The role of phonon- and photon-coupled interactions in electron pairing in solid state materials

► Diamagnetic currents in cyclobutadiene dianion cannot be explained by the BCS theory. ► The photon-coupled interactions play a role in the attractive electron-electron interactions. ► The photon-coupled interactions do not play an essential role in the decision of the Tc. ► The phonon-coupled interactions play a role in the decision of the Tc. ► Large energy gap is formed by quantization of orbitals by nature in small sized molecules.