Positronium in a cavity in a high-<i>T</i><sub>c</sub> superconductor

The behavior of positronium localized in a microvoid in a high-temperature superconductor (HTSC) environment is examined. The interaction of such a positronium with conduction electrons is treated as arising from the Ps ↔ e⁺ + e^- process, which is mathematically described here in terms of the well-known Anderson model for a magnetic impurity in a normal metal. In this model, the interaction underlying this process is due to the positronium state hybridization with the states of conduction electrons and the state of the positronium remaining within the void. Similarly to the case of a magnetic impurity, the density of states of an interacting positronium exhibits a ’Kondo resonance’ if the Fermi liquid surrounding the void is in the normal state. Based on experimental data on the lifetime τ₂ of void-trapped positrons, it is concluded that the hybridization interaction is much stronger than the intra-atomic relativistic electron — positron interaction in the Ps atom. The model used to describe the interacting Ps atom provides a relation between the experimental values of τ₂(T) and the properties of the electronic structure of the metal. Experimental results for ceramic HTSC samples of (Bi, Pb)₂Sr₂Ca₂Cu₃O₇ and (Bi, Pb)-2223 are interpreted, which show a sharp decrease in τ₂(T) at T = T_c, where T_c is the superconducting transition temperature. Using the adopted model, some conclusions are drawn as to how the properties of the pseudogap correlate with the experimental τ₂(T) dependence observed in (Bi,Pb)-2223 for T > T_c.