New phase line in the mixed state of Nb/NbZr multilayers

The superconducting multilayer Nb/NbZr is known to show an upturn feature in the parallel upper critical field Hc2parallel(T) at a characteristic temperature T+ with decreasing T. This phenomenon occurs due to the change of the position for nucleation of order parameter Δ(r) from Nb to NbZr layers. In order to understand the behavior of the localized Δ(r) in the mixed state, the authors have measured the critical current density Jc of Nb/NbZr as a function of H and of angle between H and film plane. In this system, the peak effect has been reported in Jc(H) above T+ by previous work. In this work, besides this peak, two kinds of kink structures in Jc(H) are observed at fields HL1 and HL2 only in the temperatures below T+. HL1(T) and HL2(T) are plotted around the extrapolated line from Hc2parallel(T) above T+. From the field dependence of the angular derivative of Jc and the discussion about the free energy in the system, the authors will show the possibility of the phase transition at these characteristic fields