Ferroelectric materials and high-Tc superconductivity

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Research into high transition temperature (high-Tc) superconductors is very important in materials science. The first objective of the present letter is to discuss the controversy concerning the interplay of ferroelectricity and superconductivity. The second objective of the present letter is to emphasize that the polarization of a high-T c ferroelectric crystal probably plays an important role in the pairing mechanism of high-T c superconductivity. The interplay of ferroelectricity and superconductivity was discussed in a review paper about A15 compounds [1]. Usually the conclusion has been that the two phenomena are mutually exclusive. In 1967, Matthias [2] conjectured that "ferroelectricity and superconductivity seems to be mutually exclusive" and that "ferroelectrics such as SrTiO3 have no chance of becoming superconducting." However, it was shown [3] that SrTiO3 becomes superconducting by carrier doping. Soon after the discovery of high-T~ superconductivity this controversy was re-opened. "Diagrammatic analysis enables one to indentify dramatic and specific parallels between ferroelectrics and superconductors," claimed Phillips [4]; Sleight [5] pointed out that "the polarizability which plays a crucial role in the ferroelectricity of BaTiO 3 may also play a crucial role in the superconductivity of the high-Tc superconductors." It has been shown [6] that ferroelectricity and excitonic superconductivity should be related to each other. There are reported experimental [7] and theoretical [8] evidence for an eventual correlation between ferroelectricity and high-T~ superconductivity. Another experimental report [9] shows a ferroelectriclike phase of the most famous high-T c superconductor YBa2Cu307 x. So far, X-ray or neutron structural determinations show that there is no indication of a polar structure in the high-Tc superconductors. However, recent observations [10-12] of the pyroelectric and piezoelectric effects in YBa2Cu3OT_ x and in La2CuO4+ x show clearly direct evidence of polar structures in these high-T c superconductors. Experiments on the AC Josephson effect [13] have revealed that the charge carrier in the superconducting current in these high-T~ superconductors has the charge 2 e or 2e (as on a Cooper pair). Therefore, there is no doubt that high-T c superconductivity is produced by paired electrons (or paired holes). However, the pairing mechanisms of high-T c superconductivity are not clear. Presently, the most important known mechanisms eventually responsible for high-T c superconductivity are the Fr6hlich electron-phonon interactions [14]; the Jahn-Teller effect [15]; excitonic mechanisms [16]; bipolaronic