Magnetic-field effects on the density of states of orthorhombic superconductors

The quasiparticle density of states in a two-dimensional d-wave superconduc tor depends on the orientation of the in-plane external magnetic field H. T his is because, in the region of the gap nodes, the Doppler shift due to th e circulating supercurrents around a vortex core depends on the direction o f H. For a tetragonal system the induced pattern is fourfold symmetric and, at zero energy, the density of states exhibits minima along the node direc tions. But YBa2C3O6.95 is orthorhombic because of the chains and the patter n becomes twofold symmetric with the position of the minima occurring when H is oriented along the Fermi velocity at a node on the Fermi surface. The effect of impurity scattering id Born's and unitary limits is discussed. [S 0163-1829(99)03941-7].