MAGNETIC-FIELD SUPPRESSION OF SUPERCONDUCTIVITY IN LAYERED HIGH-T-C MATERIALS

The dependence of the superconducting transition temperature in GdBa2C u3O6+delta single crystals on the magnetic field H applied parallel to the intercrystalline layers is studied experimentally. A nontrivial d ependence of the superconducting transition temperature on the field H is explained by magnetic field penetration into high-T-c single cryst als. The presence of a layered structure leads to the formation of a s patially periodic magnetic structure with a period varying discretely in fields H much greater than H-c1 (H-c1 is the lower critical field). Any change in the magnetic structure results in a nonmonotonic (jumpw ise) dependence of the superconducting transition temperature on the f ield H. The effect is estimated numerically by using the results of ca lculation of the critical temperature of small samples in strong magne tic fields. (C) 1998 American Institute of Physics.