Exciton transport inCu2O: Evidence for excitonic superfluidity?

Abstract

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...

Many-body theory of pump-probe spectra for highly excited semiconductors

“…In particular, the anomalous exciton transport observed in Cu 2 O (Ref. 3) and BiI 3 (Ref. 4) are the typical examples of them.…”

Exciton-phonon droplets with Bose-Einstein condensate: transport and optical properties

“…Then, such a packet can move ballistically through the whole crystal at T < T c , and the coherency of the Bose-core can be revealed by the packetpacket interaction or by stimulated scattering into the packet with the condensate. If T > T c or n x < n c (T ), the exciton-phonon packet exhibits the standard diffusive behavior [2]. Thus, the 3D droplets that supposed to contain the excitonic Bose-Einstein condensate are found in a spatially inhomogeneous state with the well-defined characteristic width L ch in the direction of motion.…”

“…can be extracted from the experiments on two packet interaction at T < T c [2]. The registered ballistic velocities of such excitonic packets turn out to be always less, but relatively close to the longitudinal sound speed of the crystal, v < c s .…”

“…This article is motivated by experimental data on the transport properties of excitons in 3D crystals, such as Cu 2 0 [2], and 2D sheets in BiI 3 [3]. It is important to "cook" the following initial conditions: a relatively dense cloud of excitons with the density of n x > n c (T ) (e.g., n c (T = 2 K) ≈ 8.7 × 10 16 cm −3 ) to be in a moving state with h k x = 0.…”

Unexpectedly slow two particle decay of ultra-dense excitons in cuprous oxide

“…Cuprous oxide is an ideal material for probing three dimensional bosonic MQP [10] and there have been both reports [11][12][13] and reinterpretations [14][15][16][17] of BEC and superfluidity. Recently, it has been proposed that an observed relaxation explosion indicates the generation of a BEC [18].…”