontryagin) density of the pseudospin. We find that as the potential strength increases, there are ground-state transitions in which the topological content of the pseudospin configuration changes. These transitions are generically weakly first order with a new quadratically dispersing mode (in addition to the linearly dispersing Goldstone mode) sometimes becoming nearly gapless near the transition. We show that this leads to strong suppressions of both the Kosterlitz-Thouless transition temperature and the interlayer tunneling strength, which we treat perturbatively. We discuss how these results might extend to the case of true disorder.

We use inelastic neutron scattering to show that for the optimally electron-doped ${\text{BaFe}}_{1.9}{\text{Ni}}_{0.1}{\text{As}}_2$ $\left(T_c=20\text{K}\right)$ iron arsenide superconductor, application of a magnetic field that partially suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. These results demonstrate that the energy of the resonance is intimately connected to the electron pairing energy, and thus indicate that the mode is a direct probe for measuring electron pairing and superconductivity in iron arsenides.