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

超导多层膜Nb/Nb_<1-X>Zr_X（x= 0.5 - 0.3）在平行上临界场H_&lt;c2//&gt;（T）中，在特征温度T^+下，随着T的降低，呈现出上弯的特征，这是由于有序参数DELTA（r）的形核位置从Nb层向Nb_<1-X>Zr_X层移动所致。我们的目的是研究涡旋结构，以了解该系统在混合态中的定域DELTA（r）的行为。为此，我们主要测量了Nb/NbZr（x= 0.5）的临界电流密度Jc随电场H和H与膜面夹角的变化。最有趣的特征是在T&gt;T^+（=0.7Tc）区域的Jc-H//曲线中观察到峰值效应，其中DELTA（r）局限于Nb层。然后，我们精确测量了Jc峰值Hp处场上方和下方Jc（H）的角度依赖性。在<c1>T&lt;T^+范围内，在H_（c2//）（T）曲线外推线附近的场H_和H_（c2//）（T）处，观察到Jc-H//曲线的两种扭折结构<L1><L2>。此外，从H_1和H_2周围平行场附近Jc角导数的测量结果<L1><L2>和涡旋态自由能的理论计算，我们揭示了在这些特征场中相变的发生。因此，在H_（c2//）&gt;H&gt;H_（c2//）范围内，提出了饼状涡旋态的可能性，<L1>在H&lt;H_（c2//）范围内，提出了阶梯状涡旋态<L2>的可能性，这一结果有助于对层状结构高温超导体的涡旋钉扎机制提供有效的信息。

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.