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arXiv:cond-mat/0005517 · Physical Review A

Adiabatic compression of a trapped Fermi gas

Luciano Viverit, S. Giorgini, Лев П. Питаевский, S. Stringari

Published Feb 5, 2001• 30 citations
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Abstract

We propose a method to reach conditions of high degeneracy in a trapped Fermi gas, based on the adiabatic transfer of a small fraction of atoms from a magnetic to a tighter optical trap. The transformation yields a large increase of the Fermi energy, without a significant change of the temperature. The large enhancement of the central density emphasizes the role of the interactions and makes the system much closer to the BCS transition. We show that, despite Pauli blocking, the thermalization processes ensuring equilibrium in the optical trap are not suppressed at low temperature.

DOI
10.1103/physreva.63.033603

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We propose a method to reach conditions of high degeneracy in a trapped Fermi gas, based on the adiabatic transfer of a small fraction of atoms from a magnetic to a tighter optical trap. The transformation yields a large increase of the Fermi energy, without a significant change of the temperature. The large enhancement of the central density emphasizes the role of the interactions and makes the system much closer to the BCS transition. We show that, despite Pauli blocking, the thermalization processes ensuring equilibrium in the optical trap are not suppressed at low temperature.
The standard approach for generating deeply degenerate quantum gases is evaporative or sympathetic cooling in a harmonic trap, after which the gas has reached its minimum entropy. All subsequent state transformations rely on adiabatic changes of a closed system, and coupling to the environment or non-adiabatic processes monotonically increase the entropy. Here, we demonstrate that this experimental paradigm can be bypassed by utilizing species-selective trapping with a low-dissipation optical tune-out trap in a dual-species mixture. We successfully reduce the entropy of a two-component fermionic quantum gas via isothermal compression within a bosonic bath, reaching deep quantum degeneracy of T/TF=0.024+0.007T/T_F = 0.024^{+0.007}T/TF​=0.024+0.007, with TFT_FTF​ the Fermi temperature. By characterizing the cross-dimensional relaxation and thermalization, we demonstrate that cooling light fermions with heavy bosons remains efficient and fast, even deep in the degenerate regime, where the thermalization time is found to be independent of T/TFT/T_FT/TF​. Our results pave the way for direct cooling within optical lattices, box traps, or other complex potentials, thereby eliminating the reliance on adiabatic state transformations to reach strongly interacting many-body regimes.
Pauli blocking of spontaneous emission by a single excited-state atom has been predicted to be dramatic at low temperature when the Fermi energy E F exceeds the recoil energy E R . The photon scattering rate of a ground-state Fermi gas can also be suppressed by occupation of the final states accessible to a recoiling atom; however, suppression is diminished by scattering events near the Fermi edge. We analyse two new approaches to improve the visibility of Pauli blocking in a trapped Fermi gas. Focusing the incident light to excite preferentially the high-density region of the cloud can increase the blocking signature by 14%, and is most effective at intermediate temperature. Spontaneous Raman scattering between imbalanced internal states can be strongly suppressed at low temperature, and is completely blocked for a final state E F > 4 E R in the high imbalance limit.
Light scattering from a neutral Fermi gas can be suppressed by occupation of the final states accessible to a recoiling atom. A related effect, Pauli blocking of spontaneous emission by a single excited-state atom, has been predicted to be dramatic at low temperature when the Fermi energy EF exceeds the recoil energy ER. However suppression of light scattering from a large ground-state ensemble is diminished by scattering events near unoccupied states. We analyze two possible methods of enhancement: using a focused light beam to excite only the high-density region of the cloud, and Raman scattering between imbalanced internal states. We find that focusing can enhance suppression by 14 % of the unblocked scattering rate, and that Raman scattering can yield a full suppression at low temperature for EF> 4ER, even in the inhomogeneous gas. ‡ Present address: Lyman Laboratories, Harvard University, Cambridge MA 02138, USAEnhanced Pauli blocking of light scattering in a trapped Fermi gas 2 1.
Summary form only given. Evaporative cooling of atoms in an optical trap has recently received renewed attention as a means for obtaining degeneracy in an optically trapped bosonic Cs vapor as well as a fermionic Li gas. We have developed a new model describing the evaporative cooling process for atoms confined in a time-dependent optical potential formed by a single focused Gaussian laser beam. We find that a substantial increase in the phase-space density can be obtained by adiabatically reducing the trap laser power and thereby the trap well depth as a function of time. Since we are specifically interested in the evaporative cooling of fermionic /sup 6/Li atoms, we have included the effect of Fermi statistics in the model. Although the collision rate is suppressed as the temperature T is reduced below the Fermi temperature T/sub F/, we find that values of T/T/sub F//spl Lt/1 can be achieved for suitable initial conditions.
We have produced an interacting quantum degenerate Fermi gas of atoms composed of two spin states of magnetically trapped 40K. The relative Fermi energies are adjusted by controlling the population in each spin state. Thermodynamic measurements reveal a resulting imbalance in the mean energy per particle between the two species, which is a factor of 1.4 at our lowest temperature. This imbalance of energy comes from a suppression of collisions between atoms in the gas due to the Pauli exclusion principle. Through measurements of the thermal relaxation rate we have directly observed this Pauli blocking as a factor of 2 reduction in the effective collision cross section in the quantum degenerate regime.
An evaporative cooling strategy that uses a two-component Fermi gas was employed to cool a magnetically trapped gas of 7 x 10(5) (40)K atoms to 0.5 of the Fermi temperature T(F). In this temperature regime, where the state occupation at the lowest energies has increased from essentially zero at high temperatures to nearly 60 percent, quantum degeneracy was observed as a barrier to evaporative cooling and as a modification of the thermodynamics. Measurements of the momentum distribution and the total energy of the confined Fermi gas directly revealed the quantum statistics.