Warm Dense Gold

Physicists at Lawrence Livermore Laboratory in California have used intense light to convert a small solid gold target into a plasma of electrons and positive ions. In the instant before the sample flies apart the physicists are able to record some surprising results. The most important finding is that even at extreme conditions of high energy density (10⁷ joules per kilogram), the gold metal still retains the band structure exhibited by all metals -- the allowed electron energies are not continuous but fall into certain allowed energy bands.

With light from a femtosecond laser falling on the sample, the Livermore scientists achieve the highest isochoric (meaning under conditions of constant density) energy density ever observed for a solid -- 10⁷ joules per kilogram.

According to one of the researchers, Andrew Ng (ng16@llnl.gov), who is the scientific director of Livermore's Jupiter Laser Facility, expressing the energy density in terms of energy per unit mass, rather the customary energy per unit volume, gives a more direct sense of the excitation energy being invested in each atom or molecule. (Higher energy densities have been achieved by imploding a target with laser light or a nuclear explosion, but the new result is the highest for a sample at its original volume.)

Furthermore, this experiment achieves a record for heating rate -- exceeding 10¹⁷ degrees Fahrenheit per second -- for the electrons in the solid; the ions forming the lattice of the solid are heavier and warm up at a much slower rate.

This work can be considered as part of an emerging new subject, "warm dense matter," at the crossroads between condensed matter physics and plasma physics. This research area, related to another topic called high energy density physics, is also of interest to workers in disciplines such as high pressure science, planetary science, geophysics, and shock compression.

Ping et al., Physical Review Letters, 30 June 2006
Contact Andrew Ng, ng16@llnl.gov