TEMPERATURE AND MAGNETIC-FIELD DEPENDENCE OF THE SUPERCONDUCTING ORDER-PARAMETER IN ZN STUDIED BY POINT-CONTACT SPECTROSCOPY

The differential resistance dV/dI vs voltage V of Zn-Ag point contacts (PC's) is measured in the superconducting state to evaluate the tempe rature and magnetic-field dependence of the superconducting order para meter Delta and Gamma = (h) over bar/tau in Zn using the modified Blon der-Tinkham-Klapwijk theory including a finite quasiparticle lifetime tau. The temperature dependence of Delta(T) of all PC's follows the BC S theory while for the magnetic field dependence of Delta(B) there are marked differences among different PC's. In one case the Delta(B) dep endence shows a first-order transition at B-c(PC)<B-c(bulk) for other PC's Delta(B) decreases continuously in a magnetic field and vanishes at B-c(PC) much greater than B-c(bulk). The parameter Gamma which desc ribes the broadening of the quasiparticle density of states (DOS) in t he superconductor is almost independent of B in the first case, in con trast to the increase in a magnetic field in the second case. Moreover , in zero field Gamma increases with PC resistance. Possible reasons f or the DOS broadening in zero field and with increasing magnetic field or increasing PC resistance are discussed.