`Electronic and Structural Properties of Fluid Selenium at High Pressures and Temperatures
------ A New Mechanism of Nonmetal-to-Metal Transition'
F.Yonezawa, H.Ohtani and T.Yamaguchi
Journal of Non-Crystalline Solids 250-252 510 (1999)
By theoretical calculations, we describe the mechanism of
the nonmetal-to-metal transition on the volume expansion
of fluid selenium (Se)
in the supercritical region.
We first show from energetic considerations that
some of the bonds in Se chains are weakened when the volume
is expanded,
and secondly we show that the bond weakening
causes a reduction
in the splitting,
$\Delta E_{\sigma*-\sigma} \equiv E_{\sigma*}-E_{\sigma}$
between the bonding level, $E_{\sigma}$,
and the anti-bonding level, $E_{\sigma*}$.
In spite of the fact that the band width, $W$, is decreased
on the volume expansion,
the degree of the reduction in
$\Delta E_{\sigma*-\sigma}$
is so large that
the ratio
$(W/\Delta E_{\sigma*-\sigma})$
increases.
At some critical volume, the weakened bonds are disrupted and
the energy gap separating
the occupied and unoccupied bands disappears,
thus bringing the system from nonmetal to metal.
This feature is in contrast with the well-established mechanisms
for the Bloch-Wilson
transition and the Mott-Hubbard transtion,
in which the volume expansion does not affect
the characteristic quantity (the level difference $\Delta E$
for the former
and the electron correlation energy, $U$, for the latter, respectively),
while the band widths are narrowed,
so that both ($W/\Delta E$) and ($W/U$) decrease and the transition is
from metal to nonmetal.
We also discuss the distribution of electrons
in the metallic phase.
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2000/10/12