Atomic-layer-resolved composition and electronic structure of the cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ from soft x-ray standing-wave photoemission

TL;DR

This study uses soft x-ray standing-wave photoemission to achieve atomic layer-specific insights into the composition and electronic structure of the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\\delta}$, revealing detailed layer interactions.

Contribution

It introduces a novel application of standing-wave photoemission for layer-resolved analysis of cuprates, providing new insights into their atomic composition and electronic interactions.

Findings

Ca presence detected in SrO layers

Layer-resolved valence structure influenced by supermodulation

Layer composition and electronic structure clarified

Abstract

A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO$_2$ layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-atomic layer depth resolution of these properties. Applying SW photoemission to Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the supermodulation structure--as determined by comparison to DFT calculations, by Ca-Sr intermixing, and by the Cu 3d-3d Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Measurements of this type for other quasi-two-dimensional materials with large-c represent a promising future direction.