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 (2.1π,2.1π) to (1.2π, 1.2π); whereas, it does not disperse much along <100> in going from (1.42π,0) to (2.85π,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.