X-ray imaging of dispersive charge modes in a doped Mott insulator near the antiferromagnet/superconductor transition

Momentum-resolved inelastic resonant x-ray scattering is used to map the doping evolution of bulk electronic modes in the doped Mott insulator class Nd2-xCexCUO4. As the doping induced antiferromagnet/superconductor (AFM/SC) transition is approached, we observe an anisotropic redistribution of the spectral weight of collective excitations over a large energy scale along the Gamma -> (pi, pi) direction, whereas the modes exhibit broadening (similar to 1 eV) with relatively little softening along Gamma -> (pi, 0) with respect to the parent Mott state (x=0). Our study reveals a closing of the charge gap in the vicinity of the zone center even though the mode softening and spectral redistribution involve an unusually large energy scale over the full Brillouin zone. The collective behavior of modes in the vicinity of the AFM/SC critical transition is demonstrated.