Angular Dependent Spectral-Weight Transfer and Evidence for Symmetry Broken In-Plane Charge Response in Ca$_{1.9}$Na$_{0.1}$CuO$_2$Cl$_2$

TL;DR

This study uses electron energy-loss spectroscopy to reveal momentum-dependent charge responses and potential nematic order in Ca$_{1.9}$Na$_{0.1}$CuO$_2$Cl$_2$, highlighting spectral-weight transfer and symmetry breaking in its electronic structure.

Contribution

It provides the first momentum-resolved evidence of spectral-weight transfer and symmetry breaking in the charge response of this cuprate, indicating fluctuating nematic order.

Findings

Presence of a plasmon peak along the diagonal of the BZ indicating metallic states.

Spectral-weight transfer from charge-transfer excitations to low-energy plasmon modes.

Evidence of symmetry breaking possibly due to fluctuating nematic order.

Abstract

We report about the energy and momentum dependent charge response in Ca$_{1.9}$Na$_{0.1}$CuO$_2$Cl$_2$ employing electron energy-loss spectroscopy. Along the diagonal of the Brillouin zone (BZ) we find a plasmon peak---indicating the presence of metallic states in this momentum region---which emerges as a consequence of substantial spectral-weight transfer from excitations across the charge-transfer (CT) gap and is the two-particle manifestation of the small Fermi pocket or arc observed with photoemission in this part of the BZ. In contrast, the spectrum along the [100]-direction is almost entirely dominated by CT excitations, reminiscent of the insulating parent compound. We argue that the observed polarization dependent shape of the spectrum is suggestive of a breaking of the underlying tetragonal lattice symmetry, possibly due to fluctuating nematic order in the charge channel. In addition we find the plasmon bandwidth to be suppressed compared to optimally doped cuprates.