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 constantsJ nn andJ nnn are calculated. Si doping effectively segments the CuO2-chains leading toJ 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 (Cu1−x Zn x GeO3, CuGe1−x Si x O3) which can explain theT SP (x) phase diagram.