Outer layers determine the parallel critical field of a superconducting multilayer

It is shown that the behavior of the parallel critical field for superconducting multilayers with two superconducting components is crucially determined by the outer layers. If the otuer layers sustain surface superconductivity not only the three-dimensional (3D) behavior at low temperatures and high fields but, more importantly, the crossover temperature from 2D to 3D behavior is changed. This phenomenon falls outside the Takahashi-Tachiki theory for such multilayers, but a qualitative explanation is given. Also, if no surface superconductivity occurs, the critical field in the 2D region is due to the outer layers and different from that of the inside layers