Evidence for superfluidity in an atom-based Fermi gas has been observed
for the first time by researchers at Duke University (including John
Thomas, 919-660-2508, jet@phy.duke.edu, and Michael Gehm, mgehm@ee.duke.edu,
919-403-5003). In essence, the researchers have observed an ultracold
gas of lithium-6 atoms acting as one big vibrating "jelly."
While the jelly-like (or "hydrodynamic") behavior could arise in ordinary
versions of ultracold lithium gases, the researchers found evidence
that their gas was a superfluid, a "perfect" jelly which vibrates for
a long time after being shaken.
The properties of the atomic jelly can provide information on other
superfluid systems (such as neutron
stars). The behavior of the jelly could even help determine whether
it's physically possible to create superconductors
which operate well above room temperature, which could lead to breakthroughs
ranging from widely available energy-saving power lines to magnetically
levitated trains. What's shared by all these systems, from a quark-gluon
plasma to neutrons in neutron stars, is that they are made of strongly
interacting pairs of "spin-up" and "spin-down" particles (spin up/down
is analogous to the atoms having bar magnets pointing in opposite directions).
To produce the observed behavior, the researchers believe that the
interaction mechanism among their lithium-6 atoms is in a weird "cross-over
regime" (see Update
671), a condition in which the atom pairs are neither molecules
(in which case they would form a molecular Bose Einstein condensate,
see Update
663) nor they type of weakly bound Cooper pairs found in conventional
superconductors.
In their experiment, the researchers cooled and trapped lithium-6 atoms
with a focused laser beam, whose electric field confined the atoms.
The researchers made sure the atoms were in a 50-50 mixture of spin-up
and spin-down states. They then used their optical system to lower the
temperature of atoms via "evaporative cooling" (i.e., allowing hotter
atoms to escape to lower the overall temperature of the gas).
Next, they tested the gas's ability to act like a vibrating "jelly."
To start vibrations in the gas, they turned off the trapping laser for
a short time, allowing the gas to expand, and then turned the laser
back on again. At this point the gas cloud was quivering, and the researchers
took a series of pictures to show these vibrations (see visuals).
They measured the cloud's frequency of vibration, as well as how long
the vibrations persist.
In one case, they adjusted the magnetic field so that the atoms were
strongly interacting. In this instance, they measured a frequency of
vibration of 2837 Hz, in very close agreement with a theoretical prediction
of 2830 Hz for a hydrodynamic Fermi gas. Lowering the temperature of
the gas caused the vibrations or "oscillations" to last for a longer
time, in contrast to an ordinary hydrodynamic gas, in which a lower
temperature would cause the oscillations to "damp" or die out more quickly.
The Duke physicists ruled out two non-superfluid scenarios for the
behavior, namely that the oscillations were caused by (1) a high rate
of atomic collisions (however, in this scenario, the oscillations would
die out more quickly as the temperature is lowered) and (2) a collisionless
gas that oscillates via mean-field interactions, the net effect of many
atom-to-atom interactions (however, the predicted vibration frequency
for this scenario differs by 500 Hz from the observations).
Still, the researchers do not have an iron-clad case for superfluidity
yet, in large part because the theory for strongly interacting superfluid
Fermi gases is incomplete. Namely, there is no prediction of how the
damping times of the vibrations should increase with decreasing temperature,
which would help to identify a "transition temperature" below which
superfluidity would occur. (In their setup, the Duke team started seeing
evidence for superfluidity at temperatures below 0.4 to 0.7 Microkelvin.)
In summary, the experiments constitute first evidence for what could
plausibly be superfluid behavior based on pairs of fermion atoms in
a gas. The photos provide macroscopic information (i.e., viewing the
overall gas that's visible to the naked eye) that complement the "microscopic"
information provided by other groups (Update
671), which probe the pairing of spin-up and spin-down atoms. (Kinast
et al., Physical Review Letters, 16 April 2004.)
Greatly improved solar cells might result from the use of a photophysical
process in which for each incident solar photon not one but two excitons
(electron-hole pairs) are created. As with photosynthesis what happens
in a solar cell is the conversion of light energy into a small current
of electrons; in plants the freed electrons helps to build glucose;
in solar cells the currents are collected in the form of electricity.
Victor Klimov and Richard Schaller at Los Alamos have enhanced the
phenomenon called "impact ionization," which can significantly improve
the efficiency of the conversion of solar energy to electrical current.
Normally, an incident photon striking a semiconductor produces an electron-hole
pair plus a bit of heat. By using sub-10-nm sized nanoparticles made
of lead and selenium atoms, the Los Alamos scientists encourage the
interaction to spawn a second exciton instead of the heat.
Although they haven't yet built a working solar cell, they are the
first to demonstrate the efficacy of getting the PbSe nanocrystals to
render more photo-current. Implementing the new process might result
in efficiency gains of more than 35% in the conversion of light to current.
(Physical Review Letters, upcoming
article; contact Victor Klimov, 505-699-7541, klimov@lanl.gov; see lab
website.)
