From: LARRY KLAES (ljk4_at_msn.com)
Date: Sun Feb 01 2004 - 06:52:40 PST
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From: physnews_at_aip.org
Sent: Friday, January 30, 2004 4:01 PM
To: ljk4_at_MSN.COM
Subject: Physics News Update 671
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 671 January 30, 2004 by Phillip F. Schewe, Ben Stein, and
James Riordon
HALFWAY ACROSS ON THE BEC-BCS PRAIRIE. Researchers in Colorado have
discovered a new form of atomic matter, a fermionic condensate
unlike anything seen before. To approach this
conceptually-difficult but physics-rich topic, we will proceed in
several parts: providing a quantum background, defining the word
"degeneracy," summarizing the new atomic state, and finally
assessing the advantages of the new state.
1. Quantum background. In exploring the exotic landscape of
quantum gases, physicists have lavished much attention on bosonic
atoms (atoms whose total spin has an integer value, such as 0 or 1
or 2). In 1995 scientists succeeded in cooling (bosonic) atoms so
that in a quantum sense the atoms began to overlap, at which point
they really could not be distinguished and had, in effect, become
part of a single quantum entity called Bose Einstein condensate
(BEC). Fermions (possessing half-integer spins, such as 1/2 or 3/2
or 9/2), whether elementary particles like electrons and quarks, or
whole atoms (and in determining whether an atom is a boson or
fermion one has to add up the spins of all its constituent protons,
neutrons, and electrons), do not act like bosons. The Pauli
exclusion principle dictates that no two identical fermions may
occupy the same quantum state. Most of chemistry here on Earth and
elsewhere is dictated by the simple Pauli rule: electrons fill
atomic orbitals in such a way that no two electrons have exactly the
same quantum values. Partially filled orbitals determine what kind
of chemical affinity that atom will have. Note that fermion atoms
are not precluded from interacting in ordinary chemical reactions
(the atoms have differing nuclear and electronic internal
configurations). But they may not enter into an extensive BEC kind
of quantum condensate where the atoms do possess the same quantum
attributes.
2. Degeneracy. Pauli is on duty at all times, but he chiefly
manifests himself in a quantum setting, such as in the orbitals
within an atom or in the chilled molasses of a microkelvin-level
atom trap. In this rarefied realm, bosons can all fall into that
singular BEC state. All having the same energy, these atoms are
said to be degenerate. With fermions, it's quite different. In a
quantum setting---whether electrons moving through a crystal or
fermion atoms chilled in a trap, fermions are obliged to fill, one
by one, all the different possible quantum energy states, starting
at the low end. On an energy level diagram, the fermions look as if
they were perching on the rungs of a ladder, filling all the rungs
singly. (The uppermost rung is called the fermi energy and the
temperature that corresponds to that energy is called the fermi
temperature.) Commonplace example: the free-roaming electrons in a
metal crystal, even at room temperature, are obliged to assume a set
of discrete quantum-allowed energies in this way. These electrons
are said to constitute a degenerate fermi gas. In the fermion
context, "degenerate" means that the particles fill up the plenum of
possible energy states. Creating such a gas of degenerate fermion
atoms proved more difficult to make than a degenerate (BEC) gas of
boson atoms. In fact, a degenerate fermi gas was first accomplished
only in 1999 (www.aip.org/enews/physnews/1999/split/pnu447-1.htm) in
an experiment by Deborah Jin and her NIST/JILA colleagues, the same
lab where the new results have been performed. By the way, although
physicists had long assumed the Pauli principle would apply to atoms
(composite objects) as well as to electrons (truly elementary
particles), it was only in recent work that this was demonstrated
experimentally.
3. New state of matter. Fermions, if you pair them, can become
bosons. And in that way, fermions can enter pairwise into a quantum
condensate. There are, however, a whole spectrum of pairing
mechanisms. At one extreme is the case where the atoms pair
strongly, after which they can (as molecules) collapse into a Bose
Einstein condensate (BEC). At the other end of the spectrum the
atoms can pair weakly, or more to the point, combine in an unbound
but correlated state analogous to the Cooper pairs of electrons that
form the essence of quantum currents in superconductors or the pairs
of helium-3 atoms that constitute a superfluid. In previous months
a number of labs have reported forming condensations of
strongly-bound molecules (see
www.aip.org/enews/physnews/2003/split/663-1.html). Now Deborah Jin
and her colleagues Cindy Regal and Marcus Greiner at NIST and the
University of Colorado report making great progress in moving across
the plain between the BEC and BCS pairing alternatives. The type of
pairing can be adjusted by subtly altering the strength of an
external magnetic field. The NIST researchers, who cool
potassium-40 atoms to microkelvin temperatures, are at the
cross-over region: they are not at the BEC regime because the
applied magnetic field would not permit the kind of pairing one
needs for a BEC condensate. Also they can affirm that they are not
in the BCS regime either because the strength of the interaction
among atoms is too strong for the kind of weak Cooper pairing that
occurs in superconductivity or helium-3 superfluids. This new
condensed form of atomic matter should not be thought of merely as a
way station between the BEC and (weak) BCS pairing alternatives, but
as a unique state in its own right. Eric Cornell (also at NIST but
not part of Jin's group), who won a Nobel prize for his part in the
discovery of BEC, describes the new NIST state as "a dramatic new
sort of fermionic condensate, basically Cooper pairing in the
strong-field limit."
4. Assessment. One of the goals in pursuing this research is the
chance to form novel types of Cooper pairs or superfluids, and
possibly to custom make different kinds of superconductivity. In
these cold fermi gases the interactions (and the strength of the
pairing) can be adjusted by turning a knob (changing the magnetic
field), which is more than you can say about conventional
superconductivity, metallic or ceramic. Here is one hint that this
work might lead to warmer, even room temperature,
superconductivity: In the new potassium fermionic condensate the
ratio of transition temperature (at which condensation of pairs
occurs) to fermi temperature is about 1 to 5. In conventional
low-temp superconductors the ratio is 1 to 1000 (or even 100,000).
Even in high-temp superconductors, the ratio is 1 to 100. (Regal et
al., Physical Review Letters, 30 January 2004; additional background
in Physics
Today, Oct 1999 and Oct 2003.)
***********
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