Doping-induced evolution of the intrinsic hump and dip energies dependent on the sample fabrication conditions in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$

Abstract

In oxygen-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, the spectra, as observed by tunneling or photoemission spectroscopic measurements, depend on the sample fabrication conditions, such as the temperature and pressure for fabricating the junction or surface. This implies that the hump and dip energies extracted from the spectra depend on the sample fabrication conditions. When the samples were fabricated at 4.2 K and/or under ultra-high vacuum (UHV), the hump energy exhibited a new step-like doping dependence and the dip energy followed the upper pseudo-gap line. As the fabrication conditions deteriorated, the hump and dip energies reproduced the previous results, in that the dip energy was significantly dependent on the sample, whereas the hump energy exhibited a smooth doping dependence. It can be concluded that the observations for the samples fabricated at 4.2 K and/or under UHV reflect the intrinsic bulk properties, whereas those for the samples fabricated under deteriorated conditions reflect degraded surface properties.