A comparison of the temperature dependence of electron-positron momentum density characteristics in Tl_2.2Ca₂Ba₂Cu ₃O_{10.3+ δ}, YBa₂Cu₃O ₇ and La_1.85Sr_0.15CuO₄

The temperature dependence between 2 and 300 K of the positron-electron annihilation lifetime and the electron momentum distribution from the Doppler-broadened annihilation gamma -ray energy spectrum were measured in the high-temperature superconducting material Tl_2.2Ca₂Ba ₂Cu₃O_{10.3+ delta}. A temperature dependence is observed for the positron lifetime, below the critical temperature (124 K), suggestive of a dramatic decrease in the positron-electron overlap resulting from superconductivity. The magnitude of the observed change in the positron lifetime is the largest that the authors have seen for any high-T_c material, although the temperature dependence of the positron lifetime is similar to what they recently reported for YBa₂Cu₃O ₇ and La_1.85Sr_0.15CuO₄. The temperature dependence of the deduced electron momentum distribution indicates a complex but small variation in the number of low-momentum electrons available to annihilate with the delocalised positrons. This new experimental result further confirms the authors earlier suggestions of a common physics involved in high-T_c superconductivity for perovskite materials and provides evidence for the formation of superconducting electron states which are anti-correlated with the delocalised positron wavefunction for all classes of the new high-T_c materials.