Probing and Manipulating Valley Coherence of Dark Excitons in Monolayer WSe_{2}.

Monolayers of semiconducting transition metal dichalcogenides are two-dimensional direct-gap systems which host tightly bound excitons with an internal degree of freedom corresponding to the valley of the constituting carriers. Strong spin-orbit interaction and the resulting ordering of the spin-split subbands in the valence and conduction bands makes the lowest-lying excitons in WX_{2} (X being S or Se) spin forbidden and optically dark. With polarization-resolved photoluminescence experiments performed on a WSe_{2} monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsic exchange interaction in combination with the applied in-plane and/or out-of-plane magnetic fields enables one to probe and manipulate the valley degree of freedom of the dark excitons.

Landau-Zener-Stueckelberg Physics with a Singular Continuum of States.

This Letter addresses the dynamical quantum problem of a driven discrete energy level coupled to a semi-infinite continuum whose density of states has a square-root-type singularity, such as states of a free particle in one dimension or quasiparticle states in a BCS superconductor. The system dynamics is strongly affected by the quantum-mechanical repulsion between the discrete level and the singularity, which gives rise to a bound state, suppresses the decay into the continuum, and can produce Stueckelberg oscillations. This quantum coherence effect may limit the performance of mesoscopic superconducting devices, such as the quantum electron turnstile.

Single Quantum Level Electron Turnstile.

We report on the realization of a single-electron source, where current is transported through a single-level quantum dot (Q) tunnel coupled to two superconducting leads (S). When driven with an ac gate voltage, the experiment demonstrates electron turnstile operation. Compared to the more conventional superconductor-normal-metal-superconductor turnstile, our superconductor-quantum-dot-superconductor device presents a number of novel properties, including higher immunity to the unavoidable presence of nonequilibrium quasiparticles in superconducting leads. Moreover, we demonstrate its ability to deliver electrons with a very narrow energy distribution.

Landau level spectroscopy of electron-electron interactions in graphene.

We present magneto-Raman scattering studies of electronic inter-Landau level excitations in quasineutral graphene samples with different strengths of Coulomb interaction. The band velocity associated with these excitations is found to depend on the dielectric environment, on the index of Landau level involved, and to vary as a function of the magnetic field. This contradicts the single-particle picture of noninteracting massless Dirac electrons but is accounted for by theory when the effect of electron-electron interaction is taken into account. Raman active, zero-momentum inter-Landau level excitations in graphene are sensitive to electron-electron interactions due to the nonapplicability of the Kohn theorem in this system, with a clearly nonparabolic dispersion relation.

Cyclotron motion in the vicinity of a Lifshitz transition in graphite.

Graphite, a model (semi)metal with trigonally warped bands, is investigated with a magnetoabsorption experiment and viewed as an electronic system in the vicinity of the Lifshitz transition. A characteristic pattern of up to 20 cyclotron resonance harmonics has been observed. This large number of resonances, their relative strengths and characteristic shapes trace the universal properties of the electronic states near a separatrix in momentum space. Quantum-mechanical perturbative methods with respect to the trigonal warping term hardly describe the data which are, on the other hand, fairly well reproduced within a quasiclassical approach and conventional band structure model. Trigonal symmetry is preserved in graphite in contrast to a similar system, bilayer graphene.

Raman spectroscopy of graphene edges.

Graphene edges are of particular interest since their orientation determines the electronic properties. Here we present a detailed Raman investigation of graphene flakes with edges oriented at different crystallographic directions. We also develop a real space theory for Raman scattering to analyze the general case of disordered edges. The position, width, and intensity of G and D peaks are studied as a function of the incident light polarization. The D-band is strongest for polarization parallel to the edge and minimum for perpendicular. Raman mapping shows that the D peak is localized in proximity of the edge. For ideal edges, the D peak is zero for zigzag orientation and large for armchair, allowing in principle the use of Raman spectroscopy as a sensitive tool for edge orientation. However, for real samples, the D to G ratio does not always show a significant dependence on edge orientation. Thus, even though edges can appear macroscopically smooth and oriented at well-defined angles, they are not necessarily microscopically ordered.

Effect of water drag on diffusion of drifting polarons in DNA.

It has been shown, theoretically and experimentally, that a hole or an excess electron on a DNA molecule in solution forms a delocalized wave function, a polaron. For an all-adenine (A) sequence or a mixed sequence of guanines (G's) and A's, calculations taking into account the polarization of the solution give the wave function spread over approximately four bases, which appears to be in agreement with experiment. The polaron may move by hopping or by drift. Drift can take place in a region with all the same bases, for example, A's, by the polaron dropping an A on the trailing edge and picking up an A on the leading edge. For drift that is not too rapid, the necessity of the polarization changing as the polaron moves exerts a drag on the polaron. We calculate the drag by using a model introduced earlier to describe the polaron. We find the drag to be proportional to the velocity of the polaron and to the orientational relaxation time of the water molecules. The drag is also a function of the Coulomb interactions of the fractional charges on the bases constituting the polaron, as modified by the polarization charge induced in the solution. The diffusion rate and mobility for all A polarons, calculated taking into account the drag, are 8 x 10(-5) cm(2)/s and 3 x 10(-3) cm(2)/(V s), respectively. We believe that in the experimental studies that have been carried out on hole propagation in a series of A's it was drift being observed rather than the hopping of a localized hole between adjacent A's, as was assumed to be the case.

Theory of electron spin resonance in bulk topological insulators Bi2Se3, Bi2Te3 and Sb2Te3.

We report a theoretical study of electron spin resonance in bulk topological insulators, such as Bi2Se3, Bi2Te3 and Sb2Te3. Using the effective four-band model, we find the electron energy spectrum in a static magnetic field and determine the response to electric and magnetic dipole perturbations, represented by oscillating electric and magnetic fields perpendicular to the static field. We determine the associated selection rules and calculate the absorption spectra. This enables us to separate the effective orbital and spin degrees of freedom and to determine the effective g factors for electrons and holes.

Self-trapping versus trapping: application to hole transport in DNA.

We address the problem of interplay between self-trapping effects and effects of an external potential, which may be relevant for many physical systems, such as polarons in solids or a Bose-Einstein condensate with attraction. If the potential consists of two different wells, the system initially localized in the shallower well may relax into the deeper well, or may not if stabilized by the self-trapping effect. We show how this picture can be applied to interpret results of recent experiments on electron transfer in the DNA molecule [Giese et al., Nature 412, 318 (2001)]. The results of our calculations agree well with the experimental findings, giving evidence that hole transport in DNA involves polaronic effects.

Kinetic theory of nonlinear diffusion in a weakly disordered nonlinear Schrödinger chain in the regime of homogeneous chaos.

We study the discrete nonlinear Schröinger equation with weak disorder, focusing on the regime when the nonlinearity is, on the one hand, weak enough for the normal modes of the linear problem to remain well resolved but, on the other, strong enough for the dynamics of the normal mode amplitudes to be chaotic for almost all modes. We show that in this regime and in the limit of high temperature, the macroscopic density ρ satisfies the nonlinear diffusion equation with a density-dependent diffusion coefficient, D(ρ) = D(0)ρ(2). An explicit expression for D(0) is obtained in terms of the eigenfunctions and eigenvalues of the linear problem, which is then evaluated numerically. The role of the second conserved quantity (energy) in the transport is also quantitatively discussed.

Local nature and scaling of chaos in weakly nonlinear disordered chains.

The dynamics of a disordered nonlinear chain can be either regular or chaotic with a certain probability. The chaotic behavior is often associated with the destruction of Anderson localization by the nonlinearity. In the present work it is argued that at weak nonlinearity chaos is nucleated locally on rare resonant segments of the chain. Based on this picture, the probability of chaos is evaluated analytically. The same probability is also evaluated by direct numerical sampling of disorder realizations and quantitative agreement between the two results is found.

Hopping between localized Floquet states in periodically driven quantum dots.

The dynamic localization in energy space, suppression of the absorption of energy from an external microwave field due to quantum interference, was analyzed recently for a closed quantum dot in the absence of electron-electron interactions. Here a weak interaction is shown to lead to a finite absorption and heating, which may be viewed as hopping between localized Floquet states. The heating rate grows together with the electronic temperature, eventually destroying the localization.

Dynamic localization and the Coulomb blockade in quantum dots under ac pumping.

We study conductance through a quantum dot under Coulomb blockade conditions in the presence of an external periodic perturbation. The stationary state is determined by the balance between the heating of the dot electrons by the perturbation and cooling by electron exchange with the cold contacts. We show that the Coulomb blockade peak can have a peculiar shape if heating is affected by dynamic localization, which can be an experimental signature of this effect.

Stationary polaron motion in a polymer chain at high electric fields.

We study the stationary motion of a polaron in a conducting polymer in the presence of a high electric field. Using the Su-Schrieffer-Heeger model plus an electric field, we find that at polaron velocities not exceeding the sound velocity, the dissipation of the electronic energy into the lattice occurs via emission of phonons with single selected wave vector. For this case the corresponding contribution to the polaron mobility can be calculated analytically. We discuss the issue of the polaron stability with respect to dissociation in a very high field at supersonic velocities.

Effect of solvation on hole motion in DNA.

An excess charge on a DNA chain in solution interacts with polar solvent molecules and mobile counterions. We give the first theoretical estimate of the resulting hole self-localization energy and calculate the corresponding contribution to hole mobility on a DNA stack consisting of a single base pair repeated.

Hole traps in DNA.

Sequences of guanines, GG and GGG, are known to be readily oxidized, forming radical cations, i.e., hole traps, on DNA. The trapping probability of GG is less than that of GGG. Lewis et al. (J. Am. Chem. Soc. 2000, 122, 12037) have used measurements on synthetic hairpins to determine the free energy liberated when a hole goes from the radical cation G(+) to GG or to GGG. They find these free energies to be of the order of thermal energy at room temperature, in contradiction to the expectation by many of much greater trap depths. We have calculated the wave function of a hole on G, on GG, and on GGG surrounded by adenines, as in the Lewis et al. experiments, using a simple tight-binding model. We find that to account for the shallow traps found by them it is necessary that the difference in ionization potentials of contiguous guanine and adenine be smaller by about 0.2 eV than the 0.4 eV found for isolated bases. Using this value and taking into account polaron formation, we find the wave functions of holes trapped on G, GG, or GGG to extend over approximately 6 sites (bases) and with energy level differences in good agreement with the values found by Lewis et al.

Dynamic localization in quantum dots: analytical theory.

We analyze the response of a complex quantum-mechanical system (e.g., a quantum dot) to a time-dependent perturbation phi(t). Assuming the dot to be described by random-matrix theory for the Gaussian orthogonal ensemble, we find the quantum correction to the energy absorption rate as a function of the dephasing time t(phi). If phi(t) is a sum of d harmonics with incommensurate frequencies, the correction behaves similarly to that for the conductivity deltasigma(d)(t(phi)) in the d-dimensional Anderson model of the orthogonal symmetry class. For a generic periodic perturbation, the leading quantum correction is absent as in the systems of the unitary symmetry class, unless phi(-t+tau)=phi(t+tau) for some tau, which falls into the quasi-1D orthogonal universality class.