We present many-body ab initio calculations of the electronic and optical properties of semiconducting zigzag carbon nanotubes under uniaxial strain. The $GW$ approach is utilized to obtain the quasiparticle band gaps and is combined with the Bethe-Salpeter equation to obtain the optical absorption spectrum. We find that the dependence of the electronic band gaps on strain is more complex than previously predicted based on tight-binding models or density functional theory. In addition, we show that the exciton energy and exciton binding energy depend significantly on strain, with variations of tens of milli-electron-volts per percent strain, but despite these strong changes the absorbance is found to be nearly independent of strain. Our results provide new guidance for the understanding and design of optomechanical systems based on carbon nanotubes.

We extend the theory of interfacial plasmon-phonon scattering to top-gated single-layer graphene. As with bottom-gated graphene, interfacial plasmon-phonon (IPP) modes are formed from the coupling between the graphene plasmon and the surface polar phonon modes in the top and bottom oxides. We study the effect of the top oxide thickness on dynamic screening and electron-IPP coupling. The remote phonon-limited electron mobility ${\mu }_{\mathrm{RP}}$ and electron scattering rates in a HfO${}_2$-covered, SiO${}_2$-supported single-layer graphene are computed at various electron densities $n$ for different dielectric thickness $t_{\mathrm{ox}}$. We find that ${\mu }_{\mathrm{RP}}$ is much more dependent on $t_{\mathrm{ox}}$ at low $n$. They also agree with the experimentally estimated room temperature ${\mu }_{\mathrm{RP}}$ from Zou et al. [Phys. Rev. Lett. 105, 126601 (2010)]. The electron density dependent ${\mu }_{\mathrm{RP}}$ is predicted to be between 5500 and $24200$ cm${}^2$V${}^{-1}$s${}^{-1}$ from $n={10}^{12}$ to ${10}^{13}$ cm${}^{-2}$ at 300 K.

Temperature dependence of remanent magnetization of Ni thin films at the interface to Cu is investigated by means of the depth-resolved x-ray magnetic circular dichroism technique. Although magnetization at the interface is smaller than that in the inner layers, no detectable difference in Curie temperature, at which remanent magnetization vanishes, is found between the interface and the inner layers. In contrast, the magnetization-temperature curves at the interface and in the inner layers show a small difference, which cannot be explained by using a simple mean-field theory if the interface layer is assumed to have the same exchange-coupling constant as that in the inner layers. It is also shown that the critical exponent for the in-plane magnetized film is significantly larger than that for the perpendicularly magnetized film due to the difference in the degree of freedom for the magnetic moment.

The recently reported nickel carbide superconductor body centered tetragonal $I4/mmm$ Th${}_2$NiC${}_2$ with $T_c=8.5$ K increasing to 11.2 K upon alloying Th with Sc is found to have very fine structure in its electronic spectrum, according to density functional based first-principles calculations. The filled Ni $3d$ band complex is hybridized with C $2p$ and Th character to and through the Fermi level $\left(E_F\right)$, and a sharply structured density of states arises only when spin-orbit coupling is included, which splits a zone-center degeneracy, leaving a very flat band edge lying at the Fermi level. The flat part of the band corresponds to an effective mass $m_z^{*}\to \infty$ with large and negative $m_x^{*}=m_y^{*}$. Although the region over which the effective mass characterization applies is less than 1$\%$ of the zone volume, it supplies on the order of half the states at (or just above) the Fermi level. The observed increase of $T_c$ by hole doping is accounted for if the reference as-synthesized sample is minutely hole doped, which decreases the Fermi level density of states and will provide some stabilization. In this scenario, electron doping will increase the Fermi level density of states and the superconducting critical temperature. Vibrational properties are presented, and enough coupling to the C-Ni-C stretch mode at 70 meV is obtained to imply that superconductivity is electron-phonon mediated.

One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N $K$ x-ray absorption (XA) and Fe $L_{2,3}$ x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state ($S=2$) from an intermediate spin state ($S=1$) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy.

Recently, generalizations of fractional quantum Hall (FQH) states known as fractional quantum anomalous Hall or, equivalently, fractional Chern insulators states, have been realized in lattice models. Ideal wave functions such as the Laughlin wave function, as well as their corresponding trial Hamiltonians, have been vital to characterizing FQH phases. The Wannier function representation of fractional Chern insulators recently proposed [X.-L. Qi, Phys. Rev. Lett. 107, 126803 (2011)] defines an approach to generalize these concepts to fractional Chern insulators. In this paper, we apply the Wannier function representation to develop a systematic pseudopotential formalism for fractional Chern insulators. The family of pseudopotential Hamiltonians is defined as the set of projectors onto asymptotic relative angular momentum components which forms an orthogonal basis of two-body Hamiltonians with magnetic translation symmetry. This approach serves both as an expansion tool for interactions and as a definition of positive-semidefinite Hamiltonians for which the ideal fractional Chern insulator wave functions are exact null-space modes. We compare the short-range two-body pseudopotential expansion of various fractional Chern insulator models at filling $\mu =\frac{1}{3}$ in phase regimes where a Laughlin-type ground state is expected to be realized. We also discuss the effect of inhomogeneous Berry curvature which leads to components of the Hamiltonian that can not be expanded into pseudopotentials, and elaborate on their role in determining low-energy theories for fractional Chern insulators. Finally, we generalize our Chern pseudopotential approach to interactions involving more than two bodies with the goal of facilitating the identification of non-Abelian fractional Chern insulators.

The Wigner-function formalism is a well-known approach to model charge transport in semiconductor nanodevices. The primary goal of the present article is to point out and explain the intrinsic limitations of the conventional quantum-device modeling based on such a Wigner-function paradigm, providing a definite answer to open questions related to the application of the conventional spatial boundary condition scheme to the Wigner transport equation. Our analysis shows that (i) in the absence of energy dissipation (coherent limit) the solution of the Wigner equation equipped with given boundary conditions is not unique, and (ii) when dissipation and decoherence phenomena are taken into account via a relaxation-time approximation, the solution, although unique, is not necessarily a physical Wigner function.

We employ inelastic helium atom-surface scattering to measure the low-energy phonon dispersion along high-symmetry directions on the surface of the topological insulator Bi${}_2$Te${}_3$. Results indicate that one particular low-frequency branch experiences noticeable mode softening attributable to the interaction between Dirac fermion quasiparticles and phonons on the surface. This mode softening constitutes a renormalization of the real part of the phonon self-energy. We obtain the imaginary part, and hence lifetime information, via a Hilbert transform. In doing so, we are able to calculate an average branch-specific electron-phonon coupling constant $\langle {\lambda }_{\nu }\rangle$ $=$ 1.44.

Gapless surface states on topological insulators are protected from elastic scattering on nonmagnetic impurities, which makes them promising candidates for low-power electronic applications. However, for widespread applications, these states should remain coherent and significantly spin polarized at ambient temperatures. Here, we studied the coherence and spin structure of the topological states on the surface of a model topological insulator, Bi${}_2$Se${}_3$, at elevated temperatures in spin- and angle-resolved photoemission spectroscopy. We found an extremely weak broadening and essentially no decay of spin polarization of the topological surface state up to room temperature. Our results demonstrate that the topological states on surfaces of topological insulators could serve as a basis for room-temperature electronic devices.

We performed tunneling spectroscopy of a carbon nanotube quantum dot (QD) coupled to a metallic reservoir either in the normal or in the superconducting state. We explore how the Kondo resonance, observed when the QD's occupancy is odd and the reservoir is normal, evolves towards Andreev bound states (ABS) in the superconducting state. Within this regime, the ABS spectrum observed is consistent with a quantum phase transition from a singlet to a degenerate magnetic doublet ground state, in quantitative agreement with a single-level Anderson model with superconducting leads.

The competition between on-site Coulomb repulsion and Rashba spin-orbit coupling (RSOC) is studied on two-leg ladders by numerical techniques. By studying persistent currents in closed rings by exact diagonalization, it is found that the contribution to the current due to the RSOC $V_{\mathrm{SO}}$, for a fixed value of the Hubbard repulsion $U$ reaches a maximum at intermediate values of $V_{\mathrm{SO}}$. By increasing the repulsive Hubbard coupling $U$, this spin-flipping current is suppressed and eventually it becomes opposite to the spin-conserving current. The main result is that the spin accumulation defined as the relative spin polarization between the two legs of the ladder is enhanced by $U$. Similar results for this Hubbard-Rashba model are observed for a completely different setup in which two halves of the ladders are connected to a voltage bias and the ensuing time-dependent regime is studied by the density matrix renormalization group technique. The combined effect between $V_{\mathrm{SO}}$ and $U$ is also interesting, leading to a strong enhancement of antiferromagnetic order, which in turn may explain the observed behavior of the spin-flipping current. The implications of this enhancement of the spin-Hall effect with electron correlations for spintronic devices is discussed.

Ferroelectric surfaces provide a promising method for modifying surface reactions via an external electric field, which can potentially provide an avenue for tunable molecular binding and surface catalysis. Using first-principles density functional theory, we investigate how the properties of the PbTiO${}_3$ surface vary with polarization and how these changes affect CO${}_2$ and H${}_2$O adsorption. We find that the polarized stoichiometric surfaces cancel the depolarizing field with an electronic reconstruction, which has a large effect on molecular binding energies. However, thermodynamically, the system will instead cancel the depolarizing field by adjusting the surface stoichiometry. Variation of the polarization and the environmental conditions can thus be used to systematically tune the surface chemistry over a wide range. In addition, we consider the addition of several different catalytic monolayers to the PbTiO${}_3$ surface, and we find that additional surface layers can be used to modify the binding of molecules to the surface while still responding to the polarization of the substrate.

We explore thermoelectric spin transport and spin dependent phenomena in a noncollinear quantum dot spin valve setup. Using this setup, we demonstrate the possibility of a thermoelectric excitation of single spin dynamics inside the quantum dot. Many-body exchange fields generated on the single spins in this setup manifest as effective magnetic fields acting on the net spin accumulation in the quantum dot. We first identify generic conditions by which a zero bias spin accumulation in the dot may be created in the thermoelectric regime. The resulting spin accumulation is then shown to be subject to a fieldlike spin torque due to the effective magnetic field associated with either contact. This spin torque that is generated may yield long-time precession effects due to the prevailing blockade conditions. The implications of these phenomena in connection with single spin manipulation and pure spin current generation are then discussed.

Density functional theory calculations are used to investigate the electronic structures of localized states at reconstructed armchair graphene edges. We consider graphene nanoribbons with two different edge types and obtain the energy band structures and charge densities of the edge states. By examining the imaginary part of the wave vector in the forbidden energy region, we reveal the decay behavior of the wave functions in graphene. The complex band structures of graphene in the armchair and zigzag directions are presented in both tight-binding and first-principles frameworks.

The high-pressure structural behavior of californium has been studied experimentally and theoretically up to 100 GPa. A valence change from divalent to trivalent forms was observed under modest pressure revealing californium to be the only actinide to exhibit more than one metallic valence at near to ambient conditions as is the case for cerium in the lanthanide series. Three metallic valencies and four different crystallographic phases were observed in californium as a function of pressure. High-pressure techniques, synchrotron radiation, and ab initio electronic structure calculations of total energies were used to investigate the material and to determine the role which californium's 5$f$ electrons play in influencing these transitions. The crystallographic structures observed are similar to those found in the preceding actinide elements, curium and americium, with the initially localized 5$f$ states becoming completely delocalized under the influence of high pressure.

The all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation was employed to investigate the ideal tensile strengths of elemental V and Mo solids, and V- and Mo-based random solid solutions. Under uniaxial $\left[001\right]$ tensile loading, the ideal tensile strength of V is $11.6$ GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ideal tensile strengths are $26.7$ and $37.6$ GPa for V in the $\left[111\right]$ and $\left[110\right]$ directions, respectively. The ideal strength of Mo is $26.7$ GPa in the $\left[001\right]$ direction and decreases when a few percent of Tc is introduced in Mo. For the V-based alloys, Cr increases and Ti decreases the ideal tensile strength in all principal directions. Adding the same concentration of Cr and Ti to V leads to ternary alloys with similar ideal strength values as that of pure V. The alloying effects on the ideal strength are explained using the electronic band structure.

Electron-magnon scatterings are very important for many effects in spintronics and therefore an ab initio treatment of these processes is highly desirable. Based on the spin-density functional electron theory, an operator for the electron-magnon scattering is constructed in a second-quantization formalism for crystal electron states which are represented by linear-muffin-tin-orbital basis functions. An outlook is given as to how this operator can be used to investigate the possible contribution of these scattering processes to the ultrafast demagnetization of films after exposure to a fs optical laser pulse.

Theory predicts that a strongly injection-locked spin-torque oscillator (STO) should show a characteristic ringing frequency both on its approach to the locked state and under the influence of thermal noise. While experiments have so far failed to detect such ringing, we here show numerically and analytically how current modulation of injection-locked STOs can excite the ringing frequency in a resonant manner, and hence increase the experimental sensitivity. The complexity of such dynamics leads to a nonlinear resonance, which can even unlock the STO as a function of the modulation strength. The results presented here offer a plausible method for experimentally measuring the ringing frequency of STOs. Moreover, the onset of unlocking also provides a measure for the maximum modulation strength that can be applied to phase-locked STOs.

Interlayer tunneling spectra, the Josephson critical current density $J_c$, and the normal tunneling resistance have been measured simultaneously for intrinsic Josephson junctions of Bi${}_{1.9}$Pb${}_{0.1}$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ (BiPb2212) in the underdoped region using a small mesa structure ($S=1\times 1{\mu \mathrm{m}}^2$) and a short-pulse technique. $J_c$ was significantly enhanced when Bi was partially replaced with Pb. By comparing $J_c$ with the value estimated by the Ambegaokar-Baratoff formula, it was found that superfluid density doubles in the Pb-containing sample. In addition, Pb raises the superconducting coherence peak without changing the value of the superconducting gap $\Delta$. Based on a model that accounts for the inhomogeneity of $\Delta$ in $\mathit{k}$ space, we argue that the superfluid density increases because Pb substitution increases the homogeneity of $\Delta \left(\mathit{k}\right)$.

We study the elastic properties of incommensurate solid ${}^4$He in the limit of zero temperature. Specifically, we calculate the pressure dependence of the five elastic constants ($C_{11}$, $C_{12}$, $C_{13}$, $C_{33}$, and $C_{44}$), longitudinal and transversal speeds of sound, and the $T=0$ Debye temperature of incommensurate and commensurate hcp ${}^4$He using the diffusion Monte Carlo method. Our results show that under compression, the commensurate crystal is globally stiffer than the incommensurate, however at pressures close to melting (i.e., $P\sim 25$ bar) some of the elastic constants accounting for strain deformations of the hcp basal plane ($C_{12}$ and $C_{13}$) are slightly larger in the incommensurate solid. Also, we find that upon the introduction of tiny concentrations of point defects, the shear modulus of ${}^4$He ($C_{44}$) undergoes a small reduction.