Investigation of continuum lowering in solid silicon under extreme pressure

Optical and X-ray transition energies are generally reduced with increasing temperature and pressure in plasmas. This phenomenon, called ‘continuum lowering,’ is important in the high-energy density (HED) regime. To understand the relative contributions of pressure and temperature, we used density-functional theory to study diamond silicon at low temperature but under extreme pressure, up to 150 Mbar and 12× density. When the structure is compressed to 1.36× density, the system turns metallic. We used a semicore pseudopotential because we found that the 2s and 2p wavefunctions of adjacent atoms overlap under extreme pressure, altering the bandstructure. As a benchmark, we calculated deformation potentials from low pressure in good agreement with literature results. We investigated soft X-ray transitions by calculating the absorption spectrum with the random phase approximation in the BerkeleyGW code. From these results and the Kohn-Sham potential, we found opposite behavior to the expectation from continuum lowering in plasmas. We compare to various models of continuum lowering. This investigation gives insight into the mechanisms of continuum lowering, and can help in the interpretation of experimental spectra of matter under HED conditions.