Electronic excitations in a one-dimensional J=$\frac{1}{2}$ antiferromagnet observed by Raman scattering

A major focus of experimental interest in low-dimensional iridium oxides has been to explore the interplay of quantum fluctuation, spin-orbit coupling and electron correlation effects. Ba$_5$CuIr$_3$O$_{12}$ serves as an ideal playground and contains weakly-interacting chains along which J=$\frac{1}{2}$ iridium ions are antiferromagnetically coupled. The intercept obtained by extrapolating high-temperature inverse static magnetic susceptibility is around -270\,K but no magnetic ordering occurs down to 4\,K\footnote{G. R. Blake et al., Chem. Mater. 10, 3536 (1998)}, indicating quanum fluctuation induced magnetic frustration. Here, we report polarized inelastic light-scattering spectrum of Ba$_5$CuIr$_3$O$_{12}$ as function of exciting laser energy and temperature. Low-energy magnetic response below 100\,meV only appears when the system is excited along the chain direction, corresponding to local antiferromagnetic correlations. On the other hand, the spectral lineshape of high-energy d-d electronic transitions at 570\,meV sheds light on the competition between localizing and itinerant tendency of electrons on iridium extended 5d orbits.