NSLS-II Friday Lunchtime Seminar

Different from the simple one-band Hubbard model prediction, it has been recognized that the cuprate superconductors are electron-hole asymmetric. Recent RIXS work on the electron-doped Nd2-xCexCuO4 by K. Ishii et al. [1] and W. S. Lee et al. [2] reported a hardening of the spin excitations and the emergence of a charge excitation mode. Both these observations are in distinct contrast to that reported on the hole doped side, and brought attention again to the profound electron-hole asymmetry issue in the cuprates. Taking the advantage of a La2-xCexCuO4 combi-film, namely a film with large range doping gradient distribution, we studied the evolution of the charge and magnetic excitations from optimal- to over-doping systematically in fine steps. Our results establish the universality of the previous observation for the electron-doped cuprates. And more importantly, the doping dependent evolutions show that the magnetic and charge excitations are not the two faces of a coin. Rather, the spin-correlation roots in the short range correlation, and the charge-fluctuation stems from long range Coulomb interaction. [1] K. Ishii et al, Nat. Commun. 5, 3714 (2014). [2] W. S. Lee et al, Nat. Phys. 10, 883 (2014).