Unconventional superconductivity after the BCS paradigm and empirical rules for the exploration of high temperature superconductors

TL;DR

This paper reviews unconventional superconductivity beyond the BCS theory, highlighting new pairing mechanisms and proposing empirical rules to guide the discovery of high-temperature superconductors.

Contribution

It summarizes recent progress in understanding non-BCS superconductivity and introduces empirical guidelines for exploring high-temperature superconductors.

Findings

Many superconductors violate BCS predictions.

Alternative pairing mechanisms are being explored.

Empirical rules can aid in discovering new high-temperature superconductors.

Abstract

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.