Intra- versus interlayer pairing in copper oxide superconductors: Response to a magnetic field

We have investigated the response of layered superconductors to an external magnetic field using the semiclassical phase approximation. The linearized gap equations have been derived, and solved numerically to calculate the upper critical field Hc2(T) for layered superconductors with one or two layers per unit cell for both intralayer s-wave and interlayer BCS-like pairing mechanisms. In the weak-hopping limit the equations reduce to the Lawrence-Doniach form, and for general hopping the appropriate gap equations are derived and numerically analyzed. One encounters the familiar dimensional crossover in the Hc2(T) curve for weak hopping. A different type of dimensional crossover can occur in the two-layer case with unequal intralayer or interlayer coupling strengths, such that at the crossover temperature, the magnetic field suppresses the superconductivity in the weakly coupled layers while leaving the strongly coupled layers superconducting. The effect is enhanced by unequal hopping strengths. The flux lattice consists of alternating superconducting and normal layers