In the framework of the Ginzburg-Landau model, it is shown that narrow superconducting films with width $w\simeq 3–8\xi \left(T\right)$ [$\xi \left(T\right)$ is a temperature-dependent coherence length] exhibit unusual transport properties. In the absence of bulk pinning, its critical current $I_c$ nonmonotonically depends on perpendicular magnetic field $H$ and has one minima (dip) and one maxima (peak) at some magnetic fields. At currents $I\ll I_c\left(H\right)$, the finite magnetoresistance $R\left(H\right)$ of such a sample due to thermoactivated vortex hopping via edge barriers also shows both local maxima (peak) and minima (dip) nearly at the same magnetic fields. In narrower films, such an effect is absent due to absence of the vortices and in wider films the effect is weaker due to increased vortex-vortex interaction. The finite length of the film produces additional periodic variation in both $I_c\left(H\right)$ and $R\left(H\right)$ because of discrete changes in the number of the vortices, which is superimposed on the above-mentioned nonmonotonic dependence. The obtained results are directly related to many experiments on narrow superconducting films/bridges where such nonmonotonic dependencies $I_c\left(H\right)$ and $R\left(H\right)$ were observed.

In tetragonal ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature $T=5$ K [Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A$ $=$ Ca, Sr, Ba) parent compounds of high-$T_{\mathrm{c}}$ superconductors. This surprising discovery suggests that ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ may also be a good parent compound for high-$T_{\mathrm{c}}$ superconductivity. Here structural and thermal expansion, electrical resistivity $\rho$, angle-resolved photoemission spectroscopy (ARPES), heat capacity $C_{\mathrm{p}}$, magnetic susceptibility $\chi$, ${}^{75}$As NMR, and neutron diffraction measurements of ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The $c$-axis thermal expansion coefficient ${\alpha }_c$ is negative from 7 to 300 K, whereas ${\alpha }_a$ (the $a$-axis thermal expansion coefficient) is positive over this $T$ range. The $\rho \left(T\right)$ shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy $E_{\mathrm{F}}$. The band calculations exhibit an extremely sharp peak in the density of states $\mathcal{D}(E\approx E_{\mathrm{F}})$ arising from a flat $d_{x^2-y^2}$ band, where the $x$ and $y$ axes are along the $a$ and $b$ axes of the Co square lattice, respectively. A comparison of the Sommerfeld coefficient of the electronic specific heat with $\chi (T\to 0)$ suggests the presence of strong ferromagnetic itinerant spin correlations, which on the basis of the Stoner criterion predicts that ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ should be an itinerant ferromagnet, in conflict with the magnetization data. The $\chi \left(T\right)$ does have a large magnitude, but also exhibits a broad maximum at $\approx 115$ K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we suggest that metallic ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ has a gapless quantum spin-liquid ground state.

We propose an easy-to-build easy-to-detect scheme for realizing Majorana fermions at the ends of a chain of magnetic atoms on the surface of a superconductor. Model calculations show that such chains can be easily tuned between trivial and topological ground states. In the latter, spatially resolved spectroscopy can be used to probe the Majorana fermion end states. Decoupled Majorana bound states can form even in short magnetic chains consisting of only tens of atoms. We propose scanning tunneling microscopy as the ideal technique to fabricate such systems and to probe their topological properties.

We theoretically investigate the $T$-$B$ phase diagram of cuprates building on the $\mathrm{SU}\left(2\right)$ pseudogap order recently obtained from the spin-fermion model. At low temperatures, our analysis reveals an almost temperature-independent critical magnetic field $B_{\mathrm{co}}$ at which $d$-wave superconductivity switches to a phase of charge order. For temperatures beyond a certain value of the order of the zero-field superconducting transition temperature $T_c$, the critical field $B_{\mathrm{co}}\left(T\right)$ sharply grows. We compare our results derived within the effective $\mathrm{SU}\left(2\right)$ nonlinear $\sigma$ model with the phase diagram recently obtained by sound velocity measurements by LeBoeuf et al. [Nat. Phys. 9, 79 (2013)].

We have studied the pressure dependence of the superconducting transition temperature $T_c$ of the filled skutterudite YFe${}_4$P${}_{12}$ under various quasihydrostatic pressures up to 8 GPa. DC magnetization measurements up to 1.2 GPa in a piston-cylinder cell yields a linear increase of $T_c$ with a positive coefficient $d{T}_c$/$dP$ $=$ 1 K/GPa. Resistivity measurements up to 8 GPa in a cubic anvil apparatus show that $T_c$ increases to 9.3 K at 8 GPa, but the slope decreases gradually with increasing pressure. Besides the previously reported LaFe${}_4$P${}_{12}$, YFe${}_4$P${}_{12}$ becomes the second filled skutterudite superconductor that exhibits a positive pressure coefficient $d{T}_c$/$dP$ in a broad pressure range. The observed large $d{T}_c$/$dP$ > 0 of YFe${}_4$P${}_{12}$ can be rationalized by our band-structure calculations that show an enhancement of density of states at Fermi energy and the Hopfield parameter under pressure at least up to 8 GPa. On the other hand, similar calculations predict a $d{T}_c$/$dP$ < 0 for YRu${}_4$P${}_{12}$.

Cu${}_x$Bi${}_2$Se${}_3$ has been proposed as a potential topological superconductor characterized by an odd-parity full bulk superconducting gap and zero-energy surface Andreev bound states (Majorana fermions). A predicted consequence of such Majorana fermions is a peak in the zero-energy density of states, which should lead to a persistent zero bias conductance peak (ZBCP) in Andreev reflection (AR) or tunneling experiments. Here, we employ a newly developed nanoscale AR spectroscopy method to study normal metal/superconductor (N-S) devices featuring Cu${}_x$Bi${}_2$Se${}_3$. The results show that a ZBCP can be tuned in or out from Cu${}_x$Bi${}_2$Se${}_3$ samples depending on the N-S barrier strength. While the appearance of ZBCP may be traced to different origins, its absence under finite barrier strength represents the absence of zero-energy Majorana fermions. The present observations thus call for a reexamination of the intriguing superconductivity in Cu${}_x$Bi${}_2$Se${}_3$.

It is well known that one signature of the three-dimensional electron topological insulator is the Witten effect: if the system is coupled to a compact electromagnetic gauge field, a monopole in the bulk acquires a half-odd-integer polarization charge. In the present work, we propose a corresponding phenomenon for the topological insulator of bosons in three dimensions protected by particle number conservation and time-reversal symmetry. We claim that although a monopole inside a topological insulator of bosons can remain electrically neutral, its statistics are transmuted from bosonic to fermionic. We demonstrate that this “statistical Witten effect” directly implies that if the surface of the topological insulator is neither gapless, nor spontaneously breaks the symmetry, it necessarily supports an intrinsic two-dimensional topological order. Moreover, the surface properties can not be fully realized in a purely two-dimensional system. We also confirm that the surface phases of the bosonic topological insulator proposed by Vishwanath and Senthil [Phys. Rev. X 3, 011016 (2013)] provide a consistent termination of a bulk exhibiting the statistical Witten effect. In a forthcoming paper, we will provide an explicit field-theoretic, lattice-regularized construction of the three-dimensional topological insulator of bosons, employing a parton decomposition and subsequent condensation of parton-monopole composites.

It is well known that at zero temperature and in the absence of electron-phonon coupling, the presence of an electric field leads to localization of carriers residing in a single band of finite bandwidth. We implement the self-consistent Born approximation (SCBA) to study the effect of weak electron-phonon coupling on the motion of a carrier in a biased system. At moderate and strong electron-phonon coupling, we supplement the SCBA, describing the string of phonons left behind by the carrier, with the momentum average approximation to describe the phonon cloud that accompanies the resulting polaron. We find that coupling to the lattice delocalizes the carrier, as expected, although long-lived resonances resulting from the Wannier-Stark states of the polaron may appear in the spectrum in certain regions of the parameter space. The approach we propose here can also be used to implement and check the validity of simple variational approximations.

Fermi liquid theory is the basic paradigm within which we understand the normal behavior of interacting electron systems, but quantitative values for the parameters that occur in this theory are currently unknown in many important cases. One such case is the two-dimensional homogeneous electron gas (2D HEG), which is realized in a wide variety of semiconductor devices. We have used quantum Monte Carlo (QMC) methods to calculate the Landau interaction functions between pairs of quasiparticles. We use these to study the Fermi liquid parameters, finding that finite-size effects represent a serious obstacle to the direct determination of Fermi liquid parameters in QMC calculations. We have used QMC data in the literature for other properties of the 2D HEG to assemble a set of “best available” values for the Fermi liquid parameters.

We identify an important issue in defect studies using hybrid functionals. When modeling a defect, which is supposedly an isolated system, with a finite-size supercell, the inclusion of a fraction of the Hartree-Fock interaction results in a strong cell-size dependence and an extremely slow convergence of the calculated defect properties, especially for shallow defect. These behaviors may give rise to a number of errors in calculated defect properties, including the deepening of transition level and overstabilization of shallow defects. Numerical results from hybrid functional calculations for a diverse array of systems can be understood within the Hartree-Fock theory of an electron-gas model, indicating that the long-range exchange is the main cause for the errors in the calculated defect properties within hybrid functionals.

We investigate the dynamical dielectric function of a monolayer of molybdenum disulfide within the random phase approximation. While in graphene damping of plasmons is caused by interband transitions, due to the large direct band gap in monolayer MoS${}_2$ collective charge excitations enter the intraband electron hole continuum similarly to the situation in two-dimensional electron and hole gases. Since there is no electron-hole symmetry in MoS${}_2$, the plasmon energies in $p$- and $n$-doped samples clearly differ. The breaking of spin degeneracy caused by the large intrinsic spin-orbit interaction leads to a beating of Friedel oscillations for sufficiently large carrier concentrations, for holes as well as for electrons.

Time-resolved photoluminescence experiments on a coupled system of Stranski-Krastanov quantum dots (SK QDs) overgrown by a stack of submonolayer (SML) depositions reveal an acceleration of the carrier dynamics in the SML stack mediated by distance-dependent transfer processes. The coupling between an SML stack and an SK QD layer increases with decreasing spacer thickness $d$, which separates the two different nanostructure systems. This control parameter $d$ as well as the optical transition energies of the SML stack can be adjusted via the growth process. The observed photoluminescence dynamics of the combined SK QD and SML system is well described by a rate equation model which includes both pure zero-dimensional and zero-dimensional–two-dimensional states.

In this paper, we study the superconductivity of nonrelativistic fermions at finite-density coupled to a transverse $U\left(1\right)$ gauge field, with the effective interaction including the Landau-damping. This model, first studied by Holstein, Norton, and Pincus [Phys. Rev. B 8, 2649 (1973)] has been known as an example of a non-Fermi liquid, i.e., a metallic state in which the decay rate of a quasiparticle is large compared to the characteristic quasiparticle energy; other examples of the non-Fermi liquid includes the two dimensional (2d) electron gas in a magnetic field at $\nu =1/2$ and the normal state of optimally doped cuprate superconductors. Our study thus addresses the question of whether or not non-Fermi liquids, like Fermi liquids, are unstable towards the formation of superconductivity. The results are (i) the non-Fermi liquid is stable against superconductivity below a critical gauge coupling, (ii) above this critical coupling, the ground state is an unconventional superconductor with angular momentum $\ell \ge 2$. Our results are obtained from a solution of the Dyson-Nambu equation. Note that in this problem there is a quantum critical point between a non-Fermi liquid state and the superconducting state, as the critical coupling is nonzero. This is in contrast to a weakly coupled metal, which exhibits superconductivity for infinitesimally weak interaction regardless of its sign.

In this paper, we present measurements of the chemical potential of single-layer graphene as a function of carrier density and temperature, including near the Dirac point. Far from the charge neutrality point, the graphene is homogenous with a single carrier type. However, as the Dirac point is approached, puddles form, and electrons and holes coexist. Hall effect analyses based on two charge carriers are not adequate in this regime. Hence, a new methodology is introduced, and by using the chemical potential and the transport data self-consistently, we were able to extract the density of each carrier. We obtained very good agreement with a recent theory that assumes a Gaussian distribution of the electrostatic disorder potential. Surprisingly, the temperature dependence of the minimum conductivity of graphene was primarily attributed to the temperature dependence of the disorder potential itself through the carrier charge densities, and not to the temperature dependence of the carrier's mobility. .

We investigate nonlinear heat properties in mesoscopic conductors using a scattering theory of transport. Our approach is based on a leading-order expansion in both the electrical and thermal driving forces. Beyond linear response, the transport coefficients are functions of the nonequilibrium screening potential that builds up in the system due to interactions. Within a mean-field approximation, we self-consistently calculate the heat rectification properties of a quantum dot attached to two terminals. We discuss nonlinear contributions to the Peltier effect and find departures from the Wiedemann-Franz law in the nonlinear regime of transport.

We present a microscopic theory of neutral excitons and charged excitons (trions) in monolayers of transition metal dichalcogenides, including molybdenum disulfide. Our theory is based on an effective mass model of excitons and trions, parameterized by ab initio calculations and incorporating a proper treatment of screening in two dimensions. The calculated exciton binding energies are in good agreement with high-level many-body computations based on the Bethe-Salpeter equation. Furthermore, our calculations for the more complex trion species compare very favorably with recent experimental measurements and provide atomistic insight into the microscopic features which determine the trion binding energy.

The behavior of hydrogen on the 110 surfaces of III-V semiconductors is examined using ab initio density functional theory. It is confirmed that adsorbed hydrogen should lead to a charge accumulation layer in the case of InAs, but shown here that it should not do so for other related III-V semiconductors. It is shown that the hydrogen levels due to surface adsorbed hydrogen behave in a material dependent manner related to the ionicity of the material, and hence do not line up in the universal manner reported by others for hydrogen in the bulk of semiconductors and insulators. This fact, combined with the unusually deep $\Gamma$ point conduction band well of InAs, accounts for the occurrence of an accumulation layer on InAs(110) but not elsewhere. Furthermore, it is shown that adsorbed hydrogen should be extremely hard to distinguish from native defects (particularly vacancies) using scanning tunneling and atomic force microscopy, on both InAs(110) and other III-V (110) surfaces.

Electromagnetic scattering theory has been applied to calculate polarizabilities of finite-length multiwall carbon nanotubes (MWCNTs) in microwave and subterahertz ranges. The influence of the length and diameter of a MWCNT and electron relaxation time on the regime of the MWCNT interaction with an electromagnetic field has been analyzed. Significant screening effect, due to the strong depolarizing field, determines electromagnetic response of the MWCNTs field in a wide gigahertz frequency range. The main features of the gigahertz spectra of effective permittivity and electromagnetic interference shielding efficiencies of a MWCNT-based composite observed previously in experiments have been systematized and theoretically described.

First-principles calculations have been performed for americium (Am) metal using the generalized gradient approximation $+$ orbital-dependent onsite Coulomb repulsion via Hubbard interaction (GGA+$U$) and hybrid density functional theory (HYB-DFT) methods to investigate various ground state properties and pressure-induced structural transitions. Both methods yield equilibrium volume and bulk modulus in good agreement with the experimental results. The GGA+spin orbit coupling+$U$ method reproduced all structural transitions under pressure correctly, but the HYB-DFT method failed to reproduce the observed Am-I to Am-II transition. Good agreement was found between calculated and experimental equations of states for all phases, but the first three phases need larger $U$ (α) parameters (where α represents the fraction of Hartree-Fock exchange energy replacing the DFT exchange energy) than the fourth phase in order to match the experimental data. Thus, neither the GGA+$U$ nor the HYB-DFT methods are able to describe the energetics of Am metal properly in the entire pressure range from 0 GPa to 50 GPa with a single choice of their respective $U$ and α parameters. Low binding-energy peaks in the experimental photoemission spectrum at ambient pressure relate, for some parameter choices, well to peak positions in the calculated density of states function of Am-I.