R. K. Moudgil

Coupled electron–hole quantum well structure: mass asymmetry and finite width effects

We investigate the role of many-body correlations in determining the ground-state behaviour of the coupled electron?hole quantum well structure by including the mass asymmetry and the finite width of wells. The correlations (both the intra-?and inter-well) are treated beyond the static local-field theories by employing the dynamical self-consistent mean-field approximation of Hasegawa and Shimizu. The mass asymmetry is seen to introduce a marked change in the ground state of the electron?hole system as compared to the recent corresponding results on the mass-symmetric electron?hole bilayer. First, the critical density for the liquid?Wigner crystal phase transition is greatly enhanced (e.g., by a factor of about 4 for a GaAs/GaAlAs based system). Second, there is a change in the role played by the electron?hole correlations. The Wigner crystal phase is now found to be stable below a critical density only at sufficiently large separation between the wells. The build-up of electron?hole correlations with diminishing inter-well spacing tends to favour the charge-density-wave phase over the Wigner crystal state, with the result that the former always prevails in the sufficiently close approach of wells. This result differs strikingly from the corresponding studies on the mass-symmetric system, since the electron?hole correlations are predicted here to always support, at sufficiently small well spacing, the Wigner crystal phase below a critical density and the charge-density-wave phase at relatively higher densities. Further, we find that the inclusion of the finite width of layers results in lowering of the critical density for Wigner crystallization.

Exchange and correlation effects on density excitation spectra of metallic quantum wires at finite temperature

We have studied the effect of exchange and correlations on the density excitation spectra of metallic quantum wires at finite temperature. The correlations are treated by incorporating the first-order self and exchange contributions into the random-phase approximation (RPA). Numerical results are presented for the spectra of the density response function and the plasmon dispersion for the gold wire on Si(557) substrate-a system studied recently by Nagao et al. (2006 Phys. Rev. Lett. 97 116802) for plasmons using electron energy loss spectroscopy. Our results for plasmons are found to agree with the experimental data. Though the first-order correction is small at currently accessible wire parameters, it becomes significant with increasing coupling parameter r(s). The effect of temperature on plasmons is found to be small for the wire system investigated experimentally. However, temperature has a significant effect on the spectra of the response function. We have also calculated the static structure factor, the pair-correlation function and the correlation energy at zero temperature in the first-order theory to check its applicability in dealing with correlations. Results are compared directly with the available Monte Carlo simulation data. It is found that the static correlation functions improve significantly over the RPA with the increase of r(s). On the other hand, the correlation energy shows very good agreement for r(s) ≤ 5 and wire widths b ≥ a(0). For smaller b, the agreement is good up to relatively smaller r(s).

Spin polarized and density modulated phases in symmetric electron-electron and electron-hole bilayers.

We have studied symmetric electron-electron and electron-hole bilayers to explore the stable homogeneous spin phase and the feasibility of inhomogeneous charge-/spin-density ground states. The former is resolved by comparing the ground-state energies in states of different spin polarizations, while the latter is resolved by searching for a divergence in the wavevector-dependent static charge/spin susceptibility. For this endeavour, we have used the dielectric approach within the self-consistent mean-field theory of Singwi et al. We find that the inter-layer interactions tend to change an abrupt spin-polarization transition of an isolated layer into a nearly gradual one, even though the partially spin-polarized phases are not clearly stable within the accuracy of our calculation. The transition density is seen to decrease with a reduction in layer spacing, implying a suppression of spin polarization by inter-layer interactions. Indeed, the suppression shows up distinctly in the spin susceptibility computed from the spin-polarization dependence of the ground-state energy. However, below a critical layer spacing, the unpolarized liquid becomes unstable against a charge-density-wave (CDW) ground state at a density preceding full spin polarization, with the transition density for the CDW state increasing on further reduction in the layer spacing. Due to attractive e-h correlations, the CDW state is found to be more pronounced in the e-h bilayer. On the other hand, the static spin susceptibility diverges only in the long-wavelength limit, which simply represents a transition to the homogeneous spin-polarized phase.

Long-wavelength instability in a double-electron-quantum-wire structure

We investigate theoretically the possibility of finding a charge-density-wave (CDW) instability in a zero-temperature double-electron-quantum-wire structure. Intrawire and interwire correlations are treated on the same footing within the self-consistent-field approximation of Singwi and co-workers. To check for the CDW instability, the static density susceptibility is calculated over a wide range of wire parameters (electron number density, wire size, and wire spacing). We find that the double-quantum-wire structure may become unstable against a long-wavelength CDW instability for sufficiently low electron density and narrow wire size, in the close proximity of two wires.

Finite-T correlations and free exchange-correlation energy of quasi-one-dimensional electron gas

We have studied the effect of temperature on static density–density correlations and plasmon excitation spectrum of quasi-one-dimensional electron gas (Q1DEG) using the random phase approximation (RPA). Numerical results for static structure factor, pair-correlation function, static density susceptibility, free exchange-correlation energy and plasmon dispersion are presented over a wide range of temperature and electron density. As an interesting result, we find that the short-range correlations exhibit a non-monotonic dependence on temperature T, initially growing stronger (i.e. the pair-correlation function at small inter-electron spacing assuming relatively smaller values) with increasing T and then weakening above a critical T. The cross-over temperature is found to increase with increasing coupling among electrons. Also, the q = 2kF peak in the static density susceptibility χ(q,ω = 0,T) at T = 0 K smears out with rising T. The free exchange-correlation energy and plasmon dispersion show a significant...

Coulomb drag in electron-hole bilayer: Mass-asymmetry and exchange correlation effects

Motivated by a recent experiment by Zheng et al. [App. Phys. Lett. 108, 062102 (2016)] on coulomb drag in electron-hole and hole-hole bilayers based on GaAs/AlGaAs semiconductor heterostructure, we investigate theoretically the influence of mass-asymmetry and temperature-dependence of correlations on the drag rate. The correlation effects are dealt with using the Vignale-Singwi effective inter-layer interaction model which includes correlations through local-field corrections to the bare coulomb interactions. However, in this work, we have incorporated only the intra-layer correlations using the temperature-dependent Hubbard approximation. Our results display a reasonably good agreement with the experimental data. However, it is crucial to include both the electron-hole mass-asymmetry and temperature-dependence of correlations. Mass-asymmetry and correlations are found to result in a substantial enhancement of drag resistivity.Motivated by a recent experiment by Zheng et al. [App. Phys. Lett. 108, 062102 (2016)] on coulomb drag in electron-hole and hole-hole bilayers based on GaAs/AlGaAs semiconductor heterostructure, we investigate theoretically the influence of mass-asymmetry and temperature-dependence of correlations on the drag rate. The correlation effects are dealt with using the Vignale-Singwi effective inter-layer interaction model which includes correlations through local-field corrections to the bare coulomb interactions. However, in this work, we have incorporated only the intra-layer correlations using the temperature-dependent Hubbard approximation. Our results display a reasonably good agreement with the experimental data. However, it is crucial to include both the electron-hole mass-asymmetry and temperature-dependence of correlations. Mass-asymmetry and correlations are found to result in a substantial enhancement of drag resistivity.

Plasmons in a semiconductor electron quantum wire at finite temperature in the random phase approximation

In this paper, we explore the effect of temperature and electron density on the plasmon dispersion of a semiconductor electron quantum wire using the random phase approximation. The transverse motion of the wire electrons is assumed to be confined by a symmetric harmonic potential. Numerical results are reported for the dispersion of plasmons over a wide range of temperature and electron density. The plasmon frequency is found to have a blue shift with increase in temperature and/or the electron density parameter rs.In this paper, we explore the effect of temperature and electron density on the plasmon dispersion of a semiconductor electron quantum wire using the random phase approximation. The transverse motion of the wire electrons is assumed to be confined by a symmetric harmonic potential. Numerical results are reported for the dispersion of plasmons over a wide range of temperature and electron density. The plasmon frequency is found to have a blue shift with increase in temperature and/or the electron density parameter rs.

Role of temperature on static correlational properties in a spin-polarized electron gas

We have studied the effect of temperature on the static correlational properties of a spin-polarized three-dimensional electron gas (3DEG) over a wide coupling and temperature regime. This problem has been very recently studied by Brown et al. using the restricted path-integral Monte Carlo (RPIMC) technique in the warm-dense regime. To this endeavor, we have used the finite temperature version of the dynamical mean-field theory of Singwi et al, the so-called quantum STLS (qSTLS) approach. The static density structure factor and the static pair-correlation function are calculated, and compared with the RPIMC simulation data. We find an excellent agreement with the simulation at high temperature over a wide coupling range. However, the agreement is seen to somewhat deteriorate with decreasing temperature. The pair-correlation function is found to become small negative for small electron separation. This may be attributed to the inadequacy of the mean-field theory in dealing with the like spin electron corre...

Spin dependent correlations in a homogeneous electron gas at finite temperature

We have studied theoretically the magnetic structure factor of a three-dimensional homogeneous electron gas at finite temperature. The spin density response function has been derived using the Singwi-Tosi-Land-Sjolander (STLS) theory that incorporates the correlation effects through spin anti-symmetric local field correction factor. The numerical results so obtained are compared against the recent path-integral Monte Carlo Simulation data of Brown et al. in the warm-dense regime for various temperature values. We find almost exact agreement at small temperature for different coupling parameter (rs) values. However, with increasing temperature and decreasing density, there has been observed noticeable disagreement with simulation results. This is attributed to the known failure of the STLS theory in dealing separately with the spin-resolved correlation functions.

Dynamical correlation effects on structure factor of spin-polarized two-dimensional electron gas

We report a theoretical study on static density structure factor S(q) of a spin-polarized two-dimensional electron gas over a wide range of electron number density rs. The electron correlations are treated within the dynamical version of the self-consistent mean-field theory of Singwi, Tosi, Land, and Sjolander, the so-called qSTLS approach. The calculated S(q) exhibits almost perfect agreement with the quantum Monte Carlo simulation data at rs=1. However, the extent of agreement somewhat diminishes with increasing rs, particularly for q around 2kF. Seen in conjunction with the success of qSTLS theory in dealing with correlations in the unpolarized phase, our study suggests that the otherwise celebrated qSTLS theory is not that good in treating the like-spin correlations.