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 ${\text{Ca}}_{11}{\text{Sr}}_3{\text{Cu}}_{24}{\text{O}}_{41}$. We find a strong anisotropy of the loss function for momentum transfers along the $a$ and $c$ crystallographic axes, 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-wavelength limit agree well with expectations based upon the band structure of the two-leg ladder while the observed quasilinear plasmon dispersion might be related to the peculiar properties of underdoped cuprates in general.

• Received 15 October 2010

DOI:https://doi.org/10.1103/PhysRevB.82.245110