Temperature dependence of the positron annihilation lifetime in single-crystal Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We report our measurements of the temperature dependence of the positron lifetime spectrum in single-crystal Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. Contrary to earlier measurements in sintered powders, we observe only a single lifetime component $\tau$ for temperatures in the range $5<T<\mathrm{}297\mathrm{}$ K, exhibiting a constant value ($\tau \approx 176$ ps) above $T_c$ and increasing monotonically below $T_c$ 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.