Upper critical fields in high-Tcsuperconductors

Since the discovery of high-temperature superconductivity in cuprates, understanding the unconventional pairing mechanism has remained one of the most significant challenges. The upper critical field (Hc2) is an essential parameter for obtaining information on the pair-breaking mechanism, coherence lengthξ, and pairing symmetry, all of which are crucial for understanding unconventional superconducting mechanisms. Here, we provide a brief review of studies onHc2in several representative series of cuprate, iron-based, and nickelate superconductors. By comparing the behavior ofHc2as a function of temperature, doping concentration, and anisotropy across these three major classes of superconductors, we hope to contribute to a better understanding of the complex pairing interactions in high-temperature superconductors.