KOHN ANOMALIES IN SUPERCONDUCTORS

The detailed behavior of phonon dispersion curves near momenta which s pan the electronic Fermi sea in a superconductor is presented. An anom aly, similar to the metallic Kohn anomaly, exists in a superconductor' s dispersion curves when the frequency of the phonon spanning the Ferm i sea exceeds twice the superconducting energy gap. This anomaly occur s at approximately the same momentum but is stronger than the normal-s tate Kohn anomaly. It also survives at finite temperature, unlike the metallic anomaly. Determination of Fermi-surface diameters from the lo cation of these anomalies, therefore, may be more successful in the su perconducting phase than in the normal state. However, the superconduc tor's anomaly fades rapidly with increased phonon frequency and become s unobservable when the phonon frequency greatly exceeds the gap. This constraint makes these anomalies useful only in high-temperature supe rconductors such as La1.85Sr0.15CuO4.