ANOMALOUS NEUTRON-SCATTERING NEAR THE SUPERCONDUCTING PHASE-TRANSITION

THE inelastic scattering of low-energy neutrons near a second-order phase transition can provide a description of the spontaneous fluctuations in space and time of the physical parameter which, at low temperatures, enters a state of long-range order. Often the order parameter is coupled to the neutron directly, as in magnetic or lattice relaxation phenomena; as in the case of superconductivity, discussed here, the coupling is indirect. In either case, inelastic neutron diffraction measures the amplitude of the equilibrium fluctuations on an absolute scale, which may be compared with the predictions of models based on linear response theory 1. To this end, we report an investigation of the temperature dependence of small-angle neutron scattering in homogeneous bulk samples of the high-temperature superconductor YBa2Cu3O7-delta. An anomalous increase in the scattering intensity occurs near the superconducting phase transition, and at lower temperatures. These effects seem to be larger than can be accounted for in elementary models of the normal and superconducting states.