Collective Effects in Finite Wavelength Spectra of Atoms.

Werner Brandt, Stig Lundqvist

Kazumasa Tsutsui, Yusuke Kato, Takafumi Kita

We study an interacting spinless Bose-Einstein condensate to clarify theoretically whether the spectra of its quasiparticles (one-particle excitations) and collective modes (two-particle excitations) are identical, as concluded by Gavoret and Nozières [Ann. Phys. 28, 349 (1964)]. We derive analytic expressions for their first and second moments so as to extend the Bijl-Feynman formula for the peak of the collective-mode spectrum to its width (inverse lifetime) and also to the one-particle channel. The obtained formulas indicate that the width of the collective-mode spectrum manifestly vanishes in the long-wavelength limit, whereas that of the quasiparticle spectrum apparently remains finite. We also evaluate the peaks and widths of the two spectra numerically for a model interaction potential in terms of the Jastrow wave function optimized by a variational method. It is thereby found that the width of the quasiparticle spectrum increases towards a constant as the wavenumber decreases. This marked difference in the spectral widths implies that the two spectra are distinct. In particular, the lifetime of the quasiparticles remains finite even in the long-wavelength limit.

Bernard Roulet, Ph. Nozières

Wen-bing Shen, Xiangming Hu, Jun Xu

We present a scheme for sub-half-wavelength localization based on phase-dependent coherent effects on resonance fluorescence. A three-level atom in the Λ configuration is considered. Two metastable states are so separated from each other that the fluorescence spectra from the different transitions do not overlap. Two dipole-allowed transitions and one dipole-forbidden transition are driven by a standing-wave laser field, a travelling-wave laser field and a microwave field, respectively. The spectrum of fluorescence from the standing-wave coupled transition is dependent not only on the position but also on the collective phase of all applied fields. When the phase is varied and fluorescence photons are detected, 50% probability is achieved in a half-wavelength region. This scheme is as efficient as previous ones based on coherence-controlled Autler–Townes spectra.

Moumita Indra, Dwipesh Majumder

We have studied collective spin density excitations of fractional quantum Hall effect (FQHE) in rotating Bose-Einstein condensation for the three filling fractions of first series of Jain's composite fermion sequences. We have considered, short-ranged contact interactions between the Bose atoms as well as long range Coulomb interactions to compare the nature of the spectras with FQHE of electrons. Using Monte Carlo method for finite but large number of particles, the lowest order collective modes of spin-reversed sectors is calculated here, by computing the energy differences of the respective excitons from the fully polarized ground states.

F. J. Kok