Ajr. Dasilva et Lm. Falicov, MANY-BODY CALCULATION OF THE MAGNETIC, OPTICAL, AND CHARGE-TRANSFER SPECTRA OF SOLID OXYGEN IN THE ALPHA-PHASE AND BETA-PHASE, Physical review. B, Condensed matter, 52(4), 1995, pp. 2325-2335
The electronic spectra of alpha and beta solid O-2 were calculated in
a full many-body approach for a cluster consisting of four O-2 molecul
es with periodic-boundary conditions. By including only the partially
occupied pi orbitals (16 spin orbitals, 8 electrons) the basis set con
sists of 12 870 many-electron states. The resulting spectra contains t
hree basic types of excitations at very different energy ranges: (a) 8
1 states corresponding to the ground state and magnetic excitations (m
agnons) within 100 meV; (b) 1215 states of neutral molecular excitatio
ns (excitons) with energies from about 1 to 8 eV; and (c) 11 574 charg
e-transfer states with excitation energies up to 60 eV. Analysis of th
e properties of the ground states in both alpha and beta solid O-2 has
been carried out. The lowest many-body states, those that correspond
to neutral, unexcited molecules, describe accurately the magnetic exci
tations of both solids. The two phases have very different spectra res
ulting, even at very low temperatures, in a sizeable difference in the
(magnetic) entropy. The calculated entropy difference at the observed
alpha-beta phase transition temperature agrees surprisingly well with
the experimentally measured heat of transformation.