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.)