Scientists at JILA, working with Italian theorists, have
discovered another notable similarity between ultracold atomic gases and
high-temperature superconductors, suggesting there may be a relatively
simple shared explanation for equivalent behaviors of the two very
NIST Fellow Deborah Jin in her laboratory at JILA where she studies
Described in Nature Physics,* the new research lends more
support to the idea that JILA studies of superfluidity (flow with zero
friction) in atomic gases may help scientists understand far more
complicated high-temperature superconductors, solids with zero
resistance to electrical current at relatively high temperatures. Known
high-temperature superconductors only superconduct well below room
temperature, but a detailed understanding of how the materials work may
one day lead to practical applications such as more efficient
transmission of electricity across power grids.
JILA is operated jointly by the National Institute of Standards and
Technology (NIST) and the University
of Colorado at Boulder.
The JILA group studies how atoms in a Fermi gas** behave as they
"cross over" from acting like a Bose Einstein condensate, in which atom
pairs form tightly bound molecules, to behaving like pairs of separated
electrons in a superconductor. In the new study, JILA scientists applied
a technique they developed in 2008 <http://www.nist.gov/public_affairs/releases/ultracold_080608.cfm>
to explore subtle energy properties of ultracold atoms. The technique
is an adaptation of photoemission spectroscopy, long used to probe the
energy of electrons in materials. A superconductor research group
recently used electron photoemission spectroscopy to find evidence of
electron pairing above the critical temperature where the material
switches from a superconductor to a regular conductor. Why this duality
occurs is a subject of debate.
The JILA scientists performed comparable measurements for an
ultracold gas of potassium atoms at and above temperatures where
superfluidity disappears. Like the superconductor group, the JILA team
found evidence of atom pairing above the critical temperature. This
demonstrates the existence of a so-called "pseudo-gap region" where the
system retains some pairs of correlated fermions but not all
characteristics of superfluidity. The findings were made possible in
part by significant improvements in the signal strength of the atom
photoemission spectroscopy technique since 2008.
"What makes this really interesting is that the two systems are
actually very different, with the high-temperature superconductor being
much more complicated than atomic gases," says NIST/JILA Fellow Deborah
Jin. "The observation of similar behavior with similar measurements
suggests that having a pseudogap phase does not require complicated
explanations, such as lattice effects, two-dimensionality, or exotic
Co-authors of the new paper are theorists from the Universita di
Camerino in Italy.
The research was funded by the National Science Foundation.
* J.P. Gaebler, J.T. Stewart, T.E. Drake and D.S. Jin, A. Perali, P.
Pieri and G.C. Strinati. 2010. Observation of pseudogap behavior in a
strongly interacting Fermi gas. Nature Physics. Posted online July 4.
** A Fermi gas is a collection of noninteracting particles called
fermions, one of two categories of fundamental particles found in nature
(bosons are the other). Identical fermions cannot occupy the same place
at the same time.
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