Suppression of Spectral Gap and Flat Bands on a Cuprate Superconductor Side-Surface

TL;DR

This study uses angle-resolved photoemission to investigate the electronic structure of cuprate superconductor side surfaces, revealing that disorder suppresses flat bands and related orders, which are theoretically expected to be present.

Contribution

First momentum-resolved investigation of cuprate side surfaces showing disorder as the main factor suppressing flat bands and topological states.

Findings

Superconducting gap is suppressed on the side surface.

Expected zero-energy flat band peaks are also suppressed.

Bulk inhomogeneities broaden flat-band states beyond detection.

Abstract

Side surfaces of cuprate superconductors are expected to display a suppressed $d$-wave order parameter and zero-energy topological flat bands with a large density of states, making them susceptible to symmetry broken orders. Yet such surfaces have never been investigated with momentum-resolved, surface-sensitive probes, because high-temperature superconductors rarely cleave along them. Using focused-ion-beam milling to define a controlled breaking point, we expose pristine (110) side surfaces of overdoped La$_{2-x}$Sr$_x$CuO$_4$ ($x=0.22$) suitable for angle-resolved photoemission. We observe the suppression of the superconducting spectral gap within our energy resolution ($\sim 4~\mathrm{meV}$), and surprisingly, the expected zero-energy flat band peak is also suppressed, despite the high topographic quality of the surface. Self-consistent Bogoliubov--de~Gennes calculations show that the measured geometric roughness of the cleaved surface is too weak to eliminate these modes. The calculations further demonstrate that bulk inhomogeneities characteristic of high-temperature superconductors, modelled as moderate Anderson-type disorder, can broaden the flat-band states beyond detectability. Our results provide the first momentum-resolved view of the electronic structure on a cuprate side surface and reveal disorder as the key factor currently preventing appearance of flat bands and their associated correlated orders.