Optimizing the Critical Temperature and Superfluid Density of a Metal-Superconductor Bilayer

TL;DR

This study uses numerical simulations to show that hybridizing a strongly interacting superconductor with a metal can enhance its critical temperature and superfluid density, with optimal effects at specific interaction strengths.

Contribution

It provides numerical evidence that metal-superconductor bilayers can boost $T_c$ through hybridization, especially in the strongly interacting regime, confirming theoretical predictions.

Findings

Increasing interlayer hybridization can nonmonotonically increase $T_c$ in strongly interacting regimes.

Optimal $T_c$ is achieved at a specific hybridization level, comparable to single-layer maximum.

Enhancement in superfluid stiffness underpins the $T_c$ improvements.

Abstract

A promising path to realizing higher superconducting transition temperatures $T_c$ is the strategic engineering of artificial heterostructures. For example, quantum materials could, in principle, be coupled with other materials to produce a more robust superconducting state. In this work, we add numerical support to the hypothesis that a strongly interacting superconductor weakened by phase fluctuations can boost its $T_c$ by hybridizing the system with a metal. Using determinant quantum Monte Carlo (DQMC), we simulate a two-dimensional bilayer composed of an attractive Hubbard model and a metallic layer in two regimes of the interaction strength $-|U|$. In the strongly interacting regime, we find that increasing the interlayer hybridization $t_\perp$ results in a nonmonotonic enhancement of $T_c$, with an optimal value comparable to the maximum $T_c$ observed in the single-layer attractive Hubbard model, confirming trends inferred from other approaches. In the intermediate coupling regime, when $-|U|$ is close to the value associated with the maximum $T_c$ of the single-layer model, increasing $t_\perp$ tends to decrease $T_c$, implying that the correlated layer was already optimally tuned. Importantly, we demonstrate that the mechanism behind these trends is related to enhancement in the superfluid stiffness, as was initially proposed by Kivelson [Physica B: Condensed Matter 318, 61 (2002)].