Some Theories of High Temperature Superconductivity
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SOME THEORIES OF HIGH TEMPERATURE SUPERCONDUCTIVITY MARVIN L. COHEN Department of Physics, University of California and Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720 ABSTRACT A brief review is given of some historical aspects of theoretical research on superconductivity including a discussion of BCS theory and some theoretical proposals for mechanisms which can cause superconductivity at high temperatures. BACKGROUND After Onnes' discovery of superconductivity in lead in 1911, theoretical research concentrated on explaining the zero resistance state. There was little success in obtaining even a macroscopic explanation inthis period before quantum theory was developed. The discovery of the Meissner effect in 1933 broadened the focus of the theoretical work and London in 1935 produced a successful but limited macroscopic description of superconductivity. London's theory was refined and extended by Ginzburg and Landau in 1950, and this macroscopic description is still used without major modification. The influence of the Meissner effect experiment on macroscopic theory has a parallel in the development of the microscopic theory. The discovery of the isotope effect in 1950 established the concept of a close connection between lattice vibrations and superconductivity. Early theories by Frolich and Bardeen for electron self-energies evolved into pairing mechanisms after Cooper's illustration in 1955 of the importance of paired electrons. Finally in 1957, Bardeen, Cooper and Schrieffer[1] gave a microscopic explanation of superconductivity. Two basic features of the BCS theory were the formation of paired electronic states in momentum space with the pairing attraction caused by phonon exchange. In real space the pairs overlapped with as many as 106 pairs between the mates of a given pair for a typical metal like Pb. The next 30 years brought refinement of BCS notably by Gor'kov and Eliashberg and with the possible exception of heavy fermion superconductors it was felt that the BCS theory with its refinements explained the existence of superconductivity in all materials. Despite the apparent success of the BCS mechanism of pairing via phonons, several other schemes were suggested during this period. In general, it was felt that BCS-like pairs were a necessity for superconductivity, but that the pairing itself could be caused by other excitations such as electronic or magnetic excitations. There was little experimental evidence that these theoretical proposals were justified, and systematic searches based on the theoretical models were rare. The 1986 discovery by Bednorz and MUller[2] of high temperature superconductivity (HTS) in oxides and the increase in the transition temperature Tc above the boiling point of nitrogen[3] disrupted the theoretical community. Researchers who earlier had claimed that the modern version of the BCS theory was all encompassing and one of the greatest successes of condensed matter theory began questioning the applicability of the theory to the HTS o
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