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.