Exciton correlations in coupled quantum wells and their luminescence blue shift

Laikhtman, B.; Rapaport, Ronen

doi:10.1103/physrevb.80.195313

Cited by 124 publications

(189 citation statements)

References 61 publications

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“…Indeed, this is the predicted behavior of dipolar excitons at low temperatures [20]. Hence, our findings could be considered as an indication that the exciton liquid at low temperatures undergoes a BEC.…”

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confidence: 75%

Experimental Study of the Exciton Gas-Liquid Transition in Coupled Quantum Wells

et al. 2018

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We study the exciton gas-liquid transition in GaAs/AlGaAs coupled quantum wells. Dipolar excitons in coupled quantum wells (CQW) offer an interesting test bed for studying collective effects of an interacting quantum degenerate system [1, 2]. Their relatively light mass, which is smaller than that of a free electron, implies that the necessary conditions for achieving quantum degeneracy can occur already at cryogenic temperatures, and their strong dipole-dipole interaction may give rise to formation of ordered phases. Extensive attempts have been made over the past two decades to observe these phases and to determine the phase diagram of this system [3][4][5][6][7][8][9][10][11][12].In recent years there are mounting evidences for a phase transition that occurs at low temperatures in this system, yet its nature and thermodynamics remain open questions.Many of the studies are performed in a trap geometry, which confines the excitons to a narrow region around the illuminated spot [13,14], and evidences for condensation are found through photoluminescence (PL) anomalies: The appearance of spontaneous coherence [7], onset of non-radiative recombination ("PL darkening") [9] and large blueshift of the PL energy [10,11]. An alternative approach to study this phase transition uses an open geometry, where photo-excited carriers are free to move away from the illumination spot, and their diffusion is limited only by the mesa boundary. We have recently studied the behavior of indirect excitons in such an open geometry, and found an abrupt phase transition at a critical temperature and excitation power density [12]. The PL separates into two spatial regions, one which consists of electron-hole plasma and another that has a set of properties of a high density liquid.In this work we investigate this phase transition using spatially resolved PL and resonant Rayleigh scattering (RRS). Measuring the threshold power density as a function of temperature we determine the phase diagram of the system over the temperature range 0.1 -4.8K. Pump probe measurements, in which the liquid is created by a focused pump beam and studied by a much weaker probe, reveal that the liquid is dark and diffuses to large distances away from the illuminated spot, filling the entire area of the mesa at low temperatures. We find that the RRS spectrum narrows significantly and becomes uniform over macroscopic distances at the liquid phase, indicating that the disorder in the sample is effectively screened.The sample structure is identical to that used in [12] and consists of two GaAs quantum wells with well widths of 12 and 18 nm, separated by a 3-nm Al 0.28 Ga 0.72 As barrier. Top and bottom n-doped layers allow the application of voltage that shifts the energy levels of the wide well (WW) relative to the narrow well (NW) [15]. To create the liquid we apply voltages exceeding -2.4V and excite the system with a laser diode at energy of 1.590eV, focused to a Gaussian spot with 22µm half width at half maximum (HWHM) [16]. Figure 1 shows the evolution...

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confidence: 75%

Experimental Study of the Exciton Gas-Liquid Transition in Coupled Quantum Wells

et al. 2018

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“…Because they are aligned dipoles, the excitons in this system have an intermediately strong interparticle interaction: much weaker than liquid helium but much stronger than polaritons 12,13 and other particles with short-range interaction. The research toward excitonic condensation in this system cannot yet claim such unambiguous success as that of polariton condensation 14 .…”

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confidence: 98%

Strain-induced darkening of trapped excitons in coupled quantum wells at low temperature

et al. 2011

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In GaAs/AlGaAs coupled quantum wells, strain-induced traps may be used to confine excitons in in-plane, harmonic traps. Using these traps, we have pursued Bose-Einstein condensation (BEC) of long-lived, spatially-indirect excitons. Here, we report a remarkable transition of the indirect exciton luminescence pattern with increasing strain, increasing exciton density, and decreasing temperature, to a spatial pattern exhibiting a large dark spot at trap center, where we expect the exciton density to be maximum. The mechanism of particle loss is ruled out as an explanation for this dark spot. While the onset criteria are approximately consistent with the conditions for BEC of a weakly interacting gas, the conspicuous proximity in energy of the indirect light-hole states suggests that an explanation employing the single-particle physics of light-hole/heavy-hole mixing may explain the phenomenon. The effect of the strain is modeled, and the resulting landscape of indirect exciton spin states is discussed. The relative oscillator strengths of these states are predicted by an exact numerical solution of the two-particle Schrodinger equation for electrons and holes in coupled quantum wells and an electric field. The contrast in oscillator strengths is sufficient to produce this luminescence pattern, but this analysis suggests a strongly diminished lifetime as stress is increased. The opposite lifetime dependence is observed experimentally. Additionally, the temperature dependence eludes explanation by this mechanism.

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“…With an exciton density well above 10 10 cm −2 and a temperature in the sub-Kelvin regime, the thermal de Broglie wavelength of the excitons exceeds the inter-excitonic distance 16 . Therefore, one would expect a condensation to the quantum mechanical ground state 12,27 . In a theoretical work, it was predicted that the quantum mechanical ground state is formed out of excitons which do not couple to light 29 , and the condensate is therefore dark.…”

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confidence: 99%

Confocal shift interferometry of coherent emission from trapped dipolar excitons

Repp

Schinner

Schubert

et al. 2014

Applied Physics Letters

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We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.In 1962 a Bose-Einstein-condensation of excitons was predicted by Blatt 1 and Moskalenko 2 . Dipolar excitons, where the composing electrons and holes are located in adjacent quantum wells of a semiconductor heterostructure 3,4 , are particularly suited to probe such a condensate. Due to the reduced overlap of the electron and hole wave functions, the lifetime of these quasi-particles is strongly increased. This enables them to cool down to the lattice temperature and to relax to their quantum mechanical ground state. To reach a Bose-Einstein condensate, a high particle density and low temperatures are a prerequisite, as demonstrated for related quasi-particles, namely polaritons 5 . For dipolar excitons, a condensation has been recently reported 6-8 . In particular, shiftinterferometry was used to study the spatial coherence of the excitonic emission which is considered as an indication of a condensed excitonic phase 6,[8][9][10] . Yet, there has been a controversial discussion about the impact of a limited spatial resolution on the ability to resolve the spatial coherence of the excitonic emission 11,12 . Here, we introduce a fiber-based confocal interferometer which operates down to cryogenic temperatures. We demonstrate that the approach allows measuring the coherence variation of the photoluminescence (PL) of trapped, dipolar excitons with a high spatial resolution. We extract a coherence length on the micrometer scale which is consistent with earlier reports 8 . However, we also observe that this length does not exceed the point spread function of our confocal set-up with two objectives even at the lowest temperature of 250 mK. In turn, we interpret the data such that a temporal coherence dominates the excitonic emission in our samples. We begin with the interferometer. It comprises two confocal objectives which can be positioned individually by piezo-positioners [Fig.

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Exciton correlations in coupled quantum wells and their luminescence blue shift

Cited by 124 publications

References 61 publications

Experimental Study of the Exciton Gas-Liquid Transition in Coupled Quantum Wells

Experimental Study of the Exciton Gas-Liquid Transition in Coupled Quantum Wells

Strain-induced darkening of trapped excitons in coupled quantum wells at low temperature

Confocal shift interferometry of coherent emission from trapped dipolar excitons

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