Signatures of bilayer splitting in the<i>c</i>-axis optical conductivity of double layer cuprates

Abstract

We report on the infrared studies of the interlayer response for a series of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ high-${T}_{c}$ superconductors with Pr, Ni, and Zn dopants, as well as for the optimally doped crystals of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{z}.$ These experimental results have motivated us to reexamine some of the long-standing issues in the interlayer electrodynamics of cuprates. Among them are the origins of the anomalous resonance specific to the conductivity of materials with more than one ${\mathrm{CuO}}_{2}$ plane per unit cell, as well as the microscopic roots of the notorious ``semiconducting'' behavior seen in a variety of cuprates. Our data for ${\mathrm{Pr}}_{y}{\mathrm{Y}}_{1\ensuremath{-}y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ samples indicate that the suppression of the superfluid density (and normal-state conductivity) with $\stackrel{\ensuremath{\rightarrow}}{Y}\mathrm{Pr}$ substitution occurs primarily due to changes in the electronic state of ${\mathrm{CuO}}_{2}$ planes and not because of the fragmentation of Cu-O chains. We also show that the transverse Josephson plasma model proposed to explain the anomalous mode in the interlayer response is not fully consistent with the totality of the experimental data for double-layered materials. We discuss alternative/complimentary scenario assigning this feature of the c-axis conductivity to lifted degeneracy between bonding and antibonding bands associated with the two constituents of the ${\mathrm{CuO}}_{2}$ bilayer (a so-called bilayer splitting effect).