Challenging the nature of low energy plasmon excitations in CaC$_6$ using electron energy-loss spectroscopy

TL;DR

This study uses electron energy-loss spectroscopy to investigate low energy plasmon excitations in CaC$_6$, revealing hybridization effects and challenging existing theories about its superconducting coupling mechanisms.

Contribution

It provides experimental evidence of intraband and acoustic plasmons in CaC$_6$, and questions the role of electronic mechanisms in its superconductivity.

Findings

Observation of an intraband plasmon at 3.5 eV with negative dispersion

Detection of a weak, linearly dispersing excitation around 1.2 eV

Optical limit at ~1 eV contradicts theoretical predictions

Abstract

The nature of low energy plasmon excitations plays an important role in understanding the low energy electronic properties and coupling mechanism of different superconducting compounds such as CaC$_6$. Recent ab-initio studies predict a charge carrier intraband plasmon in keeping with a low energy acoustic plasmon. Here, we have studied the low-energy electronic excitations of CaC$_6$ using high-resolution electron energy-loss spectroscopy in transmission at low temperatures. The analysis of the core-level excitations leads to the conclusion that hybridization between graphite and calcium states plays an essential role in this graphite intercalated compound. Regarding the low energy plasmon excitation, we observe the formation of an intraband (charge carrier) plasmon with a negative dispersion at about 3.5 eV in sound agreement with the theory. Finally, a weak excitation around 1.2 eV with an almost linear dispersion relation can be observed as predicted for an acoustic plasmon that may mediate the superconducting coupling in CaC$_6$. However its optical limit at ~1 eV challenges the theoretical predictions and safely rules out an electronic superconducting coupling mechanism in CaC$_6$.