FOURFOLD SYMMETRY OF D-WAVE SUPERCONDUCTIVITY

Fourfold symmetry latent in d-wave superconductivity is revealed clear ly in the vortex state. In a magnetic field (B) over right arrow paral lel to the (c) over right arrow-axis, a square lattice tilted by pi/4 from the (a) over right arrow-axis is the most stable for T <0.8T(c). Such a square lattice, though elongated in the (b) over right arrow-di rection, is seen recently by a small angle neutron scattering experime nt and by STM imaging both in YBCO. When (B) over right arrow is in th e a-b plane, the upper critical field and other physical properties ex hibit fourfold symmetry as function of theta, the angle which (B) over right arrow makes from the (a) over right arrow-axis.