Program

I.3 -- Invited, NCTP-40

Date: Tuesday, 28 July 2015

Time: 10h30 - 11h00

Microscopic model for scattering of ultracold paraexcitons in cuprous oxide

Hoàng Ngọc Cầm, Ngô Đức Thiện

Institute of Physics, VAST

With their unique properties, paraexcitons in high-purity cuprous oxide have been considered an attractive candidate for observing excitonic Bose Einstein condensation (BEC). Despite a lot of experimental effort to create a state of quantum degeneracy in the system, conclusive evidence for an excitonic BEC has not been obtained. The main reason for this failure is proved to be an efficient collision-induced loss process, whose nature still remains an open question. To gain insight into the mechanism of the decay, we develop a microscopic model for the para-para scattering at ultralow temperatures. In connection with the existence of the orthoexciton along with the paraexciton as internal states of the 1s exciton and their coupling by the exchange interaction, the para-para scattering is a two-channel problem. That is, two orthoexcitons, which are 24 meV higher than the threshold energy of two incoming paraexcitons, form for the latter a closed channel. Employing a reasonable approximation for the nonlocal exchange exciton-exciton interaction, we calculate the scattering potential in the open and closed channels as well as the coupling potential up to the third order in the small mass ratio. This allows us to compute the background s-wave scattering length, which entirely describes the entrance para-para collision at ultralow temperatures. Further, applying a localization procedure to the obtained ortho-ortho interaction potential, we compute the position of the quasibiexciton level in the closed channel, at which a resonance scattering of the incoming paraexcitons takes place. As any resonance scattering includes an inelastic component, the resonance paraexciton scattering is probably associated with the particles loss in experiments on BEC in cuprous oxide. Along with an empirical parameter for the lifetime of the metastable biexciton state, the computed quasibiexciton position makes it possible to calculate the rate of the decay process as well as the total s-wave scattering length of paraexcitons. These theoretical results are considered in relation to recent experiments on excitonic BEC.

Presenter: Hoang Ngoc Cam