Upper limit of the transition temperature of superconducting materials

Why are the transition temperatures (T _c) of superconducting materials so different? The answer to this question is not only of great significance in revealing the mechanism of high-T _c superconductivity but also can be used as a guide for the design of new superconductors. However, so far, it is still challenging to identify the governing factors affecting the T _c. In this work, with the aid of machine learning and first-principles calculations, we found a close relevance between the upper limit of the T _c and the energy-level distribution of valence electrons. It implies that some additional inter-orbital electron-electron interaction should be considered in the interpretation of high-T _c superconductivity.