Phonon Anomalies Induced by Superconductivity

We calculate the electronic polarizability in the superconducting state near extremum vectors ${\vec Q}_0$ of the Fermi surface. A pole appears in the polarizability at frequencies $ω$ near the superconducting gap $2Δ$ which leads to a sharp peak at $ω$ just below $2 Δ$ in the lattice vibrations near ${\vec Q}_0$. A second order transition to a charge density wave state below the superconducting transition is shown to be unlikely. The results are compared with the recent inelastic neutron scattering measurements in compounds, $ANi_2B_2C$, $A=Y$ or $Lu$.

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