Temperature dependence of the surface conductivity of YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7&#8722;&#948;</SUB> accompanying the change in the concentration of dimerized oxygen holes

The intensity of inelastically scattered electrons measured by electron energy loss spectroscopy has been employed to monitor the surface conductivity of YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>6.9</SUB> as a function of temperature. The study shows a drastic change in surface conductivity precedes the superconducting transition at 90K. The increase in surface conductivity is accompanied by the formation of dimerized holes in the oxygen derived p-band. This phenomenon is not observed in the non-superconducting YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>6.2