Superconductiong fluctuations in the magnetoconductivity of YBa2Cu3O7-[delta]and other high-temperature superconductors

by Axnäs, Johan

Superconducting uctuations | the traces of superconductivity abovethe transition temperature Tc | are important for a number of problems in hightemperature superconductors (HTS). For example, they provide a powerful tool to deduce information about microscopic parameters, such as the coherence lengths and the phase-breaking time. Fluctuations can also explain or contribute to the understanding of several anomalies that have been observed in HTS. In the present thesis the focus is on uctuation e ects on the magnetoconductivity [ (B) = (B) (0)] above Tc in elds up to B = 12 T, mainly in YBa2Cu3O7 , but also in Hg,Tl-based superconductors. The thesis includes both experimental and theoretical results and deals with several subjects: (i) The e ects of Zn and Fe doping of YBa2Cu3O7 . Magnetoconductivity experiments were used to study the evolution of phase-breaking time and coherence lengths with Zn doping. It was suggested that Zn causes pair breaking. For Fe doping, the standard theory could not reproduce experimental results, in particular an observed change of sign in about 5 K above Tc. (ii) Anomalies in the conduction perpendicular to Cu-O planes. We reported the rst observation of a sign change above Tc in the magnetoresistivity of YBa2Cu3O7 , and explained this as a contribution from uctuations in the normal quasiparticle density of states (DOS). We also pointed out problems with an unknown cuto in the theoretical calculations. This led theorists to propose a regularization, which we later applied. (iii) In-plane magnetoconductivity at temperatures far above Tc. We showed that literature data as well as our own data could be well described by uctuations alone, with DOS and Maki-Thompson (MT) terms included, in contrast to previous claims that MT terms were absent. De ciencies in uctuation theories at high temperatures were addressed in a theoretical article. (iv) In-plane anisotropy and the role of the chains in YBa2Cu3O7 . The in-plane magnetoconductivity for several di erent eld directions was measured. We found no signi cant anisotropy in the magnetoconductivity for B k crystal c axis, suggesting negligible uctuations in the chains. (v) Magnetoconductivity ofHg,Tl-1223 superconductors. Some of the rst results were reported, but any theoretical explanation seemed to require rather unexpected assumptions.