Figure 3: Inealstic neutron-scattering study of lattice dynamics

(a) Polarization patterns for high-frequency Σ₁ Co–O bond-stretching phonon modes. The dashed lines indicate a coupling to charge stripes (left) or to CBCO (right). (b,c) Some representative phonon scans of La_1.6Sr_0.4CoO₄ measured at (3−q 3−q 0) and for La_1.6Sr_0.4NiO₄ measured at (3+q 3+q 0) corresponding to the focusing sides in both compounds. Clearly, the opposite |q| dependence of phonon peak positions can be observed in nickelates and cobaltates. (d) Inelastic neutron-scattering intensity as a function of energy and momentum transfer for La_1.6Sr_0.4CoO₄. The white-shaded area is biased by a spurious peak and also measured at the defocusing side. An anomalous phonon softening is clearly observeable at ζ=0.5 rather than at positions indicative for charge stripes (arrows). (e) The corresponding inelastic neutron signal of a 40% Sr-doped LSNO reference sample with robust diagonal CSO; grey hexagons indicate the fitted phonon dispersion. Unlike in La_1.6Sr_0.4CoO₄, a phonon softening is observeable around propagation vectors connected to CSO and not for ζ→0.5 where even an upturn is observeable. (f) Comparison of the high-frequency Σ₁ phonon dispersions of La_2−xSr_xCoO₄ for different x. The black dash-dotted line indicates the dispersion of La₂CoO₄ shifted to higher energies. Intensity error bars are statistical error bars calculated by the square root of intensity. Other error bars are s.d. obtained from Gaussian peak fits.