UPWARD CURVATURE OF HC2 IN HIGH-TC SUPERCONDUCTORS - POSSIBLE EVIDENCE FOR S-D PAIRING

A mechanism is proposed to explain the upward curvature of the plot of thermodynamic Hc2 versus temperature in the high-Tc superconductors, as observed in dc magnetization measurements. If a multicomponent order parameter is assumed, the internal degrees of freedom of this parameter change along the Hc2(T) curve. In a superconductor with s-d mixing, a pure d wave at low fields will turn into a superposition of s and d at high fields, with a corresponding change in slope of Hc2. In a pure d-wave superconductor belonging to a two-dimensional representation, an orthorhombic distortion can produce the curvature. At low fields the slope of the curve is dominated by coupling to the orthorhombic distortion; at high fields the coupling of the supercurrent to the field is more important. Thus the curvature in Hc2(T) can be interpreted as evidence for d-wave pairing or for a mixture of s and d. The consequences of these two possibilities for other experiments are discussed. In particular, the mixed s and d picture can be reconciled with penetration-depth measurements, and therefore seems favored. © 1990 The American Physical Society.