Effect of Elastic Strain on the Superconducting Critical Temperature of Evaporated Tin Films

Abstract

The superconducting critical temperature of a large number of tin films on soda lime glass substrates are reported. The critical temperatures depend upon the stress arising in the films when they are constrained to follow the contraction of the glass on cooling to liquid helium temperature. It is shown that the crystalline orientation of the films is significant in determining the magnitude of the stress and the variation in critical temperature relative to bulk tin. For films with the tetrad axis in the plane, the stresses along that axis at helium temperature can reach 4.5×103 atm and the critical temperature may increase by 0.23°K. For both diad axes in the plane, the stress and the change in critical temperature is much less. The piezoresistivity constants along the tetrad and diad axes of single crystals were measured, π33 = 9.8±0.5ρ3×10−6/atm, π11 = −7.0±0.6ρ1×10−6/atm. The temperature dependent part of the resistivity of films at 77°K is increased above the values of bulk metal in accordance with a piezoresistive effect. For films much thinner than 300A, the critical temperature is observed to increase with decreasing thickness. For these thin films the constraint of thermal contraction imposed by the substrate appears to be insufficient to provide the entire increase in critical temperature.