Zeeman and orbital effects of an in-plane magnetic field in cuprate superconductors

We discuss the effects of a magnetic field applied parallel to the Cu-O (ab ) plane of the high T-c cuprate superconductors. After briefly reviewing th e Zeeman effect of the field, we study the orbital effects, using the Lawre nce-Doniach model for layered superconductors as a guide to the physics. We argue that the orbital effect is qualitatively different for in-plane and interlayer mechanisms for superconductivity. In the case of in-plane mechan isms, interlayer couplings may be modeled as a weak interlayer Josephson co upling, whose effects disappear as H --> infinity; in this case Zeeman domi nates the effect of the field. In contrast, in the interlayer mechanism the Josephson coupling is the driving force of superconductivity, and we argue that the in-plane field suppresses superconductivity and provides an upper bound for H-c2 which we estimate very crudely. (C) 2000 American Institute of Physics. [S0021-8979(00)33208-X].