Polarized and Unpolarized Superfluid States

In many areas of science, such as the study of chemical reactions among atoms or the nuclear interactions among protons in collisions at an accelerator, the strength of the interaction between species is imposed by nature. In some cases, however, the researcher has some control over the interaction strength and can contrive exotic states of matter thereby. An important example of this dexterity is the study of nanokelvin fermionic atoms (atoms with a half-integral total spin value, such as 1/2).

The Pauli exclusion principle forbids Fermi particles from distilling into a monolithic quantum fluid like a Bose-Einstein condensate (BEC). Paired up, however, fermions become bosons (integral-spin objects) and can condense. Fermi atoms, such as lithium-6, can marry in a variety of ways and this is what over the past few years has made them valuable to physicists in their effort to intervene in the basic interactions among particles. Typically the pairing is induced by adjusting an external magnetic field. The result can be a chemical bond: lithium atoms become tight diatomic molecules which then condense into a molecular BEC.

Alternatively, the atoms might form weakly bound Cooper pairs of large size (many times the average interatomic spacing). Or the pairing can be some kind of in-between state. This in-between pairing regime is poorly understood but intensely interesting since discoveries there may offer great insights into basic condensed matter interactions. Theorists say that one potential route to discovering of exotic new atomic condensates is the use of unbalanced clouds with an excess of spin-up or spin-down atoms.

Such systems are relevant to the study of magnetized superconductors and possibly even to pairing in cold quark matter at the centers of neutron stars. Recently, these systems have moved within reach of experiments, since in addition to being able to vary interaction strength, experimenters using cold atoms can vary the relative number of spin-up and spin-down atoms. One result of such an imbalance can be a separation into a two-phase gas consisting of a superfluid core of paired atoms surrounded by a normal-fluid mantle consisting of unpaired atoms.

Randy Hulet (randy@rice.edu) and his colleagues at Rice University and Utrecht University have now, for the first time, found evidence for two distinct superfluid regimes in an imbalanced gas of fermionic lithium-6 atoms. At lower temperatures, a sharp boundary between the fully-paired superfluid core and the excess unpaired atoms is observed, as expected for a first-order phase transition (the kind of transition -- such as water changing to ice -- in which the internal energy of the substance takes a discontinuous jump). At a slightly higher temperature, the fully paired core and the normal fluid mantle are separated by a diffuse mixed-phase which is also superfluid. Moreover, while the higher-temperature gas maintains the long cigar shape (aspect ratio of 30) imposed by the fields of the atom trap, the superfluid core of the lower-temperature gas, under the action of surface tension between the superfluid and normal phases, deforms towards a more spherical shape (aspect ratio as small as 2).

Partridge et al., Physical Review Letters, 10 November 2006 Contact Randy Hulet
Rice University
randy@rice.edu