A detailed muon spin relaxation ($\mu$SR) and magnetization study is presented of the paramagnetic-to-ferromagnetic transition in the compound ${\mathrm{SrFe}}_{0.80}{\mathrm{Co}}_{0.20}{\mathrm{O}}_3$. The critical exponents derived from the static critical analysis are close to the theoretical predictions for the Heisenberg model in three dimensions. However, a small drift toward mean-field values is interpreted as arising from the presence of long-range dipolar interactions between the Fe(IV) centers. The evolution of spin dynamics across the transition determined from the $\mu$SR study is consistent with this interpretation. $\mu$SR and magnetization data also provide evidence of an inhomogeneous magnetic state both above and below $T$${}_c$, placing this system in line with other double-exchange materials such as manganites and cobaltites where spontaneous electronic and magnetic phase separation appears conspicuous and explains much of the encountered experimental phenomenology.

Transport measurements on few-layer graphene (FLG) are important because they interpolate between the properties of single-layer graphene (SLG) as a true two-dimensional material and the three-dimensional bulk properties of graphite. In this article we present four-probe local charge transport and nonlocal spin-valve and spin-precession measurements on lateral spin field-effect transistors on FLG. We study systematically the charge- and spin-transport properties depending on the number of layers and the electrical back gating of the device. We explain the charge-transport measurements by taking the screening of scattering potentials into account and use the results to understand the spin data. The measured samples are between 3 and 20 layers thick, and we include in our analysis our earlier results of the measurements on SLG for comparison. In our room-temperature spin-transport measurements we manage to observe spin signals over distances up to 10 $\mathrm{\mu }\mathrm{m}$ and measure enhanced spin-relaxation times with an increasing number of layers, reaching ${\tau }_s~500$ ps as a maximum, about $4$ times higher than in SLG. The increase of ${\tau }_s$ can result from the screening of scattering potentials due to additional intrinsic charge carriers in FLG. We calculate the density of states of FLG using a zone-folding scheme to determine the charge-diffusion coefficient $D_C$ from the square resistance $R_S$. The resulting $D_C$ and the spin-diffusion coefficient $D_S$ show similar values and depend only weakly on the number of layers and gate-induced charge carriers. We discuss the implications of this on the identification of the spin-relaxation mechanism.

We report the observation of a remarkably strong microwave photoresistivity effect in a high-mobility two-dimensional electron system subject to a weak magnetic field and low temperature. The effect manifests itself as a giant microwave-induced resistivity peak which, in contrast to microwave-induced resistance oscillations, appears only near the second harmonic of the cyclotron resonance and only at sufficiently high microwave frequencies. Appearing in the regime linear in microwave intensity, the peak can be more than an order of magnitude stronger than the microwave-induced resistance oscillations and can not be explained by existing theories.

We evaluate the performances of ab initio $\mathit{GW}$ calculations for the ionization energies and highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of 13 gas phase molecules of interest for organic electronic and photovoltaic applications, including the ${\mathrm{C}}_{60}$ fullerene, pentacene, free-base porphyrins and phtalocyanine, PTCDA, and standard monomers such as thiophene, fluorene, benzothiazole, or thiadiazole. Standard $G$${}_0$$W$${}_0$ calculations, that is, starting from eigenstates obtained with local or semilocal functionals, significantly improve the ionization energy and band gap as compared to density functional theory Kohn-Sham results, but the calculated quasiparticle values remain too small as a result of overscreening. Starting from Hartree-Fock-like eigenvalues provides much better results and is equivalent to performing self-consistency on the eigenvalues, with a resulting accuracy of 2%–4$\%$ as compared to experiment. Our calculations are based on an efficient Gaussian-basis implementation of $\mathit{GW}$ with explicit treatment of the dynamical screening through contour deformation techniques.

Laser-induced voltages in incline-oriented thin films of thermoelectric Ca${}_x$CoO${}_2$ have been investigated at four laser wavelengths. Large film in-plane voltage signals, several tens of volts, were identified at each wavelength regardless of the occurrence of interband optical transition. The thermal model based on heat conduction equation demonstrates that this transverse voltage is closely related to heat conduction normal to the film surface and, hence, that it is associated with a phenomenon called the off-diagonal thermoelectric effect. However, distinct discrepancies between the experiment and the thermal model were identified for short laser wavelengths at which interband transition occurred. We discuss the discrepancies in terms of a photovoltaic effect (the photo-Dember effect), showing that the laser-induced voltage may not be purely thermal in origin when it is accompanied by interband transition.

The electronic conductance of a benzene molecule connected to gold electrodes via thiol, thiolate, or amino anchoring groups is calculated using nonequilibrium Green functions in combination with the fully self-consistent GW approximation for exchange and correlation. The calculated conductance of benzenedithiol and benzenediamine is one-fifth that predicted by standard density functional theory (DFT), in very good agreement with experiments. In contrast, the widely studied benzenedithiolate structure is found to have a significantly higher conductance due to the unsaturated sulfur bonds. These findings suggest that more complex gold-thiolate structures where the thiolate anchors are chemically passivated by Au adatoms are responsible for the measured conductance. Analysis of the energy level alignment obtained with DFT, Hartree-Fock, and GW reveals the importance of self-interaction corrections (exchange) on the molecule and dynamical screening at the metal-molecule interface. The main effect of the GW self-energy is to renormalize the level positions; however, its influence on the shape of molecular resonances also affects the conductance. Non-self-consistent ${\text{G}}_0{\text{W}}_0$ calculations, starting from either DFT or Hartree-Fock, yield conductance values within 50% of the self-consistent GW results.

The Zn-paratacamite family, Zn${}_x$Cu${}_{4-x}$(OH)${}_6$Cl${}_2$ for $x⩾$ 0.33, is an ideal system for studying spin-$\frac{1}{2}$ frustrated magnetism in the form of antiferromagnetic Cu${}^{2+}$ kagome planes. Here we report a new synthesis method by which high-quality millimeter-sized single crystals of Zn-paratacamite have been produced. These crystals have been characterized by metal analysis, x-ray diffraction, neutron diffraction, and thermodynamic measurements. The $x=1$ member of the series displays a magnetic susceptibility that is slightly anisotropic at high temperatures with ${\chi }_c>{\chi }_{\mathit{ab}}$. Neutron and synchrotron x-ray diffraction experiments confirm the quality of these $x=1$ single crystals and indicate no obvious structural transition down to temperatures of $T=2$ K.

We report very low-temperature magnetoconductance $\Delta G$ measurements on graphene devices with the magnetic field $H$ applied parallel to the carbon sheet. The $\Delta G(H)$ signal depends on the gate voltage $V_g$ and its sign is related to the universal conductance fluctuations. When the magnetic field is swept at fast rates, $\Delta G$ displays hysteresis loops evident for different sizes and at different transport regimes of the devices. We attribute this to the magnetization reversal of paramagnetic centers in the graphene layer, which might originate from defects in our devices.

We report on low-temperature scanning tunneling microscopy and spectroscopy (STM) studies of the electronic structure of single-crystalline ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$. Multiple superconducting gaps are observed in the density of states (DOS) and the sizes of the two dominant gaps ${\Delta }_L$ and ${\Delta }_S$ are 7.6 and 3.3 meV, respectively. The flat bottom of the DOS spectra near zero bias indicates the nodeless feature of the gaps, while the global fitting to the spectra definitely requires the anisotropy. The nodeless gaps with finite anisotropy revealed in our STM data agree well with the expectations of an extended $s$-wave superconductivity.

Full counting statistics concerns the stochastic transport of electrons in mesoscopic structures. Recently it has been shown that the charge transport statistics for noninteracting electrons in a two-terminal system is always generalized binomial: it can be decomposed into independent single-particle events, and the zeros of the generating function are real and negative. Here we investigate how the zeros of the generating function move into the complex plane due to interactions and demonstrate that the positions of the zeros can be detected using high-order factorial cumulants. As an illustrative example we consider electron transport through a Coulomb blockade quantum dot for which we show that the interactions on the quantum dot are clearly visible in the high-order factorial cumulants. Our findings are important for understanding the influence of interactions on counting statistics, and the characterization in terms of zeros of the generating function provides us with a simple interpretation of recent experiments, where high-order statistics have been measured.

We investigate magnetotransport in a ferromagnetic-normal-ferromagnetic graphene junction where a gate electrode is attached to the normal segment. It is shown that the charge conductance can be maximal at an antiparallel configuration of the magnetizations. Moreover, we demonstrate that both the magnitude and the sign of the spin-transfer torque can be controlled by means of the gate voltage in the normal segment. In this way, the present system constitutes a spin-transfer torque transistor. These anomalous phenomena are attributed to the combined effect of the exchange field and the Dirac dispersion of graphene. Our prediction opens up the possibility of moving domain walls parallel or antiparallel to the current in a controllable fashion by means of a local gate voltage.

We study topological insulators characterized by the integer topological invariant $\mathbb{Z}$, in even and odd spatial dimensions. These are well understood in the case when there are no interactions. We extend the earlier work on this subject to construct their topological invariants in terms of their Green’s functions. In this form, they can be used even if there are interactions. Specializing to one and two spatial dimensions, we further show that if two topologically distinct topological insulators border each other, the difference of their topological invariants is equal to the difference between the number of zero-energy boundary excitations and the number of zeros of the Green’s function at the boundary. In the absence of interactions Green’s functions have no zeros; thus, there are always edge states at the boundary, as is well known. In the presence of interactions, in principle, Green’s functions could have zeros. In that case, there could be no edge states at the boundary of two topological insulators with different topological invariants. This may provide an alternative explanation to the recent results on one-dimensional interacting topological insulators.

We present a microscopic discussion of a nano-sized structure which uses the quantization of energy levels and the physics of single charge Coulomb interaction to achieve an optimal conversion of heat flow to directed current. In our structure the quantization of energy levels and the Coulomb blockade lead to the transfer of quantized packets of energy from a hot source into an electric conductor to which it is capacitively coupled. The fluctuation-generated transfer of a single energy quantum translates into the directed motion of a single electron. Thus in our structure the ratio of the charge current to the heat current is determined by the ratio of the charge quantum to the energy quantum. An important novel aspect of our approach is that the direction of energy flow and the direction of electron motion are decoupled.

We present neutron diffraction measurements on single-crystal samples of nonsuperconducting Ba(${\mathrm{Fe}}_{1-x}{\mathrm{Cr}}_x$)${}_2$${\mathrm{As}}_2$ as a function of Cr doping for $0⩽x⩽0.47$. The average spin-density-wave moment is independent of concentration for $x⩽0.2$ and decreases rapidly for $x⩾0.3$. For concentrations in excess of 30% chromium, we find a new competing magnetic phase consistent with G-type antiferromagnetism which rapidly becomes the dominant magnetic ground state. Strong magnetism is observed for all concentrations measured, naturally explaining the absence of superconductivity in the Cr-doped materials.

We study Andreev reflection at an interface between a half metal and a superconductor with spin-orbit interaction. While the absence of minority carriers in the half metal makes singlet Andreev reflection impossible, the spin-orbit interaction gives rise to triplet Andreev reflection (i.e., the reflection of a majority electron into a majority hole or vice versa). As an application of our calculation, we consider a thin half-metal film or wire laterally attached to a superconducting contact. If the half metal is disorder free, an excitation gap is opened that is proportional to the spin-orbit interaction strength in the superconductor. For electrons with energy below this gap a lateral half-metal–superconductor contact becomes a perfect triplet Andreev reflector. We show that the system supports localized Majorana end states in this limit.

It is shown that within the Ginzburg-Landau (GL) approximation the order parameters ${\Delta }_1(\mathbf{r},T)$ and ${\Delta }_2(\mathbf{r},T)$ in two-band superconductors vary on the same length scale, the difference in zero-$T$ coherence lengths ${\xi }_{0\nu }~\hslash v_{F\nu }/{\Delta }_{\nu }(0)$, $\nu =1,2$ notwithstanding. This amounts to a single physical GL parameter $\kappa$ and the classic GL dichotomy: $\kappa <1/\sqrt{2}$ for type I and $\kappa >1/\sqrt{2}$ for type II.

The unconventional superconductivity in the newly discovered iron-based superconductors is intimately related to its multiband/multiorbital nature. Here we report the comprehensive orbital characters of the low-energy three-dimensional electronic structure in BaFe${}_{1.85}$Co${}_{0.15}$As${}_2$ by studying the polarization and photon-energy dependence of angle-resolved photoemission data. While the distributions of the $d_{\mathit{xz}}$, $d_{\mathit{yz}}$, and $d_{3z^2-r^2}$ orbitals agree with the prediction of density functional theory, those of the $d_{\mathit{xy}}$ and $d_{x^2-y^2}$ orbitals show remarkable disagreement with theory. Our results point out the inadequacy of the existing band structure calculations and, more importantly, provide a foundation for constructing the correct microscopic model of iron pnictides.

Single crystals of CaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ are found to exhibit the tetragonal (T) to collapsed-tetragonal (cT) transition at $T_{\mathrm{cT}}\lesssim 100$ K for $x>0.05$. The $c$-axis shrinks by $~9\%$ below $T_{\mathrm{cT}}$, which substantially diminishes the interband nesting between the hole and electron bands. In sharp contrast to the superconducting T phase of $A$Fe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ ($A=$ Ba, Sr), where the anomalous non-Fermi liquid transport properties are observed, the resistivity, Hall coefficient, and magnetoresistance data in the Ca-based system all indicate that the standard Fermi liquid behaviors are recovered abruptly below $T_{\mathrm{cT}}$, and the superconductivity disappears completely. The simultaneous disappearance of the superconductivity and the non-Fermi liquid transport enlightens the essential role of interband-associated fluctuations to the superconductivity in Fe pnictides.

We report magnetic quantum oscillations in magnetic fields extending to $~$60 T in single crystals of the body-centered tetragonal antiferromagnet CePt${}_2$In${}_7$—recently discovered to exhibit pressure-induced superconductivity at $T_{\mathrm{c}}\approx$ 2.1 K. Despite the two-dimensionality of its Fermi surface, the microscopic electronic properties of layered CePt${}_2$In${}_7$ are revealed to be more similar to those of cubic CeIn${}_3$ than those of layered CeRhIn${}_5$. A significant field-induced change in the Fermi surface occurs below $H_{\mathrm{m}}\approx$ 45 T in both CePt${}_2$In${}_7$ and CeIn${}_3$, where it is broken into small pockets with field-dependent effective masses—signaling partial 4$f$-electron involvement in the Fermi surface for $H<H_{\mathrm{m}}$. Since CePt${}_2$In${}_7$ and CeIn${}_3$ differ only in the dimensionality of their Ce sublattices, an ideal pair of compounds for investigating the effect of dimensionality on superconductivity is realized.

We use scanning superconducting quantum interference device (SQUID) microscopy to investigate the behavior of vortices in the presence of twin boundaries in the pnictide superconductor Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$. We show that the vortices avoid pinning on twin boundaries. Individual vortices move in a preferential way when manipulated with the SQUID: They tend to not cross a twin boundary, but rather to move parallel to it. This behavior can be explained by the observation of enhanced superfluid density on twin boundaries in Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$. The observed repulsion from twin boundaries may be a mechanism for enhanced critical currents observed in twinned samples in pnictides and other superconductors.