Anisotropy in quasiparticle density of states in the vortex state of an orthorhombic d-wave superconductor

The quasiparticle density of states is calculated in, the vortex: state of an orthorhombic d-wave superconductor as a function of the orientation of t he external magnetic field (H) in the CuO2-plane. Because of the existence of chains oriented along the y-axis in YBCO, its in-plane properties are hi ghly anisotropic and in the superconducting state a predominantly d(x2-y2) order parameter will also contain a subdominant s-wave admixture allowed by the orthorhombic crystal space group. In a simple anisotropic effective ma ss model for the band structure and a dfs gap we compute the quasiparticle density of states as a function of the orientation (gamma) of H with respec t to the a-axis in the CuO2 plane. As a function of gamma we find a two fol d pattern of behavior with minima at the angles where the Fermi velocity at the nodal momentum is in the direction of the external field. For an untwi nned single crystal the resulting anisotropy of the specific heat at low te mperatures should be measurable. It is greatly reduced in twinned samples.