Number 734, June 22, 2005
by Phil Schewe and Ben Stein
Superfluidity in an Ultracold Gas of Fermion Atoms
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) T`
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 to 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. (Deccaet al., Physical Review Letters, 24 June 2005F
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