The prevalent concept is that a topological insulator is made of thick layers of atoms so that its surfaces, where electrical conduction occurs, can be distinguished from the insulating bulk of the interior. One important reason is that the thin film topological insulator may cause the opening of energy band gaps by quantum tunneling between the two surfaces. Inspired by an experiment on antimony thin films, we use density functional calculations to investigate the electronic structure of the four-bilayer Sb film and find that adsorptions of nonmagnetic impurity atoms of hydrogen and 3$d$ transition-metal atoms on the film all close the energy gap of the free-standing film and facilitate the formation of Dirac cones that preserve time-reversal symmetry. However, magnetic atoms of the 3$d$ transition metals do just the opposite. The results suggest the counterintuitive concept of achieving topological conduction by doping nonmagnetic impurity atoms on thin films of topological insulators.

We investigate the nonequilibrium phenomena through a quantum dot coupled to normal and superconducting leads using a weak-coupling continuous-time Monte Carlo method. Calculating the time evolution of the particle number, double occupancy, and pairing correlation at the quantum dot, we discuss how the system approaches the steady state. We also deduce the steady current through the quantum dot beyond the linear-response region. It is clarified that the interaction decreases the current in the Kondo-singlet dominant region. On the other hand, when the quantum dot is tightly coupled to the superconducting lead, the current is increased by the introduction of a Coulomb interaction, which originates from the competition between the Kondo and proximity effects. Transient currents induced by the interaction quench are also addressed.

Electrical-field control of the carrier density of topological insulators (TIs) has greatly expanded the possible practical use of these materials. However, the combination of low-temperature local probe studies and a gate tunable TI device remains challenging. We have overcome this limitation by scanning tunneling microscopy and spectroscopy measurements on in situ molecular-beam epitaxy grown Bi${}_2$Se${}_3$ films on SrTiO${}_3$ substrates with prepatterned electrodes. Using this gating method, we are able to tune the Fermi level of the top surface states within a range of ≈250 meV on a 3-nm-thick Bi${}_2$Se${}_3$ device. We report field effect studies of the surface-state dispersion, band gap, and electronic structure at the Fermi level.

$AA$-stacked bilayer graphene supports Fermi circles in its bonding and antibonding bands, which coincide exactly, leading to symmetry breaking in the presence of electron-electron interactions. We analyze a continuum model of this system in the Hartree-Fock approximation, using a self-consistently screened interaction that accounts for the gap in the spectrum in the broken symmetry state. The order parameter in the ground state is shown to be of the Ising type, involving transfer of charge between the layers in opposite directions for different sublattices. We analyze the Ising phase transition for the system and argue that it continuously evolves into a Kosterlitz-Thouless transition in the limit of vanishing interlayer separation $d$. The transition temperature is shown to depend only on the effective spin stiffness of the system even for $d>0$, and an estimate of its value suggests that the transition temperature is of the order of a few degrees Kelvin.

We demonstrate that the first-principles based adiabatic continuation approach is a very powerful and efficient tool for constructing topological phase diagrams and locating nontrivial topological insulator materials. Using this technique, we predict that the ternary intermetallic series Li${}_2{M}^{\prime }X$, where $M^{\prime }=\text{Cu}$, Ag, Au, or Cd and $X=\text{Sb}$, Bi, or Sn, hosts a number of topological insulators with remarkable functional variants and tunability. We also predict that several III-V semimetallic compounds are topologically nontrivial. We construct a topological phase diagram in the parameter space of the atomic numbers of atoms in Li${}_2{M}^{\prime }X$ compounds, which places a large number of topological materials presented in this work as well as in earlier studies within a single unified topological framework. Our results demonstrate the efficacy of adiabatic continuation as a useful tool for exploring topologically nontrivial alloying systems and for identifying new topological insulators even when the underlying lattice does not possess inversion symmetry, and the approaches based on parity analysis are not viable.

We present a theory for the Kondo spin-$\frac{1}{2}$ effect in strongly correlated quantum dots. The theory is applicable at any temperature and voltage. It is based on a quadratic Keldysh effective action parametrized by a universal function. We provide a general analytical form for the tunneling density of states through this universal function for which we propose a simple microscopic model. We apply our theory to the highly asymmetric Anderson model with $U=\infty$ and describe its strong-coupling limit, weak-coupling limit, and crossover region within a single analytical expression. We compare our results with a numerical renormalization group in equilibrium and with a real-time renormalization group out of equilibrium and show that the universal shapes of the linear and differential conductance obtained in our theory and in these theories are very close to each other in a wide range of temperatures and voltages. In particular, as in the real-time renormalization group, we predict that at the Kondo voltage the differential conductance is equal to $2/3$ of its maximum.

We explore theoretically the spin transport in nanostructures consisting of a gold quantum dot bridging nonmagnetic electrodes and two Mn${}_{12}$-Ph single molecule magnets (SMMs) that are thiol bonded to the dot but are not in direct contact with the electrodes. We find that reversal of the magnetic moment of either SMM by the application of a magnetic field leads to a large change in the resistance of the dot, i.e., a strong spin-valve effect. We show that this phenomenon arises from the following physical principle: The spin-dependent molecular orbitals that extend over the dot and both SMMs change drastically when the magnetic moment of either SMM is reversed, resulting in a large change in the conduction via those orbitals. The same physics may also be responsible for the spin-valve phenomena discovered recently in carbon nanotube quantum dots with rare-earth SMMs [M. Urdampilleta et al., Nat. Mater. 10, 502 (2011)].

Layer-layer coupling plays a critical role in defining the physical properties of few-layer graphene. With respect to vibrations, for each layer thickness $N,$ interlayer coupling creates a set of $N-1$ out-of-plane layer breathing modes (ZO${}^{\prime }$) with finite frequencies at the zone center. Unlike the widely studied intralayer vibrations, the properties of these layer breathing modes (LBMs) are defined by the layer-layer interactions. Here we report the observation of distinct LBMs for Bernal stacked graphene of layer thicknesses $N$ $=$ 2–20 through measurement of electronically resonant overtone Raman bands. The Raman bands exhibit multipeak features that are unique for graphene samples of each layer thickness. The frequencies can be described well using a simple linear-chain model based on nearest-neighbor couplings between the layers.

The coherent potential approximation has historically allowed the efficient study of disorder effects over a variety of solid state systems. Its original formulation is, however, limited to a single-site or uncorrelated model of local substitutions. This neglects the effects of correlation and short-range ordering, often found in real materials. Recent theoretical work has shown one possible way to systematically address such shortcomings for simple materials with only one element per unit cell. We briefly review the basic ideas of such development within the framework of multiple scattering theory and suggest its generalization to materials with complex lattices. We validate our extension through a systematic comparison with a classic ${\text{Cu}}_{1-c}{\text{Zn}}_c$ reference test case and propose, for further illustration of local environment effects, the example of the yttria-stabilized cubic phase of zirconia, re-examined through various techniques for the first-principles treatment of disorder.

The performance of hydrogenated amorphous silicon (a-Si:H) solar cells is severely affected by the light-induced formation of metastable defects in the material (Staebler-Wronski effect). The common notion is that the dangling-bond (db) defect, a threefold coordinated silicon atom, plays a key role in the underlying mechanisms. To support the characterization of this defect by electron paramagnetic resonance (EPR), we present in this work a first-principles study of the EPR parameters for a structural ensemble of the db defect. We show that the a-Si:H dangling bond is a network defect for which charge and spin localization substantially depend on the actual coordination of the db atom and the local geometric and electronic structure of the immediate surrounding. It consequently differs by its very nature from its crystalline counterpart, which is typically related to the presence of a vacancy. The application of hydrostatic strain to our models yields further insights into the dependence of the hyperfine interaction on the structural characteristics of the defect. The observed trends are shown to result from the interplay between delocalization and $sp$ hybridization.

The thermal conductivity of diameter and polytype modulated SiC nanowires is predicted using nonequilibrium molecular dynamics. For the polytype modulated nanowires, the two main SiC polytypes, zinc blende ($3C$) and wurtzite ($2H$) were considered. We show that the thermal conductivity of the diameter modulated nanowires may be even smaller than that of the constant diameter nanowire with the small section. This remarkable reduction in thermal conduction is attributed to a significant thermal boundary resistance displayed by the constriction, as measured by independent molecular-dynamics simulations. The constriction resistance is related to the confinement of low-frequency modes, as shown by vibrational density-of-states calculations. We used Monte Carlo simulations to conclude that the value of the constriction resistance may be explained by the specular reflections of this class of modes on the surface surrounding the constriction.

We investigate theoretically two-dimensional (2D) electronic transport in Si:P $\delta$-doped layers limited by charged-dopant scattering. Since the carrier density is approximately equal to the dopant impurity density, the density dependent transport shows qualitatively different behavior from that of the well studied 2D Si MOSFETs (metal-oxide-semiconductor field-effect transistors) where the carrier density is independent of the impurity density. We find that the density dependent mobility of the Si:P system shows nonmonotonic behavior which is exactly opposite of the nonmonotonicity observed in Si MOSFETs—in the Si:P system the mobility first decreases with increasing density and then it increases slowly with increasing density above a typical density ${10}^{14}$ cm${}^{-2}$ (in contrast to Si MOSFETs where the mobility typically increases with density first and then slowly decreases at high density as surface roughness scattering dominates). In the low-density limit (or strong screening limit) mobility decreases inversely with increasing density, but in the high-density limit (or weak-screening limit) it slowly increases due to the finite width effects of the 2D layer. In the intermediate density regime ($1/a<2k_F<q_{TF}$, where $a$, $k_F$, and $q_{TF}$ are the confinement width of the $\delta$ layer, Fermi wave vector, and Thomas-Fermi screening wave vector, respectively) the density dependent mobility is approximately a constant at the minimum value. However, the calculated mean free path increase monotonically with density. We also compare the transport scattering time relevant to the mobility and the single-particle relaxation time relevant to the quantum level broadening, finding that the transport scattering time could be much larger than the single-particle scattering time unlike in Si MOSFETs where they are approximately equal.

Thermal boundary resistance dominates the overall resistance of nanosystems. This effect can be utilized to improve the figure of merit of thermoelectric materials. It is also a concern for thermal failures in microelectronic devices. The interfacial resistance depends sensitively on many interrelated structural details including material properties of the two layers, the system dimensions, the interfacial morphology, and the defect concentrations near the interface. The lack of an analytical understanding of these dependencies has been a major hurdle for a science-based design of optimum systems on a nanoscale. Here we have combined an analytical model with extensive, highly converged direct-method molecular dynamics simulations to derive analytical relationships between interfacial thermal boundary resistance and structural features. We discover that thermal boundary resistance linearly decreases with total interfacial area that can be modified by interfacial roughening. This finding is further elucidated using wave-packet analysis and local density of state calculations.

We present high-resolution single-crystal time-of-flight neutron scattering measurements on the candidate quantum spin liquid pyrochlore Tb${}_2$Ti${}_2$O${}_7$ at low temperature and in a magnetic field. At $\sim 70$ mK and in zero field, Tb${}_2$Ti${}_2$O${}_7$ reveals diffuse magnetic elastic scattering at ($\frac{1}{2}$,$\frac{1}{2}$,$\frac{1}{2}$) positions in reciprocal space, consistent with short-range correlated regions based on a two-in, two-out spin ice configuration on a doubled conventional unit cell. This elastic scattering is separated from very low-energy magnetic inelastic scattering by an energy gap of $\sim$0.06–0.08 meV. The elastic signal disappears under the application of small magnetic fields and upon elevating temperature. Pinch-point–like elastic diffuse scattering is observed near (1,1,1) and (0,0,2) in zero field at $\sim 70$ mK, in agreement with Fennell et al. [Phys. Rev. Lett. 109, 017201 (2012)], supporting the quantum spin ice interpretation of Tb${}_2$Ti${}_2$O${}_7$.

We report the structural and magnetic properties of rare-earth substituted spin-chain compounds Ca${}_{2.75}$$R$${}_{0.25}$Co${}_2$O${}_6$ ($R$ $=$ Dy and Lu). The Rietveld refinement of neutron and x-ray powder diffraction patterns confirms the single-phase formation of both compounds in the rhombohedral structure (space group $R$$\overline{3}$$c$). The derived values (from the analysis of the neutron diffraction patterns at 50 K) of the bond-valence sum indicate a reduction in the oxidation state of the cobalt ions at the trigonal prism (TP) site (6$a$) with $R$ substitution, which is further supported by low temperature neutron diffraction [where a zero value of the ordered moment at the 6$b$ site and a reduction in the values of the maximum ordered moment at the TP site have been observed] and dc magnetization studies. In the neutron diffraction patterns, additional Bragg peaks appear for both compounds below Néel temperature ($T_N$) of $\sim$16 K, indicating the onset of an antiferromagnetic ordering of cobalt spin chains on the triangular lattice. The magnetic structure corresponds to a spin density wave (SDW) structure [with a propagation vector $\mathit{k}=\{0,0,1.02\}$], having $c$ axis as a direction of both moment and modulation. For both compounds, the refined values of the ordered moment at the 18$e$, 6$b$, and 6$a$ sites are $\sim 0.03\left(2\right),0.02\left(2\right)$, and 4.2 (2) $\mu$${}_B$, respectively. Unlike the parent compound Ca${}_3$Co${}_2$O${}_6$, no temperature dependence as well as no time dependence in the intensity of the strongest antiferromagnetic reflection (10 $\tau$), corresponding to the propagation vector $k$ $=$ {0, 0, 1.02}, has been observed down to 1.5 K confirming that the SDW structure is stabilized by the substitution with rare-earth ions. The stabilization of the SDW structure and the observed decrease in the values of $T$${}_N$ could be due to a decrease in the value of positive FM intrachain exchange interaction $J$ with the rare-earth substitution in a system with competing intrachain and interchain exchange interactions.

We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$, a material that qualifies as an archetypical clean superconductor, containing only sparse strong pointlike pins [in the sense of C. J. van der Beek et al., Phys. Rev. B 66, 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ single crystals. Analysis reveals that the pinning force and energy distributions depend on the P content $x$. However, they are always much narrower than in Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching $T_c$ in BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers $\overline{\mathcal{L}}\sim 90$ nm, increasing with increasing P content $x$. This evolution can be understood as being the consequence of the P dependence of the London penetration depth. Further salient features are a wide vortex free “Meissner belt”, observed at the edge of overdoped crystals, and characteristic chainlike vortex arrangements, observed at all levels of P substitution.

Proximity and coupling effects between contiguous regions of ${}^4$He are expected to occur over a distance on the order of the temperature-dependent correlation length $\xi \left(t\right)$. We present recent measurements of ${}^4$He confined in arrays of ${\left(2\mu \mathrm{m}\right)}^3$ boxes linked through a ${}^4$He film. The specific heat and superfluid fraction of the ${}^4$He in these geometries reveal effects which occur at distances much larger than expected. These effects include enhancements in specific heat and superfluid fraction as well as shifts in the temperatures of the superfluid onset and specific-heat maximum. Our analysis gives evidence for the relevance of the finite-size correlation length $\xi (t,L)$ in explaining these effects even though the effects occur at distances over an order of magnitude larger than $\xi (t,L)$. These results can be used to deduce the finite-size scaling locus for $\xi (t,L)$. The spatial distance over which coupling and proximity effects are observed raises questions regarding the physical mechanism for these effects and the interpretation of $\xi \left(t\right)$. Lastly, these results are discussed in connection to other measurements of ${}^4$He in confined geometries, and the relevance to other critical systems.

We report an angle-dependent study of the magnetic torque $\tau \left(\theta \right)$ within the vortex state of single-crystalline LaO${}_{0.9}$F${}_{0.1}$FeAs and SmO${}_{0.9}$F${}_{0.1}$FeAs as a function of both temperature $T$ and magnetic field $H$. Sharp peaks are observed at a critical angle ${\theta }_c$ at either side of $\theta ={90}^{\circ }$, where $\theta$ is the angle between $H$ and the interplanar $c$ axis. ${\theta }_c$ is interpreted as the critical depinning angle where the vortex lattice, pinned and locked by the intrinsic planar structure, unlocks and acquires a component perpendicular to the planes. We observe a series of smaller replica peaks as a function of $\theta$ and as $\theta$ is swept away from the planar direction. These suggest commensurability effects between the period of the vortex lattice and the interplanar distance leading to additional kinked vortex configurations.

Using a combination of density functional theory (DFT), classical potentials, molecular dynamics, and nudged elastic band (NEB) calculations, we explore the diffusion of xenon in uranium dioxide (UO${}_2$). We compare migration barriers of empirical potentials with DFT by performing NEB calculations and subsequently we use the DFT-validated empirical potentials to calculate vacancy clusters, with and without xenon, to determine the migration path and barrier of xenon in bulk UO${}_2$. We find the following: (i) Two empirical potentials out of four tested agree qualitatively with DFT derived energetics for Schottky defect migration; (ii) through the use of molecular dynamics with empirical potentials, we have found a path for the diffusion of xenon-tetravacancy clusters (Xe$+2V_{\mathrm{U}}+2$$V_{\mathrm{O}}$); (iii) this path has an energy barrier significantly lower than previously reported paths by nearly 1 eV; (iv) we examine the physical contributions to the migration pathway and find the barrier is largely electrostatic and that xenon contributes very little to the barrier height; (v) once a uranium vacancy attaches to a xenon-Schottky defect, the resulting xenon-tetravacancy cluster is strongly bound; and (vi) as xenon in a tetravacancy, a xenon-double Schottky defect can diffuse in a concerted manor with a comparable barrier to xenon in a tetravacancy, but two of the oxygen vacancies are only weakly bound to the defect.

The chiral helimagnet Cr${}_{1/3}$NbS${}_2$ has been investigated by magnetic, transport, and thermal properties measurements on single crystals and by first-principles electronic structure calculations. From the measured field and temperature dependence of the magnetization for fields applied perpendicular to the $c$ axis, the magnetic phase diagram has been constructed in the vicinity of the phase transition. A transition from a paramagnetic to a magnetically ordered phase occurs near 120 K. The transition is found to strongly affect the electrical transport. The resistivity decreases sharply upon cooling near 120 K, and the spin reorientation from the helimagnetic ground state to the commensurate ferromagnetic state is evident in the magnetoresistance. At high fields, a large magnetoresistance ($55\%$ at 140 kOe) is observed near the magnetic transition temperature. Heat capacity and electronic structure calculations show that the density of states at the Fermi level is low in the magnetically ordered state.