Abstract
The unusual softening of the ${\mathit{B}}_{1\mathit{g}}$-like phonon of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ is studied as a function of ${\mathit{T}}_{\mathit{c}}$ by substituting up to 6% nickel for Cu. The abrupt onset and small temperature range over which the softening occurs in undoped ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ are modified upon doping with the softening occurring well above ${\mathit{T}}_{\mathit{c}}$ and continuing smoothly to 10 K when ${\mathit{T}}_{\mathit{c}}$ is reduced to 71 K. The phonon linewidth in the doped films shows no anomalies, regardless of Ni concentration. In contrast, the self-energies of the ${\mathit{A}}_{\mathit{g}}$ modes associated with the plane copper [Cu(2)] and bridging oxygen (${\mathrm{O}}_{4}$) atoms reveal normal thermal behavior for all films. We conclude that an additional mechanism, besides strong coupling of phonons to superconducting electrons, contributes to the ${\mathit{B}}_{1\mathit{g}}$-phonon anomalies.
Published Version
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