FOURFOLD SYMMETRY OF VORTEX STATE OF D-WAVE SUPERCONDUCTIVITY

Fourfold symmetry inherent in d-wave superconductivity is most clearly seen in the presence of a magnetic field. In a magnetic field paralle l to the c axis a square vortex lattice tilted by pi/4 from the a axis is shown to be the most stable for T < 0.88T(c). Such a square lattic e, though elongated in the b direction, is seen recently by a small an gle neutron scattering experiment and by STM imaging both in YBCO mono crystals. When the magnetic field B is in the a-b plane, the upper cri tical field and the torque in the a-b plane exhibit fourfold symmetry, which is seen by a recent torque magnetometry again in a YBCO monocry stal. Also the electron density of states at low temperatures reflects this fourfold symmetry even in a low magnetic field.