In several parent iron-pnictide compounds the resistivity has an extended range of linear magnetic field dependence. We argue that there is a simple and natural explanation of this behavior. Spin density wave transition leads to Fermi-surface reconstruction corresponding to strong modification of the electronic spectrum near the nesting points. It is difficult for quasiparticles to pass through these points during their orbital motion in magnetic field, because they must turn sharply. As the area of the Fermi surface affected by the nesting points increases proportionally to magnetic field, this mechanism leads to the linear magnetoresistance. The crossover between the quadratic and linear regimes takes place at the field scale set by the SDW gap and scattering rate.

We describe a chip-based, solid-state analog of cavity-QED utilizing acoustic phonons instead of photons. We show how long-lived and tunable acceptor impurity states in silicon nanomechanical cavities can play the role of a matter nonlinearity for coherent phonons just as, e.g., the Josephson qubit plays in circuit QED. Both strong coupling (number of Rabi oscillations $\lesssim$100) and strong dispersive coupling (0.1–2 MHz) regimes can be reached in cavities in the 1–20-GHz range, enabling the control of single phonons, phonon-phonon interactions, dispersive phonon readout of the acceptor qubit, and compatibility with other optomechanical components such as phonon-photon translators. We predict explicit experimental signatures of the acceptor-cavity system.

We investigate the nonadiabatic processes occurring during the manipulations of Majorana qubits in one-dimensional semiconducting wires with proximity-induced superconductivity. Majorana qubits are usually protected by the excitation gap. Yet, manipulations performed at a finite pace can introduce both decoherence and renormalization effects. Although exponentially small for slow manipulations, these effects are important as they may constitute the ultimate decoherence mechanism. Moreover, as adiabatic topological manipulations fail to produce a universal set of quantum gates, nonadiabatic manipulations might be necessary to perform quantum computation.

We study the static and dynamic behavior of charge ordering within a $d$-wave pair pseudogap (pg) scenario. This is addressed using a density-density correlation function derived from the standard pg self-energy $\Sigma$ and compatible with the longitudinal and transverse sum rules. The broadening factor $\gamma$ in $\Sigma$ reflects the breaking of pairs into constituent fermions. We apply this form for $\Sigma$ (derived elsewhere for high fields) to demonstrate the existence of quantum oscillations in a non-Fermi liquid pg state. Our conclusion is that the pseudogap-induced pair breaking, via $\gamma$, allows the underlying fermiology to be revealed; $d$-wave (as distinct from $s$-wave) pairing in YBCO, with finite $\omega$ and $\gamma$ enables antinodal fluctuations, despite their competition in the static and superconducting limits.

The electronic structure and lattice dynamics for simple cubic (sc) P are calculated over the pressure range 0–70 GPa from first principles using the local-density approximation. The $R$ phonon mode is found to be unstable below 20 GPa in the harmonic approximation, but may be stable down to a pressure less than 20 GPa when anharmonicity is considered. The electron-phonon coupling is calculated for pressures above 20 GPa, and the superconducting transition temperature $T_c$ is found to decrease with increasing pressure throughout this pressure range. The result is in agreement with experimental results above 30 GPa. In contrast to experiment, no evidence for a decrease in $T_c$ with decreasing pressure below 30 GPa is found. The structural transition from rhombohedral $A7$ to sc is also investigated. An interesting two-step transition is found to occur theoretically which may have relevance for the pressure dependence of $T_c$. Possible explanations for the discrepancy with experiment are discussed.

We report current-driven scanning tunneling spectroscopy (CDSTS) measurements at very low temperatures on vortices in 2H-NbSe${}_2$. We find that a current produces an increase of the density of states at the Fermi level in between vortices and a reduction of the zero-bias peak at the vortex center. This occurs well below the depairing current. We conclude that a supercurrent affects the low-energy part of the superconducting gap structure of 2H-NbSe${}_2$.

Previous work on nodal $d$-wave superconductors has shown that a Fulde-Ferrell-Larkin-Ovchinnikov- (FFLO-) like superconducting (SC) state, which is modulated along the film plane, can be realized with no magnetic field when quasiparticles acquire an additional linear term in the wave vector in their dispersion. In the present work, the stability of such a modulated SC state in an artificial film against an applied magnetic field is studied. As a reflection of the presence of two FFLO-like states of different origins, one close to zero field and the other at the high-field end, in a single field vs temperature phase diagram of thin films, the conventional SC state, which is uniform along the film plane, generally tends to appear as a reentrant ordered phase bounded by the normal phase in lower fields.

We study the photocurrent response of topological insulator surface states to circularly polarized light for arbitrary oblique incidence. We describe the surface states within a Dirac model, including several perturbations such as hexagonal warping, nonlinear corrections to the mode velocity, and applied magnetic fields. We find that the photogalvanic current is strongly suppressed for the usual orbital coupling, prompting us to include the weaker Zeeman coupling. We find that the helicity-independent photocurrent dominates over the helicity-dependent contributions.

We present a numerical method based on real-space renormalization that outputs the exact ground space of “frustration-free” Hamiltonians. The complexity of our method is polynomial in the degeneracy of the ground spaces of the Hamiltonians involved in the renormalization steps. We apply the method to obtain the full ground spaces of two spin systems. The first system is a spin-1/2 Heisenberg model with four-spin cyclic-exchange interactions defined on a square lattice. In this case, we study finite lattices of up to 160 spins and find a triplet ground state that differs from the singlet ground states obtained by Batista and Trugman, [Phys. Rev. Lett. 93, 217202 (2004)]. We characterize such a triplet state as consisting of a triplon that propagates in a background of fluctuating singlet dimers. The second system is a family of spin-1/2 Heisenberg chains with uniaxial exchange anisotropy and next-nearest-neighbor interactions. In this case, the method finds a ground-space degeneracy that scales quadratically with the system size and outputs the full ground space efficiently. Our method can substantially outperform methods based on exact diagonalization and is more efficient than other renormalization methods when the ground-space degeneracy is large.

We consider iron impurities in the noble metals gold and silver and compare experimental data for the resistivity and decoherence rate to numerical renormalization group results. By exploiting non-Abelian symmetries, we show improved numerical data for both quantities as compared to previous calculations [Costi et al., Phys. Rev. Lett. 102, 056802 (2009).], using the discarded weight as criterion to reliably judge the quality of convergence of the numerical data. In addition, we also carry out finite-temperature calculations for the magnetoresistivity of fully screened Kondo models with $S=\frac{1}{2}$, 1, and $\frac{3}{2}$, and compare the results with available measurements for iron in silver, finding excellent agreement between theory and experiment for the spin-$\frac{3}{2}$ three-channel Kondo model. This lends additional support to the conclusion of Costi et al. that the latter model provides a good effective description of the Kondo physics of iron impurities in gold and silver.

We propose an alternative approach to self-consistency and conservation laws in the theory of nonequilibrium Green's functions (NEGF's), which provides an infinite family of conserving but, generally, non-self-consistent approximations. Within any $\Phi$-derivable approximation the associated Born series for the NEGF is shown to be conserving. Expectation values calculated from the Born series are then used to build a Padé table of approximations, while conservation laws are naturally preserved. We implement this technique for the $\Phi$-derivable self-consistent Born approximation (SCBA), for which we obtain a recursion relation that yields the Born series for the NEGF up to any desired order. The expectation values calculated from the Born series are then postprocessed to build a Padé table of conserving approximations. The calculation of the SCBA photocurrent in a biased molecular junction model provides an example where, in addition to conservation laws, a substantial convergence acceleration relative to standard techniques is achieved. The present reformulation of the SCBA might aid convergence to the fully self-consistent results in a wide variety of problems.

Using time- and angle-resolved two-photon photoemission spectroscopy, we investigate the energy- and momentum-dependent ultrafast electron dynamics in the Rashba spin-split quantum-well nanofilm Bi/Cu(111). We find an expected increase of electron lifetimes towards the band bottom due to a competition of intra- and interband scattering processes. In addition, we find an unexpected peculiar decrease of the lifetimes around the intersection of the split bands. We compare the experimental results with calculated lifetimes due to electron-electron scattering in a model system of a 2D electron gas including a Rashba interaction term and an effective statically screened Coulomb interaction. Although the Rashba model reproduces the increase of lifetimes towards the band bottom well, there is no indication of the experimentally observed decrease around the intersection point in this simple model system. To investigate spin-orbit coupling effects, beyond those contained in a pure Rashba model, we introduce a phenomenological $k$-dependent spin mixing that leads to a “spin hot spot.” It is shown that such a mixing would strongly increase the electron-electron scattering rate around the band intersection and thus improves the agreement with experiment.

The magnetism in ∼1 at$\%$ Mn-doped nanometric Bi${}_2$Se${}_3$ epitaxial layers has been studied by ferromagnetic resonance (FMR), magnetometry, and polarized neutron reflectivity (PNR) measurements. The FMR results reveal the formation of an intrinsic homogeneous FM bulk phase, as confirmed by the PNR measurements. Its critical temperature is low: $T_c$ ≈ 6 K; no surface magnetic phase is detected above this temperature. The FMR measurements at $T$ $=$ 4.2 K show a single uniform mode spectrum with a strong uniaxial in-plane/out-of-plane symmetry. The uniaxial anisotropy field at $T$ $=$ 4 K is $H_a$ ∼ −2500 Oe, corresponding to a uniaxial anisotropy constant of −9 × 10${}^3$ erg/cm${}^3$. The easy axis of magnetization is oriented in the film plane for temperatures between 4 K and $T_c$. Conduction electron-mediated FM in this (very) diluted magnetic semiconductor is considered as the most probable origin of the FM phase formation.

Polariton lasing has recently been observed in strongly coupled crystalline anthracene microcavities. A simple model is developed describing the onset of the nonlinear threshold based on a master equation including the relevant relaxation processes and employing realistic material parameters. The mechanism governing the buildup of the polariton population, namely, bosonic stimulated scattering from the exciton reservoir via a vibronically assisted process, is characterized and its efficiency calculated on the basis of a microscopic theory. The role of polariton-polariton bimolecular quenching is identified and temperature-dependent effects are discussed.

We study the details of electronic transport related to the atomistic structure of silicon quantum dots embedded in a silicon dioxide matrix using ab initio calculations of the density of states. Several structural and composition features of quantum dots (QDs), such as diameter and amorphization level, are studied and correlated with transport under transfer Hamiltonian formalism. The current is strongly dependent on the QD density of states and on the conduction gap, both dependent on the dot diameter. In particular, as size increases, the available states inside the QD increase, while the QD band gap decreases due to relaxation of quantum confinement. Both effects contribute to increasing the current with the dot size. Besides, valence band offset between the band edges of the QD and the silica, and conduction band offset in a minor grade, increases with the QD diameter up to the theoretical value corresponding to planar heterostructures, thus decreasing the tunneling transmission probability and hence the total current. We discuss the influence of these parameters on electron and hole transport, evidencing a correlation between the electron (hole) barrier value and the electron (hole) current, and obtaining a general enhancement of the electron (hole) transport for larger (smaller) QD. Finally, we show that crystalline and amorphous structures exhibit enhanced probability of hole and electron current, respectively.

We discuss how excitons can affect the generation of coherent radial breathing modes in the ultrafast spectroscopy of single-wall carbon nanotubes. Photoexcited excitons can be localized spatially and give rise to a spatially distributed driving force in real space which involves many phonon wave vectors of the exciton-phonon interaction. The equation of motion for the coherent phonons is modeled phenomenologically by the Klein-Gordon equation, which we solve for the oscillation amplitudes as a function of space and time. By averaging the calculated amplitudes per nanotube length, we obtain time-dependent coherent phonon amplitudes that resemble the homogeneous oscillations that are observed in some pump-probe experiments. We interpret this result to mean that the experiments are only able to see a spatial average of coherent phonon oscillations over the wavelength of light in carbon nanotubes and the microscopic details are averaged out. Our interpretation is justified by calculating the time-dependent absorption spectra resulting from the macroscopic atomic displacements induced by the coherent phonon oscillations. The calculated coherent phonon spectra including excitonic effects show the experimentally observed symmetric peaks at the nanotube transition energies, in contrast to the asymmetric peaks that would be obtained if excitonic effects were not included.

The energy spectra and wave functions of bound excitons in important two-dimensional (2D) graphene derivatives, i.e., graphyne and graphane, are found to be strongly modified by quantum confinement, making them qualitatively different from the usual Rydberg series. However, their parity and optical selection rules are preserved. Thus a one-parameter modified hydrogenic model is applied to quantitatively explain the ab initio exciton spectra, and allows one to extrapolate the electron-hole binding energy from optical spectroscopies of 2D semiconductors without costly simulations. Meanwhile, our calculated optical absorption spectrum and enhanced spin singlet-triplet splitting project graphyne, an allotrope of graphene, as a candidate for intriguing energy and biomedical applications.

We describe the topological surface states of Bi${}_4$Se${}_3$, a compound in the infinitely adaptive Bi${}_2$-Bi${}_2$Se${}_3$ natural superlattice phase series, determined by a combination of experimental and theoretical methods. Two observable cleavage surfaces, terminating at Bi or Se, are characterized by angle-resolved photoelectron spectroscopy and scanning tunneling microscopy, and modeled by ab initio density functional theory calculations. Topological surface states are observed on both surfaces, but with markedly different dispersions and Kramers point energies. Bi${}_4$Se${}_3$ therefore represents the only known compound with different topological states on differently terminated, easily distinguished and stable surfaces.

Fundamental gap renormalization due to electronic polarization is a basic phenomenon in molecular crystals. Despite its ubiquity and importance, all conventional approaches within density-functional theory completely fail to capture it, even qualitatively. Here, we present a new screened range-separated hybrid functional, which, through judicious introduction of the scalar dielectric constant, quantitatively captures polarization-induced gap renormalization, as demonstrated on the prototypical organic molecular crystals of benzene, pentacene, and C${}_{60}$. This functional is predictive, as it contains system-specific adjustable parameters that are determined from first principles, rather than from empirical considerations.

In order to determine the superconducting paring state in the ferromagnetic superconductor UCoGe, the ${}^{59}$Co NMR Knight shift, which is directly related to the microscopic spin susceptibility, was measured in the superconducting state under magnetic fields perpendicular to the spontaneous magnetization axis ${}^{59}{K}^{a,b}$. ${}^{59}{K}^{a,b}$ shows to be almost constant, but does not decrease below a superconducting transition. These behaviors as well as the invariance of the internal field at the Co site in the superconducting state exclude the spin-singlet pairing, and can be interpreted with the equal-spin pairing state with a large exchange field along the $c$ axis, which was studied by Mineev [Phys. Rev. B 81, 180504 (2010)].