Particle-hole excitations in insulating antiferromagnet Ca2CuO2Cl2

We briefly report results of an x-ray scattering study of particle-hole excitations in a cuprate insulator. The excitation spectrum shows a broad feature around 5.7 eV and a dispersive feature around 2.5 eV energy-loss. The low energy feature disperses upward about 0.8 eV along [110] having momentum-transfers (Q) from (2.1 pi ,2.1 pi) to (1.2 pi, 1.2 pi); whereas, it does not disperse much along [100] in going from (1.42 pi ,0) to (2.85 pi ,0) within the level of energy resolution of 0.56 eV. This dispersive behavior of particle-hole excitations is mostly determined by the underlying strong magnetic correlations.