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date: 19 August 2019

(p. xi) Plan of the Book and Acknowledgments

(p. xi) Plan of the Book and Acknowledgments

Plan of the book

This Oxford Handbook is about the remarkable materials that are now commonly referred to as mesoscopic superconductors. The word “meso” has a Greek root, and refers to the “middle”; in this case, the meso level operates between the bulk and atomic levels. In fact, mesoscopic superconductors are, for all practical purposes, tiny: often fabricated at dimensions that range from about a nanometer to about a few micrometers. When the size of the superconductor is reduced to this level, its properties are dramatically changed. For simplicity, we have called them small superconductors. The subject matter would be of interest to graduates and final-year undergraduates reading physics, materials science, and engineering with a focus on nanoscience and nanotechnology. It is also likely to be directly relevant to specialists and trained electronic engineers interested in the possible new generation of ultra-small high-sensitivity superconducting probes.

The theoretical and experimental research studies in mesoscopic superconductivity have been groundbreaking. Further, in a world obsessed with miniaturization of electronic device technology, mesoscopic superconductors are acquiring even greater relevance and timeliness. But here, miniaturization is not meant to be just a choice of fashion; it is, in fact, an inescapable commitment, if one is to exploit the quantum effects that manifest themselves only at the nano levels. This Handbook is meant to provide a lens into what might emerge as a giga world of nano superconductors. Chapters contributed by a host of eminent frontrunners in the field investigate the novel and intriguing features and theoretical underpinnings of the phenomenon of mesoscopic superconductivity, offer accounts of the latest fabrication methods and characterization tools, and discuss the opportunities and challenges associated with technological advances. In its totality, the book addresses the current status and great promise of small superconductors in the theoretical, experimental, and technological spheres.

This Handbook comprises 19 chapters describing cutting-edge developments in research and applications of small superconductors. The chapters are grouped and presented in three parts. The distribution is kept near-even with 7 chapters in Part I, 6 in Part II, and 6 in Part III. The first part carries an extended introduction and relates to the developments in basic research of small superconductors. Part II is materials-specific, while Part III reviews the current progress in their device technology. However, perhaps it is worth pointing out that the progress (p. xii) described in most of the topics presented in the three parts is of recent origin. Consequently, the boundaries separating some of them are as yet not sufficiently sharp, and therefore the pertinent chapters could arguably have been grouped differently in the three categories from how we have presented them below.

Part I

Introduction and Basic Studies

Part I starts with an introductory chapter (Narlikar) whose prime motivation is to make the interested reader, new to this important area, familiar with some of the basic definitions, prominent characteristics, and important effects manifested by small superconductors. We have therefore briefly introduced some of the mainstay topics of the field which include various size effects, surface effects, electron-mean-free-path effects, different types of phase slips, unusual vortex states, a variety of proximity effects, and some of the experimental techniques commonly used in the synthesis of small superconductors. This should allow the reader to understand the specialized and more comprehensive chapters covering various novel facets of this evolving field which this book is all about. The next two chapters discuss the intriguing vortex matter of small superconductors with numerous vortex states that do not exist in bulk superconductors. Here (Chapter 2), Roditchev and his collaborators have studied, using STM/STS techniques, the organization of vortex cores at different levels of confinement. They show that the sample size and shape govern the vortex distribution and pinning, leading to ultra-dense configurations unachievable in bulk superconductors. These authors further present the peculiar features of vortices in atomically thin superconductors which exhibit mixed Abrikosov–Josephson vortices. In Chapter 3, Kokubo, Okayasu, and Kadowaki present a state-of-the-art overview of their recent work on the multi-vortex state carried out on mesoscopic superconducting dots of different geometrical shapes using a scanning SQUID microscope. They discuss the formation of multiple shell structures, their sequential filling, and the commensurability effect, which hold immediate relevance to future developments of magnetic flux-based superconducting mesoscopic instrumentation and quantum computers. Cuevas et al. in Chapter 4 describe the proximity effect on small length and energy scales in novel low-dimensional systems, studied with the combination of in situ fabricated superconducting nanostructures and STM/STS techniques. Gonnelli and his collaborators, in Chapter 5, have emphasized the potential of the point contact Andreev reflection spectroscopy (PCARS) technique for measuring the symmetry of the energy gap and other key parameters of various 0-, 1-, and 2-dimensional systems. Topological superconductors and Majorana fermions form the exciting theme of Chapter 6 by Li and Jia. The topological superconductor has led to new insights about mesoscopic superconductivity and has revealed that some superconductors can support Majorana fermions (p. xiii) and non-Abelian statistics. The closing chapter of Part I by Gariglio and his co-authors focuses on surface and interface superconductivity, another pivotal area of mesoscopic superconductivity. The current experimental status and theoretical understanding of the field have been reviewed and the perspective on unconventional phenomena occurring at the surfaces and interfaces is discussed.

Part II

Materials Aspects

The contents of Part II focus on mesoscopic materials, their synthesis, and their properties, although as mentioned earlier some of the chapters included in this part could just as well have been in the previous one. It starts with Chapter 8 where Karapetrov and his collaborators present the mesoscopic effects in superconducting (SC)/ferromagnetic (FM) hybrids, an area which has matured during the last 10 years. The authors have discussed interesting situations that lead to spontaneous formation of flux vortices pinned to the magnetic domain structure. Formation of vortex–antivortex molecules, vortex chains, disordered vortex matter, etc., which until recently were only theoretically predicted, have now been experimentally corroborated through local scanning probe microscopy (SPM) techniques. In Chapter 9, the focus is on HTS cuprates. Asai has theoretically considered the intrinsic Josephson junction (IJJ) model to discuss the THz emission from these materials in the mesoscopic state. The intense emission is ascribed to the strong excitation of transverse Josephson plasma waves in IJJs under a DC bias. The chapter further discusses the recent theoretical and experimental studies aimed towards realizing practical THz sources based on mesoscopic HTS cuprates. In the next chapter, André Müller and his coauthors present their work on micromagnetic measurements on electrochemically grown mesoscopic superconductors, namely lead and tin. The technique allows fabricating samples in a variety of shapes and sizes in the mesoscopic regime. Its particularly remarkable feature is that one can use the technique to grow composite core–shell structures of the two superconductors and subsequently study their mutual proximity interaction. Using the micro-Hall probes and by looking at only one sample at a time, the authors have been able to observe individual flux lines entering and leaving the sample and discuss their size-dependent behavior near Tc. Growth, structure, and properties of nanowires of high-temperature cuprate superconductors form the mainstay of Chapter 11 by Koblischka. He has described two methods, namely the templating technique and electrospinning method, for preparing nanowires and nanobelts of HTS cuprates such as YBCO, NdCO, LSCO, and Bi2212. The methods yield continuous long lengths, albeit of somewhat larger thickness of 100–250 nm, suitable for applications. In Chapter 12, Han Zhang has related mesoscopic structural features of the HTS crystal structures to understand their characteristic high T c and anisotropy by developing model calculations. As the (p. xiv) closing chapter of Part II, Ortiz and his collaborators, in Chapter 13, look into the practical problem of thermally driven high-speed flux avalanches occurring in superconducting thin films. The thin films are synthesized with artificial pins in the form of sub-micrometric antidots, which though enhancing the critical current give rise to large-scale flux avalanches, adversely affecting the superconducting device performance. The authors have discussed the vortex dynamics and avalanche morphologies in relation to antidot geometries and the current crowding effects. These considerations provide them with some useful insights to meet and overcome the above challenge.

Part III

Device Technology

In this last part of the Handbook we have six contributions describing the progress in the technological development of small superconductors. As stated at the beginning, in technology, the prime motivation in making superconductors small in size has been to meet the challenge posed by the ever-increasing demand of device miniaturization and to improve their capabilities in terms of resolution and sensitivity. By using a combination of a superconductor and a ferromagnet one can have Cooper pairs in the triplet state in which the electron spins are aligned like a ferromagnet. The superconducting spin currents generated in such a situation are sensitive to the magnetization direction of a ferromagnet. These two features together constitute the basis of the exciting new field of superconducting spintronics, covered by Blamire and Robinson in Chapter 14. This way, as the authors discuss, within a single circuit, in principle, one can integrate the data storage capabilities of magnetics with the low-energy dissipation of superconducting electronics. The chapter reviews the current status of this new field, drawing attention to the critical issues and developments needed for its application to low-power quantum computing. In Chapter 15, Weides focuses on Josephson junction (JJ) barriers where the mesoscopic superconductivity resides and which are the most intriguing components of JJ-devices. The author has critically discussed a host of different barriers made from insulators, metals, semiconductors, magnets, and nanowires and new developments in device technology are identified. In Chapter 16, Nygård and his co-authors present an overview of their studies of hybrid superconducting devices based on quantum wires, in the form of semiconductor nanowires or carbon nanotubes, which are coupled to superconducting electrodes. These coupled structures serve as highly tunable mesoscopic systems whose characteristics are sensitively dependent on the strength of coupling between the two. Weak coupling results in quantum dots while strong coupling gives rise to quantized supercurrents in a one-dimensional nanowire. The authors discuss a host of exciting possibilities such as topological superconductivity, (p. xv) superconductivity-enhanced quantum dot spectroscopy, and non-local signals in Cooper pair splitter devices that emerge primarily due to changes in the coupling strength between the superconductor and quantum wire. In the next chapter Gallop and Hao review the recent progress made in superconducting nanodevices by presenting details of the fabrication methods developed for superconducting nanowires and nanoscale JJs based on different barrier materials that also include the recent microbridges and weak link methods. Future potential of these nanodevices is assessed in the light of improvements in nanoscale fabrication and manipulation techniques and their likely impacts on future quantum technology and measurement are evaluated. In Chapter 18, Bylander presents the very important topic of superconducting quantumbits. During the last few years there has been extraordinary progress in this area, although the construction of a universal quantum computer still remains a big challenge. In this chapter, Bylander starts with the basics of modern superconducting qubit devices and their architectures, and reviews major improvements in circuit designs, materials, and experimental tools. This way we learn about the experimental state of the art, including the latest research directions pursued in this highly competitive area of small superconductors. Finally, this brings us to the last chapter of Part III, Chapter 19, by María José Martínez-Pérez et al., where the authors describe the nanoSQUID for investigating small magnetic systems. The authors discuss how the already very high sensitivity of conventional SQUIDs gets further enhanced by reducing the size of the SQUID loop down to the nanoscale. At the same time they draw attention to the practical constraints and challenges encountered in using the nanoSQUID technology to study small spin systems ranging from magnetic nanoparticles to molecule magnets. To achieve a proper alignment of the magnetic nanoentity with the nanoSQUID as the measuring device remains a non-trivial problem with the new technology.

The contents of the Handbook, as mentioned at the beginning, should interest advanced-level science and engineering students and further reach out to the nanotechnology experts from electronic industries, interested to know the current status of the theory, manufacture, and future of mesoscopic superconductors. Various reviews and overviews might also answer the queries and curiosities of non-specialists interested in nanoscale superconductivity. In doing so, the present volume will offer to all types of the mentioned readership the opportunity to engage with the cutting edge of research in arguably the most exciting nanolevel discipline of Physics, Materials Science, and Engineering of today and tomorrow.

Acknowledgments

At the outset, I remain grateful to all the contributors of this Handbook, from more than 15 countries, for their enthusiastic participation and sustained (p. xvi) cooperation all along without which this project could not have been feasible. I am thankful to the Indian National Science Academy, New Delhi and its Science Promotion program for supporting my superconductor research and the preparation of the present book. Thanks are extended to the Director of UDCSR, for making available various facilities and resources of the Consortium. I am especially grateful to my colleague Dr. U.P. Deshpande for his help and suggestions. Helpful assistance provided by Arjun Sanap is acknowledged. I am thankful also to my former colleagues at NPL, New Delhi, in particular Drs. Hari Kishan, Anurag Gupta, and V.P.S. Awana, for many useful discussions and for drawing my attention to some of the recent publications. A substantial part of the work of preparing the present book was carried out during my various extended visits to Hamburg, Germany where I am grateful to Professor Dr. Kurt Scharnberg of the Theoretical Physics Group of the University of Hamburg for many stimulating discussions and very useful suggestions in preparation of this book. I am further thankful to him and to Professor Dr. Roland Wiesendanger of the Nanoscience Center and the Physics Department of the University of Hamburg, for their courtesy in providing the office space and the basic infrastructure facilities. Several useful suggestions and enthusiastic support that came from Ms. Julia Kramer and Mr. Olaf Kruithoff are heartily acknowledged. I remain thankful to Professor Dr. B.A. Glowacki, Head of the Applied Superconductivity and Cryoscience Group of the Department of Materials Science and Metallurgy at Cambridge University, for many years of interaction and his vast experience on nanostructured superconductors which was invaluable in the present project.

Very special thanks are due to my wife Dr. Aruna Narlikar for her invaluable help, patience, and constant support. Her valued suggestions and unmatched cooperation throughout during the preparation of the book were most admirable. It is worth mentioning that the original suggestion of the present handbook devoted to nanoscale superconductors came from our daughter, Professor Dr. Amrita Narlikar, who had an active role also in my drafting the OUP proposal for the present book. I am thankful to her for this and for her sustained enthusiasm in helping me in numerous ways, including in getting me the references and publications that I often needed rather urgently. At OUP, Oxford, the superb cooperation and the efficient help provided by Ms. Ania Wronski were instrumental in overcoming the unexpected hurdles and in meeting the time targets. Along with her I would like to record my personal thanks to Dr. Sonke Adlung at OUP for his experienced advice and timely suggestions at all stages of the project. It has been a pleasure working with him, as always.

June 2016

A.V. Narlikar