Lakshmi N. Pandey

Intersubband Transitions in Quantum-Well Heterostructures with Delta-Doped Barriers

A single quantum well doped by a negative delta‐function potential (δ‐potential) in the barrier regions is analyzed in terms of the optical transitions in between subbands. The first two states of the quantum well do not change at all as a function of the strength of the δ‐potential up to a certain value, whereas the third one gets lowered almost exponentially. An important point is that the δ‐potential brings a state from the continuum to the bound region. There is a range of the strength of the δ‐potential during which the transition energy for the first to second state decreases rapidly, and at a certain strength the first and second states disappear, and the third and fourth, which have been brought from the continuum, take their places. The oscillator strengths of the allowed transitions have a kind of oscillatory behavior in that range.

Wavefunctions and minimum uncertainty states of the harmonic oscillator with an exponentially decaying mass

Exact solutions to the harmonic oscillator with an exponentially decaying mass are explicitly represented in terms of Bessel functions. The dynamical invariant quantity of the system has the form of a rosette-shaped orbit in phase space. From this, it is confirmed that this system is bounded. By using the invariant operator represented in terms of lowering and raising operators, we have obtained wavefunctions and the propagator. Finally, minimum uncertainty states and conditions are evaluated by using other operators which are obtained from the above ones.

Diffusion of Impurity Excitations in Superfluid and Solid 3He–4He Mixtures

Starting from the system of linearized kinetic equations, the exact expressions for the spin and mass diffusion coefficients and thermal diffusion ratio in quantum mixtures of helium isotopes with any degree of degeneration is obtained. From the general expression the various limiting expressions have been obtained, which are determined by various relations between collision rates of quasiparticles. The results of the calculations are compared with available experimental data. It is shown that in the low-temperature region, where an impuriton gas is one-component, the relaxation of concentration of3He in superfluid3He–4He mixtures is determined by the acoustic and dissipative collective modes with an effective diffusion coefficient. From the general relation, the expression for a spin diffusion coefficient in solid3He–4He quantum mixtures is derived. The comparison of the results obtained with the experimental data makes it possible to find a numerical value for the width of the impuriton energy zone.

Enhanced Nonlinear-Optical Responses of Disordered Clusters and Composites

Clusters and nanocomposites belong to so-called nanostructured materials. Properties of such materials may be dramatically different from those of bulk materials with identical chemical composition. Confinement of atoms, electrons, phonons, electric fields, etc., in a small spatial region modifies spectral properties (shifts quantum levels, changes transition probabilities), and also changes the interaction between the constituent particles. In this paper we concentrate on an important source of the modification of properties, namely on local fields.

Normal mode analysis for a comparative study of relaxation processes of charge transfer and photoexcitation in C60

In this article we employ a normal mode analysis to the relaxation processes of charge transfer and photoexcitation in C60. It is found that only a few Hg modes, the so-called dominant modes, play a critical role in the relaxation, which may imply that only a few Hg modes can have a strong electron–phonon coupling constant. This is consistent with recent experimental results, although previous calculations reported that almost all the Hg modes have roughly the same coupling constant. Those dominant modes control mainly the early and the later behaviors and determine the relaxation times of the processes. The difference between the relaxation times of charge transfer and photoexcitation can also be understood from the dominant modes. We discover that the eigenvectors of the dominant modes are very unique, i.e., if the bond lengths are altered along the eigenvectors, one can observe a change in the lattice by a typical laminar distortion structure.

Propagator of a time-dependent unbound quadratic Hamiltonian system

SummaryThe propagator for a time-dependent unbound quadratic Hamiltonian system is explicitly evaluated using the path integral method. Two time-invariant quantities of the system are found where these invariants determine whether or not the system is bound. Several examples are considered to illustrate that the propagator obtained for the unbound systems is correct.

First viscosity of dilute3He-4He mixtures below 0.6 K

Starting with the Boltzmann transport equation, the first viscosity of dilute3He-4He mixtures for various3He concentrations x is evaluated up to around T ≅ 0.6 K by including the contribution from three-phonon processes (3PP) in the anomalous elementary excitation spectrum of liquid4He. Due to 3PP, the characteristic time τη for3He viscosity at high temperatures, i.e., T⩾2TF where TF is the3He Fermi temperature, is evaluated as 5 × 10−12/xT, which is smaller than the value estimated by Rosenbaum et al. This is interpolated with τη in the degenerate (quantum) region, T≪TF. The obtained viscosities are in better agreement with experimental results than those of Baym and Saam, whose theory does not include 3PP. However, at very low concentrations there exists a discrepancy between the present theory and experiments, so that an alternate treatment should be considered.

Dwell time in doped double‐barrier heterostructures

A time‐dependent Schrodinger equation has been solved numerically for a double‐barrier and a quantum‐well resonant tunnelling structure. Special emphasis has been paid to the system where barriers are doped specially by negative delta‐function potentials (δ potentials) which broaden the widths of the resonances and in turn decrease the dwell times. The strengths of the delta functions could be such that they may form bound states in the barrier regions, but the states bound to δ potentials are very shallow. Delta‐function potentials are replaced by equivalent barriers of different heights and widths which are easy to incorporate into the numerical calculation of the propagation of the wave packet, and the corresponding physical structures can be conveniently fabricated. It is found that for a certain strength of the δ potential or parametric value of the equivalent barriers in the barriers of the resonant tunneling structure, there are three resonance states very close together. The square of the wave fun...

Light-induced electron transfer counter to an electric-field force in an asymmetric double quantum well

Abstract Electron transfer counter to an electric-field force is predicted for an asymmetrical double quantum well subjected to a dc bias in response to optical (far IR) excitation of an intersubband electronic transition. This transfer exhibits a resonance enhancement when the bias electric field aligns the excited levels in the wide and narrow wells. The transfer effect is driven by the quantum- mechanical delocalization caused by the coherent resonant tunneling which prevails over the electric force. The effect brings about a photoinduced increase of the potential difference at the double well and a transient electric current opposite to the direction favored by the bias.

Theoretical Study of Phase-Shifted Quantum Beats in Time-Resolved Luminescence Spectra from a Biased Asymmetric Double Quantum Well

Abstract Analytical expressions have been developed for time-dependent luminescence intensities for a practically feasible asymmetric double quantum well under bias and numerically demonstrated to elucidate the characteristic of π-phase-shifted quantum beats. It is clear analytically as well as numerically that the magnitude of the tunneling interaction can be quantitatively estimated by the beat modulation depth.