Metallization of Warm Dense ${\mathrm{SiO}}_2$ Studied by XANES Spectroscopy

We investigate the evolution of the electronic structure of fused silica in a dense plasma regime using time-resolved x-ray absorption spectroscopy. We use a nanosecond (ns) laser beam to generate a strong uniform shock wave in the sample and a picosecond (ps) pulse to produce a broadband x-ray source near the Si $K$ edge. By varying the delay between the two laser beams and the intensity of the ns beam, we explore a large thermodynamical domain with densities varying from 1 to $5\mathrm{g}/{\mathrm{cm}}^3$ and temperatures up to 5 eV. In contrast to normal conditions where silica is a well-known insulator with a wide band gap of 8.9 eV, we find that shocked silica exhibits a pseudogap as a semimetal throughout this thermodynamical domain. This is in quantitative agreement with density functional theory predictions performed using the generalized gradient approximation.

Figure 1

Experiment setup, shocked target geometry, and corresponding SOP typical image. The emission of the four transparent layers can be clearly followed.

Figure 2

On the left, density and temperature conditions inferred from the shock timing and velocity $\mathrm{SOP}/\mathrm{VISAR}$ data coupled to the hydrodynamic simulations. The different colors refer to different laser intensities. The pink dots represent the conditions at which the ab initio simulations were performed. The orange shaded areas represent the conditions at which the measurements and calculations were compared. The right-hand figures show a typical $\rho$ and T spatial profile along the shock direction of $2\text{−}\mu \mathrm{m}$ probed ${\mathrm{SiO}}_2$ as given by the Lagrangian code MULTI.

Figure 3

Si $K$ edge XANES spectra measured (left) and calculated (right) near solid density.

Figure 4

On the left, total densities of states at $2\mathrm{g}/{\mathrm{cm}}^3$: the gap of the insulator at 0 K disappears and becomes a pseudogap with the temperature. On the right, densities of state at $5\mathrm{g}/{\mathrm{cm}}^3$: occupied (full line) and total (dashed line). The populated states above the Fermi level increase with the temperature, like in a metal.

Figure 5

Variation in temperature of the Si $K$ edge XANES spectrum around the $5\mathrm{g}/{\mathrm{cm}}^3$ isochore given by the calculations (right) and observed experimentally (left). The spectra are normalized to the x-ray absorption value far above the $K$ edge and offset for clarity.