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Title: An experimental study of the magnetotransport properties of several new superconductors
Author: Fisher, I. R.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1997
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The magnetotransport properties of three new superconducting compounds (Y1-xCaxBa2Cu3O7-δ, TI2Ba2CuO6+δ and RNi2B2C) have been investigated. The study has principally involved measurements of the anisotropic resistivity, magnetoresistance, and Hall effect of these compounds. Results for sintered and thin film samples of Y1-xCaxBa2Cu3O7-δ are presented. The Hall coefficient of the fully oxygenated overdoped material is smaller in magnitude and has a weaker temperature dependence than that of optimally doped material, while the resistivity has an upward curvature and can be well fitted by A + BT + CT2. Despite these large changes in ρ and RH, cot(θH) shows remarkably little variation with doping, as is also the case for underdoped YBa2Cu3O7-δ. By fitting the small downturn in the resistivity above Tc to 3D Gaussian fluctuation theory, it is possible to estimate the c-axis coherence length ξc, and it is shown that the planar hole concentration has a much stronger effect on ξc than had previously been thought. In addition, it is shown that overdoping Y1-xCaxBa2Cu3O7-δ causes the mixed-state resistivity to mainly shift rather than broaden in applied magnetic fields. Measurements of the c-axis magnetoresistance of single crystals of TI2Ba2CuO6+δ(TI-2201) reveal a striking four-fold angular dependence as the magnetic field is rotated in the ab-plane. A metallic c-axis resistivity (dρc/dT > 0) indicates that the out-of-plane transport may be coherent, and based on this assumption a simple model with an anisotropic relaxation time τ is developed. Calculations based on this model using Boltzmann transport theory are used to interpret the data and obtain values of the mean free path for carrier motion parallel and perpendicular to the CuO2 planes. Although the ab-plane value is consistent with other estimates, the c-axis mean free path is smaller than the distance between adjacent CuO2 planes (in the experimental temperature range). In addition, it is shown that changes in the anisotropy in τ with temperature cannot fully account for the observed temperature dependence of the in-plane Hall coefficient.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available