Study of the phonon anomalies in the cuprate superconductor LSCO

The possible roles of the pseudogap phenomena, of dynamic charge correlations and of the electronphonon coupling in driving high-temperature superconductivity in the lamellar cuprates continue to be debated. Neutron and X-ray scattering experiments have revealed a significant anomalous dispersion of the Cu-O bond stretching phonons, consistent with strong electron-phonon coupling. It appears that these anomalies are related to dynamic charge instabilities within the quintessential CuO2 planes, i.e., charge-spin ‘stripe’ or charge density wave (CDW) correlations. The recent discovery of CDW correlations in numerous cuprates via hard X-ray and soft resonant X-ray scattering constitutes a significant development in this regard, as it demonstrates a universal tendency toward some form of charge order. The planar wave vectors of the phonon anomalies and the CDW order are very similar, and the CDW correlations appear below a distinct characteristic temperature TCDW that is comparable to pseudogap temperature T* (as obtained from, e.g., charge transport experiments). Since the phonon anomalies may be viewed as a particularly sensitive observable of the underlying charge-order correlations, more studies of the phonon anomaly and of its connection to pseudogap phenomena are highly desirable to yield deep insight into the extent and role of such correlations throughout the cuprate phase diagram. Here we propose to use state-of-the-art inelastic X-ray scattering (IXS) with high energy resolution to monitor in detail the temperature dependence of Cu-O bond-stretching phonons across T* in LSCO. It is generally overlooked that, for the archetypal cuprate La2-xSrxCuO4 (LSCO), T* may be rather large, e.g., as large as 400 K at optimal doping (x=0.15) and even larger at lower doping. Interestingly, while it was noted that the phonon anomaly weakens at the highest measured temperature of 330 K, measurements of superconducting samples have not yet been carried out across T*. It is a distinct possibility that the onset of the anomalies associated with dynamic charge correlations tracks the pseudogap temperature. This possibility is tested here.