Pseudogap problem in high-temperature superconductors

It has been more than 30 years since the discovery of high-temperature superconductors (HTSCs). The number of articles published on the subject is huge, but the cause of the high superconducting transition temperature $T_{\rm c}$ is still an open question. The so-called ‘pseudogap’ HTSC phase—an anomalous ‘normal’ state—turned out to be as complicated a problem as high-temperature superconductivity itself. Its role remains unexplained. We discuss here only key experimental results with the aim to demonstrate the complexity and intricacy arising in the explanation of problems related to the nature of this unique phenomenon.