WHEN measuring the critical currents in thin films with a view to making comparisons with theoretical predictions, it is advisable to use specimens with 'compensated geometry'. In this way there are no additional uncertainties due to non-uniformity of the current density distribution. This property of the geometry is difficult to adopt in the case of alloy films because the usual method of fabrication, which consists of rotating a cylindrical substrate in the vapour, would probably give rise to a concentration gradient in the sample. In view of this, the present work, which sets out to evaluate the current carrying ability of thin strips, has made reference to the theory with the use of work by Bowers. 1 In this the current distribution across a film strip is estimated.
Previous measurements on critical currents in thin films have been rather inconclusive although a tendency exists for the Ginzburg-Landau 2 theory to be favoured. In support of this theory, one can refer to the d.c. experiments of Ginzburg and Shal'nikov 3 who used compensated geometry and obtained magnitudes for the current which were a factor of 5 below the theoretical predictions. With similar geometry, only this time using 10 -9 s pulses, Hagedorn 4 obtained agreement with the Silsbee criterion using the critical field of the bulk material. These two results imply different dependencies upon temperature, the former indicates that je, the critical current density, varie~ near Te as AT 3/2, where AT = Te -T, while the latter provides a linear dependence. It should be pointed out here, however, that only a few of Hagedorn's results were obtained near Te. Measurements by Mercereau and Hunt s have further indicated the three-halves dependence to be applicable, but in this experiment the specimen was a cylinder trapping flux so that uniform current distribution is not obvious. Some work on film strips by Glover and Coffey 6 in which corrections were applied for non-uniform current flow also claim to support the Ginzburg-Landau dependence.
In alloy films, with the compensated geometry arrangement, Meikeljohn 7 obtained a temperature dependence of the critical current which varied with the alloy concentration. This feature is not expected theoretically and is rather suggestive of the difficulties t At present: l~cole de Physique, Universit6 de Gen6ve, Switzerland.