Warm dense gold

At the crossroads between condensed matter and plasmas lies warm dense matter, for which the thermal energy is comparable to the electrons’ Fermi energy and the ionic cores are strongly correlated. Difficult to study either theoretically or experimentally, warm dense matter is a largely uncharted frontier. Now, physicists at Lawrence Livermore National Laboratory have used intense femtosecond laser pulses to turn a thin film of gold into a warm dense state and have measured a key parameter of the sample: the frequency-dependent dielectric function, which contains information about the density of states and band structure. After an initial femtosecond laser pulse heated the gold to energy densities as high as 10⁷ J/kg, the team sent in a second pulse, spanning the energy range from 1.5 to 2.8 eV, to probe the gold’s dielectric response. The results show an initial transient response that settles after about a picosecond into a quasi-steady-state response. Contributions from both interband and intraband transitions are evident in the dielectric function. By varying the energy of the initial pulse and the delay between the initial and probe pulses, the researchers could follow the evolution of the density of states. Furthermore, they could use the AC response to derive the DC conductivity and other material parameters. Such studies of nonequilibrium, transient states in warm dense matter will provide insights into other areas of physics, including shock physics, inertial confinement fusion, and astrophysics. (Y. Ping et al., Phys. Rev. Lett. 96 , 255003, 2006 http://dx.doi.org/10.1103/PhysRevLett.96.255003 .)