Plasmons and Interband Transitions of Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$ investigated by Electron Energy-Loss Spectroscopy

TL;DR

This study uses electron energy-loss spectroscopy to analyze the electronic structure and elementary excitations of Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$, revealing anisotropic loss functions and a linear plasmon dispersion linked to its unique ladder structure.

Contribution

It provides new insights into the anisotropic electronic excitations and plasmon behavior in Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$, highlighting the relation to its band structure and underdoped cuprate properties.

Findings

Strong anisotropy in loss function along different crystallographic axes

Observation of a linear plasmon dispersion parallel to ladder legs

Charge carrier plasmon position matches band structure predictions

Abstract

Electron energy-loss spectroscopy studies have been performed in order to get a deeper insight into the electronic structure and elementary excitations of the two-leg ladder system Ca$_{11}$Sr$_3$Cu$_{24}$O$_{41}$. We find a strong anisotropy of the loss function for momentum transfers along the a and c-crystallographic axis, and a remarkable linear plasmon dispersion for a momentum transfer parallel to the legs of the ladders. The investigated spectral features are attributed to localized and delocalized charge-transfer excitations and the charge carrier plasmon. The charge carrier plasmon position and dispersion in the long wave-length limit agree well with expectations based upon the band structure of the two-leg ladder, while the observed quasi-linear plasmon dispersion might be related to the peculiar properties of underdoped cuprates in general.