  Superconducting state is achieved through quantum condensation of Cooper
pairs which are new types of charge carriers other than single electrons in
normal metals. The theory established by Bardeen-Cooper-Schrieffer (BCS) in
1957 can successfully explain the phenomenon of superconductivity in many
single-element and alloy superconductors. Within the BCS scheme, the Cooper
pairs are formed by exchanging the virtual vibrations of lattice (phonons)
between two electrons with opposite momentum near the Fermi surface. The BCS
theory has dominated the field of superconductivity over 64 years. Many
superconductors discovered in past four decades, such as the heavy Fermion
superconductors, cuprates, iron pnictide/chalcogenide and nickelates seem,
however, to strongly violate the BCS picture. The most important issue is that,
perhaps the BCS picture based on electron-phonon coupling are the special case
for superconductivity, there are a lot of other reasons or routes for the
Cooper pairing and superconductivity. In this short overview paper, we will
summarize part of these progresses and try to guide readers to some new
possible schemes of superconductivity after the BCS paradigm. We also propose
several empirical rules for the exploration of high-temperature unconventional
superconductors.

Superconductivity