Cu-induced surface disruption of ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$

Interface reactions for vapor-deposited Cu on ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ were investigated with x-ray photoemission and inverse photoemission. Strong Cu-O reactions indicate oxygen removal and the $\mathrm{Cu} {2p}_{\frac{3}{2}}$ emission shows conversion of the superconductor ${\mathrm{Cu}}^{2+}$ configuration to ${\mathrm{Cu}}^{1+}$ within 40-50 Å of the surface. Valence-band results show a loss of the $\mathrm{Cu} {3d}_{x^2-y^2}-\mathrm{O} {2p}_{x,y}$ electronic states near the Fermi level, and we conclude that the surface layer is not superconducting. Changes in the occupied and empty electronic states of La at low Cu coverages provide additional evidence of Cu-induced reaction. A metallic overlayer begins to form at ∼2 Å nominal Cu coverage as these surface reactions become diffusion limited at room temperature.