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Abstract

This thesis presents the results of transverse-field muon spin rotation (TF-/Li+SR) measurements of the magnetic penetration depth in the a6-plane (i.e. Aa&), for the vortex state of high quality single crystals of YBaaCusOg.gs. In particular, the low-temperature dependence of Aa(, was determined in an effort to clarify the nature of the pairing mechanism in the YBa2Cu306.95 compound. These results should be more reliable than previous jU+SR studies on powders and crystal mosaics, due to the employment of a novel low-background apparatus, as well as to improvements in sample quality and in the fitting procedure. A strong linear temperature dependence for 1/A^, was found to exist below 50K for applied magnetic fields of 0.5T and 1.5T. This linear temperature dependence contradicts the consensus of previous / i+SR studies which suggested a behaviour consistent with conventional s-wave pairing of carriers in the superconducting state. The presence of a linear term in the data reported here, supports recent microwave cavity measurements in zero field and indicates the existence of a more unconventional pairing state. In addition, a possible field dependence for Aa& at low temperatures was indicated by the data, with Xab(T = 0) having a range of 1347-145lA and 1437-1496A for the 0.5T and 1.5T data, respectively. The range of these values was determined by fitting the data several different ways. For each type of analysis, Aof,(0) was found to be greater and the linear term was stronger in the 0.5T data. Furthermore, the 1.5T data appear to agree better with the microwave cavity measurements. Included in this thesis is a qualitative description of the conventional s-wave pairing state and a proposed