MRSej, Vol. 6, No.1, pp.103-118, 2004

NONUNIFORM SUPERCONDUCTING STATES
AND UMKLAPP PROCESSES
IN FERROMAGNET–SUPERCONDUCTOR NANOSTRUCTURES

M.G. Khusainov

Kazan state university, Kazan, Russia

Kazan state technical university, branch "Vostok", Chistopol', Russia

For the layered ferromagnetic metal/superconductor (FM/S) structures new boundary-value problem, which takes into account a competition between the one-dimensional (1D) and three-dimensional (3D) realizations of the Larkin-Ovchinnikov-Fulde-Ferrell states, is derived. Superconductivity in the FM/S structures proves to be a superposition of the BCS pairing with zero total momentum in the S layers and the FFLO paring with nonzero pair momentum k in the FM layers. It is shown that processes of transition and mutual transformation of the BCS and FFLO pairs at the FM/S boundary occur as the Umklapp processes during which the coherent pair momentum k is conserved with exactness up to the reciprocal LOFF lattice vector G . These Umklapp processes can occur both in normal (1D states) and in the tangent (3D states) directions with respect to the FM/S interface. It is found that nonmonotonic behavior of the critical temperature Tc in the FM/S billayers is caused by the oscillations of the Cooper pairs flux through the S/FM boundary due to 3D-1D-3D phase transition cascade and switching between normal and tangent Umklapp processes. For the FM/S superlattices the existence of new $\pi$-magnetic 0$\pi$ and $\pi\pi$ LOFF states, which at certain conditions can have a much higher Tc than earlier known 0-magnetic 00 and $\pi$0 LOFF states, is discovered. The Tc nonmonotony in the FM/S superlattices may be due to the 3D(0$\pi$)-1D( $\pi\pi$)-3D( $\pi\pi$) phase transitions cascade at small S interlayer thickness or due to another chain of the 3D(00)-1D($\pi$0)-3D($\pi$0) transitions at larger S interlayer thickness.