Three dimensional collective charge excitations in electron-doped cuprate superconductors

TL;DR

This study uses resonant inelastic x-ray scattering to reveal three-dimensional collective charge excitations, specifically acoustic plasmons, in electron-doped cuprate superconductors, highlighting the role of out-of-plane charge dynamics in high-temperature superconductivity.

Contribution

It provides the first direct characterization of out-of-plane charge dynamics and identifies the acoustic plasmon as a key excitation in layered cuprates.

Findings

Discovered a dispersive collective mode with 3D character.

Mode periodicity matches interplane Coulomb interactions.

Supports the role of acoustic plasmons in superconductivity.

Abstract

High temperature cuprate superconductors consist of stacked CuO2 planes, with primarily two dimensional electronic band structures and magnetic excitations, while superconducting coherence is three dimensional. This dichotomy highlights the importance of out-of-plane charge dynamics, believed to be incoherent in the normal state, yet lacking a comprehensive characterization in energy-momentum space. Here, we use resonant inelastic x-ray scattering (RIXS) with polarization analysis to uncover the pure charge character of a recently discovered collective mode in electron-doped cuprates. This mode disperses along both the in- and, importantly, out-of-plane directions, revealing its three dimensional nature. The periodicity of the out-of-plane dispersion corresponds to the CuO2 plane distance rather than the crystallographic c-axis lattice constant, suggesting that the interplane Coulomb interaction drives the coherent out-of-plane charge dynamics. The observed properties are hallmarks of the long-sought acoustic plasmon, predicted for layered systems and argued to play a substantial role in mediating high temperature superconductivity.