Physics
News Update
The American Institute of Physics Bulletin of Physics
News
Number
447 (Story #1), September 9, 1999 by Phillip F. Schewe
and Ben Stein
A "FERMI-DEGENERATE" ATOMIC GAS, a gas of
fermion atoms (atoms composed of an odd total number of constituents--electrons,
protons, and neutrons, each of which has half-integer spin) which essentially
overlap with one another, has been created for the first time, promising
tabletop insights into the basic properties of neutron stars, superfluid helium
and all forms of superconductivity. Preparing this gas of fermions requires the
exact same conditions as for preparing a Bose-Einstein condensate (BEC) of boson
atoms, atoms composed of an even number of constituents with half-integer spin.
One must cool a gas of atoms to the point that they exhibit wavelike properties
and pack them densely enough so that the average distance between atoms is
comparable to their "deBroglie wavelength." At this point, individual atoms
become impossible to distinguish. If the atoms are bosons, they fall
collectively into the lowest-energy (ground) state to form a BEC (Update
233). If the atoms are fermions, however, this cannot happen. The Pauli
exclusion principle prohibits two fermions from occupying the same state.
Instead, the fermions dutifully occupy different quantum states on the lowest
available energy levels, just as water fills a bottle from the bottom up to some
top level. (See figures at Physics News Graphics.) This
ensemble of atoms is called a "quantum degenerate gas" owing to the fact that
the differences between bosons and fermions only become important in this
low-temperature, high-density regime. A Fermi degenerate gas has more energy
than predicted by classical physics, because fermions have to occupy higher and
higher energy levels once the lower ones get filled up. Achieving this state has
been difficult because cooling fermions is more difficult than cooling bosons:
placed in a trap made with magnetic fields, fermions in similar states tend to
repel each other and avoid the energy-transferring collisions required for
"evaporative cooling." To combat this, researchers in Colorado (Deborah Jin,
303-492-5735, NIST/University of Colorado) prepared potassium-40 atoms in two
different states of spin, a quantity which describes how the atoms respond to an
external magnetic field. The two species could collide with one another and this
enabled evaporative cooling to occur. Then, one spin species was removed by a
radio-frequency field, leaving about a million of atoms in the other spin
species for study. The Colorado group deduced their temperature to be
approximately 290 nanokelvins--the lowest ever recorded for a gas of fermions.
They witnessed that the fermion nature of the atoms dramatically inhibited
evaporative cooling. This is due in part to the Fermi pressure--the repulsion of
atoms in the trap--which resists the compression necessary for effective
evaporative cooling. (Therefore, this system can provide insights into how the
fermions that make up white dwarfs and neutron stars remain buoyant instead of
collapsing by the force of gravity.) In the future, researchers hope to study
superconductivity by forming Cooper pairs with the fermions, at even lower
temperatures than presently achieved. Creating such a "Fermi superfluid" will
enable investigations into all forms of superfluidity and superconductivity.
(DeMarco and Jin, Science, 10
September 1999.) Other groups are pursuing these and similar states with other
fermion atoms (Phys. Rev.
Focus, 24 May 1999).
