Charge dynamics in low dimensional prototype correlated systems: A view with high energy X-rays.

摘要

The electronic structure of Mott systems continues to be an unsolved problem in physics despite more than half-century of intense research efforts. Well-developed momentum-resolved spectroscopies such as photoemission and neutron scattering cannot directly address problems associated with the full Mott gap as angle-resolved photoemission probes the occupied states and neutrons do not couple to the electron's charge directly. Our observation of dispersive particle-hole pair excitations across the charge gap (effective Mott gap) in several low dimensional prototype Mott insulators using high resolution inelastic x-ray scattering suggests that the excitations across the gap are highly anisotropic and momentum dependent. The results indirectly provide some information about the momentum dependence of unoccupied states in these correlated systems. The x-ray scattering results are complementary to the electron scattering results by the possibility of studying the excitations in the high momentum transfer regimes (near the zone boundaries and corners). This is clearly demonstrated in case of studying plasmons near the wave vector regime where Landau damping starts to dominate. X-ray scattering also allows one to probe the symmetry characters of localized electrons and the excitations through the strong polarization dependence of scattering near a core resonance. Given its deeply bulk-sensitive and weak-coupling nature and the ability to probe dispersive behavior of charge fluctuations over several Brillouin zones, inelastic x-ray scattering shows the promise to become an important experimental tool for studying the electronic structure of complex insulators.