arXiv subjects

V. Gasparian

Publications and source records attributed to V. Gasparian.

12 recordsLinked to original sources

arXiv2016

Bloch states in light transport through a perforated metal

Light transport in a metal with two-dimensional hole arrays is considered. Analytical expression for a transmission coefficient in periodic, isolated and disordered cases are derived, assuming the existence of waveguide modes transverse tunneling in two-dimensional plane perpendicular to traveling direction of light. The one dimensional case of periodic holes, due to its simplicity, is investigated in detail. In the dilute metal regime, when metal fraction is small, our numerical study of the transmission coefficient of central diffracted wave indicates the existence of a minimum which is completely independent of an incident wavelength. Further increasing of metal fraction leads to the unusual monotonic increasing of central diffracted wave transmission. The role of the surface plasmons is discussed.

cond-mat.mes-hall↗

arXiv2015

Localization length calculations in alternating metamaterial-birefringent disordered layered stacks

A detailed theoretical and numerical analysis of the localization length in alternating metamaterial-birefringent random layered stacks, under uncorrelated thickness-disorder, has been performed. Similar structures have recently been reported to suppress the Brewster delocalization for $\it{p}$-polarized light, when "standard" isotropic layers (with positive index of refraction) are considered instead of metamaterial layers, providing a generic means to produce polarization-insensitive, broadband reflections. However, this enhancement of localization is valid for short wavelengths $λ$ compared to the mean layer thickness $a_0$. At higher wavelengths, we recover the Brewster anomalies for $\it{p}$-polarized states impeding a remarkable localization of light. To achieve a better localization for a wider range of wavelengths, we replaced the conventional isotropic layers by negative-index metamaterials presenting low losses and constant index of refraction over the near-infrared range. As a result, our numerical calculations exhibit a linear dependence of the localization length with $λ$ (in the region $5 <λ/a_0 < 60$) reducing the Brewster anomalies in more than two orders of magnitude with respect to the standard isotropic scheme at oblique incidence. This enhancement of localization is practically independent of the thickness disorder kind and is also held under weak refractive-index disorder.

physics.optics↗

arXiv2015

Tunneling time calculations for general finite wavepackets based on the presence time formalism

We analyze the tunneling time problem via the presence time formalism. With this method we reproduce previous results for very long wavepackets and we are able to calculate the tunneling time for general wavepackets of arbitrary shape and length. The tunneling time for a general wavepacket is equal to the average over the energy components of the standard phase time. With this approach we can also calculate the time uncertainty. We have checked that the results obtained with this approach agree extremely well with numerical simulations of the wavepacket evolution.

quant-ph↗

arXiv2015

Green functions of interacting systems in the strongly localized regime

We have developed an approach to calculate the single-particle Green function of a one-dimensional many-body system in the strongly localized limit at zero temperature. Our approach, based on the locator expansion, sums the contributions of all possible forward scattering paths in configuration space. We demonstrate for fermions that the Green function factorizes when the system can be splited into two non interacting regions. This implies that for nearest neighbors interactions the Green function factorizes at every link connecting two sites with the same occupation. As a consequence we show that the conductance distribution function for interacting systems is log-normal, in the same universality class as for non-interacting systems. We have developed a numerical procedure to calculate the ground state and the Green function, generating all possible paths in configuration space. We compare the localization length computed with our procedure with the one obtained via exact diagonalization. The latter smoothly converges to our results as the disorder increases.

cond-mat.mes-hall↗

arXiv2013

Inverse Smith-Purcell effect near rough surfaces

Absorption of a photon by an electron moving parallel to a rough surface is studied.In the weak scattering regime we have evaluated the absorption probability of absorption of a single photon. It is shown the absorption probability with diffusional contribution becomes large by a lin/l factor compared to the analogous result with the single scattering contribution. The maximum of probability takes place for the infrared wavelengths and strongly depends on the particle energy. We also discuss the case of two-dimensional periodical surface profile and indicate optimal conditions for maximal absorption probability. The results can be used in electron energy gain spectroscopy and in laser-driven acceleration.

cond-mat.mes-hall↗

arXiv2013

Faraday Rotation in a Disordered Medium

The Faraday rotation angle is calculated in a diffusive regime in a three dimensional disordered slab. It is shown that it is an oscillating function of the magnetic field or the mediums internal properties, and is proportional to the ratio of the inelastic mean free path to the mean free path, that is to the average number of photon scatterings. The maximum rotation is achieved at frequencies when the photons elastic mean free path is minimal. We have obtained the rotation angle of polar backscattered light taking into account the maximally crossed diagrams. The latter leads to an ellipticity in the backscattered wave that can serve as precursor of weak localization.

cond-mat.dis-nn↗

arXiv2009

The effective spin concept to analyze coherent charge transport in mesoscopic systems

An effective spin concept is introduced to examine the mathematical and physical analogy between phase coherent charge transport in mesoscopic systems and quantum operations on spin based qubits. When coupled with the Bloch sphere concept, this isomorphism allows formulation of transport problems in a language more familiar to researchers in the field of spintronics and quantum computing. We exemplify the synergy between charge tunneling and spin qubit unitary operations by recasting well-known problems of tunneling through a delta scatterer, a resonant tunneling structure, a superlattice structure, and arrays of elastic scatterers, in terms of specific unitary operations (rotations) of a spinor on the Bloch sphere.

cond-mat.mes-hall↗

arXiv2006

Global Partial Density of States: Statistics and Localization Length in Quasi-one Dimensional disordered systems

We study the distributions functions for global partial density of states (GPDOS) in quasi-one-dimensional (Q1D) disordered wires as a function of disorder parameter from metal to insulator. We consider two different models for disordered Q1D wire: a set of two dimensional $δ$ potentials with an arbitrary signs and strengths placed randomly, and a tight-binding Hamiltonian with several modes and on-site disorder. The Green functions (GF) for two models were calculated analytically and it was shown that the poles of GF can be presented as determinant of the rank $N\times N$, where $N$ is the number of scatters. We show that the variances of partial GPDOS in the metal to insulator crossover regime are crossing. The critical value of disorder $w_c$ where we have crossover can be used for calculation a localization length in Q1D systems.

cond-mat.dis-nn↗

arXiv2001

Fluctuations of the correlation dimension at metal-insulator transitions

We investigate numerically the inverse participation ratio, $P_2$, of the 3D Anderson model and of the power-law random banded matrix (PRBM) model at criticality. We found that the variance of $\ln P_2$ scales with system size $L$ as $σ^2(L)=σ^2(\infty)-A L^{-D_2/2d}$, being $D_2$ the correlation dimension and $d$ the system dimension. Therefore the concept of a correlation dimension is well defined in the two models considered. The 3D Anderson transition and the PRBM transition for $b=0.3$ (see the text for the definition of $b$) are fairly similar with respect to all critical magnitudes studied.

cond-mat.dis-nn↗

arXiv2001

Anomalously large critical regions in power-law random matrix ensembles

We investigate numerically the power-law random matrix ensembles. Wavefunctions are fractal up to a characteristic length whose logarithm diverges asymmetrically with different exponents, 1 in the localized phase and 0.5 in the extended phase. The characteristic length is so anomalously large that for macroscopic samples there exists a finite critical region, in which this length is larger than the system size. The Green's functions decrease with distance as a power law with an exponent related to the correlation dimension.

cond-mat.dis-nn↗

arXiv1996

Partial Densities of States, Scattering Matrices, and Green's Functions

The response of an arbitrary scattering problem to quasi-static perturbations in the scattering potential is naturally expressed in terms of a set of local partial densities of states and a set of sensitivities each associated with one element of the scattering matrix. We define the local partial densities of states and the sensitivities in terms of functional derivatives of the scattering matrix and discuss their relation to the Green's function. Certain combinations of the local partial densities of states represent the injectivity of a scattering channel into the system and the emissivity into a scattering channel. It is shown that the injectivities and emissivities are simply related to the absolute square of the scattering wave-function. We discuss also the connection of the partial densities of states and the sensitivities to characteristic times. We apply these concepts to a delta-barrier and to the local Larmor clock.

cond-mat↗

arXiv1995

Dimensional effects in photoelectron spectra of Ag deposits on GaAs(110) surfaces

It is shown that the peak structure observed in angle-resolved photoelectron spectra of metallic deposits can only be unambiguously associated to single electronic states if the deposit has a two dimensional character (finite along one spatial direction). In one and zero dimensions the density of states shows peaks related to bunches of single electron states (the finer structure associated to the latter may not always be experimentally resolved). The characteristics of the peak structure strongly depend on the band dispersion in the energy region where they appear. Results for the density of states and photoemission yield for Ag crystallites on GaAs(110) are presented and compared with experimental photoelectron spectra.

cond-mat↗