skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Effects of density imbalance on the BCS-BEC crossover in semiconductor electron-hole bilayers

Abstract

We study the occurrence of excitonic superfluidity in electron-hole bilayers at zero temperature. We not only identify the crossover in the phase diagram from the BCS limit of overlapping pairs to the BEC limit of nonoverlapping tightly bound pairs but also, by varying the electron and hole densities independently, we can analyze a number of phases that occur mainly in the crossover region. With different electron and hole effective masses, the phase diagram is asymmetric with respect to excess electron or hole densities. We propose, as the criterion for the onset of superfluidity, the jump of the electron and hole chemical potentials when their densities cross.

Authors:
;  [1];  [1];  [2]
  1. Dipartimento di Fisica, Universita di Camerino, I-62032 Camerino (Italy)
  2. (Australia)
Publication Date:
OSTI Identifier:
20957774
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. B, Condensed Matter and Materials Physics; Journal Volume: 75; Journal Issue: 11; Other Information: DOI: 10.1103/PhysRevB.75.113301; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ALUMINIUM COMPOUNDS; BCS THEORY; BOSE-EINSTEIN CONDENSATION; DENSITY; EFFECTIVE MASS; ELECTRON DENSITY; ELECTRONS; GALLIUM ARSENIDES; HOLES; LAYERS; PHASE DIAGRAMS; SEMICONDUCTOR MATERIALS; SUPERFLUIDITY

Citation Formats

Pieri, P., Strinati, G. C., Neilson, D., and School of Physics, University of New South Wales, Sydney 2052. Effects of density imbalance on the BCS-BEC crossover in semiconductor electron-hole bilayers. United States: N. p., 2007. Web. doi:10.1103/PHYSREVB.75.113301.
Pieri, P., Strinati, G. C., Neilson, D., & School of Physics, University of New South Wales, Sydney 2052. Effects of density imbalance on the BCS-BEC crossover in semiconductor electron-hole bilayers. United States. doi:10.1103/PHYSREVB.75.113301.
Pieri, P., Strinati, G. C., Neilson, D., and School of Physics, University of New South Wales, Sydney 2052. Thu . "Effects of density imbalance on the BCS-BEC crossover in semiconductor electron-hole bilayers". United States. doi:10.1103/PHYSREVB.75.113301.
@article{osti_20957774,
title = {Effects of density imbalance on the BCS-BEC crossover in semiconductor electron-hole bilayers},
author = {Pieri, P. and Strinati, G. C. and Neilson, D. and School of Physics, University of New South Wales, Sydney 2052},
abstractNote = {We study the occurrence of excitonic superfluidity in electron-hole bilayers at zero temperature. We not only identify the crossover in the phase diagram from the BCS limit of overlapping pairs to the BEC limit of nonoverlapping tightly bound pairs but also, by varying the electron and hole densities independently, we can analyze a number of phases that occur mainly in the crossover region. With different electron and hole effective masses, the phase diagram is asymmetric with respect to excess electron or hole densities. We propose, as the criterion for the onset of superfluidity, the jump of the electron and hole chemical potentials when their densities cross.},
doi = {10.1103/PHYSREVB.75.113301},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 11,
volume = 75,
place = {United States},
year = {Thu Mar 15 00:00:00 EDT 2007},
month = {Thu Mar 15 00:00:00 EDT 2007}
}
  • The BCS to Bose-Einstein condensation (BEC) crossover of electron-hole (e-h) pairs in optically excited semiconductors is studied using the two-band Hubbard model with both repulsive and attractive interactions. Applying the self-consistent t-matrix approximation combined with a local approximation, we examine the properties of a normal phase and an excitonic instability. The transition temperature from the normal phase to an e-h pair condensed one is studied to clarify the crossover from an e-h BCS-like state to an excitonic Bose-Einstein condensation, which takes place on increasing the e-h attraction strength. To investigate effects of the repulsive interaction and the e-h mass difference,more » we calculate the transition temperature for various parameters of the interaction strengths, the e-h particle density, and the mass difference. While the transition temperature in the e-h BCS regime is sufficiently suppressed by the repulsive interaction, that of the excitonic BEC is largely insensitive to it. We also show quantitatively that in the whole regime the mass difference leads to large suppression of the transition temperature.« less
  • We address the thermodynamics, density profiles, and superfluid density of trapped fermions undergoing BCS-BEC crossover. We consider the case of zero and finite population imbalance and apply the local density approximation (LDA) to include the trap potential. Our approach represents a fully self-consistent treatment of 'pseudogap effects'. These effects reflect the distinction between the pair formation temperature T* and the pair condensation temperature T{sub c}. As a natural corollary, this temperature difference must be accommodated in the fermionic excitation spectrum E{sub k} to reflect the fact that fermions are paired at and above T{sub c}. It is precisely this naturalmore » corollary which has been omitted from all other many body approaches in the literature. At a formal level, we show how enforcing this corollary implies that pairing fluctuation or self-energy contributions enter into both the gap and the number equations; this is necessary in order to be consistent with a generalized Ward identity. At a less formal level, we demonstrate that we obtain physical results for the superfluid density n{sub s}(T) at all temperatures. In contrast, previous work in the literature has led to nonmonotonic, or multivalued or discontinuous behavior for n{sub s}(T). Because it reflects the essence of the superfluid state, we view the superfluid density as a critical measure of the physicality of a given crossover theory. In a similarly unique fashion, we emphasize that in order to properly address thermodynamic properties of a trapped Fermi gas, a necessary first step is to demonstrate that the particle density profiles are consistent with experiment. Without a careful incorporation of the distinction between the pairing gap and the order parameter, the LDA-derived density profiles (of the unpolarized gas) tend to exhibit sharp features at the condensate edge, which are never seen experimentally in the crossover regime. The lack of demonstrable consistency between theoretical and experimental density profiles, along with problematic behavior found for the superfluid density, casts doubt on previous claims in the literature concerning quantitative agreement between thermodynamical calculations and experiment.« less
  • In this paper we present a very general theoretical framework for addressing fermionic superfluids over the entire range of BCS to Bose Einstein condensation (BEC) crossover in the presence of population imbalance or spin polarization. Our emphasis is on providing a theory which reduces to the standard zero temperature mean-field theories in the literature, but necessarily includes pairing fluctuation effects at nonzero temperature within a consistent framework. Physically, these effects are associated with the presence of preformed pairs (or a fermionic pseudogap) in the normal phase, and pair excitations of the condensate, in the superfluid phase. We show how thismore » finite T theory of fermionic pair condensates bears many similarities to the condensation of point bosons. In the process we examine three different types of condensate: the usual breached pair or Sarma phase and both the one- and two-plane-wave Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states. The last of these has been discussed in the literature albeit only within a Landau-Ginzburg formalism, generally valid near T{sub c}. Here we show how to arrive at the two-plane-wave LOFF state in the ground state as well as at general temperature T.« less
  • No abstract prepared.
  • We theoretically investigate a Bose-condensed exciton gas out of equilibrium. Within the framework of the combined BCS-Leggett strong-coupling theory with the non-equilibrium Keldysh formalism, we show how the Bose-Einstein condensation (BEC) of excitons is suppressed to eventually disappear, when the system is in the non-equilibrium steady state. The supply of electrons and holes from the bath is shown to induce quasi-particle excitations, leading to the partial occupation of the upper branch of Bogoliubov single-particle excitation spectrum. We also discuss how this quasi-particle induction is related to the suppression of exciton BEC, as well as the stability of the steady state.