A Review of the Fundamental Principles and Research Progress in Superconductors

Since the discovery of superconductors in 1911, their zero-resistance conductivity and perfect diamagnetism have significantly advanced science and technology. This paper reviews the key principles and parameters of superconductors, focusing on three critical values: critical temperature (Tc), critical magnetic field (Hc), and critical current density (Jc). Superconductors are categorized as low-temperature (LTS) or high-temperature (HTS) based on Tc, and as type I or type II based on magnetic flux penetration. Type I superconductors, such as aluminum, completely expel external magnetic fields, while type II superconductors, such as YBCO, allow partial flux penetration, enabling flux pinning and stable levitation. The Meissner effect is fundamental to superconductivity, with type I superconductors exhibiting only the perfect Meissner state and the normal state, whereas type II superconductors also feature a mixed state. Additionally, this paper discusses BCS theory, which explains Cooper pair formation in conventional superconductors but fails to fully describe high-temperature superconductors due to their strong coupling nature. Recent advancements, such as textured YBCO tapes with aligned grains, have significantly improved superconducting performance. Future breakthroughs will require exploring new mechanisms beyond existing theories to advance next-generation superconducting technologies.