Unusual behavior of pseudogap in high-temperature superconductors under the influence of external factors. Part I (Review article)

Within the framework of the developed theoretical approach, the temperature dependences of the resistivity ρ(T), excess conductivity σ′(T), and pseudogap Δ*(T) in well-structured YBa2Cu3O7–δ (YBCO) thin films are analyzed. It is shown that the characteristic temperature, Tpair, corresponding to the BEC–BCS crossover in high-temperature superconductors, remains virtually constant regardless of the change in the number of charge carriers nf or the superconducting transition temperature, Tc. The results were compared with those obtained under the influence of magnetic PrBa2Cu3O7−δ (PrBCO) layers on YBCO–PrBCO superlattices, magnetic PrBCO impurities on Y1–xPrxBa2Cu3O7–δ single crystals, and an external magnetic field on YBCO thin films. It has been established that the interaction of magnetism and superconductivity significantly affects both the value of Tpair and the shape of Δ*(T), pointing to the superconducting nature of the pseudogap, which is discussed in detail.