Evidence for a stable excitonic ground state in a spatially separated electron-hole system

Phase diagram of a symmetric electron-hole bilayer system: a variational Monte Carlo study

We study the phase diagram of a symmetric electron-hole bilayer system at absolute zero temperature and in zero magnetic field within the quantum Monte Carlo approach. In particular, we conduct variational Monte Carlo simulations for various phases, i.e. the paramagnetic fluid phase, the ferromagnetic fluid phase, the anti-ferromagnetic Wigner crystal phase, the ferromagnetic Wigner crystal phase and the excitonic phase, to estimate the ground-state energy at different values of in-layer density and inter-layer spacing. Slater-Jastrow style trial wave functions, with single-particle orbitals appropriate for different phases, are used to construct the phase diagram in the (r _s , d) plane by finding the relative stability of trial wave functions. At very small layer separations, we find that the fluid phases are stable, with the paramagnetic fluid phase being particularly stable at [Formula: see text] and the ferromagnetic fluid phase being particularly stable at [Formula: see text]. As the layer spacing increases, we first find that there is a phase transition from the ferromagnetic fluid phase to the ferromagnetic Wigner crystal phase when d reaches 0.4 a.u. at r _s   =  20, and before there is a return to the ferromagnetic fluid phase when d approaches 1 a.u. However, for r _s   <  20 and [Formula: see text] a.u., the excitonic phase is found to be stable. We do not find that the anti-ferromagnetic Wigner crystal is stable over the considered range of r _s and d. We also find that as r _s increases, the critical layer separations for Wigner crystallization increase.

Diffusion Monte Carlo study of excitons and biexcitons in a mass-asymmetric electron-hole bilayer

We employed the diffusion Monte Carlo method, under fixed node approximation, to investigate the various ground state properties of a mass-asymmetric electron-hole bilayer system. Particularly, we calculated the ground state energy, the condensation fraction c, and the pair correlation function g(r) at density r_s = 5 a.u. for the inter-planer distance d ≤ 0.5 a.u. Based on the characteristics of condensate faction and pair correlation functions, we found the phase transition from the excitonic fluid phase to the biexcitonic fluid phase at d = 0.24 a.u. We also analytically calculated the critical layer separation d_crit for biexciton stability, the value for which was almost double the d = 0.24 a.u.

Unconventional pairing in excitonic condensates under spin-orbit coupling

It is shown that Rashba and Dresselhaus spin-orbit couplings enhance the conclusive power in the experiments on the excitonic condensate by at least three low temperature effects. First, spin-orbit coupling facilitates the photoluminescence measurements via enhancing the bright contribution in the otherwise dominantly dark ground state. The second is the presence of a low temperature power law dependence of the specific heat and weakening of the second order transition at the critical temperature. The third is the appearance of the nondiagonal elements in the static spin susceptibility.

Excitonic condensation under spin-orbit coupling and BEC-BCS crossover

The condensation of electron-hole pairs is studied at zero temperature and in the presence of a weak spin-orbit coupling (SOC) in coupled quantum wells. Under realistic conditions, a perturbative SOC can have observable effects in the order parameter of the condensate. First, the fermion exchange symmetry is absent. As a result, the condensate spin has no definite parity. Additionally, the excitonic SOC breaks the rotational symmetry yielding a complex order parameter in an unconventional way; i.e., the phase pattern of the order parameter is a function of the condensate density. This is manifested through finite off-diagonal components of the static spin susceptibility, suggesting a new experimental method to confirm an excitonic condensate.

Prediction of an excitonic ground state in InAs/InSb quantum dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the InAs/InSb quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap E(g) between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome E(g), leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by approximately 15 meV) than the dot without excitons.

Spontaneous Bose coherence of excitons and polaritons

In the past decade, there has been an increasing number of experiments on spontaneous Bose coherence of excitons and polaritons. Four major areas of research are reviewed here: three-dimensional excitons in the bulk semiconductor Cu2O, two-dimensional excitons in coupled quantum wells, Coulomb drag experiments in coupled two-dimensional electron gases, and polaritons in semiconductor microcavities. The unifying theory of all these experiments is the effect of spontaneous symmetry breaking in the Bose-Einstein condensation phase transition.

Excitonic condensation in a symmetric electron-hole bilayer

Using diffusion Monte Carlo simulations we have investigated the ground state of a symmetric electron-hole bilayer and determined its phase diagram at T = 0. We find clear evidence of an excitonic condensate, whose stability however is affected by an in-layer electronic correlation. This stabilizes the electron-hole plasma at large values of the density or interlayer distance, and the Wigner crystal at low density and large distance. We have also estimated pair correlation functions and low-order density matrices to give a microscopic characterization of correlations as well as to try and estimate the condensate fraction.

Prospecting for the superfluid transition in electron-hole coupled quantum wells using coulomb drag

I investigate the possibility of using Coulomb drag to detect a precursor of the predicted (but as yet not definitively observed) superfluid transition in electron-hole coupled quantum wells. The drag transresistivity rho(21) is shown to be significantly enhanced above the mean-field transition temperature T(c) and to diverge logarithmically as T-->T(+)(c), due to electron-hole pairing fluctuations which are somewhat analogous to the Maki-Thompson contribution to conductivity in metals above the superconducting T(c). The enhancement in rho(21) is estimated to be detectable at temperatures much higher than T(c).