Enhanced Superconducting Diode Effect due to Coexisting Phases

The superconducting diode effect refers to an asymmetry in the critical supercurrent $J_c\left(\hat{n}\right)$ along opposite directions, $J_c\left(\hat{n}\right)\ne J_c(-\hat{n})$. While the basic symmetry requirements for this effect are known, it is, for junction-free systems, difficult to capture within current theoretical models the large current asymmetries $J_c\left(\hat{n}\right)/J_c(-\hat{n})$ recently observed in experiment. We here propose and develop a theory for an enhancement mechanism of the diode effect arising from spontaneous symmetry breaking. We show—both within a phenomenological and a microscopic theory—that there is a coupling of the supercurrent and the underlying symmetry-breaking order parameter. This coupling can enhance the current asymmetry significantly. Our work might not only provide a possible explanation for recent experiments on trilayer graphene but also pave the way for future realizations of the superconducting diode effect with large current asymmetries.