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Physics News Update
Number 827, June 6, 2007 by Phil Schewe and Ben Stein

Universal Behavior In Strongly Interacting Fermions

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 short-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

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