Resistive transitions at magnetic field and critical field anisotropy in metall-oxide compounds

Resistive transitions R(T) at magnetic field and the upper critical field are studied on YBa2Cu3Ox and LaSrCuO4 polycrystals grown by using the melt and ceramics techniques. It is found that for the melt samples in fairly high fields the dependences R(T) can be filled in the unique universal curve described by percolated theory for polycrystals with a random distribution of anisotropy axes in angle. A procedure is proposed which makes it possible to eliminate the effect of weak bonds in the ceramics samples on the dependences Hc2(T) to be measured. The procedure permits the irreproductibility of Hc2(T)-values to be resolved which can be attributed to the difference in microstructure of the samples prepared by different techniques. By using percolated theoretical results, the parameters dHcparallelorparallel/dT and dHcperpendicularorperpendicularto/dT featuring the anisotropy in single crystals of the above high-temperature compounds are measured. The magnetic field-induced anisotropy of the polycrystal resistance in the superconducting transition region is studied. It is related to the anisotropy of normal resistance of single-crystal grains that constitute a polycrystal