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arXiv:cond-mat/0203317 · Physical Review A

Raman coupler for a trapped two-component quantum-degenerate Fermi gas

Sierk Pötting, M. Cramer, Weiping Zhang, Pierre Meystre

Published Jun 19, 2002• 5 citations
arXivHTMLPDF

Abstract

We investigate theoretically the Raman coupling between two internal states of a trapped low-density quantum-degenerate Fermi gas. In general, the trap frequencies associated with the two internal states can be different, leading to the onset of collapses and revivals in the population difference $\ensuremath{\Delta}N$ of the two internal states. This behavior can be changed drastically by two-body collisions. In particular, we show that under appropriate conditions they can suppress the dephasing leading to the collapse of $\ensuremath{\Delta}N,$ and restore almost full Rabi oscillations between the two internal states. These results are compared and contrasted to those for a quantum-degenerate bosonic gas.

DOI
10.1103/physreva.65.063620

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We investigate theoretically the Raman coupling between two internal states of a trapped low-density quantum-degenerate Fermi gas. In general, the trap frequencies associated with the two internal states can be different, leading to the onset of collapses and revivals in the population difference \ensuremathΔN\ensuremath{\Delta}N\ensuremathΔN of the two internal states. This behavior can be changed drastically by two-body collisions. In particular, we show that under appropriate conditions they can suppress the dephasing leading to the collapse of \ensuremathΔN,\ensuremath{\Delta}N,\ensuremathΔN, and restore almost full Rabi oscillations between the two internal states. These results are compared and contrasted to those for a quantum-degenerate bosonic gas.
We experimentally demonstrate the relation of Raman coupling strength with the external bias magnetic field in degenerate Fermi gas of 40 K atoms. Two Raman lasers couple two Zeeman energy levels, whose energy splitting depends on the external bias magnetic field. The Raman coupling strength is determined by measuring the Rabi oscillation frequency. The characteristics of the Rabi oscillation is to be damped after several periods due to Fermi atoms in different momentum states oscillating with different Rabi frequencies. The experimental results show that the Raman coupling strength will decrease as the external bias magnetic field increases, which is in good agreement with the theoretical prediction.
Two internal hyperfine states of a Bose-Einstein condensate in a dilute magnetically trapped gas of 87Rb{}^{87}\mathrm{Rb}87Rb atoms are strongly coupled by an external field that drives Rabi oscillations between the internal states. Due to their different magnetic moments and the force of gravity, the trapping potentials for the two states are offset along the vertical axis, so that the dynamics of the internal and external degrees of freedom are inseparable. The rapid cycling between internal atomic states in the displaced traps results in an adiabatic transfer of population from the condensate ground state to its first antisymmetric topological state. This has a pronounced effect on the internal Rabi oscillations, modulating the fringe visibility in a manner reminiscent of collapses and revivals. We present a detailed theoretical description based on zero-temperature mean-field theory.
Abstract. The inuence of s- and p-wave interactions on trapped degenerate one and two-component Fermi gases is investigated. The energy functional of a multicomponent Fermi gas is derived within the Thomas-Fermi approximation including the s- and p-wave terms of an eective contact interaction. On this basis the stability of the dilute gas against mean-eld induced collapse due to attractive interactions is investigated and explicit stability conditions in terms of Fermi momentum and s- and p-wave scattering lengths are derived. Furthermore the spatial separation in a two-component Fermi gas is discussed, explicit conditions for the onset of component demixing are given, and the density distributions in the separated phase are calculated. The ndings are summarized in a zero-temperature phase diagram for the degenerate two-component Fermi gas. It is shown that the p-wave interaction has signicant inuence on the phase diagram of the degenerate Fermi gas and causes new phenomena like absolute stabilization against collapse and component separation. It may therefore be useful in the context of the envisioned BCS transition in trapped atomic Fermi gases. PACS numbers: 03.75.Fi, 34.20.Cf, 32.80.Pj 1.
The influence of s- and p-wave interactions on trapped degenerate one- and two-component Fermi gases is investigated. The energy functional of a multicomponent Fermi gas is derived within the Thomas-Fermi approximation, including the s- and p-wave terms of an effective contact interaction. On this basis the stability of the dilute gas against mean-field induced collapse due to attractive interactions is investigated and explicit stability conditions in terms of Fermi momentum and s- and p-wave scattering lengths are derived. Furthermore, the spatial separation in a two-component Fermi gas is discussed, explicit conditions for the onset of component demixing are given, and the density distributions in the separated phase are calculated. The findings are summarized in a zero-temperature phase diagram for the degenerate two-component Fermi gas. It is shown that the p-wave interaction has a significant influence on the phase diagram of the degenerate Fermi gas and causes new phenomena such as absolute stabilization against collapse and component separation. It may therefore be useful in the context of the envisioned BCS transition in trapped atomic Fermi gases.
We review our recent experimental realization and investigation of a spin-orbit (SO) coupled Bose-Einstein condensate (BEC) and quantum degenerate Fermi gas. By using two counter-propagating Raman lasers and controlling the different frequency of two Raman lasers to engineer the atom-light interaction, we first study the SO coupling in BEC. Then we study SO coupling in Fermi gas. We observe the spin dephasing in spin dynamics and momentum distribution asymmetry of the equilibrium state as hallmarks of SO coupling in a Fermi gas. To clearly reveal the property of SO coupling Fermi gas, we also study the momentum-resolved radio-frequency spectroscopy which characterizes the energy-momentum dispersion and spin composition of the quantum states. We observe the change of fermion surfaces in different helicity branches with different atomic density, which indicates that a Lifshitz transition of the Fermi surface topology change can be found by further cooling the system. At last, we study the momentum-resolved Raman spectroscopy of an ultracold Fermi gas.
The collective excitations of two-component degenerate Fermi gases in a harminic trap with a quartic distortion are investigated.Using variational approaches,the coupled equations of motions for the center-of-mass coordinate and width of two-component degenerate Fermi gases are derived.Then two low-energy excitation modes are obtained analytically.The freqency shift,collapse and revival of the collective excitations of two-component degenerate Fermi gases induced by the anharmonic distortion under different values of the effective s-and p-wave interactions are discussed,respectively.