Nodeless high-T$_c$ superconductivity in highly-overdoped monolayer CuO$_2$

TL;DR

This paper demonstrates that a monolayer CuO$_2$ grown on a cuprate superconductor exhibits nodeless high-temperature superconductivity due to a unique two-orbital electronic structure, suggesting new pathways for high-$T_c$ superconductor design.

Contribution

It reveals that heavily overdoped monolayer CuO$_2$ can host nodeless $s$-wave superconductivity driven by spin-orbital interactions, a novel finding in cuprate research.

Findings

Heavily overdoped CuO$_2$ monolayer has a unique two-orbital Fermi surface.

The superconductor exhibits extended $s$-wave pairing symmetry.

Pairing gap size is comparable to bulk $d$-wave gap.

Abstract

We study the electronic structure and superconductivity in CuO$_2$ monolayer grown recently on $d$-wave cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. Density functional theory calculations indicate significant charge transfer across the interface such that the CuO$_2$ monolayer is heavily overdoped into the hole-rich regime yet inaccessible in bulk cuprates. We show that both the Cu $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals become important and the Fermi surface contains one electron and one hole pocket associated with the two orbitals respectively. Constructing a minimal correlated two-orbital model for the $e_g$ complex, we show that the spin-orbital exchange interactions produce a nodeless superconductor with extended $s$-wave pairing symmetry and a pairing energy gap comparable to the bulk $d$-wave gap, in agreement with recent experiments. The findings point to a direction of realizing new high-$T_c$ superconductors in ozone grown transition-metal-oxide monolayer heterostructures.