On the Nature of the Superconducting Transition in YBCO

Center for Neutron Scattering, Oak Ridge National Laboratory* Japan Atomic Energy Agency** KEK*** University of Warwick**** National Institute of Standards and Technology*****
â—‹Mohana Yethiraj* David K. Christen* Masatoshi Arai** Yokoo Tetsuya*** Sonya Crowe**** Paul McK. Donald**** Lionel Porcar*****

Small angle neutron scattering (SANS) is used to probe the flux line lattice (FLL) in type-II superconductors since the neutron has a magnetic moment and can scatter from the magnetic contrast arising from a flux line.

In the high-Tc superconductor YBCO, a transition was observed from a hexagonal FLL at low magnetic field (parallel to the c-axis) to a square configuration at high fields. Also seen was a rapid decrease in the Bragg intensity at low temperature (T). Since the s-wave theory for conventional superconductors predicts little variation in the T-dependence of the Bragg intensity between 0K and ~Tc/3, it has been the general belief that both the symmetry change and the T-dependence behaviour was due to the d-wave nature of high-Tc superconductivity.

However, we observed that the fall-off in intensity with increasing temperature depended on the strength of the applied external field and that excellent fits to this T-dependence could be obtained by simply multiplying the temperature dependence of the familiar Ginzburg-Landau two-fluid model, appropriate for high-kappa materials conventional superconductors, by an exponential factor exp(-aT), with the field-dependent variable 'a' being the only free parameter. Further, comparing the YBCO results to those for another high-Tc material (La-Sr)2CuO4, we see the orientation of the square FLL is not related to the directions along which nodes exist in the superconducting gap. Finally, the phase diagram of the hexagonal and square FLL arrangement in YBCO is similar to that observed in V3Si, a conventional superconductor for which non-local effects play a significant role. These observations to strongly oppose a d-wave origin for the square-hexagonal FLL symmetry transition in the high-Tc superconductors.