VORTEX STATE OF A D-WAVE SUPERCONDUCTOR IN HIGH-T-C CUPRATES

We study theoretically the vortex state of a d-wave superconductor in a magnetic field parallel to the c axis. We find, except in the immedi ate vicinity of the superconducting transition temperature, that a squ are lattice tilted by pi/4 from the a axis gives the most stable confi guration, which provides the simplest interpretation of a recent small angle neutron scattering experiment by Keimer et al. and a more recen t scanning tunnel microscope experiment by Maggio-Aprile et al. The re sidual density of states is shown to follow a root B law in a wide fie ld region consistent with an earlier prediction by Volovik. The effect of d-s mixture is also studied. The most important effect appears to be a suppression of the upper critical field at low temperatures by 5- 10 %.