Minimal model to describe the magnetism of CuGeO3

We show that to describe properly the low-energy excitations of CuGeO3 one must include the effects of a transverse antiferromagnetic coupling, which is estimated to be J(perpendicular to)=0.15J. Owing to this coupling the frustration in the chains is significantly lower than recent one-dimensional (1D) estimates based on purely one-dimensional arguments, we find J(2)=0.2J Furthermore, we have found a strong modulation of the nearest-neighbor coupling due to the static distortion delta=0.065, which is five times higher than that previously deduced from a 1D chain approach. Our set of parameters gives (i) a value of the distortion which agrees well with some recent estimate for a lower bound we are able to perfectly reproduce, (ii) the dispersions, and (iii) the experimental susceptibility at both high and low temperatures. By performing density matrix renormalization-group (DMRG) calculations for two coupled chains we have analyzed the effect of the transverse coupling on the ratio of singlet to triplet gaps. The ratio is very sensitive to the parameters and the universality reported in the strict one-dimensional case is lost. As an additional point, we provide a simple picture to explain the interesting feature observed in recent inelastic neutron-scattering experiments: the existence of a second branch of excitations. [S0163-1829(99)05445-4].