Optical response of high-$T_c$ cuprates: possible role of scattering rate saturation and in-plane anisotropy

Abstract

We present a generalized Drude analysis of the in-plane optical conductivity $σ_{ab}$($T$,$ω$) in cuprates taking into account the effects of in-plane anisotropy. A simple ansatz for the scattering rate $Γ$($T$,$ω$), that includes anisotropy, a quadratic frequency dependence and saturation at the Mott-Ioffe-Regel limit, is able to reproduce recent normal state data on an optimally doped cuprate over a wide frequency range. We highlight the potential importance of including anisotropy in the full expression for $σ_{ab}$($T$,$ω$) and challenge previous determinations of $Γ$($ω$) in which anisotropy was neglected and $Γ$($ω$) was indicated to be strictly linear in frequency over a wide frequency range. Possible implications of our findings for understanding thermodynamic properties and self-energy effects in high-$T_c$ cuprates will also be discussed.