A Snapshot View of High Temperature Superconductivity 2002

Ivan K. Schuller, Arun Bansil, Dimitri N. Basov, Malcolm R. Beasley, Juan C. Campuzano, Jules P. Carbotte, Robert J. Cava, George Crabtree, Robert C. Dynes, Douglas Finnemore, Theodore H. Geballe, Kenneth Gray, Laura H. Greene, Bruce N. Harmon, David C. Larbalestier, Donald Liebenberg, M. Brian Maple, William T. Oosterhuis, Douglas J. Scalapino, Sunil K. Sinha, Zhixun Shen, James L. Smith, Jerry Smith, John Tranquada, Dale J. van Harlingen, David Welch

This report outlines the conclusions of a workshop on High Temperature Superconductivity held April 5-8, 2002 in San Diego. The purpose of this report is to outline and highlight some outstanding and interesting issues in the field of High Temperature Superconductivity. The range of activities and new ideas that arose within the context of High Temperature Superconductors is so vast and extensive that it is impossible to summarize it in a brief document. Thus this report does not pretend to be all-inclusive and cover all areas of activity. It is a restricted snapshot and it only presents a few viewpoints. The complexity and difficulties with high temperature superconductivity is well illustrated by the Buddhist parable of the blind men trying to describe “experimentally” an elephant. These very same facts clearly illustrate that this is an extremely active field, with many unanswered questions, and with a great future potential for discoveries and progress in many (sometimes unpredictable) directions.

It is very important to stress that independently of any current or future applications, this is a very important area of basic research.

Basic research in high temperature superconductivity, because the complexity of the materials, brings together expertise from materials scientists, physicists and chemists, experimentalists and theorists. Much of the research in High Tc superconductivity has spilled over to other areas of research where complex materials play an important role such as magnetism in the manganites, complex oxides, two and one dimensional magnets, etc. Applications could greatly benefit from the discovery of new superconductors which are more robust and allow easier manufacturing. Perhaps this is not possible since a naive inspection of superconductors seems to indicate that the higher the Tc the more complex the material. An excellent review where many target needs for applications have been outlined is an NSF report of ~5 years ago. Many of the comments made there regarding applied needs, are still valid.

It is important to realize that this field is based on complex materials and because of this materials science issues are crucial. Microstructures, crystallinity, phase variations, nonequilibrium phases, and overall structural issues play a crucial role and can strongly affect the physical properties of the materials. Moreover, it seems that to date there are no clear-cut directions for searches for new superconducting phases, as shown by the serendipitous discovery of superconductivity in MgB2. Thus studies in which the nature of chemical bonding and how this arises in existing superconductors may prove to be fruitful. Of course, enlightened empirical searches either guided by chemical and materials intuition or systematic searches using well-defined strategies may prove to be fruitful. It is interesting to note that while empirical searches in the oxides, gave rise to many superconducting systems, similar (probable?) searches after the discovery of superconductivity in MgB2 have not uncovered any new superconductors. Anyhow, this illustrates that superconductivity is pervasive in many systems and thus future work should not be restricted to a particular type of materials systems. See Chapter II.

Research in the electronic properties of High Tc superconductors has proven to be particularly fruitful. This has lead to improvements in electronic structure techniques which unquestionably have an effect on other fields. The improvement on real and reciprocal space resolution uncovered many interesting properties. However, it is not clear at the present time whether many of these properties are related in some essential way to superconductivity or they are just accidentally present. It seems that the presence of competing phenomena is present in most high temperature superconductors. Thus it is natural to investigate systems which are close to some form of instability such as the metal-insulator transition, magnetic phases, electronic instabilities such as stripe phases, etc. Comparisons of classical infrared spectroscopy, and photoemission measurements with tunneling may prove to be fruitful. In particular, mapping with high resolution (in real and reciprocal space) the electronic structure may prove to hold some of the keys to the mechanism of superconductivity. To make these useful, issues such as surface contamination, surface segregation, and in general heterogeneity of the materials close to surfaces or interfaces must be addressed, and are particularly important in these very short coherence length superconductors. This is particularly important for surface sensitive probes such as photoemission. Several techniques such as Raman scattering, NMR and muon spin depolarization are not addressed in this snapshot, although they give valuable information and are heavily researched. Complementary measurements are particularly useful if a whole battery of tests, in the same sample, which are structurally characterized in detail, are performed. The quality of samples on the other hand, must be well established by structural criteria which are well defined a-priori and not based on circular or theoretical arguments. See Chapter III.

The properties of High Temperature Superconductors in a magnetic field have proven to be particularly interesting. A myriad of new phases have been uncovered in the vortex system and have lead to the establishment of a very complex phase diagram the details of which are still being established. The presence of many phases and the interaction/competition/closeness to magnetic phases allows for much new research using artificially structured pinning. New lithography and preparation techniques allow modifications and confinement of these materials in length scales approaching the superconducting coherence length and certainly the penetration depth. Moreover, novel imaging techniques are arising which can give detailed microscopic images of the vortex system. This of course can provide the microscopic picture of the magnetic state of high temperature superconductors and will probably also help improvements on their use. See Chapter IV.

Many basic research studies and a large number of applications require the High Temperature Superconductors to be in proximity with other materials. Thus issues of proximity effects, spatial variations close to an interface or surface, structural and materials variations are particularly important in thin film and/or nanoscopic structures. For this purpose it is important to investigate the mutual interaction between superconductors and other materials. This requires careful preparation and detailed characterization of inhomogeneous materials, together with superconducting measurements as a function of well-defined structural parameters. This may also allow addressing issues such as the importance of the proximity to other ordered phases such as magnetic and electronic inhomogeneities which are naturally existent or are artificially engineered. It is not even clear in the various models of high temperature superconductivity or even experimentally how the proximity effect occurs. What is the dependence of the order parameter in an ordinary or magnetic metal, or a low temperature superconductor when in proximity with a d-wave superconductor? See Chapter V.

Contrary to low temperature superconductors, high Tc ones have received very little attention under nonequilibrium (time dependent, strongly driven, exposed to varying radiations, etc.) conditions. This may prove to be a very interesting and novel direction for ceramic oxides. These types of studies may hold important clues to the mechanism of superconductivity, may unravel new physics and are important in many applications. For instance, simple issues such as the microscopic nature or even existence of critical slowing down close to the superconducting phase transition has not been firmly established. See Chapter VI.

The theory of high temperature superconductivity has proven to be elusive to date. This is probably as much caused by the fact that in these complex materials it is very hard to establish uniquely even the experimental phenomenology, as well as by the evolution of many competing models, which seem to address only particular aspects of the problem. The Indian story of the blind men trying to characterize the main properties of an elephant by touching various parts of its body seems to be particularly relevant. It is not even clear whether there is a single theory of superconductivity or whether various mechanisms are possible. Thus it is impossible to summarize, or even give a complete general overview of all theories of superconductivity and because of this, this report will be very limited in its theoretical scope. The general view point (determined by majority vote) seems to be that low temperature superconductors are phonon mediated whereas high Tc ones are somehow unconventional and anisotropic, although the origin of the anisotropy remains controversial. Because of this, numerical studies in well-defined theoretical models may prove to be particularly illuminating and may help uncover the essence of superconductivity. Particularly, understanding and further developing the t-J model looks like a promising numerical direction. Electronic structure calculations combined with well developed methodologies seem to explain quantitatively many aspects of superconductors with moderate Tcs. How far can these type of approaches be pushed? Could they in fact explain ab-initio superconductivity in some of the cuprates? Moreover, first principle electronic calculations may be very useful in providing parameters for model hamiltonians. Another approach which at least allows parametrizing in some useful way the properties of superconductors has also been used. How far can these type of models go and how universally can they explain the (superconducting or normal) properties is not clear at this stage. There are several important issues which must be kept in mind. It may be that there is a theoretical model which has the essence of the problem in it and it either has not yet been developed or has not yet percolated to the conscience of the community. Moreover, it seems that to date no theory has been developed which has predictive power as far as materials system are concerned. Since purely theoretical approaches have difficulties so far in identifying a clear avenue for search, empirical studies in which materials parameters and properties are correlated with superconducting properties may prove useful. This may serve at a later stage as a test ground for theories. Comparisons of theoretical ideas which rely only on the layered material of high Tc ceramics, with artificially engineered layered superlattices should not be neglected and may prove to be useful. See Chapter VII.

Finally, there seems to be still much work needed to understand in detail the connections, control and effect of defects on high temperature superconductivity. This of course is very important for applications, particularly those which require high critical currents such as power applications. Moreover, the intrinsic brittleness highlights that understanding and controlling the mechanical properties while not directly related to superconductivity, is a very important and promising new area of research, especially in connections with large scale applications. See Chapter VIII.

In the rest of this paper we will expand on these issues and attempt to outline some well defined promising directions of research. The focus is mostly on basic research challenges and opportunities, which hold back progress.

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(c) 2007 Ivan K. Schuller       -       designed by Thomas Gredig