Spatially indirect excitons in coupled quantum wells

Lai, Chih-Wei Eddy

doi:10.2172/887430

Cited by 2 publications

(3 citation statements)

References 235 publications

(255 reference statements)

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“…However since we observe a linear dependance of the peak energy with V g both on the ring and inside it although the charges distribution is very different we can conclude that if there are some additional effects of the gate voltage on the internal electric field, these are probably not dominant. increase of the spectral width with the bath temperature is quite common in excitonic systems and usually signs a homogeneous broadening due to increased elastic collisions with the phonon bath, between excitons and between excitons and the free charges in the quantum well [78,88,[90][91][92]. The strong drop of the integrated intensities when the bath temperature increases confirms the loss of signal which has been previously seen on the measurements of the direct photoluminescence (see fig.III.12).…”

Section: Iiic1 Influence Of Experimental Parameterssupporting

confidence: 69%

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Experimental evidence for the quantum condensation of ultracold dipolar excitons

Alloing

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In this thesis, we report experimental evidence of a "gray" condensate of excitons, as predicted theoretically by M. Combescot et al. Most importantly, the condensate is characterized by the macroscopic population of dark excitons coherently coupled to a weak population of bright excitons through fermion exchanges. Such quantum condensation results from the excitons internal structure, with a dark i.e. optically inactive ground state. It is actually very similar to what occurs in the phases of superfluid 3He or in the more recent spinor condensates of ultracold atomic Bose gases. While it is our belief that such a "gray" condensate will eventually be observed in other excitonic systems, our study focus on its appearance together with the macroscopic auto-organization of dipolar excitons. Precisely we emphasize fragmented exciton rings in an electrically biased GaAs single quantum well. This very striking pattern was first observed independently by the groups of L. Butov and D. Snoke. It was interpreted as the result of an ambipolar diffusion of carriers in the quantum wells. The fragmentation of the macrosopic ring observed at low temperature by Butov and coworkers, and the subsequent evidence for long-range spatial coherence together with complex pattern of polarization, led Butov et al. to interpret the fragmentation as an evidence for the transition to a quantum regime where coherent exciton transport dominates. Our experiments led us to a very different interpretation. Indeed, we show that for our sample the formation of the fragmented ring is dominated by the diffusion of dipolar excitons in an optically induced electrostatic landscape. This potential landscape arises from the modulation of the internal electric field by excess charges injected in the QW by the same excitation beam which induces the ring. Dipolar excitons then explore a potential landscape characterized by a wide anti-trap inside the ring and more strikingly by microscopic traps distributed along the circumference of the ring. There, i.e. in the outside vicinity of the ring, a confining potential is responsible for the formation of "islands" where the population of dark excitons is dominant. Due to the low energy splitting between the bright and dark excitonic states in our sample, the observation of a dominant population of dark excitons signals that excitons condense in the low-lying dark states. To confirm this interpretation we show that the weak photoluminescence emitted in the outer vicinity exhibits macroscopic spatial coherence, up to 10 times larger than the de Broglie wavelength. Islands of extended coherence are in fact identified and quickly disappear upon increase of the bath temperature. This leads to an evolution of the coherence length strongly dependent on the temperature. Finally, we show that the photoluminescence emitted in the vicinity of the fragmented ring is dominantly linearly polarized and also organized in islands outside the ring. All these observations confirm the predicted signatures of a "gray" condensate, as formulated by M. and R. Combescot. En aquesta tesis, mostrem evidència experimental d'un condensat "gris" d'excitons, tal com prediu la teoria de M. Combescot et al. En particular, el condensat està caracteritzat per la població macroscòpica d'excitons foscos acoblats coherentment a una població baixa d'excitons brillants a través d'intercanvis fermiònics. Aquesta condensació quàntica es dóna com a resultat de l'estructura interna dels excitons, amb un estat fonamental fosc i.e. òpticament inactiu. És de fet molt similar al que passa en les fases de 3He superfluid o en els més recents condensats d'espinors de gasos atòmics ultrafreds de Bose. Encara que nosaltres creiem que un condensat "gris" serà eventualment observat en altres sistemes excitònics, el nostre estudi es focalitza en la seva manifestació juntament amb l'auto-organització macroscòpica d'excitons dipolars. Precisament, ens centrem en els anells excitònics fragmentats en un sol pou quàntic elèctricament polaritzat. Aquest sorprenent patró va ser observat independentment per primer cop pels grups de L. Butanov i D. Snoke. Va ser interpretat com el resultat d'una difusió ambipolar de portadors en pous quàntics. La fragmentació de l'anell macroscòpic observada a baixes temperatures per Butov i els seus col.laboradors, i la posterior evidència de coherència espacial de llarg abast juntament amb un patró de polarització complex, va portar a Butov et al. a interpretar la fragmentació com una evidència de la transició cap al règim quàntic en el que domina el transport coherent d'excitons. El nostre experiment ens va portar cap una interpretació molt diferent. En efecte, mostrem que per la nostra mostra la formació d'anells fragmentats és dominada per la difusió d'excitacions dipolars en un perfil electrostàtic òpticament induït. Aquest perfil de potencial sorgeix de la modulació del camp elèctric intern per un excés de càrregues injectades en el PQ pel mateix feix d'excitació que indueix l'anell. Les excitacions dipolars exploren per tant un perfil de potencial caracteritzat per una anti-trampa ampla dins de l'anell i més sorprenentment per trampes microscòpiques distribuïdes al llarg de la circumferència de l'anell. Allà, i.e. en la proximitat exterior de l'anell, un potencial de confinament és el responsable de la formació d'"illes" on la població d'excitons foscos és dominant. Degut a la baixa separació energètica entre els estats excitònics brillant i fosc en la nostra mostra, l'observació d'una població dominant d'excitons foscos senyala que els excitons es condensen en els estats foscos de més baixa energia. Per tal de confirmar aquesta interpretació, mostrem que la dèbil fotoluminescència emesa en la proximitat exterior exhibeix coherència espacial macroscòpica, fins a 10 vegades major que la longitud d'ona de de Broglie. Illes de coherència ampliada són de fet identificades i desapareixen ràpidament en incrementar la temperatura del focus. Això porta cap a una evolució de la longitud de coherència que depèn fortament de la temperatura. Finalment, mostrem que la fotoluminescència emesa en la proximitat de l'anell fragmentat està dominantment polaritzada linealment i organitzada també en illes fora de l'anell. Totes les observacions confirmen les senyals característiques previstes per un condensat "gris", tal com està formulat en la teoria desenvolupada per M. i R. Combescot

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Section: Iiic1 Influence Of Experimental Parameterssupporting

confidence: 69%

“…This measurement thus confirms that for V g < −1 the photoluminescence comes from dipolar excitons (or from a gas of dipolar excitons in interactions with free charges). This general behavior for excitons in SQW (or DQW) is very common and was also described in [69,[88][89][90].…”

Section: Iia Samplementioning

confidence: 70%

Experimental evidence for the quantum condensation of ultracold dipolar excitons

Alloing

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“…This large spatial dipole moment of indirect excitons which are responsible for the strong exciton-exciton repulsion may prevent the formation of bi-excitons and excitonic molecules [16]. At low temperatures, the dipole interaction is mutually repulsive and hence favors for the stability of the exciton and screens in-plane disorder more effectively [18].…”

Section: Effect Of Electric Fieldmentioning

confidence: 99%

Electrically induced direct to indirect exciton transition in CdTe/CdMnTe concentric double quantum ring rooted in SiO₂ matrix

Sherly,

Nithiananthi

2024

Phys. Scr.

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Effect of electric field and ring dimension on the localization of direct and indirect exciton formed inside a CdTe/CdMnTe Concentric Double Quantum Ring (CDQR) has been studied using variational technique. The whole double ring structure is embedded inside a SiO2 matrix to enhance exciton localization. Electric field applied along the axial (z) axis tilts the band structure which promotes the carrier localization more in the edges than in the center of the ring. Thus, the stronger electric field gives a triangular confinement to the carrier in the edges which enhances the carrier stability. Along with the effect of electric field the ring dimension which determines the stability of the exciton retains has been estimated through the mean square distance. The effect of the location of the constituent carriers on the excitonic stability and the efficient carrier transfer from inner ring to outer ring has been calculated for both symmetric and asymmetric ring dimensions. Height of the ring has been varied to understand its effect on the localization of both direct and indirect exciton.

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Spatially indirect excitons in coupled quantum wells

Abstract: at low temperature and screens in-plane disorder more effectively. All these features

Cited by 2 publications

References 235 publications

Experimental evidence for the quantum condensation of ultracold dipolar excitons

Experimental evidence for the quantum condensation of ultracold dipolar excitons

Electrically induced direct to indirect exciton transition in CdTe/CdMnTe concentric double quantum ring rooted in SiO₂ matrix

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Spatially indirect excitons in coupled quantum wells

Abstract: at low temperature and screens in-plane disorder more effectively. All these features

Cited by 2 publications

References 235 publications

Experimental evidence for the quantum condensation of ultracold dipolar excitons

Experimental evidence for the quantum condensation of ultracold dipolar excitons

Electrically induced direct to indirect exciton transition in CdTe/CdMnTe concentric double quantum ring rooted in SiO2 matrix

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Electrically induced direct to indirect exciton transition in CdTe/CdMnTe concentric double quantum ring rooted in SiO₂ matrix