Resistive transitions in a magnetic field and critical field anisotropy in metal-oxide compounds
Abstract
Resistive transitions R(T) in a magnetic field and the upper critical field are studied in YBa2Cu3Ox. and LaSrCuO4 polycrystals grown from the melt and by the ceramic technique. It is found that for samples grown from the melt, the dependences R(T) in fairly high fields can be fitted into a single universal curve described by the percolation theory for polycrystals with a random angular distribution of anisotropy axes. A method is proposed for eliminating the weak bond effect in ceramic samples on the Hc2(T) dependences being measured. This procedure helps remove the irreproducibility of Hc2 values caused by a difference in the microstructure of samples prepared by different methods. The results of the percolation theory are used in the experimental determination of parameters dHc2‖/dT and dHc2⊥/dT characterizing the anisotropy in the single crystals of the above-mentioned high-temperature superconductors. The magnetic-field induced anisotropy in the resistance of polycrystals is studied in the region of the superconducting transition. It is found that this anisotropy is related to the anisotropy in the normal resistance of single-crystal grains constituting the polycrystal.