INELASTIC-NEUTRON-SCATTERING INVESTIGATION OF THE SPIN-PEIERLS SYSTEMCUGEO3

We have investigated the spin dynamics of the spin-Peierls system CuGe O3 by inelastic neutron scattering. The measurements have been perform ed as a function of wave vector, temperature, and magnetic field, for fields perpendicular to the chain axis (up to 10 T). Our neutron resul ts confirm the occurrence of a crystalline distortion below T-SP appro ximate to 14.2 K, corroborated by the appearance of superlattice peaks indexed with a commensurate wave vector k(SP) = (1/2,0,1/2). At low t emperature, the spin-excitation spectrum exhibits a well defined energ y gap Delta approximate to 2 meV approximate to 1.66 kT(SP) at the ant iferromagnetic point k(AF) = (0,1,1/2), distinct from k(SP). The exper imental results for T much less than T-SP are quantitatively understoo d from an alternating-exchange Hamiltonian with an exchange parameter (2J(1)) approximate to 10.6 meV and an alternation parameter alpha = J (2)/J(1) approximate to 0.92, despite the existence of sizable interch ain couplings both along the alpha and b axes (J'(alpha)/J(1) approxim ate to 0.011 and J'(b)/J(1) approximate to 0.11, respectively). We pre sent the temperature dependence of the spin dynamics on both sides of T-SP. Our results confirm the persistence of a pseudogap in the excita tion spectrum for q=k(AF), but only in the immediate vicinity of the s pin-Peierls transition temperature. The pseudogap vanishes rapidly wit h T, following decreasing size of dimerized segments. The stronger ine lasticity observed above T-SP at q=k(SP) has been attributed to an eff ect of the dispersion of magnetic excitations due to interchain coupli ngs J'(alpha) and J'(b). Under a magnetic field applied perpendicular to the chain axis, the excited tripler is split into three components, with gap values depending linearly on the field. The spin-Peierls tra nsition temperature TSP decreases with increasing field, following a q uadratic dependence on H, in agreement with theoretical as well as oth er experimental results. We have determined the field dependence of th e dimerization peak intensities up to 10 T, which show very small chan ges. In particular, no trace of pretransitional incommensurability cou ld be detected, at least up to 0.77H(c).