Room Temperature Superconductivity

Abstract

Superconductivity was discovered by Kamerlingh Onnes in 1911. For one century superconductivity has been a great challenge to theoretical physics. The first successful set of phenomenological equations for superconducting metals was given by F. London in 1935. Yet, in 1950, almost 40 years after the discovery of this phenomenon, there was not any adequate microscopic theory of superconductivity. However, by 1935, single elements necessary to a successful theory to explain superconductivity was known to theorists. The peculiar condensation of a Bose-Einstein gas was predicted by Einstein in 1925. The idea that pairs of fermions can combine to form bosons has been known since 1931. In 1950 the most relevant ideas of superconductivity has been summarized by F. London in his famous book “Superfluids”, volume 1. At last, BCS theory (Bardeen et al., 1957) was the first successful theory to explain the microscopic mechanisms of superconductivity in metals and alloys. Practical applications of superconductivity are steadily improving every year. However, the actual use of superconducting devices is limited by the fact that they must be cooled to low temperatures to become superconducting. For example, superconducting magnets used in most particle accelerators are cooled with liquid helium, that is, it is necessary to use cryostats that should produce temperatures of the order of 4 K. Helium is a very rare and expensive substance. On the other hand, because helium reserves are not great, the world's supply of helium can be wasted in a near future. Thus, because liquid nitrogen is not expensive and the reserves of nitrogen could not be wasted, it is important to use high-Tc superconductors cooled with liquid nitrogen. Superconductors with critical temperatures greater 77 K may be cooled with liquid nitrogen. We know that BCS theory (Bardeen et al., 1957) explains the microscopic mechanisms of superconductivity in metals. According to BCS theory, electrons in a metallic superconductor are paired by exchanging phonons. According to many researchers (De Jongh, 1988; Emin, 1991; Hirsch, 1991; Ranninger, 1994), BCS theory is not appropriate to be applied to explain the mechanisms of superconductivity in oxide superconductors. Nevertheless, other models relying on a BCS-like picture replace the phonons by another bosons, such as: plasmons, excitons and magnons, as the mediators causing the attractive interaction between a pair of electrons and many authors claim that superconductivity in the oxide superconductors can be explained by the conventional BCS theory or BCS-like theories (Canright & Vignale, 1989; Tachiki & Takahashi, 1988; Takada, 1993). Copper oxide superconductors are the most important high-Tc superconductors. The discovery of a room temperature superconductor should trigger a great technological