Bosons in a Lattice: Exciton-Phonon Condensate in Cu2O

Abstract

We explore a nonlinear field model to describe the interplay between the ability of excitons to be Bose-condensed and their interaction with other modes of a crystal. We apply our consideration to the long-living para-excitons in Cu2O. Taking into account the exciton-phonon interaction and introducing a coherent phonon part of the moving condensate, we derive the dynamic equations for the exciton-phonon condensate. These equations can support localized solutions, and we discuss the conditions for the moving inhomogeneous condensate to appear in the crystal. We calculate the condensate wave function and energy, and a collective excitation spectrum in the semiclassical approximation; the inside-excitations were found to follow the asymptotic behavior of the macroscopic wave function exactly. The stability conditions of the moving condensate are analyzed by use of Landau arguments, and Landau critical parameters appear in the theory. Finally, we apply our model to describe the recently observed interference and strong nonlinear interaction between two coherent exciton-phonon packets in Cu2O.

arXiv comment34 pages, LaTeX, four figures (.ps) are incorporated by epsf. Submitted to Phys. Rev. B

References in corpus (10)

• Theory of Bose-Einstein condensation in trapped gases
• Conserving and Gapless Approximations for an Inhomogeneous Bose Gas at Finite Temperatures
• Condensate fraction and critical temperature of a trapped interacting Bose gas
• On Critical Velocities in Exciton Superfluidity
• Fine structure of excitons in Cu${}_2$O
• Spin degree of freedom in two dimensional exciton condensates
• Quantum saturation and condensation of excitons in Cu${}_2$O: a theoretical study
• A new method for detection of exciton Bose condensation using stimulated two-photon emission
• Moving Stationary State of Exciton-Phonon Condensate in Cu2O
• On Reduction of Critical Velocity in a Model of Superfluid Bose-gas with Boundary Interactions