Spin Proximity Effect

It is well known that spin-orbit coupling introduces a spin-triplet component into the spin states of a BCS superconductor. Since most superconductors are comprised of relatively heavy elements, spin-orbit coupling is always present. Indeed, there are only two well documented purely spin-singlet superconductors, Al and Be. These light elements have a negligible intrinsic spin-orbit scattering rate and a corresponding exponentially small spin susceptibility in the superconducting state. For this reason they make ideal candidates for studying the effects of interface spin-orbit coupling that is induced by impurity coatings, such as Au or Pt.

In the graphic on the left we depict one of several issues addressed in a series of experments in which the spin susceptibitlity of the superconducting state is probed via parallel field measurements on Be/Au bilayers. A small coverage of Au on the surface of a pure spin-singlet superconductor such as Be mixes the spin-states near the interface but has virtually no effect on the gap. Indeed, by the usual proximity effect we can assume the monolayer of Au is superconducting. One outstanding question is how does the superconductor accomodate a mixed-spin boundary condition at the interface with the spin-singlet eigenstates that exist well away from the interface. Clearly there must be a healing length, though little is known about its properties.

In the plot on the left we show the parallel critical field normalized by the gap value as a function of Be thickness for Be/Au bilayers (circles), a Be/Pb bilayer (crossed box), and pure Be films (triangles), all taken at 60 mK. The Au thickness is 0.5 nm in all the bilayers. The long dashed line represents the theoretical orbital critical field which is quite high due to the thinness of the films. Clearly the critical field of films with Be thicknesses less than
5 nm is Zeeman mediated, thus giving us a direct probe of the spin susceptibility of the system. The solid line is a guide to the eye. The horizontal dashed line represents the Clogston critical field.

The linear scaling behavior of the circular symbols is unusual and cannot be explained in terms of standard impurity scattering models. Note that the parallel critical field of the thinnest bilayers is almost an order of magnitude greater than the Clogston limit.

Read more:

Interface Spin Orbit Coupling Be/Au Bilayers