Upper critical fields in high-$ T_{\rm{c}} $ superconductors

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 ($H_{\rm{c2}}$) is an essential parameter for obtaining information on the pair-breaking mechanism, coherence length $\xi$, and pairing symmetry, all of which are crucial for understanding unconventional superconducting mechanisms. Here, we provide a brief review of studies on $H_{\rm{c2}}$ in several representative series of cuprate, iron-based, and nickelate superconductors. By comparing the behavior of $H_{\rm{c2}}$ as 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.