Momentum density and phase maps of a two-dimensional trapped Bose-Einstein condensate excited by a red laser

Abstract

We investigate numerically the momentum density and phase maps $-$in coordinate and momentum space$-$ of a two dimensional Bose-Einstein condensate (BEC) excited by a moving red-detuned laser potential. The BEC is confined in a harmonic trap cutoff by hard walls. The system and excitation scheme are as in our previous work (Roger R. Sakhel {\it et al.} to appear in J. Low Temp. Phys. (2013)); but with twice the number of particles and interaction strength. We solve the time-dependent Gross-Pitaevskii equation numerically using the split-step Crank-Nicolson method in real time. It is demonstrated that the red-detuned laser has a phase-imprinting effect like a repulsive potential barrier. Signatures of excitations are extracted from the dynamics of the momentum densities and phase maps. Further, a new phase is defined in momentum space, which is used to reveal excitations. Therefore, phase maps in coordinate space and momentum space are compared for different BEC evolution times. We argue, that this momentum-space phase is especially important with regard to the studies of BEC momentum distributions. In addition, this work presents a new method of BEC interferometry and should contribute to the ongoing research in that field. One of our significant findings is the presence of substantial differences betwteen the momentum density obtained by a Fourier transform (FT) of the spatial density distribution and the one obtained from the modulus of the wavefunction in momentum space; the latter is obtained by a FT of the spatial wavefunction.