PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 734  June 22, 2005
by Phillip F. Schewe, Ben Stein
                
SUPERFLUIDITY IN AN ULTRACOLD GAS OF FERMION ATOMS has been demonstrated in
an experiment at MIT, where an array of vortices has been set in motion in
a molecular Bose Einstein condensate (BEC) of paired lithium-6 atoms. 
There have been previous hints of superfluidity in Li-6, for example,
(http://www.aip.org/pnu/2004/split/681-1.html) but the presence of vortices
observed in the new experiment clinches the case since vortices manifest
the most characteristic feature of superfluidity, namely persistent
frictionless flow.  Wolfgang Ketterle and his MIT colleagues use laser
beams to hold the chilled atoms in place and separate laser beams to whip
up the vortices.  In general the quantum behavior of bosonic atoms (those
whose total internal spin---the spin of the nucleus added to that of the
electron retinue---is an integral number of units) and fermi atoms (those
with a half-integral-valued total spin) is very different.  Gaseous Li-6
represents only the second known superfluid among fermi atoms, the other
being liquid helium-3. (Superconductivity is also a  form of fermion
superfluidity, but in this case the constituents are charged particles,
electrons, unlike the neutral atoms used in the experiments described
here.)  There are great advantages in dealing with a neutral superfluid in
dilute gas form rather than in liquid form: in the gas phase (with a
material density similar to that of the interstellar medium), inter-atomic
scattering is simpler; furthermore, the strength of the pairing interaction
can be tuned at will using an imposed external magnetic field.  According
to Ketterle, one of those who won a Nobel prize for his pioneering work
with boson BECs, the study of fermionic superfluidity is much richer than
for bosons: control over forces will permit researchers to vary the
strength and nature of the pairing (fermi atoms must pair up before falling
into BEC form) and to load atoms into an optical lattice.  Additional
pairing mechanisms can also be explored.  One further superlative: the
ultracold lithium gas represents, in a narrow sense, the first
"high-temperature" superfluid.  Consider the ratio of the
critical temperature (Tc) at which the superfluid transition takes place to
the fermi temperature (Tf), the temperature (or energy, divided by
Boltzmann's constant) of the most energetic particle in the ensemble.  For
ordinary superconductors, Tc/Tf is about 10^-4; for superfluid helium-3 it
is 10^-3; for high-temp superconductors 10^-2; for the new lithium
superfluid it is 0.3.  (Zwierlein et al., Nature, 23 June 2005)

GRAVITY IS NORMAL DOWN TO THE 100-nm LEVEL.  Gravity at the level of
planets is well studied, and was known accurately even in Newton's day. 
This is owing to the fact that the other physical forces, such as the
strong and weak nuclear forces, don't operate over such great distances,
and electromagnetic forces between immense far-apart, electrically-neutral
objects like planets are dilute.  Gravity at shorter lengths, by contrast,
is harder to measure, partly because all the other forces are in full play.
 Furthermore, theories of particle interactions hypothesizing the existence
of additional spatial dimensions suggest that the strength of gravity will
depart from Newton's famous inverse-square formulation.  To test these
propositions, various tabletop setups have been devised to probe gravity
below the micron level.  One previous experiment, conducted by Eric
Adelberger's group at the University of Washington, ruled out extra gravity
components having a strength comparable to conventional gravity down t
o a size scale of about 100 microns
(http://www.aip.org/pnu/2000/split/pnu483-1.htm).  A new experiment,
carried out by a Indiana/Purdue/Lucent/Florida/Wabash collaboration
examines a shorter distance scale---100 nm---but is able to rule out only
corrections to gravity that are, in fact, a trillion times larger than
gravity itself.  Nevertheless, such measurements help to constrain the
general pursuit of unified theories of particle physics, including
explanations of gravity.  The sort of "Yukawa" corrections being
sought are analogous to the force proposed by Hideki Yukawa in the 1930s to
explain how mesons transmit the nuclear force between nucleons and would
come about because of transmission of the presumed force particles
associated with the hypothetical extra dimensions.  The present
measurements improve the exclusion of such corrections by a factor of ten.
According to Ricardo Decca of Indiana University-Purdue University
(rdecca{at}iupui.edu, 317-278-7123), the sensitivity of the apparatus should
grow by a factor of a hundred over the next year.  The size of the sample
is smaller here than in many other tabletop gravity experiments.  The
flea-sized torsional apparatus must operate with such concern for forces
acting over small distances that one of the chief goals here is reducing
the background produced by the Casimir force---a quantum effect in which
two very close objects are drawn together because of the way they exclude
vacuum fluctuations (that is, the spontaneous creation of pairs of virtual
particles) from occurring in a slender volume of space---between a flat
plane and sphere lying only 200 nm apart.  (Decca et al., Physical Review
Letters, 24 June 2005)

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