Fluctuation Mechanisms In Superconductors

Holger Bartolf discusses state-of-the-art detection concepts based on superconducting nanotechnology as well as sophisticated analytical formulæ that model dissipative fluctuation-phenomena in superconducting nanowire single-photon detectors. Such knowledge is desirable for the development of advanced devices which are designed to possess an intrinsic robustness against vortex-fluctuations and it provides the perspective for honorable fundamental science in condensed matter physics. Especially the nanowire detector allows for ultra-low noise detection of signals with single-photon sensitivity and GHz repetition rates. Such devices have a huge potential for future technological impact and might enable unique applications (e.g. high rate interplanetary deep-space data links from Mars to Earth).

Since the discovery in 1986 of high temperature superconductors by J. G. Bednorz and K. A. Müller, a considerable progress has been made and several important scientific problems have emerged. Within this NATO Advanced Study Institute our intention was to focus mainly on the controversial topic of the symmetry of the superconducting gap and given the very short coherence length, the role of fluctuations. The Institute on ‘The Gap Symmetry and Fluctuations in High- Superconductors’ took place in the “Institut d’Etudes Scientifiques de Cargèse” in Corsica, France, between 1 - 13 September 1997. The 110 participantsfrom 18 countries (yet 30 nationalities) including 23 full time lecturers, have spent two memorable weeks in this charming Mediterranean resort. All lecturers were asked to prepare pedagogical papers to clearly present the central physical idea behind specific model or experiment. The better understanding of physics of high temperature superconductivity is certainly needed to guide the development of applications of these materials in high and weak current devices.

This NATO Advanced Research Workshop was held at a time when there was little consensus as to the mechanism for high temperature superconductivity, in the context of a world undergoing major changes in its political alignments and sense of the possibility for the future. It was characterized by generosity in the sharing of our uncertainties and speculations, as was appropriate for both the subject matter and the context. The workshop was organized, of necessity around the experimental work, as is this volume. Where the theoretical work is directly relevant to particular experiments, it is included in the appropriate sections with them. Most of the participants felt strongly that magnetic fluctuations played an important role in the mechanism for high T c, although with the exception of the IlS R work reported by Luke showing results inconsistent with the anyon picture, and the work on flux phases by Lederer, the mechanism remained an issue in the background. A major focus was the phenomenological interpretation of the NMR data.

These Proceedings of a NATO-ARW (HTECH ARW 96 00 52) held at the International Center for Theoretical Physics, Trieste, Italy from Aug 5 till Aug 9, 1996 resulted from many discussions between various workers, concerning the need for a gathering of all (if possible) who were concerned about the subject of superconductivity fluctuations in High critical Temperature Superconductors (HTS). It appeared to many that the Skocpol-Tinkham work of 1975 had to be revitalized in view of the discovery of the new superconducting ceramics and the enormous amount of work having already taken place. The study of HTS is one of the most prominent research subject in solid state sciences. The understanding of the role of fluctuations is also thought to be necessary before technological applications since the fluctuations may destroy the superconducting state. The workshop discussions have touched upon (i) Superconducting fluctuations in the vicinity of the critical transition, (ii) Superconductivity fluctuations near the percolation transition, and (iii) Fluctuations of the vortex lattice at the lattice melting temperature. These topics served as initiators for a very great amount of discussions with many comments from the audience. More than forty "long lectures" and two "poster sessions" were held. Private discussions going unrecorded but obviously took place at many locations : lecture halls, staircases, cafetaria, bedrooms, bars, beach, . . .

The Novel Mechanisms of Superconductivity Conference was initially conceived in the early part of 1986 as a small, 2-1/2 day workshop of 40-70 scientists, both theorists and experimentalists interested in exploring the possible evidence for exotic, non phononic superconductivity. Of course, the historic discoveries of high temperature oxide superconductors by Bednorz and Mftller and the subsequent enhancements by the Houston/Alabama groups made such a small conference impractical. The conference necessarily had to expand, 2-1/2 days became 4-1/2 days and superconductivity in the high Tc oxides became the largest single topic in the workshop. In fact, this conference became the first major conference on this topic and thus, these proceedings are also the first maj or publication. However, heavy fermion, organic and low carrier concentration superconductors remained a very important part of this workshop and articles by the leaders in these fields are included in these proceedings. Ultimately the workshop hosted rearly 400 scientists, students and media including representatives from the maj or research groups in the U.S., Europe, Japan and the Soviet Union.

The book includes 17 chapters written by noted scientists and young researchers and dealing with various aspects of superconductivity, both theoretical and experimental. The authors tried to demonstrate their original vision and give an insight into the examined problems. A balance between theory and experiment was preserved at least from the formal viewpoint (9 and 8, respectively). The readers should be warned that many of the problems studied here are far from being solved and are treated on the basis of competing viewpoints. The reason is that such is the state of the art! Science of superconductivity develops rapidly and new unexpected discoveries are expected in the nearest future.

Superconductivity in materials without inversion symmetry in the respective crystal structures occurs in the presence of antisymmetric spin-orbit coupling as a consequence of an emerging electric field gradient. The superconducting condensate is then a superposition of spin-singlet and spin-triplet Cooper pairs. This scenario accounts for various experimental findings such as nodes in the superconducting gap or extremely large upper critical magnetic fields. Spin-triplet pairing can occur in non-centrosymmetric superconductors in spite of Anderson’s theorem that spin-triplet pairing requires a crystal structure that exhibits inversion symmetry. This book, authored and edited by leading researchers in the field, is both an introduction to and overview on this exciting branch of novel superconductors. Its self-contained and tutorial style makes it particularly suitable for self-study and as source of teaching material for special seminars and courses. At the same time it constitutes an up-to-date and authoritative reference for anyone working in this exciting field.