The origin of high-temperature superconductivity is still not clear, but it seems that instead of electron– phonon attraction mechanisms, as in conventional superconductivity, one is dealing with genuine electronic mechanisms (e.g. by antiferromagnetic correlations), and instead of conventional, purely s-wave pairing, more exotic pairing symmetries are thought to be involved (d -wave in the case of the cuprates; primarily extended s -wave, but occasionally d -wave, in the case of the iron-based superconductors).