A Proximity Effect in Pure HTS/Ferromagnet Contacts

The proximity effect is theoretically studied for thin and massive pure ferromagnet/high-temperature superconductor (F/HTS) contacts. It is shown that there is no continuous matching between the s-wave and d-wave pair amplitudes at the HTS/F boundary. In thin HTS/F nanostructures, the exchange field strongly suppresses the averaged sand d-pairing and leads to the competition between BCS and FFLO scenarios of superconductivity. On the contrary, in massive HTS/F contact, the exchange field has no effect on the critical temperature T
_c
^d
of d-wave pairing. This is due to the lack of d-wave pairing in F layers, resulting in full internal reflection of d-wave pairs even from the perfectly transparent HTS/F boundary. The s-wave superconductivity localized at the HTS/F interface is predicted.Spontaneous symmetry change of the order parameter at the HTS/F interfaces from initially d-wave type to s-type becomes possible for massive HTS/F contact. It qualitatively agrees with experimental observations of the mixture of sand d-wave components at the Ag/BiSrCaCuO and Fe/Ag/BiSrCaCuO interfaces.