Central mode and spin confinement near the boundary of the superconducting phase in Y Ba2 Cu3 O6.353 (Tc =18 K)

We have mapped the neutron scattering spin spectrum at low energies in Y Ba2 Cu3 O6.353 (Tc =18 K), where the doping ∼0.06 is near the critical value (pc =0.055) for superconductivity. No coexistence with long-range-ordered antiferromagnetism is found. The spins fluctuate on two energy scales: one a damped spin response with a 2 meV relaxation rate and the other a central mode with a relaxation rate that slows to less than 0.08 meV below Tc. The spectrum mirrors that of a soft mode driving a central mode. Extremely short correlation lengths, 42±5 Å in-plane and 8±2 Å along the c direction, and isotropic spin orientations for the central mode indicate that the correlations are subcritical with respect to any second-order transition to Néel order. The dynamics follows a model where damped spin fluctuations are coupled to the slow fluctuations of regions with correlations shortened by the hole doping.