A marked reduction in the velocity dispersion of excitons propagating through a thick CU2O crystal at low temperatures and high densities is attributed to the onset of a superfluid excitonic phase.PACS numbers: 71.35.+Z Superfluidity in liquid "^He is intimately connected with Bose-Einstein condensation. However, because of the high density of atoms in the liquid, it is not easy to disentangle the effects due to particle-particle interactions (required for superfluidity) from the pure quantum statistical eff'ects leading to Bose-Einstein condensation (BEC) [1]. For instance, it is known that below 1 K, the total mass of the liquid is superfluid yet only about 10% of the particles are found to be in the condensed state. It is therefore important to find other, more dilute systems where the connection between Bose statistics and superfluidity can be studied as a function of particle density.It has been recognized in the past decade that excitonic particles in semiconductors can display the quantum statistical properties expected from bosons [2]. In particular, it has been shown recently that exciton densities exceeding the critical density for BEC can be obtained in CU2O [3]. An important question concerns exciton transport properties under such conditions. Does the excitonic gas become superfluid, a regime characterized by a dragfree motion of part of the fluid? For excitons propagating freely inside a crystal, this corresponds to the disappearance of scattering of excitons with acoustic phonons, a process which represents the main source of drag in their motion. The study of superfluid motion and its relation with BEC is particularly interesting in an excitonic fluid since the particle density necessary for BEC can be much lower than in liquid He, due to the very small mass of excitonic particles, thereby minimizing the effects from interactions. For example, in the case of excitons in CU2O with a Bohr radius QB =0.7 nm, the critical density He for Bose condensation is 10^^ cm "^ at r = 2 K so that Hcai^^ and one truly deals with a weakly interacting Bose gas.There are some indications that a drastic reduction of friction in the motion of excitons takes place at high density in CU2O. Anomalies of transport have been observed through time and spatially resolved measurements of the exciton luminescence at early times following particle creation [3]. At high densities, the front of the excitonic cloud expands ballistically at near sonic velocity away from the surface where it is initially located. By contrast, similar measurements at low densities reveal a normal diffusive regime l4l. It has been proposed in Ref.[3] that this behavior signals the onset of superfluidity. Recently, Link and Baym [5] have come to a similar conclusion from a numerical simulation of the hydrodynamics of the excitonic gas at early times. They concluded that the rapid initial expansion of the exciton gas indicates a substantial reduction of drag, supporting an interpretation in terms of superfluidity. However, another interpretation of the...

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International audienceAmplification of a directed beam of excitons by stimulated scattering is observed in Cu2O at 2 K. It indicates that an excitoner, the radiationless excitonic counterpart of a laser, can be realized in semiconductors

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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.