Temperature dependences of the 340-, 440-, and 500-cm−1 Raman modes of YBa2Cu3Oy for 6.7≲ y≲7.0

Abstract

The temperature dependences of the linewidths and frequencies of the 340-, 440-, and 500-${\mathrm{cm}}^{\mathrm{-}1}$ Raman-active phonons have been measured in lightly twinned single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$ with different oxygen concentrations y. The observed superconductivity-induced changes in the linewidths and frequencies that occur for temperatures below the critical temperature have been found to depend on the oxygen concentration within the range 6.7≲y≲7.0, with the most sensitive dependence occurring in the 340-${\mathrm{cm}}^{\mathrm{-}1}$ phonon-linewidth variations near y≊7.0. The linewidth and frequency dependences of the 340-${\mathrm{cm}}^{\mathrm{-}1}$ ${\mathit{B}}_{1\mathit{g}}$ mode for T<${\mathit{T}}_{\mathit{c}}$ suggest that the redistribution of electronic states below ${\mathit{T}}_{\mathit{c}}$ is very sensitive to the oxygen or hole concentration within this range. Estimates for the superconducting energy gap 2Δ have been obtained from comparisons of the temperature dependence of the 340-${\mathrm{cm}}^{\mathrm{-}1}$ mode with strong-coupling theory and these indicate that 2Δ increases as the oxygen or hole concentration y decreases from y=7.0 where 2Δ≊5.4 k${\mathit{T}}_{\mathit{c}}$. The linewidth and frequency of the 440-${\mathrm{cm}}^{\mathrm{-}1}$ ${\mathit{A}}_{\mathit{g}}$ mode are, however, relatively insensitive to variations in y and it is suggested that the 440- and 500-${\mathrm{cm}}^{\mathrm{-}1}$ phonons couple to a different set of electrons than does the 340-${\mathrm{cm}}^{\mathrm{-}1}$ mode.