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arXiv:cond-mat/0410704

Laser cooling all the way down to Fermi superfluid

Jacek Dziarmaga, Maciej Lewenstein

Published Oct 27, 2004• 0 citations
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Abstract

Numerical simulations for realistic experimental parameters demonstrate that laser cooling on the attractive side of the Feshbach resonance can drive fermions much below the superfluid transition. For the assumed set of experimental parameters the transition takes place at $0.35T_F$, and laser cooling can drive the system down to at least $0.085T_F$ in a time of a few seconds. Superfluid growth is self-consistently included in simulations.

DOI
10.48550/arxiv.cond-mat/0410704

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Numerical simulations for realistic experimental parameters demonstrate that laser cooling on the attractive side of the Feshbach resonance can drive fermions much below the superfluid transition. For the assumed set of experimental parameters the transition takes place at 0.35TF0.35T_F0.35TF​, and laser cooling can drive the system down to at least 0.085TF0.085T_F0.085TF​ in a time of a few seconds. Superfluid growth is self-consistently included in simulations.
On the repulsive side of the Feshbach resonance fermionic atoms can bind into diatomic molecules which at low enough temperature condense into molecular Bose-Einstein condensate. Sufficiently close to the resonance the molecules are bound so weakly that a significant fraction of unbound fermionic atoms survives even below the temperature of condensation. In this paper we show that laser cooling of the unbound fermionic atoms can reduce temperature of mixture of atoms and molecules below the condensation temperature. Numerical simulations for reasonable experimental parameters show that it is possible to achieve molecular condensate after a few seconds of laser cooling.
Besides the enormous opportunities offered by spectroscopy and atom interferometry, laser cooling has opened up the possibility of experimentally realizing and manipulating quantum degenerate gases: Bose–Einstein condensates and degenerate Fermionic systems. This chapter starts with an illustration of key experimental techniques, such as magnetic trapping, evaporative cooling, and Feshbach resonances. It then turns to the physics of atomic Bose–Einstein condensates, discussing their superfluid behaviour, their coherence properties, and their application to precise measurements. It also describes the experimental investigation of ultracold Fermi gases and fermionic superfluidity, as well as ongoing research into ultracold polar molecules.
We investigate trapped resonant fermions with unequal populations within the local density approximation above the superfluid transition temperature. By tuning the attractive interaction between fermions via Feshbach resonance, the system evolves from weakly interacting fermi gas to strongly interacting fermi gas, and finally becomes a Bose-Fermi mixture. The density profiles of fermions are examined and compared with experiments. We also point out the simple relationships between the local density, the axial density, and the gas pressure within the local density approximation.
Near Feshbach resonances, n | a | 3 ≫ 1, systems of Bose and Fermi particles become strongly interacting/dense. In this unitary limit both bosons and fermions have very different properties than in a dilute gas, e.g., the energy per particle approaches a value ℏ 2 n 2/3 / m times a universal many-body constant. Calculations based upon an approximate Jastrow wavefunction can quantitatively describe recent measurements of trapped Bose and Fermi atoms near Feshbach resonances. The pairing gap between attractive fermions also scales as Δ ∼ ℏ 2 n 2/3 / m near Feshbach resonances and is a large fraction of the Fermi energy—promising for observing BCS superfluidity in traps. Pairing undergoes several transitions depending on interaction strength and the number of particles in the trap and can also be compared with pairing in nuclei.
Near Feshbach resonances, n|a|3 ≫ 1, systems of Bose and Fermi particles become strongly interacting/dense. In this unitary limit both bosons and fermions have very different properties than in a dilute gas, e.g., the energy per particle approaches a value ℏ2n2/3/m times a universal many-body constant. Calculations based upon an approximate Jastrow wavefunction can quantitatively describe recent measurements of trapped Bose and Fermi atoms near Feshbach resonances. The pairing gap between attractive fermions also scales as Δ ∼ ℏ2n2/3/m near Feshbach resonances and is a large fraction of the Fermi energy—promising for observing BCS superfluidity in traps. Pairing undergoes several transitions depending on interaction strength and the number of particles in the trap and can also be compared with pairing in nuclei.