We show that the analytic bond-order potentials (BOPs) may be used to reproduce the density of states and energy of recursion-based methods for close-packed atomic configurations. In this way, we demonstrate that the analytic BOPs can efficiently recast the numerical bond-order potentials in a polynomial approximation. By introducing damping factors for the expansion coefficients in analogy to the kernel polynomial method, negative regions of the density of states are removed such that the analytic BOPs may be applied also to open systems with band gaps. By estimating higher moments from the termination of the Lanczos recursion chain, we, furthermore, achieve a faster convergence than the usual kernel polynomial method at a negligible additional computational cost.

We present measurements of the electrical and thermal transport properties of individual Ni nanowires as a function of the applied magnetic field, recorded in the temperature range between 78 and 380 K. In analogy to the anisotropic magnetoresistance (AMR) effect observed in ferromagnetic conductors, we find that the thermal resistance of Ni nanowires depends on the angle between magnetization vector and current direction. This anisotropic magnetothermal resistance effect turns out to be weaker than the AMR effect in Ni nanowires over the temperature range investigated. As a consequence, also the Lorenz number is found to be anisotropic with respect to the magnetization direction. To explain our observation, we propose a simple model that considers spin mixing due to electron-magnon scattering.

We have performed an isovalent substitution study in a layered titanium oxypnictide system BaTi${}_2$(Sb${}_{1-x}$Bi${}_x$)${}_2$O ($0⩽x⩽0.40$) by measurements of x-ray diffraction, electrical resistivity, and magnetic susceptibility. The parent compound BaTi${}_2$Sb${}_2$O is confirmed to exhibit superconductivity at 1.5 K as well as charge- or spin-density wave (CDW/SDW) ordering below 55 K. With the partial substitution of Sb by Bi, the lattice parameters $a$, $c$, and $c/a$ all increase monotonically, indicating a negative chemical pressure and lattice distortion for the (super)conducting Ti${}_2$Sb${}_2$O layers. Bi doping elevates the superconducting transition temperature to its maximum $T_c=3.7$ K at $x=0.17$, and then $T_c$ decreases gradually with further Bi doping. A metal-to-nonmetal transition takes place around $x=0.3$, and superconductivity at $\sim$1 K survives on the nonmetal side. The CDW/SDW anomaly, in comparison, is rapidly suppressed by Bi doping, and vanishes for $x⩾0.17$. The results are discussed in terms of negative chemical pressure and the disorder effect.

To guide the search for the Majorana fermion, we theoretically study superconductor/topological-insulator/superconductor (S/TI/S) junctions in an experimentally relevant regime. We find that the striking features present in these systems, including the doubled periodicity of the Andreev bound states (ABSs) due to tunneling via Majorana states, can still be present at high electron densities. We show that via the inclusion of magnetic layers, this 4$\pi$ periodic ABS can still be observed in three-dimensional (3D) topological insulators, where finite angle incidence usually results in the opening of a gap at zero energy and hence results in a 2$\pi$ periodic ABS. Furthermore, we study the Josephson-junction characteristics and find that the gap size can be controlled and decreased by tuning the magnetization direction and amplitude. These findings pave the way for designing experiments on S/3DTI/S junctions.

A theoretical study of the dynamical dielectric response of bulk lead at low energies is presented. The calculations are performed with full inclusion of the electronic band structure calculated by means of a first-principles pseudopotential approach. The effect of inclusion of the spin-orbit interaction in the band structure on the excitation spectra in Pb is analyzed, together with dynamical exchange-correlation and local-field effects. In general, results show significant anisotropy effects on the dielectric response of bulk Pb. At small momentum transfers along three different high-symmetry directions, the calculated excitation spectra present several peaks with strong acousticlike dispersion in the energy range below 2 eV. The analysis shows that only one of such modes existing at momentum transfers along the $\Gamma$–$K$ direction can be interpreted as a true acousticlike plasmon, whereas all other modes are originated from the enhanced number of intraband electron-hole excitations at corresponding energies. Comparison with available optical experimental data shows good agreement.

We consider a junction between two topological insulators and calculate the properties of the interface states with an effective low-energy Hamiltonian for topological insulators with a single cone on the surface. This system bears a close resemblance to bilayer graphene, as both result from the hybridization of Dirac cones. We find gapless interface states not only when the helicity directions of the topological surface states are oppositely oriented, but they can also exist if they are equally oriented. Furthermore, we find that the existence of the interface states can be understood from the closing of the bulk gap when the helicity changes orientation. Recently superluminal tachyonic excitations were also claimed to exist at the interface between topological insulators. However, here we show that these interface states do not exist.

We numerically study the thermoelectric transport properties based on the Haldane model of the three-dimensional topological insulator (3DTI) thin film in the presence of an exchange field $g$ and a hybridization gap $\Delta$. The thermoelectric coefficients exhibit rich behaviors as a consequence of the interplay between $g$ and $\Delta$ in the 3DTI thin film. For $\Delta =0$ but $g\ne 0$, the transverse thermoelectric conductivity ${\alpha }_{xy}$ saturates to a universal value $1.38k_{B}e/h$ at the center of each Landau level (LL) in the high-temperature regime, and displays a linear temperature dependence at low temperatures. The semiclassical Mott relation is found to remain valid at low temperatures. If $g=0$ but $\Delta \ne 0$, the thermoelectric coefficients are consistent with those of a band insulator. For both $g\ne 0$ and $\Delta \ne 0$, ${\alpha }_{xy}$ saturates to a universal value $0.69k_{B}e/h$ at the center of each LL in the high-temperature regime. We attribute this behavior to the split of all the LLs, caused by the simultaneous presence of nonzero $g$ and $\Delta$, which lifts the degeneracies between Dirac surface states.

We investigate experimentally and theoretically the resonant emission of single InAs/GaAs quantum dots in a planar microcavity. Due to the presence of at least one residual charge in the quantum dots, the resonant excitation of the neutral exciton is blocked. The influence of the residual doping on the initial quantum dots charge state is analyzed, and the resonant emission quenching is interpreted as a Coulomb blockade effect. The use of an additional nonresonant laser in a specific low power regime leads to the quantum dots neutralization and allows an efficient optical gating of the exciton resonant emission. A detailed population evolution model, developed to describe the quantum dot neutralization and the optical gate effect, perfectly fits the experimental results in the steady-state and dynamical regimes of the optical gate with a single set of parameters. We deduce that ultraslow Auger- and phonon-assisted capture processes govern the quantum dot neutralization with relaxation times in the 1–$100\mu \mathrm{s}$ range. We conclude that the optical gate acts as a very sensitive probe of the quantum dots population relaxation in an unprecedented slow-capture regime.

We study the full-counting statistics of charges transmitted through a single-level quantum dot weakly coupled to a local Einstein phonon which causes fluctuations in the dot energy. An analytic expression for the cumulant generating function, accurate up to second order in the electron-phonon coupling and valid for finite voltages and temperatures, is obtained in the extended wide-band limit. The result accounts for nonequilibrium phonon distributions induced by the source-drain bias voltage, and concomitantly satisfies the fluctuation theorem. Extending the counting field to the complex plane, we investigate the locations of possible singularities of the cumulant generating function, and exploit them to identify regimes in which the electron transfer is affected differently by the coupling to the phonons. Within a large-deviation analysis, we find a kink in the probability distribution, analogous to a first-order phase transition in thermodynamics, which would be a unique hallmark of the electron-phonon correlations. This kink reflects the fact that although inelastic scattering by the phonons once the voltage exceeds their frequency can scatter electrons opposite to the bias, this will never generate current flowing against the bias at zero temperature, in accordance with the fluctuation theorem.

Patterned structures of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ that exhibit a decrease in their electrical conductivity below 30 K recover their higher conductivity upon warming in a thermally activated process. Two dominant energy barriers $E_b$ are identified: $E_{b1}=0.224\pm 0.003$ eV related to conductivity recovery near $70\mathrm{K}$ and $E_{b2}=0.44\pm 0.015$ eV related to conductivity recovery near $160\mathrm{K}$. We discuss possible linkage to structural defects such as dislocations and twin boundaries.

Anomalous, vacancy-induced anisotropic thermal conductivity is one of the important properties in the recently discovered high-$ZT$ In${}_4$Se${}_{3-x}$ compound. In this work, the lattice thermal conductivity of In${}_4$Se${}_{3-x}$ is investigated using equilibrium molecular dynamics simulation, the point-defect model, and ab initio calculations. The charge density distribution shows highly anisotropic structure with strong bonding along In-Se-In chain direction. Se vacancy strongly suppresses the phonon propagation along the chain direction, with little change in other directions. We show that suppressed long-range acoustic phonon transport caused by the vacancy results in anisotropic change of lattice conductivity. We suggest controlling of vacancy in intrinsic low-dimensional compounds is critical in achieving optimal lattice thermal conductivity and other thermoelectric properties.

The variational cluster approximation is used to study the ground state of the spin-asymmetric Hubbard model at half filling with geometrical frustration defined on the square lattice. We show that the three phases with Néel, collinear, and disorder states appear in the parameter space and we determine the orders of the phase transitions between the three phases. We corroborate quantitatively that the asymmetry due to the spin-dependent hopping parameters, which gives rise to the Ising anisotropy in spin space, suppresses the quantum fluctuations of the system and destabilizes its disordered spin state induced by the geometrical frustration.

The effect of hyperfine interaction (HFI) on the recently discovered room-temperature defect-enabled spin-filtering effect in GaNAs alloys is investigated both experimentally and theoretically based on a spin Hamiltonian analysis. We provide direct experimental evidence that the HFI between the electron and nuclear spin of the central Ga atom of the spin-filtering defect, namely, the ${\mathrm{Ga}}_i^{}$ interstitials, causes strong mixing of the electron spin states of the defect, thereby degrading the efficiency of the spin-filtering effect. We also show that the HFI-induced spin mixing can be suppressed by an application of a longitudinal magnetic field such that the electronic Zeeman interaction overcomes the HFI, leading to well-defined electron spin states beneficial to the spin-filtering effect. The results provide a guideline for further optimization of the defect-engineered spin-filtering effect.

Formation energies of native defects in Bi${}_2$(Te${}_x$Se${}_{3-x}$), with comparison to ideal Bi${}_2$Te${}_2$S, are calculated in density-functional theory to assess transport properties. Bi${}_2$Se${}_3$ is found to be $n$ type for both Bi- and Se-rich growth conditions, while Bi${}_2$Te${}_3$ changes from $n$ to $p$ type going from Te- to Bi-rich conditions, as observed. Bi${}_2$Te${}_2$Se and Bi${}_2$Te${}_2$S are generally $n$ type, explaining observed heavily doped $n$-type behavior in most samples. A (0/−) transition level at 16 meV above valence-band maximum for Bi on Te antisites in Bi${}_2$Te${}_2$Se is related to the observed thermally active transport gap causing a $p$-to-$n$ transition at low temperature. Bi${}_2$(Te${}_x$Se${}_{3-x}$) with $x>2$ are predicted to have high bulk resistivity due to effective carrier compensation when approaching the $n$-to-$p$ crossover. Predicted behaviors are confirmed from characterization of our grown single crystals.

We investigate the emission properties of excitons in GaAs nanowires containing quantum disks formed by structural alternation between the zinc-blende and wurtzite phases, by means of temperature-dependent photoluminescence. At 10 K the emission from an ensemble of disks is distributed in a band of full width at half maximum ∼30 meV, whereas the emission linewidth for a single disk is 700 μeV. While the disk ensemble emission exhibits an S-shaped temperature dependence, the emission from single quantum disks follows the temperature dependence of the band gap over the whole temperature range. This indicates that intradisk exciton localization on impurities is negligible and that increasing the temperature induces a transfer of excitons from narrow to thick disks along the length of the wires. Our observations of the emission linewidth for single crystal-phase quantum disks show a scattering rate of excitons with acoustic phonons eight times larger than the values usually reported for (Al,Ga)As/GaAs quantum wells. This large scattering rate demonstrates that the electron effective mass in wurtzite GaAs is much heavier than in zinc-blende GaAs and is evidence of coupling between the Γ${}_7$ and Γ${}_8$ conduction bands of wurtzite GaAs.

We study the light-matter coupling of microcavity photons and an interband transition in a one-dimensional (1D) nanowire. Due to the Van Hove singularity in the density of states, resulting in a resonant character of the absorption line, the achievement of strong coupling becomes possible even without the formation of a bound state of an electron and a hole. The calculated absorption in the system and corresponding energy spectrum reveal anticrossing behavior characteristic of the formation of polariton modes. In contrast to the case of conventional exciton polaritons, the formation of 1D Van Hove polaritons will not be restricted to low temperatures and can be realized in any system with a singularity in the density of states.

We report on resistivity and Hall measurements performed on a series of narrow mesa devices fabricated from ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ single interface heterostructure with a bridge width range of 1.5–10 microns. Upon applying back-gate voltage of the order of a few volts, a strong increase in the sample resistance (up to factor of 35) is observed, suggesting a relatively large capacitance between the Hall bar and the gate. The high value of this capacitance is due to the device geometry, and can be explained within an electrostatic model using the Thomas-Fermi approximation. The Hall coefficient is sometimes a nonmonotonic function of the gate voltage. This behavior is inconsistent with a single conduction-band model. We show that a theoretical two-band model is consistent with this transport behavior, and indicates a metal-to-insulator transition in at least one of these bands.

The recent discovery of the diamondlike C${}_3$B and C${}_5$B compounds has raised hopes of revealing interesting properties and also elicits questions about the stability of such compounds. Using our implementation of the evolutionary global space-group optimization method, we have found ordered structural models for C${}_3$B (layered hexagonal) and C${}_5$B (diamondlike) with lower energies than previously obtained and revealing unusual layer-stacking sequences. The compounds are less stable than a mixture of freestanding lowest-energy phases of B, C, and C${}_4$B, thus C${}_3$B and C${}_5$B are not ground-state structures. Nevertheless, disordered diamondlike C${}_3$B and C${}_5$B can be formed exothermically at high temperature in the reaction [graphitelike C${}_3$B] $+$ 2C → [diamondlike C${}_5$B] and [graphitelike C${}_3$B] → [diamondlike C${}_3$B]. Thus, the disorder on the C and B sites of diamondlike C${}_3$B and C${}_5$B is responsible for the observed phases.