Anisotropy of Superconducting Energy Gap in Aluminium Determined by Ultrasonic Method

Abstract

The attenuation of 10 MHz ultrasound in aluminum has been measured by the pulse reflection method at temperatures between 0.4 and 1.2 K. Three cylindrical single-crystal specimens of high purity and perfection, having 99.9999% of purity and low density of dislocations, were used and their crystallographic orientations were [100], [110] and [111]. The temperature dependence of attenuation in the superconducting state was analyzed on the basis of the BCS theory, and the apparent superconducting energy gap was determined for the above three sound propagation directions. By using these results, the energy gap anisotropy in the electronic k-space was derived. The gap value ranged between 3.56 and 3.14 in units of (1/2)k_BT_c. The gap was largest for the [100] direction, somewhat smaller for the [110] direction, and especially small for the [111] direction. These conclusions were in agreement with existing experimental and theoretical results.