Local pairs in high-temperature superconductors: The concept of pseudogap

Abstract

The origin of the high-temperature superconductivity of cuprates remains a subject of debate after almost four decades of investigation. One of the main questions — what is the physics behind the mechanism of superconducting pairing, which makes it possible to obtain real Cooper pairs at temperatures much higher than 100 K, remains open. It is believed that the answer can be obtained by the studying the properties of cuprates in the normal state above Tc, where the pseudogap (PG) opens at T* >> Tc. The number of papers devoted to the study of PG is extraordinarily large, but its physics also remains in question. In cuprates, the question arises whether the pseudogap phase arises exclusively due to antiferromagnetic fluctuations, spin or charge density waves, or whether it can be explained by the formation below T* of specific paired fermions, the so-called local pairs. We review studies on both approaches to PG that should clarify this issue. In addition to theoretical considerations, we analyze and discuss various experimental results of fluctuation conductivity and PG measurements, as well as magnetic measurements, in an attempt to prove the decisive role of local pairs in the formation of the PG state. Accordingly, various types of supposed magnetic interactions can be considered as possible mechanisms of superconducting pairing in high-temperature superconductors.