Magnetic Anisotropy in the High Transition Temperature Superconductor Yttrium BARIUM(2) COPPER(3) OXYGEN(7-DELTA)

The strong anisotropy found in the electronic structure of YBa_2Cu_3 O_{7-delta} resulting from the existence of both the CuO_2 planes within the a,b plane and the CuO chains along the b axis has brought strong interest regarding the effect of the electronic structure anisotropy on the superconducting properties in this material. Such superconducting properties include both the intrinsic type (such as H _{c1}, H_ {c2}, H_{ c}, lambda,xi ...) and the non-intrinsic type (such as flux pinning and j_{c} ). The availability of twinned and more recently, untwinned single crystals of YBa_2Cu _3O_{7-delta } has made it possible for such anisotropic properties to be investigated. Magnetization critical currents (j_{c}) were measured for H//c and H//a,b in order to investigate the characteristics of flux pinning. The temperature dependence of j_{c} was measured up to 77 K, to determine the viability of the material for application use at such high temperatures. Irradiation of the single crystals with high energy neutrons is also done to investigate the possibility of using such a method for increasing j_{c} . Intrinsic superconducting parameters are investigated by measuring the magnitude and anisotropy of the lower critical field H_{c1} and comparing it to the measurements of the upper critical field H_{c2} . From these values the magnitude and anisotropy of the other characteristic values such as lambda, kappa and H_{c} are estimated using an anisotropic Ginzburg -Landau model. The temperature dependence of H _{c1} was found to give unusual results, which differed with the direct temperature dependent measurements of the magnetic penetration depth lambda. The lower critical field was measured on an untwinned single crystal of YBa _2Cu_3O_ {7-delta}, to investigate both the a-b anisotropy and the effects of the twin boundaries on the measurements of H_{c1 }.