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Skriven 2005-06-22 19:28:09 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 734
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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@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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