Enhanced superconducting gap in the outer CuO$_2$ plane of the trilayer cuprate (Hg,Re)Ba$_2$Ca$_2$Cu$_3$O$_{8+\delta}$

TL;DR

This study uses angle-resolved photoemission spectroscopy to reveal that the outer CuO$_2$ plane in a high-$T_c$ Hg-based trilayer cuprate exhibits an unusually large superconducting gap, which is key to its record transition temperature.

Contribution

First momentum-resolved observation of superconducting gaps in both inner and outer CuO$_2$ planes of a Hg-based trilayer cuprate, highlighting the significance of the outer plane's enhanced pairing energy.

Findings

Outer CuO$_2$ plane has a larger superconducting gap than inner plane.

Superconducting gap in the inner plane is comparable to lower-$T_c$ cuprates.

Enhanced pairing in the outer plane is crucial for the high $T_c$ in this material.

Abstract

We report the first observation of a momentum-resolved superconducting gap in the Hg-based trilayer cuprate, which holds the highest record of superconducting transition temperature ($T_\mathrm{c}$) at ambient pressure. By angle-resolved photoemission spectroscopy utilizing a micro-focused beam, clear quasiparticle dispersions originating from the inner and outer CuO$_2$ planes (IP and OP, respectively) were separately identified. The magnitude of the superconducting gap for the IP was comparable to that of the Bi-based trilayer cuprate with a lower $T_\mathrm{c}$. In contrast, the superconducting gap for the OP was significantly larger than that of the Bi-based one. While strong pairing in the IP has been highlighted as the key element of trilayer cuprates, the present results suggest that the enhanced pairing energy in the OP is essential for the highest $T_\mathrm{c}$ at ambient pressure realized in the Hg-based trilayer cuprate.