Angular dependences of critical field of superconducting superlattices. Theory

The angular dependences of the upper critical field strength H(Θ) for a superlattice formed by thin superconducting layers with Josephson layers between them are studied theoretically. The function is singular in the low angle region and at all temperatures. Formulas are derived for the quantity β(H)=dH/d|Θ|Θ=Θ which characterizes the singularity. The form of the dependence H(Θ) for all angles in the region of ''two-dimensional'' behaviour of the superlattice (low temperatures) is elucidated, and it is shown that the variation of H(Θ) is more abrupt than expected from the Tinkham formula for thin films. The effect of nonideality of the superlattice structure on H(Θ) is investigated