Observation of Fermi Pressure in a Gas of Trapped Atoms

Abstract

We report the attainment of simultaneous quantum degeneracy in a mixed gas of bosons (lithium-7) and fermions (lithium-6). The Fermi gas has been cooled to a temperature of 0.25 times the Fermi temperature by thermal collisions with the evaporatively cooled bosons. At this temperature, the spatial size of the gas is strongly affected by the Fermi pressure resulting from the Pauli exclusion principle and gives clear experimental evidence for quantum degeneracy.

Shell Effects in the First Sound Velocity of an Ultracold Fermi Gas

“…In the last years quantum degeneracy has been achieved also with fermionic atoms 3,4,5,6,7 . It is important to observe that the role of dimensionality in these degenerate gases has been experimentally studied until now only with bosons 8,9,10 .…”

Evaporative cooling of magnetically trapped atomic molybdenum

“…Mo has several stable isotopes, 2 fermions and 5 bosons. This opens the possiblity for sympathetic cooling [6][7][8], of both Fermi and Bose degenerate gases, and interisotope comparisons. In addition, experimental studies of cold, trapped atoms at temperatures above degeneracy can shed light on collision and atomic structure theory [9].…”

Three-fluid description of the sympathetic cooling of a boson-fermion mixture

“…Truscott et al [2] note that the cooling process seems to stall as soon as the specific heat of the bosons is smaller than the specific heat of the fermions, because the energy which can be carried away by evaporating a bosonic atom then becomes smaller than the energy of a fermionic atom. This result is confirmed by the present simulation, which explains why the final temperature is insensitive to the initial ratio of bosons to fermions.…”

“…Recent attempts to cool down a trapped gas of fermionic atoms, such as 40 K [1] or 6 Li [2,3], have reached temperatures below the Fermi temperature. Degenerate atomic Fermi gases offer the intriguing possibility to create a paired fermion state such as a BardeenCooper-Schrieffer phase in a novel and highly controllable system.…”

“…This problem was circumvented by simultaneously trapping two atom species or two different hyperfine spin states of a given atom species. In [2], 6 Li is trapped together with its bosonic isotope 7 Li. Whereas the s-wave scattering length of 6 Li is zero by symmetry, the s-wave scattering length for a collision between a 6 Li and a 7 Li atom is 2.2 nm, which is enough to thermalize both species together.…”