Elementary excitations, exchange interaction and spin-Peierls transition in CuGeO3

The microscopic description of the spin-Peierls transition in pure and doped CuGeO₃ is developed taking into account realistic details of crystal structure. It it shown that the presence of side-groups (here Ge) strongly influences superexchange along Cu-O-Cu path, making it antiferromagnetic. Nearest-neighbour and next-nearest neighbour exchange constants J _nn and J _nnn are calculated. Si doping effectively segments the CuO₂-chains leading to J _nn (Si)≃0 or even slightly ferromagnetic. Strong sensitivity of the exchange constants to Cu-O-Cu and (Cu-O-Cu)-Ge angles may be responsible for the spin-Peierls transition itself ("bond-bending mechanism" of the transition). The nature of excitations in the isolated and coupled spin-Peierls chains is studied and it is shown that topological excitations (solitons) play crucial role. Such soltons appear in particular in doped systems (Cu_{1- x} Zn _x GeO₃, CuGe_{1- x} Si _x O₃) which can explain the T _SP (x) phase diagram.