Correlation of the superconducting critical temperature with spin and orbital excitations in (Cax La1-x)(Ba1.75-xLa 0.25+x) Cu3 Oy as measured by resonant inelastic x-ray scattering

Electronic spin and orbital (dd) excitation spectra of (CaxLa1-x)(Ba1.75-xLa0.25+x)Cu3Oy samples are measured by resonant inelastic x-ray scattering (RIXS). In this compound, Tc of samples with identical hole dopings is strongly affected by the Ca/Ba substitution x due to subtle variations in the lattice constants, while crystal symmetry and disorder as measured by linewidths are x independent. We examine two extreme values of x and two extreme values of hole-doping content y corresponding to antiferromagnetic and superconducting states. The x dependence of the spin-mode energies is approximately the same for both the antiferromagnetic and superconducting samples. This clearly demonstrates that RIXS is sensitive to the superexchange J even in doped samples. A positive correlation between J and the maximum of Tc at optimal doping (Tcmax) is observed. We also measured the x dependence of the dxy→dx2-y2 and dxz/yz→dx2-y2 orbital splittings. We infer that the effect of the unresolved d3z2-r2→dx2→y2 excitation on Tcmax is much smaller than the effect of J. There appears to be dispersion in the dxy→dx2-y2 peak of up to 0.05 eV. Our fitting furthermore indicates an asymmetric dispersion for the dxz/yz→dx2-y2 excitation. A peak at ∼0.8 eV is also observed and attributed to a dd excitation in the chain layer.