VORTEX STATE IN D-WAVE SUPERCONDUCTIVITY

The vortex state in d-wave superconductivity exhibits a number of surp rising features. In a magnetic field B parallel to c we find a square vortex lattice tilted by pi/4 from the a-axis is the most stable. Such a vortex lattice though elongated in the b-direction has been seen in a recent small angle neutron scattering experiment and STM imaging bo th on YBCO monocrystals. When B is in the a-b plane the upper critical field exhibits the fourfold symmetry in theta, where theta is the ang le B makes from the a-axis. Again this fourfold symmetry is seen recen tly by a sensitive torque measurement in an untwinned YBCO monocrystal .