Electronic structures of the YBa2Cu3O7−x surface and its modification by sputtering and adatoms of Ti and Cu

Abstract

We present x-ray and inverse photoemission results for fractured surfaces of Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ before and after surface modification by Ar ion bombardment and the deposition of adatoms of Ti and Cu. Representative results are compared for samples prepared in three different ways. Two of the sample types exhibit substantial emission from grain-boundary phases because of both intergranular and transgranular fracture; they produce results that are very similar to those presented thus far in the literature. A third type was nearly free of contamination and clearly showed spectral features characteristic of the superconductor. Comparison of these nearly contamination-free valence-band results to those for clean ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_4$ shows remarkably similar x-ray photoemission spectroscopy densities of states, with subtle differences near the Fermi level and at 3 eV. Inverse photoemission results show the top of the Cu-O hybrid orbitals to be 2 eV above $E_F$ and the empty states of Y and Ba at higher energy. Comparison with one-electron densities of states shows reasonable agreement, but there are large differences within the set of calculated results, and it is unclear from the valence bands alone how to account for final-state $\mathrm{Cu} d-d$ Coulomb correlation effects (satellite features show these effects very clearly). Argon sputtering for both types of samples shows destruction of the superconductor, with differences that can be related to sample surface quality. The deposition of adatoms of Ti and Cu results in reaction associated with oxygen withdrawal from the near-surface region. Studies of the $\mathrm{Cu} {2p}_{\frac{3}{2}}$ line shape show that the deposition of as little as ∼1 monolayer equivalent of Ti or Cu reduces the formal ${\mathrm{Cu}}^{2+}$ emission within the probed volume (30-50 Å deep). Core-level analysis shows that this chemical reduction of Cu is accompanied by crystal-structure modifications as well. Studies of Cu adatom interactions reveal the progression from ${\mathrm{Cu}}^{2+}$ to ${\mathrm{Cu}}^{1+}$ and, ultimately, to Cu metal as the overlayer thickens $\mathrm{Cu} {2p}_{\frac{3}{2}}$ binding energy 932.5 eV for Cu metal, 933.1 eV for ${\mathrm{Ci}}^{1+}$, and 932.8 eV for the superconductor). Valence-band results during interface formation show the disappearance of emission near the Fermi level, consistent with the loss of ${\mathrm{Cu}}^{2+}$-O covalent bonds of the superconductor.