Theory of the spin gap in bilayer cuprates
Ioffe L. B., Larkin A. I., Millis A. J., Altshuler B. L.
A model of two planes of antiferromagnetically correlated electrons coupled together by a weak antiferromagnetic interaction of strength ñ has been formulated and solved. It is shown that in-plane antiferromagnetic correlations dramatically enhance the pairing effect of the interplane interaction. For the case in which the in-plane correlation length Ë~~È is ~ T~b we find that the interaction λ leads to spin pairing at a temperature Τ*~λ much higher than the usual BCS result, exp( —J/λ). We suggest that this is a possible explanation of the spin-gap effects observed below T*~ 150 ë in YBa2Cu306 6.