College Park, Maryland      June 6 - 10 , 2004

M4-A5 (5:15 PM): On the temperature dependence of the order parameter in YBCO

M. Yethiraj, D. K. Christen (Oak Ridge National Laboratory), D. McK. Paul, S. Crowe (University of Warwick), M. Arai (KEK - High Energy Accelerator Research Organization), L. Porcar (NIST Center for Neutron Research)

An unusual feature of the FLL in YBa2Cu3O7 is the temperature (T) dependence of the Bragg intensity. Conventional theories predict very little variation in the intensity between 0 K and ~T /3. Since the London penetration depth (of which the Bragg intensity is a measure) is an order parameter of the superconductivity, the unexpectedly rapid decrease in the intensity at low T seen in the first measurements at 0.8 Tesla were extremely interesting, particularly what it implied on the mechanism driving the transition in these materials. However, since disorder results in a decrease of the integrated Bragg intensity, detailed measurements are needed to gauge the effect of twin plane defects and lattice structural distortions on the Bragg intensity. Recent measurements of the temperature dependence of the intensity and vortex lattice structure at different fields in a clean but twinned sample of optimally doped YBCO will be discussed.

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