Enhanced superconducting gaps in the tri-layer high-$T_c$ cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$

TL;DR

This study uses angle-resolved photoemission spectroscopy to reveal that in the tri-layer cuprate Bi2223, the inner CuO2 plane exhibits a larger superconducting gap than the outer plane, with interlayer coupling enhancing the outer gap.

Contribution

It demonstrates the distinct doping levels and superconducting gaps of inner and outer CuO2 planes in Bi2223, highlighting the role of interlayer coupling and disorder effects.

Findings

Inner CuO2 plane is underdoped with a large gap (~60 meV)

Outer CuO2 plane is overdoped with a smaller gap (~43 meV)

Interlayer coupling enhances the outer plane's superconducting gap

Abstract

We have investigated the optimally doped tri-layer cuprate Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223) by angle-resolved photoemission spectroscopy, and observed energy bands and Fermi surfaces originated from the outer and inner CuO$_2$ planes (OP and IP) separately. The OP band is overdoped (hole density $\sim$ 0.26/Cu) with a large d-wave gap of $\Delta_0$ $\sim$ 43 meV while the IP is underdoped ($\sim$ 0.06/Cu) with an even large gap of $\Delta_0$ $\sim$ 60 meV. We propose that the enhancement of the $\Delta_0$ of IP is due to the minimal influence of out-of-plane disorder and that the OP gap is then enhanced through interlayer coupling, most likely a proximity effect from the IP.