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Date: Thursday, March 18, 2010
Time: 14:30
Room: WBBA/008, PSI West

Beta-NMR probe of vortex lattice disorder and surface magnetism in high-Tc superconductors YBCO
 

Dr. Hassan Saadaoui

Abstract:

Low-energy Beta-NMR has been developed at TRIUMF to study materials in a depth-resolved manner, where the implantation depth of highly spin-polarized Li-8 is controlled on a nanometer scale via electrostatic deceleration. In this talk, I will present the results of a Beta-NMR study of the magnetism near the surface of cuprate superconductors YBCO. For several advantages, the measurements of the spin resonance, i.e. magnetic field distribution, were done outside the superconductor by implanting the Li-8 beam into silver films (15 to 120 nm thick) evaporated on the superconductors. In the vortex state, the distribution is more symmetric and the dependence on the applied magnetic field is much weaker than expected from an ideal vortex lattice, indicating the existence of a disordered vortex lattice inside YBCO, where the vortex density varies across the face of the sample due to pinning at twin and grain boundaries. In the Meissner state, the distribution is found to broaden below the transition temperature Tc of YBCO. This signals the appearance of weak disordered magnetic fields near the YBCO surface. Similar results have been obtained in (110), (001), and (103)-oriented YBCO films, and place an upper limit estimated at 0.2 Gauss on the magnitude of spontaneous magnetic fields associated with a possible time-reversal symmetry breaking state in YBCO. The origin of such disordered magnetic fields will be discussed.

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