Overview of superconducting materials with Tc higher than 23 K

Abstract

Among the eight families of high-Tc superconductors, magnesium diboride (MgB2), borocarbide (YPd2B2C), and calcium (Ca) at high pressures are thought to be conventional superconductors with phonon-mediated pairing. Tc = 39 K for a simple metal MgB2 is surprising in view of the old-known ceiling of 30 K of the electron-phonon mechanism. Several circumstances in MgB2 appear to optimize the phonon-mediated pairing and to up-shift the old ceiling of Tc. These give a hint on further optimization or further enhancement of Tc. In doped bismuthates (BaBiO3), fullerides (C60), and chloronitrides (HfNCl), dominant pairing interaction is also supposed to be electron-phonon coupling. These superconductors are proximate to insulating phases. Because of this, the electron (carrier) density is significantly low as compared to that in simple metals, suggestive of contribution from electronelectron interactions which may act to strengthen electron-phonon coupling. One other family, from organic compounds, is going to join the latter group. Copper oxides and iron arsenides/selenides, as well as heavy fermions compounds, are distinct from others, and obviously belong to unconventional superconductors with dominant electron-electron interactions.