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.