Acoustic plasmons and conducting carriers in hole-doped cuprate superconductors

TL;DR

This study provides the first spectroscopic evidence of acoustic plasmons and conducting holes in hole-doped cuprate superconductors, clarifying the charge excitation behavior in these materials.

Contribution

It demonstrates the existence of acoustic plasmons and coherent hole carriers in hole-doped cuprates using momentum-resolved RIXS measurements and theoretical calculations.

Findings

Detection of acoustic plasmons in hole-doped cuprates

Evidence of coherent conducting holes in these materials

Reconciliation of previous experimental discrepancies

Abstract

The superconductivity of cuprates, which has been a mystery ever since its discovery decades ago, is created through doping electrons or holes into a Mott insulator. There, however, exists an inherent electron-hole asymmetry in cuprates. The layered crystal structures of cuprates enable collective charge excitations fundamentally different from those of three-dimensional metals, i.e., acoustic plasmons. Acoustic plasmons have been recently observed in electron-doped cuprates by resonant inelastic X-ray scattering (RIXS); in contrast, there is no evidence for acoustic plasmons in hole-doped cuprates, despite extensive measurements. This contrast led us to investigate whether the doped holes in cuprates La$_{2-x}$Sr$_x$CuO$_4$ are conducting carriers or are too incoherent to induce collective charge excitation. Here we present momentum-resolved RIXS measurements and calculations of collective charge response via the loss function to reconcile the aforementioned issues. Our results provide unprecedented spectroscopic evidence for the acoustic plasmons and long sought conducting p holes in hole-doped cuprates.