P. Bakshi

Spontaneous polarization of electrons in quantum dashes

We investigate the possibility of generating a spontaneous anti‐ferroelectric polarization in an array of elongated quantum dots, which we call ‘‘quantum dashes.’’ We suggest ways in which this phenomenon might be observed and point out possible technological applications resulting from this phenomenon.

Plasmon-based terahertz emission from quantum well structures

Growing coherent plasma oscillations (plasma instability) can be generated in quantum well structures, where a sufficient population inversion is maintained in the carrier distribution by appropriate injection–extraction configurations. The collective response characteristics of such structures have been calculated. Such structures have been grown, and their radiation emission spectra observed. The experimental results are in agreement with the theoretical predictions. The emission maximum is in the terahertz frequency range, and occurs at an intersubband plasmon frequency. This shows that population inversion is achieved in these structures.

Amplification of surface modes in type II semiconductor superlattices

We study the drift‐induced instability of surface modes in type II semiconductor superlattices, including both carrier‐phonon and carrier‐carrier scatterings. Stability‐boundary curves are obtained for typical physical parameters. Amplification is possible for experimentally achievable drift velocities.

CURRENT DRIVEN PLASMA INSTABILITY IN QUANTUM WIRES

Abstract We investigate current driven plasma instabilities in quantum wires. We find that such instabilities might be observed in such systems for experimentally achievable drift velocities. We also find, that the threshold drift velocity for this instability is proportional to the Fermi velocity, as was the case in higher dimensional systems. Lowering the Fermi energy of the plasma is thus advantageous for generating these instabilities and possible device applications.

Towards stimulated generation of coherent plasmons in nanostructures

A possible generation scheme for growing coherent plasma oscillations (plasma instability) is presented. Specific quantum well structures, where a sufficient population inversion can be maintained in the carrier distribution by appropriate injection–extraction configurations, form the basis of this scheme. Self-consistent random phase approximation calculations show that a population inversion, leading to a plasma instability, can occur in such structures. A comparison between the calculated and the observed differential conductance curves suggests that such quantum well structures could be designed as active regions for the generation of terahertz frequency radiation sources.

Molecular Dynamics Studies of Solid–Liquid Phase Transition in 2-D Yukawa Systems

We present systematic studies aimed at investigating the precise details of solid-liquid phase transition in 2-D classical many-particle systems interacting with the Yukawa potential. This is done by introducing and analyzing a variety of indicators, such as the bond angular order parameter, the angular distribution of the Einstein oscillations, local angular correlations, global positional correlations, and the variation of internal energy in the vicinity of the melting temperature. Our results consequently show rapid changes around Gamma=415 for kappamacr=2 of the investigated quantities

Phonons in Yukawa lattices and liquids

The understanding of the theoretical structure of phonon dispersion in Yukawa lattices and the relationship between these perfect lattice phonons on the one hand, and the excitations in the disordered and liquid states on the other, is an important issue in analysing experimental and simulation results on plasma crystals. As the first step in this programme, we have numerically calculated the full phonon spectrum for 2D triangular Yukawa lattices, for a wide range of κ (screening parameter) values and along different propagation angles. Earlier calculations of the excitation spectra of the 2D and 3D Yukawa liquids were based on the quasilocalized charge approximation (QLCA), whose implicit premise is that the spectrum of an average distribution (governed by the isotropic liquid pair correlation function) is a good representation of the actual spectrum. To see the implications of this model more clearly, we compare the high r (near crystallization) QLCA phonon spectra with the angle-averaged phonon spectra of the lattice phonons.

Numerical and analytical study of high-resolution limb spectral radiance from nonequilibrium atmospheres

Abstract Emission line shapes are calculated numerically for isolated optically thick infrared lines in the earthlimb as a function of tangent height using a general nonlocal thermodynamic equilibrium (non-LTE), upper-atmospheric line-by-line radiation transport code. It is shown that the exact integral form of the transport equation can be written in a form that is easily amenable to analytical approximation of high accuracy. In this form, the limb spectral radiance Iv appears as a weighted average of nu/nl, the ratio of upper-state to lower-state population density, multiplied by the absorptivity 1 - exp[-τ(v)], where τ(v) is the total optical path along the line of sight. In the wings the variation of Iv is governed by the absorptivity; in the core of the optically thick line, Iv is determined by the averaged population ratio. The analytical forms enable us to calculate all of the important features of the self-absorbed line and agree remarkably well with more time-intensive numerical calculation. These results are illustrated by calculations on the 15-μm CO2 v2 (0110-0000) vibrational transition for tangent heights ranging through the mesosphere and lower thermosphere. Even though the collision linewidth is less than 1% of the Doppler width at these altitudes, it is shown that it is essential to use the Voigt line profile in this calculation rather than the Doppler profile. Failure to do so leads to a total band radiance that is in error by up to a factor of three, as well as incorrect band shapes and line shapes.

Collective Modes in 2-D Yukawa Solids and Liquids

We report comparative studies on collective excitations in 2-D complex plasmas, in which particles interact through the Yukawa potential, encompassing both the solid and the strongly coupled liquid states. Dispersion and polarization of the collective modes in the solid state are calculated through the lattice summations, while in the liquid state, through molecular dynamics (MD) simulations in conjunction with the theoretical quasi-localized charge approximation analysis. The latter closely emulates the dispersion, resulting from an angular averaging in the lattice. In general, however, the lattice dispersion is substantially different from that of the liquid. The MD simulations show the dramatic transformation of the anisotropic phonon dispersion of the crystal lattice near the solid-liquid transition into the isotropic liquid dispersion

AMPLIFICATION OF PLASMA MODES IN SEMICONDUCTOR HETEROSTRUCTURES

Abstract We investigate current driven instabilities of plasma modes in semiconductor heterostructures. Amplification of the plasma modes becomes possible when electrons are driven parallel to the interface by a sufficiently large electric field. Effects of electron-electron and electron-phonon scatterings are included. We find that the idealized, strictly two-dimensional treatment of the charge carriers used in our previous studies is an excellent approximation if only one subband is occupied at T=0, and the frequency of the generated oscillation is much less than the intersubband separation. We also find that if more subbands are occupied, the threshold drift velocity for this instability can be significantly reduced, making it more practical for device applications.