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Register a new userComparative study of the phonons in non-superconducting BaC6 and superconducting CaC6 using inelastic x-ray scattering
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The low energy phonons of two different graphite intercalation compounds (GICs) have been measured as a function of temperature using inelastic x-ray scattering (IXS). In the case of the non-superconductor BaC6, the phonons observed are significantly higher (up to 20 %) in energy than those predicted by theory, in contrast to the reasonable agreement found in superconducting CaC6. Additional IXS intensity is observed below 15 meV in both BaC6 and CaC6. It has been previously suggested that this additional inelastic intensity may arise from defect or vacancy modes in these compounds, unpredicted by theory (dAstuto et al, Phys. Rev. B 81 104519 (2010)). Here it is shown that this additional intensity can arise directly from the large disorder of the available samples. Our results show that future theoretical work is required to understand the relationship between the crystal structure, the phonons and the superconductivity in GICs.
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We investigate the dispersion and temperature dependence of a number of phonons in the recently discovered superconductor CaC6 utilizing inelastic x-ray scattering. Four [00L] and two ab-plane phonon modes are observed, and measured at temperatures both above and below T_c. In general, our measurements of phonon dispersions are in good agreement with existing theoretical calculations of the phonon dispersion. This is significant in light of several discrepancies between experimental measurements of phonon-derived quantities and theoretical calculations. The present work suggests that the origin of these discrepancies lies in the understanding of the electron-phonon coupling in this material, rather than in the phonons themselves.
We used resonant inelastic x-ray scattering (RIXS) with and without analysis of the scattered photon polarization, to study dispersive spin excitations in the high temperature superconductor YBa2Cu3O6+x over a wide range of doping levels (0.1 < x < 1). The excitation profiles were carefully monitored as the incident photon energy was detuned from the resonant condition, and the spin excitation energy was found to be independent of detuning for all x. These findings demonstrate that the largest fraction of the spin-flip RIXS profiles in doped cuprates arises from magnetic collective modes, rather than from incoherent particle-hole excitations as recently suggested theoretically [Benjamin et al. Phys. Rev. Lett. 112, 247002(2014)]. Implications for the theoretical description of the electron system in the cuprates are discussed.
We present a resonant inelastic x-ray scattering (RIXS) study of spin and charge excitations in overdoped La1.77Sr0.23CuO4 along two high-symmetry directions. The line shape of these excitations is analyzed and they are shown to be highly overdamped. Their spectral weight and damping are found to be strongly momentum dependent. Qualitative agreement between these observations and a calculated RPA susceptibility is obtained for this overdoped compound, implying that a significant contribution to the RIXS signal stems from a continuum of charge excitations. Furthermore, this suggests that the spin-excitations in the overdoped regime can be captured qualitatively by an itinerant picture. Our calculations also predict a new low-energy spin excitation branch to exist along the nodal direction near the zone center. With the energy resolution of the present experiment, this branch is not resolvable but we show that next generation of high-resolution spectrometers will be able to test this prediction.
We present the results of a neutron scattering study of the high energy phonons in the superconducting graphite intercalation compound CaC$_6$. The study was designed to address hitherto unexplored aspects of the lattice dynamics in CaC$_6$, and in particular any renormalization of the out-of-plane and in-plane graphitic phonon modes. We present a detailed comparison between the data and the results of density functional theory (DFT). A description is given of the analysis methods developed to account for the highly-textured nature of the samples. The DFT calculations are shown to provide a good description of the general features of the experimental data. This is significant in light of a number of striking disagreements in the literature between other experiments and DFT on CaC$_6$. The results presented here demonstrate that the disagreements are not due to any large inaccuracies in the calculated phonon frequencies.
Measurements of spin excitations are essential for an understanding of spin-mediated pairing for superconductivity; and resonant inelastic X-ray scattering (RIXS) provides a considerable opportunity to probe high-energy spin excitations. However, whether RIXS correctly measures the collective spin excitations of doped superconducting cuprates remains under debate. Here we demonstrate distinct Raman- and fluorescence-like RIXS excitations of Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_{2}$O$_{8+{delta}}$ in the mid-infrared energy region. Combining photon-energy and momentum dependent RIXS measurements with theoretical calculations using exact diagonalization provides conclusive evidence that the Raman-like RIXS excitations correspond to collective spin excitations, which are magnons in the undoped Mott insulators and evolve into paramagnons in doped superconducting compounds. In contrast, the fluorescence-like shifts are due primarily to the continuum of particle-hole excitations in the charge channel. Our results show that under the proper experimental conditions RIXS indeed can be used to probe paramagnons in doped high-$T_c$ cuprate superconductors.
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