Route to high-temperature superconductivity in composite systems

Apparently, some form of local superconducting pairing persists up to temperatures well above the maximum observed T_c in underdoped cuprates; i.e., T_c 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 T_c reduced by phase fluctuations, can one design a composite system in which T_c approaches its mean-field value, T_c→T_MF≈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 T_c of the composite system can reach the upper limit T_c≈Δ₀/2.