Superconducting transition temperature, critical magnetic fields, and the structure of vanadium films

The superconducting transition temperature T/sub c/, the perpendicular critical magnetic field H/sup perpendicular//sub c/, the electrical resistance, and the structure of vanadium films obtained by ionic evaporation in an ultrahigh vacuum apparatus were investigated. A carbon sublayer and coating of 15 A thickness were applied in order to protect the samples against external influences. The critical fields were measured by the resistive method. Upon a decrease of film thickness from 2900 to 60 A a reduction of T/sub c/ from 5.1--5.2 to 2.4 K was observed, along with an increase of the residual resistivity rho/sub n/ from 4.5 to 20 ..mu..ohm-cm and an increase of the derivative vertical-bardH/sup perpendicular//sub c//dTvertical-bar near T/sub c/ from 3.6 to 6.7 kOe/K. No superconductivity was observed in a film of 30 A thickness down to 1.3 K. The type of crystal structure and the lattice constant for 125--1000 A thick films, determined by the technique of electron diffraction, are the same as those for bulk samples of vanadium; the grain size in these films amounted to 500 to 800 A. The electron density of states N (0), calculated on the basis of the data concerning vertical-bardH/sup perpendicular//sub c//dTvertical-bar and rho/sub n/ for thick filmsmore » (720 to 2900 A), agrees with the value of N (0) for bulk vanadium (if it is assumed that coefficient, which takes account of the correction due to strong coupling effects for H/sub c//sub 2/, is given by eta=1.2). Upon a reduction of the film thickness to 60 A, N (0) is reduced by approximately a factor of two. The results agree with the assumption that strong variations of N (0), having an influence on T/sub c/, can be observed in transition-metal films due to a reduction in the electron mean free path. The obtained quantitative information with regard to N (0) in thin vanadium films (d< or =250 A) is sensitive, however, to the presence of a hypothetical surface transition layer and to its properties and thickness.« less