Electronic structure and optical properties of quantum confined lead-salt nanowires

In the framework of four-band envelope-function formalism, developed earlier for spherical semiconductor nanocrystals, we study the electronic structure and optical properties of quantum-confined lead-salt (PbSe and PbS) nanowires (NWs) with a strong coupling between the conduction and the valence bands. We derive spatial quantization equations, and calculate numerically energy levels of spatially quantized states of a transverse electron motion in the plane perpendicular to the NW axis, and electronic subbands developed due to a free longitudinal motion along the NW axis. Using explicit expressions for eigenfunctions of the electronic states, we also derive analytical expressions for matrix elements of optical transitions and study selection rules for interband absorption. Next we study a two-particle problem with a conventional long-range Coulomb interaction and an interparticle coupling via medium polarization. The obtained results show that due to a large magnitude of the high-frequency dielectric permittivity of PbSe material, and hence, a high dielectric NW/vacuum contrast, the effective coupling via medium polarization significantly exceeds the effective direct Coulomb coupling at all interparticle separations along the NW axis. Furthermore, the strong coupling via medium polarization results in a bound state of the longitudinal motion of the lowest-energy electron-hole pair (a longitudinal exciton), while fast transverse motions of charge carriers remain independent of each other.Comment: Some misprints and mistakes are correcte

Brane-Induced Gravity's Shocks

We construct exact gravitational field solutions for a relativistic particle localized on a tensional brane in brane-induced gravity. They are a generalization of gravitational shock waves in 4D de Sitter space. We provide the metrics for both the normal branch and the self-inflating branch DGP braneworlds, and compare them to the 4D Einstein gravity solution and to the case when gravity resides only in the 5D bulk, without any brane-localized curvature terms. At short distances the wave profile looks the same as in four dimensions. The corrections appear only far from the source, where they differ from the long distance corrections in 4D de Sitter space. We also discover a new non-perturbative channel for energy emission into the bulk from the self-inflating branch, when gravity is modified at the de Sitter radius.Comment: 4 pages, revtex4; v4: a sign error corrected; the correction tantamount to swapping normal and self-inflating branch solutions; the only significant change is that the spectacular new instability is on the self-inflating branch in the limit of vanishing brane tension; more details available in hep-th/050203

The Biedenharn Approach to Relativistic Coulomb-type Problems

The approach developped by Biedeharn in the sixties for the relativistic Coulomb problem is reviewed and applied to various physical problems.Comment: 16 pages, 4 figure

Energy flow of moving dissipative topological solitons

We study the energy flow due to the motion of topological solitons in nonlinear extended systems in the presence of damping and driving. The total field momentum contribution to the energy flux, which reduces the soliton motion to that of a point particle, is insufficient. We identify an additional exchange energy flux channel mediated by the spatial and temporal inhomogeneity of the system state. In the well-known case of a DC external force the corresponding exchange current is shown to be small but non-zero. For the case of AC driving forces, which lead to a soliton ratchet, the exchange energy flux mediates the complete energy flow of the system. We also consider the case of combination of AC and DC external forces, as well as spatial discretization effects.Comment: 24 pages, 5 figures, submitted to Chao

The Fate of the Initial State Fluctuations in Heavy Ion Collisions. III The Second Act of Hydrodynamics

Hydrodynamical description of the "Little Bang" in heavy ion collisions is surprisingly successful, mostly due to the very small viscosity of the Quark-Gluon plasma. In this paper we systematically study the propagation of small perturbations, also treated hydrodynamically. We start with a number of known techniques allowing for analytic calculation of the propagation of small perturbations on top of the expanding fireball. The simplest approximation is the "geometric acoustics", which substitutes the wave equation by mechanical equations for the propagating "phonons". Next we turn to the case in which variables can be separated, in which case one can obtain not only the eikonal phases but also amplitudes of the perturbation. Finally, we focus on the so called Gubser flow, a particular conformal analytic solution for the fireball expansion, on top of which one can derive closed equations for small perturbations. Perfect hydrodynamics allows all variables to be separated and all equations to be solved in terms of known special functions. We can thus collect the analytical expression for all the harmonics and reconstruct the complete Green function of the problem. In the viscous case the equations still allow for variable separation, but one of the equations has to be solved numerically. We still can collect all the harmonics and show real-time perturbation evolution, observing viscosity-induced changes in the spectra and the correlation functions of secondaries. We end up by comparing the calculated angular shape of the correlation function to the STAR experimental data, and find, for sufficiently large viscosity, a surprisingly good agreement.Comment: The paper was changed after PRC referee report. It was resubmitted in this for

Electric excitation of spin resonance in antiferromagnetic conductors

Antiferromagnetism couples electron spin to its orbital motion, thus allowing excitation of electron-spin transitions by an ac electric rather than magnetic field - with absorption, exceeding that of common electron spin resonance at least by four orders of magnitude. In addition to potential applications in spin electronics, this phenomenon may be used as a spectroscopy to study antiferromagnetic materials of interest - from chromium to borocarbides, cuprates, iron pnictides, and organic and heavy fermion conductors.Comment: the journal print versio

Non-Perturbative One-Loop Effective Action for Electrodynamics in Curved Spacetime

In this paper we explicitly evaluate the one-loop effective action in four dimensions for scalar and spinor fields under the influence of a strong, covariantly constant, magnetic field in curved spacetime. In the framework of zeta function regularization, we find the one-loop effective action to all orders in the magnetic field up to linear terms in the Riemannian curvature. As a particular case, we also obtain the one-loop effective action for massless scalar and spinor fields. In this setting, we found that the vacuum energy of charged spinors with small mass becomes very large due entirely by the gravitational correction.Comment: LaTeX, 23 page

Umklapp-Assisted Electron Transport Oscillations in Metal Superlattices

We consider a superlattice of parallel metal tunnel junctions with a spatially non-homogeneous probability for electrons to tunnel. In such structures tunneling can be accompanied by electron scattering that conserves energy but not momentum. In the special case of a tunneling probability that varies periodically with period $a$ in the longitudinal direction, i.e., perpendicular to the junctions, electron tunneling is accompanied by "umklapp" scattering, where the longitudinal momentum changes by a multiple of $h/a$. We predict that as a result a sequence of metal-insulator transitions can be induced by an external electric- or magnetic field as the field strength is increased.Comment: 5 pages, 3 figure

Excitonic gap, phase transition, and quantum Hall effect in graphene

We suggest that physics underlying the recently observed removal of sublattice and spin degeneracies in graphene in a strong magnetic field describes a phase transition connected with the generation of an excitonic gap. The experimental form of the Hall conductivity is reproduced and the main characteristics of the dynamics are described. Predictions of the behavior of the gap as a function of temperature and a gate voltage are made.Comment: Revtex4, 10 pages, 4 figures, text essentially extended, one figure and references added; v3: to match PRB versio

Energy Flow Puzzle of Soliton Ratchets

We study the mechanism of directed energy transport for soliton ratchets. The energy flow appears due to the progressive motion of a soliton (kink) which is an energy carrier. However, the energy current formed by internal system deformations (the total field momentum) is zero. We solve the underlying puzzle by showing that the energy flow is realized via an {\it inhomogeneous} energy exchange between the system and the external ac driving. Internal kink modes are unambiguously shown to be crucial for that transport process to take place. We also discuss effects of spatial discretization and combination of ac and dc external drivings.Comment: 4 pages, 3 figures, submitted to PR