Central mode and spin confinement near the boundary of the superconducting phase in YBa[sub 2]Cu[sub 3]O[sub 6.353] (T[sub c] = 18 K)

Abstract

We have mapped the neutron scattering spin spectrum at low energies in YBa2Cu3O6.353 (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\ufffd5 \ufffd in-plane and 8\ufffd2 \ufffd 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 N\ufffdel 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.NRC publication: Ye