Quantum Monte Carlo Study of Superconductivity in a Composite Bilayer System

The proposition that the superconducting transition temperature Tc can be enhanced in a composite system by coupling a superconductor with a large pairing scale but strong phase fluctuations to a metal remains an interesting topic to date. To validate this prediction, we use the Determinant Quantum Monte Carlo(DQMC) method to study the composite bilayer system of a negative-U Hubbard layer coupled with a metal(U=0) layer on a square lattice. In the regime of U~W where W is the bandwidth of the metal layer, a previous study with the Dynamical Cluster Approximation(DCA) shows that T_c is enhanced for intermediate inter-layer hopping t_⊥ because of an increase in the superfluid stiffness despite a reduction in the pairing interaction. However, the DCA result depends sensitively on the cluster size for small t_⊥, makes estimations of T_c less precise. The DQMC method is sign problem free for the model, and provides an accurate measurement of physical quantities on finite clusters. By calculating the superfluid stiffness and the pairing correlations, we estimate T_c and compare with the DCA result.