Science > Physics > PHYSICS NEWS UPDATE -- Number 827 June 6, 2007 by Phillip F. Schewe,Ben Stein
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06 Jun 2007 11:13:39 AM |
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PHYSICS NEWS UPDATE -- Number 827 June 6, 2007 by Phillip F. Schewe,Ben Stein |
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 827 June 6, 2007 by Phillip F. Schewe, Ben Stein
www.aip.org/pnu
UNIVERSAL BEHAVIOR IN STRONGLY INTERACTING FERMIONS, in which a single
equation describes the energy and entropy of ultracold Fermi gases, has
been found by theorists at the University of Queensland Centre for
Quantum-Atom Optics in Australia and at Renmin University of China in
Beijing. The work provides the first comparison of different strongly
interacting fermions-that is, particles with spin one-half--which are
the building blocks of matter. Experiments, at places such as JILA,
Rice, and Duke, all explore the interactions of ultracold Fermi gases
(such as potassium-40 and lithium-6). The ultimate goal of the
experiments is to better understand interactions among fermions in
high-temperature superconductors and other complex systems such as
supernovae.
Hu, Drummond and Liu (Drummond@physics.uq.edu.au) posit the concept of
a "universal thermodynamic regime," which says that when the force
between fermions is strong enough all fermion species should behave in
essentially the same way. This would be true regardless of their mass,
density, or interaction details. There is one restriction: the forces
have to operate over a short range compared to the spacing between the
particles.
By comparison, most common quantum many-body systems (such as molecules
in a volume of water) are very complicated, and require different
theories to be worked out for every specific type of atom or particle,
usually with an enormous and complex new computer simulation for every
case. The powerful idea behind universality in this new theory is that
while physicists expect free, non-interacting fermions to be very
simple, it is now thought that universal and simple behavior can also
occur for very strong interactions as well.
This overall picture of universality is rapidly gaining widespread
acceptance, say the researchers. It can be potentially applied to
understanding matter made of quarks (such as protons). Insights into
neutron stars are possible if the theory is further developed to
account for the relativistic motions of fermions in the stars. It is
possible that the work may help understanding of high-Tc
superconductors, but additional complexities of these systems have to
be factored in order to fully comprehend them. (Hu, Drummond and Liu,
Nature Physics, June 2007.)
X-RAY TRANSPARENCY. A method to achieve electromagnetically induced
transparency (EIT) for x rays has been worked out. Normally a gas of
atoms will absorb light at a certain frequency if that frequency
corresponds to the energy needed to bridge the gap between two internal
quantum levels in the atoms in the gas. If, however, a third quantum
level exists, it might be possible to set up the quantum phenomenon
called electromagnetically induced transparency. To bring this about,
one laser beam, the pump beam, creates a coherent superposition of
levels 2 and 3, which become so-called *dressed states.* If tuned
properly, the transition pathways from level 1 to the two dressed
states will interfere destructively. After that, over a narrow range
of frequencies, absorption of a probe beam is suppressed at the
frequency corresponding to the transition from level 1 to level 2. It
just so happens that this selective transparency causes the index of
refraction to vary rapidly near that frequency, a development which
has, in turn, been exploited in slowing optical light pulses. Getting
all this to work at x-ray frequencies has been difficult since the
energy levels are broad, corresponding to very sh ort-lived vacancies
in inner electron shells lodged in somewhat heavy atoms; in neon atoms,
for example, the lifetime is about 2 femtoseconds. Theorists calculated
that to get EIT to work for x rays, a very powerful pump beam (10^12
W/cm^2) would be needed. A new study of the problem, undertaken by
Christian Buth, Robin Santra, and Linda Young at Argonne National Lab,
shows that such a powerful beam will not necessarily undo the fragile
troika of states needed for EIT to work. Santra (rsantra@anl.gov) says
that induced transparency for x rays would help in the shaping of x-ray
pulses in pending x-ray laser facilities where biomolecule imaging and
molecular movies are envisioned. Tests of the new x-ray transparency
scheme might be made soon at Berkeley's Advanced Light Source and at
Argonne's Advanced Photon Source. (Physical Review Letters, upcoming
article)
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
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