16
T. Onishi
Fig. 1.18 Superexchange
interaction between copper
atoms via oxygen anion in
two-dimensional CuO 2 layer
(Cu-O-Cu model)
MO
MO
Cu
O
Cu
O
Fig. 1.19 Cu-F shrink and
elongation in K 2 CuF 4
perovskite
Cu
F
spin of oxygen 2p x orbital. Hence, α spin of left copper 3d x
2 −y
2 orbital and β spin
of right copper 3d x
2 −y
2 orbital remain: Superexchange Rule [55]. When electron or
hole is doped in two-dimensional CuO 2 layer together with decreasing temperature,
superconductivity appears. Though many superconductor theories were proposed,
superconducting mechanism is fully unrevealed.
1.5.2.3 Magnetic Perovskite
1. Ligand Bonding Effect in Perovskite Fluoride
Ligand bonding effect [56] is observed in K 2 CuF 4 perovskite. Cu-F distance
in two-dimensional CuF 2 layer shrinks or elongates, though it is fixed along
perpendicular axis (see Fig. 1.19). As the result, orbital overlaps between copper
3d and fluorine 2p orbitals exist in specific directions (see MO1α and MO2α
in Fig. 1.20). In contrast to Cu-O-Cu model, left copper 3d z
2 −y
2 orbital is not
overlapped with bridge oxygen 2p x orbital, though there is overlap between right
copper 3d z
2 −x
2 and bridge oxygen 2p x orbitals. This is why the alternate 3d z
2 −x
2 -
type orbital ordering is caused in K 2 CuF 4 perovskite.