Observation of the exciton Mott transition in the photoluminescence of coupled quantum wells

Abstract

Indirect excitons in coupled quantum wells have long radiative lifetimes and form a cold quasitwo-dimensional population suitable for studying collective quantum effects. Here we report the observation of the exciton Mott transition from an insulating (excitons) to a conducting (ionized electron-hole pairs) phase, which occurs gradually as a function of carrier density and temperature. The transition is inferred from spectral and time-resolved photoluminescence measurements around a carrier density of 2 × 10 10 cm −2 and temperatures of 12-16 K. An externally applied electric field is employed to tune the dynamics of the transition via the quantum-confined Stark effect. Our results provide evidence of a gradual nature of the exciton Mott transition.

Bose Condensation of Long-Living Direct Excitons in an Off-Resonant Cavity

“…The considered densities are much higher than IX densities in BEC experiments in CWQs [7,10]. At the same time, much smaller exciton Bohr radius (a X B = 11 nm for 8-nm QW and a X B = 6 nm for the ultra-narrow QW) and higher binding energy of direct excitons prevents them from reaching Mott transition (which occurs in CQWs at n = 2 · 10 10 cm −2 and 12-16 K [31]). However, as we confirm below, the exciton gas at those densities is in the regime of intermediate correlations and cannot be readily described by the mean field approximation.…”

Carrier-mediated optomechanical forces in semiconductor nanomembranes with coupled quantum wells

“…The lifetime Γ −1 I of indirect EHPs in the CQWs is tunable over two orders of magnitude by means of the bias voltage V b . This feature is modeled using Fermi's golden rule, in which the carrier wave functions in the CQWs and their overlap are found by solving the one-dimensional single-particle Schrödinger equation numerically [26]. Using no free parameters the model agrees well with time-resolved photoluminescence measurements (see Appendix B) at the indirect transition wavelength, as shown in Fig.…”

“…InGaAs/GaAs/InGaAs 9/5/9 nm CQWs, whose band diagram is detailed in Ref. [26], are embedded between two AlGaAs barriers and placed offcenter, i.e. 140 nm above the bottom layer ( Fig.…”

“…Since the overlap of the carrier wave functions in the indirect state |I is small, absorption via transition |0 → |I is negligible. Therefore, carriers are only optically pumped into the direct state |D with a generation rate per unit area G = αI/hω L , where α = 0.0214 [26] is the absorption probability, I the intensity distribution of the incident laser beam and ω L = 2πc/λ. Assuming γ ph Γ D , Ω we find the rate equations transformed into frequency domain to beñ D ≈ 0,ñ…”

“…In the case of nanomembranes with lower frequencies (MHz-regime) a method to extend the carrier lifetime to match the period of oscillations needs to be devised. In this work we present a device where such a method is implemented by embedding coupled quantum wells (CQWs) in a nanomembrane and controlling the lifetime of EHPs up to 750 ns via a bias voltage [25,26]. Using amplitude-modulated above-bandgap laser light at a peak optical power of 1 µW, mechanical oscillations can be driven without the use of an optical cavity to an amplitude of 1.314(2) nm, which is orders of magnitude larger than expected from radiation pressure.…”