QUANTUM DISORDERED REGIME AND SPIN GAP IN THE CUPRATE SUPERCONDUCTORS

We discuss the crossover from the quantum critical, z=1, to the quantu m disordered regime in high-T-c materials in relation to the experimen tal data on the nuclear relaxation, bulk susceptibility, and inelastic neutron scattering. In our scenario, the spin excitations develop a g ap Delta similar to Delta(xi) = c/xi well above T-c, which is suppleme nted by the quasiparticle gap below T-c. The above experiments yield c onsistent estimates for the value of the spin gap, which for small dop ing increases as the correlation length decreases.