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

We investigate the Raman spectra in the geometry where both incident and scattered photon polarizations are parallel to the (Formula presented) direction, for a plane-chain bilayer coupled via a single-particle tunneling (Formula presented) 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 (Formula presented) and interband (Formula presented) transitions. In the small-(Formula presented) limit appropriate for underdoped samples, 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 (Formula presented) is small, the Coulomb screening (Formula presented) 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 (Formula presented).