Preface: phys. stat. sol. (b) 242/1

We have accepted with great pleasure the suggestion of the Guest Editor Miodrag Kulić to write a short preface to the special issue of this journal, which is devoted to the role played by electron–phonon interaction (EPI) in high‐temperature superconductors (HTSC). From the very beginning, it was absolutely clear to us that there is no metal in which the EPI could be ignored, and high‐temperature superconducting compounds cannot be an exception in this respect. We expressed this opinion, in particular, in our early Review Article [1] and in the talk [2] given at the Grenoble M 2 S HTSC Conference in 1994. We would like to emphasize that we were not in isolation. There have been many other researchers, some authors of this issue among them, who have also considered the EPI as an essential part of the physics of high‐temperature superconductors. However, a large part of researchers in the field, including a few famous scientists, have considered the EPI to be irrelevant to high‐temperature superconductivity. Up to now, we do not understand the scientific basis for such an opinion. Moreover, that point of view has never been shared by some other famous scientists; in this respect mention should be made of J. Friedel and A. A. Abrikosov. Turning back to physics, we would like to point out some features of high‐temperature superconducting cuprates, which should lead to the existence of a strong EPI in these materials. First of all, it is the proximity of these compounds, even in the optimally doped case, to the layered ionic crystals. This fact has been emphasized in our early publications as well as in many papers by other authors, and it is discussed in detail in the Review Article by C. Falter published in this issue. There are other approaches to the HTSC compounds, which allow to consider that a strong EPI exists. They are also based on some peculiarities in the crystalline and chemical structure of these compounds, in particular, on their multiphase nanoscale structure. This point is discussed by J. Phillips in this issue. There are also many experimental indications in favor of the existence of a strong EPI in the HTSC cuprates. For example, the behavior of the electron relaxation, the peculiarities of the phonon spectra, the interaction of the Josephson current with phonons, and the electron mass renormalization. All these phenomena have been discussed in the recent Review Articles [3, 4]. Currently, additional evidence was provided which has thrown new light on the role played by the EPI in HTSC systems. These are the ARPES experiments conducted by the Stanford group, which have given an unambiguous proof of the electron mass renormalization due to the EPI. A Review Article of this group by T. Cuk et al. is also presented in this issue. We should also mention the contribution of L. Pintschovius who presented new interesting results on the electron–phonon coupling effects observed by means of inelastic neutron scattering. A comprehensive discussion of a major part of the electron–phonon coupling effects presented in the Review Articles [1, 3] has been based on the traditional approach of the Eliashberg type. Up to now, we consider this approach to be quite suitable for pursuing a number of goals, mainly for describing properties of the normal state. Nevertheless, we do not disclaim the importance of more detailed investigations of the EPI, which take into account the strong anisotropy, the interplay between electron–phonon and electron–electron interaction, and the non‐adiabatic effects. Four Review Articles in this issue, by Schneider, by Rösch, Han, Gunnarsson and Crespi, by Kulić and Dolgov, and by Cappelluti and Pietronero are devoted to different aspects of these problems. To conclude, we would like to emphasize that the main problem related to the mechanism of superconductivity in the HTSC cuprates is the interplay between the strong EPI and the electron exchange and correlation. Unfortunately, previous work did not crack this problem and much effort should be made in the future. We hope that the publication of this issue will aid to attract the attention of many researchers to the investigation of unsolved problems of the EPI in HTSC systems.