The quasi-one-dimensional cuprate Li$_2$CuO$_2$ is a prototype edge-sharing chain compound [1]. The Cu$^{2+}$ ions in this strongly correlated material give rise to one spin ½ per CuO$_4$ plaquette with a nearest neighbor Cu-O-Cu bond angle close to 90° implying weak superexchange coupling between Cu spins. As a result, spins order antiferromagnetically between the chains below T$_N$$\sim$9K, but ferromagnetically in the chains.
We have performed Resonant Inelastic X-ray Scattering (RIXS) at Cu L$_3$ and O K-resonances at the ADRESS beamline on this compound. Our momentum resolved RIXS measurements at the Cu L$_3$-edge allow analyzing orbital excitations with high sensitivity. At the O K-edge, the RIXS spectra display a complicated interplay of low-energy excitations from charge, orbital and lattice degrees of freedom.
In particular, we discuss charge transfer related spectral components in the scenario of exotic Zhang-Rice (ZR) singlet and triplet excitations which can be reached in the final state with O K-edge RIXS [2,3]. Our recent temperature dependent measurements evidence clear opposite temperature behavior for these two features, in good agreement with optical conductivity calculations [4]. Comparing the excitation energy of both ZR features allows to directly read the binding energy of the Zhang-Rice singlet in Li$_2$CuO$_2$ from the spectra. Furthermore, this study suggests RIXS as an excellent probe for investigating local magnetic order.
References
1. R. Neudert et al., Phys. Rev. B 60, 13413, (1999).
2. T. Learmonth et al., Euro. Phys. Lett. 79, 47012, (2007).
3. K. Okada and A. Kotani, Phys. Rev. B 63, 45103, (2001).
4. J. Malek et al., Phys. Rev. B 78, 060508(R) (2008).
