Inelastic X-ray scattering study of the doping and temperature dependence of electron-phonon anomalies in Nd2-xCexCuO4

One of the central challenges in condensed matter physics is to understand the origin of high-temperature superconductivity in the cuprates. Both theoretical and experimental efforts have been directed toward discerning the rich variety of ordered states that may coexist, cooperate or compete with superconductivity. The 2012 discovery with synchrotron X-rays of CDW correlations in competition with superconductivity in hole-doped YB2Cu3O6+δ (YBCO) has attracted much attention. Subsequent X-ray experiments demonstrated the existence of CDW correlations in other hole-doped cuprates and in the archetypal electron-doped cuprate Nd2-xCexCuO4 (NCCO). Although it is now widely accepted that (short-range) charge-order correlations (stripe or CDW) are a universal property of the cuprates, it is of utmost importance to establish the common features of these correlations and their link to superconductivity and the pseudogap. Charge correlations have long been argued to manifest themselves as an anomalous lineshape and dispersion of the longitudinal Cu-O bond-stretching phonon mode in the hole-doped cuprates. In pioneering inelastic X-ray scattering (IXS) work, some of us (in collaboration with a group at the ESRF) revealed similar anomalous phonon behavior in nearly-optimally-doped NCCO (x=0.14). The two-dimensional wave vector (H,K) ≈ (0.2,0) where anomalous phonon softening was found to occur in NCCO is consistent with the recently observed CDW wave vector at the same doping level. The prior IXS phonon measurements were limited to one sample/doping level (x=0.14), and the experiment did not not exceed room temperature. In light of the extensive recent CDW results for NCCO, it is very timely to pursue a comprehensive investigation of the temperature and doping dependence of the phonon anomaly in NCCO. Interestingly, the CDW order in NCCO appears to be more robust with respect to doping and temperature than in the hole-doped cuprates, which suggests that this might also be the case for the associated phonon anomaly. We thus propose to significantly extend the prior IXS work to a wide doping and temperature range.