We consider $Z_2$ spin liquids on the kagome lattice on the verge of a valence bond solid (VBS) transition, where vortex excitations carrying $Z_2$ magnetic flux—so-called visons—condense. We show that these vison excitations can couple directly to the external electromagnetic field, even though they carry neither spin nor charge. This is possible via a magnetoelastic coupling mechanism recently identified. [Potter, Senthil, and Lee, arXiv:1301.3495; Hao, Phys. Rev. B 85, 174432 (2012)] For the case of transitions to a 36-site unit cell VBS state, the corresponding finite ac conductivity has a specific power-law frequency dependence, which is related to the crossover exponent of the quantum critical point. The visons’ contribution to the optical conductivity at transitions to VBS states with a 12-site unit cell vanishes, however.

A procedure to construct symmetry-adapted Wannier functions in the framework of the maximally localized Wannier function approach [Marzari and Vanderbilt, Phys. Rev. B 56, 12847 (1997); Souza, Marzari, and Vanderbilt, Phys. Rev. B 65, 035109 (2001)] is presented. In this scheme, the minimization of the spread functional of the Wannier functions is performed with constraints that are derived from symmetry properties of the specified set of the Wannier functions and the Bloch functions used to construct them, therefore one can obtain a solution that does not necessarily yield the global minimum of the spread functional. As a test of this approach, results of atom-centered Wannier functions for GaAs and Cu are presented.

For analyzing quantum transport in semiconductor devices, accurate electronic structures are critical for quantitative predictions. Here we report theoretical analysis of electronic structures of all III-V zinc-blende semiconductor compounds. Our calculations are from density functional theory with the semilocal exchange proposed recently [Tran and Blaha, Phys. Rev. Lett. 102, 226401 (2009)], within the linear muffin tin orbital scheme. The calculated band gaps and effective masses are compared to experimental data and good quantitative agreement is obtained. Using the theoretical scheme presented here, quantum transport in nanostructures of III-V compounds can be confidently predicted.

Photoluminescence (PL), micro-PL, and PL excitation (PLE) spectroscopy for different light polarizations have been used to investigate the electronic properties of GaAs characterized by a dominant wurtzite (WZ) phase that forms in bare GaAs and in InGaAs/GaAs heterostructure (HS) nanowires (NWs). In both cases, the GaAs luminescence exhibits very narrow emission lines, which persist up to room temperature. At 10 K, the energy of the exciton ground state recombination of GaAs NWs is equal to 1.522–1.524 eV. In HS NWs, micro-PL combined with transmission electron microscopy pinpoints the tip of the GaAs section, with a dominant WZ phase, as the origin of that emission. In PLE, two very narrow excitonic absorptions at 1.523 and 1.631 eV involve different critical points of the WZ valence band (${\Gamma }_{9v}$ and ${\Gamma }_{7vu}$). The low-energy peak shows a negligible Stokes shift with respect to PL. At 10 K, a further weak and broad PLE signal is found at 1.59 eV. The possible attribution of these lines within the present knowledge of the WZ band structure is critically discussed.

Semiconductor doping is a process of fundamental importance to semiconductor physics and solid-state electronics, but cannot be explicitly simulated from first principles due to the huge system size needed for most doping scenarios. We examine the efficacy of the simulation of doping in silicon by the inclusion of “pseudoatoms” with fractional nuclear charge, introduced via specially constructed pseudopotentials. These provide a net charge carrier concentration matching an arbitrarily chosen doping level, at no increase of the computational cost. By extending this approach to consider minute deviations from the integer charge, we demonstrate that the electron Fermi level can be set to any value within the forbidden gap, at minimal perturbation of the electronic structure. Beyond the bulk scenario, we successfully simulate the development of the space-charge region in a heavily doped p-n junction and the doping dependence of the work function of the hydrogen-passivated (semiconducting) Si(111) surface.

Weyl fermions are two-component chiral fermions in $(3+1)$ dimensions. When coupled to a gauge field, the Weyl fermion is known to have an axial anomaly, which means the current conservation of the left-handed and right-handed Weyl fermions cannot be preserved separately. Recently, Weyl fermions have been proposed in condensed-matter systems named “Weyl semimetals.” In this paper we propose a Weyl semimetal phase in magnetically doped topological insulators, and study the axial anomaly in this system. We propose that the magnetic fluctuation in this system plays the role of a “chiral gauge field” which minimally couples to the Weyl fermions with opposite charges for two chiralities. We study the anomaly equation of this system and discuss its physical consequences, including one-dimensional chiral modes in a ferromagnetic vortex line, and a novel plasmon-magnon coupling.

We find unique properties accompanying surface-enhanced fluorescence (SEF) from dye molecules adsorbed on Ag nanoparticle aggregates, which generate surface-enhanced Raman scattering. The properties are observed in excitation laser energy dependence of SEF after excluding plasmonic spectral modulation in SEF. The unique properties are large blue shifts of fluorescence spectra, deviation of ratios between anti-Stokes SEF intensity and Stokes from those of normal fluorescence, super-broadening of Stokes spectra, and returning to original fluorescence by lower energy excitation. We elucidate that these properties are induced by electromagnetic enhancement of radiative decay rates exceeding the vibrational relaxation rates within an electronic excited state, which suggests that molecular electronic dynamics in strong plasmonic fields can be largely deviated from that in free space.

We theoretically investigate light-matter interactions for chiral molecules in the presence of nonchiral nanoantennas. Isotropic nanostructures supporting optical-frequency electric or magnetic dipoles are sufficient to locally enhance the excitation of a molecule's chiral polarizability and thus its circular dichroism spectrum. However, simultaneous electric and magnetic dipoles are necessary to achieve a net, spatially averaged enhancement. Our contribution provides a theoretical framework to understand chiral light-matter interactions at the nanoscale and sets the necessary and sufficient conditions to enhance circular dichroism spectroscopy in the presence of nanoantennas. The results may lead to new, field-enhanced, chiral spectroscopic techniques.

A quantum metamaterial can be implemented as a quantum coherent one-dimensional array of qubits placed in a transmission line. The properties of quantum metamaterials are determined by the local quantum state of the system. Here we show that a spatially periodic quantum state of such a system can be realized without direct control of the constituent qubits, by their interaction with the initializing (“priming”) pulses sent through the system in opposite directions. The properties of the resulting quantum photonic crystal are determined by the choice of the priming pulses. This proposal can be readily generalized to other implementations of quantum metamaterials.

We show that the breakdown of the Wiedemann-Franz law due to electron-electron scattering in diffusive spin valves may result in a strong suppression of the Fano factor that describes the ratio between shot noise and average current. In the parallel configuration of magnetizations, we find the universal value $\sqrt{3}/4$ in the absence of a normal-metal spacer layer, but including the spacer leads to a nonmonotonous suppression of this value before reaching back to the universal value for large spacer lengths. On the other hand, in the case of an antiparallel configuration with a negligibly small spacer, the Fano factor is $\sqrt{3(1-P^2)}/4$, where $P$ denotes the polarization of the conductivities. For $P\to \pm 1$, the current through the system is almost noiseless.

Applying high-power continuous-wave resonant $s$-shell excitation to a single self-assembled InGaAs quantum dot, we demonstrate the generation of postselected single-indistinguishable photons from the Mollow triplet sidebands. A sophisticated spatial filtering technique based on a double Michelson interferometer enabled us to separate the spectrally close lying individual Mollow components and perform almost background-free two-photon interference measurements. Showing high consistency with the results of an independent determination of the emission coherence of $\sim$$250\pm 30$ ps, our analysis reveals a close to ideal visibility contrast of up to $97\%$. Due to their easy spectral tunability and their distinct advantage of cascaded-photon emission between the individual Mollow sidebands, they resemble a versatile tool for quantum information applications.

Single-particle states in electronic Bloch bands form a Riemannian manifold whose geometric properties are described by two gauge invariant tensors, one being symmetric and the other being antisymmetric, that can be combined into the so-called Fubini-Study metric tensor of the projective Hilbert space. The latter directly controls the Hall conductivity. Here we show that the symmetric part of the Fubini-Study metric tensor also has measurable consequences by demonstrating that it enters the current noise spectrum. In particular, we show that a nonvanishing equilibrium current noise spectrum at zero temperature is unavoidable whenever Wannier states have nonzero minimum spread, the latter being quantifiable by the symmetric part of the Fubini-Study metric tensor. We illustrate our results by three examples: (1) atomic layers of hexagonal boron nitride, (2) graphene, and (3) the surface states of three-dimensional topological insulators when gapped by magnetic dopants.

The mechanisms of upconverted photocurrent in InAs quantum structures embedded in Al${}_x$Ga${}_{1-x}$As were studied with simultaneous measurements of photoluminescence and photocurrent spectra. Efficient upconversion was verified in samples with and without quantum dots. The dominant upconversion process from low temperatures to room temperature was found to occur through an Auger process in disklike InAs quantum structures. The results suggest the importance of shallow energy levels, which enable upconversion and efficient carrier extraction through multiparticle interactions. The disklike structure was concluded to be a suitable intermediate-band structure in terms of the energy conversion efficiency.

We analyze the spin dynamics of an individual magnetic atom (Mn) inserted in a II-VI semiconductor quantum dot under resonant optical excitation. In addition to standard optical pumping expected for a resonant excitation, we show that for particular conditions of laser detuning and excitation intensity, the Mn spin population can be trapped in the state which is resonantly excited. This effect is modeled considering the coherent spin dynamics of the coupled electronic and nuclear spin of the Mn atom optically dressed by a resonant laser field. This “spin population trapping” mechanism is controlled by the combined effect of the coupling with the laser field and the coherent interaction between the different Mn spin states induced by an anisotropy of the strain in the plane of the quantum dot.

We study the neutral singlet excitations of the zinc-tetraphenylporphyrin and $C_{70}$-fullerene donor-acceptor complex within the many-body Green's-function $GW$ and Bethe-Salpeter approaches. The lowest transition is a charge-transfer excitation between the donor and the acceptor with an energy in excellent agreement with recent constrained density functional theory calculations. Beyond the lowest charge-transfer state, which can be determined with simple electrostatic models that we validate, the Bethe-Salpeter approach provides the full excitation spectrum. We evidence the existence of hot electron-hole states which are resonant in energy with the lowest donor intramolecular excitation and show a hybrid intramolecular and charge-transfer character, favoring the transition towards charge separation. Such findings, and the ability to describe accurately both low-lying and excited charge-transfer states, are important steps in the process of discriminating “cold” versus “hot” exciton dissociation processes.

We study the Friedel oscillations induced by a localized impurity in an anisotropic graphenelike structure. We focus on the limit when the two inequivalent Dirac points merge. We find that, in this limit, the Friedel oscillations manifest very peculiar features, such as a strong asymmetry and an atypical inverse square-root decay. Our calculations are performed using both a $T$-matrix approximation and a tight-binding exact diagonalization technique. They allow us to numerically obtain the local density of states as a function of energy and position as well as an analytical form of the Friedel oscillations in the continuum limit. The two techniques yield results that are in excellent agreement, confirming the accuracy of such methods to approach this problem.

The reflectivity of low-energy electrons from graphene on copper substrates is studied both experimentally and theoretically. Well-known oscillations in the reflectivity of electrons with energies 0–8 eV above the vacuum level are observed in the experiment. These oscillations are reproduced in theory, based on a first-principles density functional description of interlayer states forming for various thicknesses of multilayer graphene. It is demonstrated that $n$ layers of graphene produce a regular series of $n-1$ minima in the reflectance spectra, together with a possible additional minimum associated with an interlayer state forming between the graphene and the substrate. Both (111) and (001) orientations of the copper substrates are studied. Similarities in their reflectivity spectra arise from the interlayer states, whereas differences are found because of the different Cu band structures along those orientations. Results for graphene on other substrates, including Pt(111) and Ir(111), are also discussed.

We have implemented a generic method, based on the $2n+1$ theorem within density functional perturbation theory, to calculate the anharmonic scattering coefficients among three phonons with arbitrary wave vectors. The method is used to study the phonon broadening in graphite and graphene mono- and bilayers. The broadening of the high-energy optical branches is highly nonuniform and presents a series of sudden steps and spikes. At finite temperature, the two linearly dispersive acoustic branches TA and LA of graphene have nonzero broadening for small wave vectors. The broadening in graphite and bilayer graphene is, overall, very similar to the graphene one, the most remarkable feature being the broadening of the quasiacoustical Z-polarized branch. Finally, we study the intrinsic anharmonic contribution to the thermal conductivity of the three systems, within the single mode relaxation time approximation. We find the conductance to be in good agreement with experiments in the out-of-plane direction but underestimate by a factor 2 in-plane.

We discuss how entanglement of strongly correlated fermions is influenced by a superlattice structure by considering a one-dimensional Hubbard superlattice, made up of a repeated pattern of $L_U$ repulsive sites followed by $L_0$ free sites. Lanczos diagonalization of lattices up to 24 sites is used to calculate the von Neumann entropy and the negativity. The breakdown of particle-hole symmetry broadens the maxima of the entropy in the underdoped region, while the entanglement in the overdoped region is crucially influenced by the nature of the magnetic state, with dips at densities corresponding to repulsive-layer singlets and to $q=\pi$ (in units of inverse unit cell length, $L_U+L_0$) spin-density waves. At these special densities, the system is either a Mott insulator or a “compressible insulator.” We have also found that sites within the same repulsive layer (for $L_U\ge 2$) are monogamically entangled with each other, which may provide a route to control entanglement simultaneously in different length scales.

We report a study on the thermal conductivity ($\kappa$) of Tb${}_2$Ti${}_2$O${}_7$ single crystals at low temperatures. It is found that in zero field this material has an extremely low phonon thermal conductivity in a broad temperature range. The mean free path of phonons is even smaller than that of amorphous materials and is 3–4 orders of magnitude smaller than the sample size at 0.3 K. The strong spin fluctuation of the spin-liquid state is discussed to be the reason of the strong phonon scattering. The magnetic-field dependence of $\kappa$ and comparison with Y${}_2$Ti${}_2$O${}_7$ and TbYTi${}_2$O${}_7$ confirm the magnetic origin of this phonon-glass-like behavior.