Temperature dependence of the positron annihilation lifetime in single-crystal YBa2Cu3O7

We report our measurements of the temperature dependence of the positron lifetime spectrum in single-crystal YBa2Cu3O7. Contrary to earlier measurements in sintered powders, we observe only a single lifetime component for temperatures in the range 5<T<297 K, exhibiting a constant value (176 ps) above Tc and increasing monotonically below Tc to 187 ps at 5 K. Possible correlations with the superconducting gap, relaxation of the oxygen atoms, or a redistribution of the electrons are discussed.