Study of Anomalous Excitonic Transport in Cu₂O

Abstract

An experimental study of excitonic transport in Cu,O is presented. Excitons are detected several mm away from their initial location, via the photovoltaic effect. Results at low exciton densities are well explained by a diffusive motion. However, at high densities, anomalous transport, in the form of excitonic packets propagating at constant speed, is observed. Threshold conditions for the onset of ballistic packet motion are determined as a function of lattice temperature and exciton densities. They are consistent with requirements for Bose-Einstein condensation of excitons. Double pulse experiments reveal an important attraction of low density probe particles towards a packet. The packet is suggested to correspond to the superfluid component of a quantum degenerate ecxitonic fluid.Une etude experimentale du transport d'excitons dans Cu,O a basse temperature est decrite. La migration d'excitons sur des distances de plusieurs mm est observee par effet photovolta'ique. A faibles densites, les particules obtissent a une loi de diffusion classique. Par contre, i haute densite, le nuage excitonique se propage a vitesse constante sous la forme d'un paquet ballistique. Les conditions d'appdrition du paquet ballistique sont en bond accord avec celles requises pour une condensation de Bose-Einstein d'excitons. Des experiences de type pompe-sonde indiquent que les particules sondes ajouttes en prksence d'un paquet sont entrainees par celui-ci. Nous attribuons le paquet a la composante superfluide du gaz excitonique.

Photoluminescence characterization of polycrystalline n-type Cu2O films

“…Photoluminescence (PL) spectroscopy has been widely used to study exciton states, excitonic complexes, and Bose-Einstein condensation of excitons in Cu 2 O [13][14][15][16][17]. Ishizuka et al [18] used PL emission efficiency to study the effects of hydrogen plasma treatment on polycrystalline Cu 2 O films.…”

Exciton transport in Cu2O, exciton detection by field ionization in a Schottky barrier and intraexcitonic transitions

“…With intense fs pulses it was possible under two photon excitation to bleach the whole np, exciton series while the LO phonon replica of the 1s ortho exciton was hardly affected [11]. This finding has been used to monitor the propagation of excitons under spatial and temporal resolution in [12] under similar excitation conditions as in [6]. In these experiments the exciton transport could be modelled with classical diffusion and temperature dependent exciton lifetimes and diffusion constants in the range from 100 ms to 3 ms and 8 to 40 cm 2 /s, respectively.…”

“…More recently, it has been shown [8], that the Schottky barrier might be driven under the highest excitation intensities used in [6] by a factor 10 5 -10 6 into saturation, with the consequence that the data in [6] cannot be considered as conclusive for a BEC, though BEC and superfluidity might have been reached in [6]. The pros and contras have been exchanged in [8,9] and do not have to be repeated here.…”

“…In a series of beautiful experiments the problem of excitonic BEC and superfluidity has been approached in the following way [6]. The excitons are excited on one side of a brick-shaped Cu 2 O sample by a ns-laser pulse in the LO-phonon continuum of the yellow 1s ortho exciton and the arrival on the opposite side has been monitored by a current pulse produced by the field ionization of the excitons in a Cu 2 O/Cu Schottky contact.…”

Exciton Transport in Cu2O

“…The sample depth is taken l 2X2 mm, the excitation stripes of 0.5 mm thickness are separated so that x l 1 x r 2 . The calculations are made with b 1X0, D 600 cm 2 /s and for excitation wavelength 532 nm with " hw p 2X324 eV and absorption coefficient l 3000 cm À1 [11]. We can see that the exciton flux from two spatially separated but simultaneous pulses is much stronger than a simple sum of signals from a single excitation pulse.…”