Anisotropic pairing caused by unscreened long-range interactions

Anisotropy of the order parameter in high-T _c superconductors is explained by unscreened interaction of charge carriers with long-wave optical phonons. Screening is absent due to the low frequency of long-wave plasmons in layered structures.The authors are grateful to G. Deutscher, A. Bill, and E. Joon for discussions. Thanks are also due to D. Nevedrov who did the numerical calculations. The research described in this publication was made possible in part by Grant No. LL4100 from the Joint Fund Program of Estonia. This work was also supported by Estonian SF Grant No. 369.