VORTEX STATE OF SUPERCONDUCTING Nb/Nb_<1-X>Zr_X MULTILAYERS

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The superconducting multilayr, Nb/Nb_<1-X>Zr_X (x=.5-.3), is known to show an upturn feature in the parallel upper critical field H_<c2//> (T) at the characteristic temperature T^+with decreasing T.This phenomenon occurs due to the shift of the nucleated position of the order parameterDELTA (r) from Nb to Nb_<1-X>Zr_Xlayrs.In this work, our aim is to investigate the vortex structure to understand the behavior of the localizedDELTA (r) in the mixed state of this system. For this purpose, we have measured mainly the critical current density Jc of Nb/NbZr (x=.5) as a function of the field H and the various angles between H and film plane. The mostly interesting feature is that the peak effect has been observed in Jc-H// curves in the region of T>T^+ (=0.7Tc) , in which DELTA (r) localizes in Nb-layrs. Then we have precisely measured the angular dependence of Jc (H) above and below the field at Jc-peak, Hp. From these investigations, we have found the pancake vortex structure in Hp<H<H_<c2//>and the stepwise vortex structure in H_<c1><H<Hp.On the other hand, in the region of T<T^+, two kinds of kink structure in Jc-H// curves were observed at fields H_<L1> and H_<L2> which were plotted around the extrapolated line from H_<c2//> (T) curve in the range of T>T^+. Furthermore, from the measured results of angular derivative of Jc in the vicinity of the parallel fields around H_<L1> and H_<L2> and the theoretical calculation concerning the free energy in the vortex state, we have exhibited the occurrence of the phase transition at these characteristic fields. Therefore, the possibikity of the pancake vortex state has been suggested in the region of H_<c2//>>H>H_<L1> and that of the stepwise vortex state has been done in the range of H<H_<L2>.The results of this work contribute to give the efficient information on the vortex pinning mechanism of High Tc superconductor with the layred structure.