C-axis Raman spectra of a normal plane-chain bilayer cuprate and the pseudogap

TL;DR

This study analyzes the Raman spectra of a bilayer cuprate to understand how interband and intraband transitions, influenced by interlayer coupling and Coulomb interactions, reveal pseudogap features.

Contribution

It derives the Raman vertex in a bilayer cuprate model and examines the effects of interlayer tunneling and Coulomb screening on the spectra.

Findings

Interband transitions dominate Raman spectra at small interlayer tunneling.

Coulomb interactions minimally affect interband contributions when tunneling is small.

Total Raman spectra strongly depend on the interlayer coupling strength.

Abstract

We investigate the Raman spectra in the geometry where both incident and scattered photon polarizations are parallel to the $\hat{z}$-direction, for a plane-chain bilayer coupled via a single-particle tunneling $t_\perp$. The Raman vertex is derived in the tight-binding limit and in the absence of Coulomb screening, the Raman intensity can be separated into intraband ($\propto t_\perp^4$) and interband ($\propto t_\perp^2$) transitions. In the small-$t_\perp$ limit, the interband part dominates and a pseudogap will appear as it does in the conductivity. Coulomb interactions bring in a two-particle coupling and result in the breakdown of intra- and interband separation. Nevertheless, when $t_\perp$ is small, the Coulomb screening ($\propto t_\perp^4$) has little effect on the intensity to which the unscreened interband transitions contribute most. In general, the total Raman spectra are strongly dependent on the magnitude of $t_\perp$.