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

Abstract

We investigate the Raman spectra in the geometry where both incident and scattered photon polarizations are parallel to the z⁁ direction, for a plane-chain bilayer coupled via a single-particle tunneling t⊥. 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 (∝t⊥4) and interband (∝t⊥2) transitions. In the small-t⊥ 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 t⊥ is small, the Coulomb screening (∝t⊥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⊥.

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