Ultrasonic attenuation in an orthorhombic anisotropic superconductor

We show that ultrasound attenuation in a clean two-dimensional orthorhombic superconductor can be used to get information separately, on band-structur e anisotropy, and on the relative magnitude of a subdominant s-wave compone nt to the dominant d-wave gap. An angular sweep as a function of orientatio n of the in-plane attenuated sound momentum will exhibit a twofold pattern. Using a simplified one-band model of the electronic structure with anisotr opic effective masses in the plane, we illustrate that the maximum in the a ttenuation occurs when the direction of the sound is perpendicular to the d irection of the Fermi velocity at the nodes. This is not the same as the di rection of the nodes in the gap:except for the case of a circular Fermi sur face. Nevertheless, with the same analysis, nodal directions can be determi ned as well as band and gap anisotropy parameters and the ratio of gap magn itude to Fermi energy. [S0163-1829(99)01545-3].