Route to high-temperature superconductivity in composite systems

Apparently, some form of local superconducting pairing persists up to temperatures well above the maximum observed Tc in underdoped cuprates; i.e., Tc is suppressed due to the small phase stiffness. With this in mind, we consider the following question: Given a system with a high pairing scale Δ0 but with Tc reduced by phase fluctuations, can one design a composite system in which Tc approaches its mean-field value, Tc → TMF ≈ Δ0 /2 ? Here, we study a simple two-component model in which a "metallic layer" with Δ0 =0 is coupled by single-particle tunneling to a "pairing layer" with Δ0 >0 but zero phase stiffness. We show that in the limit where the bandwidth of the metal is much larger than Δ0, the Tc of the composite system can reach the upper limit Tc ≈ Δ0 /2.