Phonon Instabilities in Ca(1.4)Sr(0.6)RuO4

Phonon instabilities in Ca$_{1.4}$Sr$_{0.6}$ RuO$_{4}$ are investigated by inelastic neutron scattering techniques. Sr$_{2}$RuO$_{4}$ is an unconventional p-wave superconductor with the same structure as La$_{2}$CuO$_{4}$, the parent compound of the high T$_{c}$ superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$. By partial substitution of Ca$^{+2}$ for Sr$^{+2}$, induced structural stresses create a complex phase diagram with exotic phases. La$_{2}$CuO$_{4}$ has a temperature dependent $\Sigma _{4}$ phonon instability, correlated with the tetragonal to orthorhombic structural transition. It is anticipated that Ca$_{1.4}$Sr$_{0.6}$RuO$_{4}$ will exhibit similar behavior as a precursor to its tetragonal to orthorhombic phase transition. Indeed the instability exists, but two anomalies appear in the spectra. A new phonon mode appears mimicking the dispersion of the $\Sigma _{4}$ phonon and a new Bragg peak appears incommensurate with the tetragonal unit cell. The origins and implications of the anomalies will be discussed. Work supported by NSF- DMR 0105232, NSF-DMR0346826, and DOE DE-FG02-04ER46125. ORNL, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725