Anomalous magnetic field dependence of Y Ba2Cu3O7 - Delta d: effect of orthorhombic distortion

In this work an attempt has been made to ascertain the origin of the mixed symmetry state of the order parameters in the high-Tc superconductor Y Ba2Cu3O7 - Delta *d, with the contesting candidates being the orthorhombic distortion associated with the CuO2 planes and an s-wave order parameter component originating from the CuO chains present in the system. Due to its applicability over the entire range of applied magnetic field Hc1 < H < Hc2 the present theoretical model has been employed to study various unusual properties which are signatures of the unconventional pairing state symmetry of this material. The results presented in this paper and their agreement with the relevant experimental data suggest that the origin of the mixed symmetry state of pairing and the various experimentally observed unusual properties of Y Ba2Cu3O7 - Delta *d including the anomalous magnetic field dependence of the vortex lattice structure, unusual upward curvature of the Hc2(T) plot, etc, can be attributed to the orthorhombic distortion of the CuO2 planes present in this material. However, the effect discussed in the present work pertains to the orthorhombic distortion arising only through a second-order coupling; the distortion arising from other possible sources, namely anisotropic mixed gradient coupling, has not been taken into account here. Hence this discussion is restricted to a qualitative explanation of the effect due to orthorhombic distortion arising from the second-order coupling only.