Chemical potential jump between the hole-doped and electron-doped sides of ambipolar high-Tc cuprate superconductors

In order to study an intrinsic chemical potential jump between the hole-doped and electron-doped high-T-c superconductors, we have performed core-level x-ray photoemission spectroscopy measurements of Y0.38La0.62Ba1.74La0.26Cu3Oy (YLBLCO), into which one can dope both holes and electrons with maintaining the same crystal structure. Unlike the case between the hole-doped system La2-xSrxCuO4 and the electron-doped system Nd2-xCexCuO4, we have estimated the true chemical potential jump between the hole-doped and electron-doped YLBLCO to be similar to 0.8 eV, which is much smaller than the optical gaps of 1.4-1.7 eV reported for the parent insulating compounds. We attribute the reduced jump to the indirect nature of the charge-excitation gap as well as to the polaronic nature of the doped carriers.