Surface Superconductivity with High Transition Temperatures in Layered Ca<sub><i>n</i></sub>B<sub><i>n</i>+1</sub>C<sub><i>n</i>+1</sub> Films
Liangliang Liu, Xiaohan Liu, Peng Song, Liying Zhang, Xiaowei Huang, Weifeng Zhang, Zhenyu Zhang, Yu Jia
Abstract
Proposed by Ginzberg nearly 60 years ago, surface superconductivity refers to the emergent phenomenon that the electrons on or near the surface of a material becomes superconducting despite its bulk is nonsuperconducting. Here, based on first-principles calculations within density functional theory, we predict that the superconducting transition temperature T c at the surfaces of Ca n B n +1 C n +1 ( n = 1, 2, 3, ...) films can be drastically enhanced to ∼90 K from 8 K for bulk CaBC. Our detailed analyses reveal that structural symmetry reduction at surfaces induces pronounced carrier self-doping into the surface B–C layer of the films and shifts the σ-bonding states toward the Fermi level; furthermore, the in-plane stretching modes of the surface layers experience significant softening. These two effects work collaboratively to strongly enhance the electron–phonon coupling, which in turn results in much higher T c values than the McMillian limit. These findings point to new material platforms for realizing unusually high- T c surface superconductivity.