The semiconductor-metal transition in fluid selenium studied by first-principles molecular-dynamics simulation

The changes in the structure and the electronic states of liquid selenium d ue to the semiconductor-metal (SC-M) transition at high temperatures and pr essures are investigated by first-principles molecular-dynamics simulation using generalized-gradient-corrected density functional theory. It is found that the chain structure persists even in the metallic state, though the a verage length of the chains is decreasing with increasing temperature. From the time changes of the chain structure, it is also found that the interac tion between the Se chains is crucially important for bond breaking, and th at the bond breaking and the rearrangement of the Se chains occur more freq uently at higher temperatures. When the Se-Se bonds break the anti-bonding states above the Fermi level (E-F) are stabilized while the non-bonding sta tes below E-F become unstable, and as a result the gap disappears at high t emperatures. The eigenstates which fill up the energy gap and give rise to the metallic state of liquid Se have large amplitudes of wavefunctions near the ends of the Se chains; To understand the experimentally observed photo-induced SC-M transition of liquid Se near the triple point, the possibility of inducing bond breaking in a Se chain by exciting an electron in the HOMO (highest occupied molecul ar orbital) to the LUMO (lowest unoccupied molecular orbital) is investigat ed by first-principles molecular-dynamics simulation and such a bond breaki ng is confirmed.