`Theoretical study of expanded selenium in the supercritical region'
F.Yonezawa, H.Ohtani and T.Yamaguchi
Journal of Physics: Condensed Matter 10 11419 (1998).
By theoretical calculations, we elucidate the mechanism for
the nonmetal-to-metal (NM-to-M) transition on the density decrease
in supercritical Se. We first show from energetic considerations that
some of the bonds in Se chains are disrupted when the density
is decreased,
and secondly we clarify that the bond disruption
causes a drastic reduction
of the splitting ($E_{\sigma*-\sigma}$) between the bonding ($\sigma$)
and anti-bonding ($\sigma^*$) levels. As a consequence, the energy
gap separating
occupied and unoccupied levels decreases and eventually disappears,
thus bringing the system from nonmetallic to metallic nature.
A remarkable point is that, although the band width ($W$) reduces
on the density decrease,
the degree of the reduction in
$E_{\sigma*-\sigma}$
is so distinguished that the ratio ($W/E_{\sigma*-\sigma}$)
increases.
This feature is in contrast with the traditional Wilson
transition,
in which the decrease of density gives no significant influence
to the difference between the
corresponding energy levels
such as the spacing between the s-level and p-level
in the case of expanded Hg,
while the decrease of density narrows the band width $W$ and the ratio
$W/E_{\sigma*-\sigma}$ as well, and accordingly a metal-to-nonmetal
transition is induced.
Our calculations also show that the structural changes
such as the shortening of bond lengths take place
at the NM-to-M transition.
Back
2000/10/12