New research opportunities in superconductivity

New research opportunities in superconductivity* D.J S e a l a p i n o , D . R . C l a r k e t, J. C l a r k e , R E. S c h w a l l t , a n d D K. F i n n e m o r e * *

A.F. Clark~

Umversity of Cahfornm-Santa Barbara, Santa Barbara, CA 93106, USA t l B M T J Watson Research Center, PO Box 218, Yorktown Heights, NY 10598, USA $Office of Naval Research London, 223 Old Marylebone Road, London NWl 5TH, UK **Ames Laboratory, Iowa State Umverslty, Ames, IA 50010, USA Reported are the d~scussions held at the Workshop on Problems tn Superconductiwty held at Copper Mountain, Colorado on 10 1 2 April 1 988 The Workshop was orgamzed m four sessions addressing fundamentals, matermls, electromcs and large-scale apphcatmns, wtth D J Scalapmo, D R Clarke, J Clarke and R E Schwall as discussion leaders, and A F Clark and D K Fmnemore as co-chmrmen The report prowdes insight into the opportumtles discussed by the Workshop members, identffms barners to developmental progress and recommends specific areas of superconductwtty research

Keywords superconduct,ng electromcs, h,gh Tc superconduct,wty, large-scale apphcat=ons, superconducting materials, rewew

Introduction

Brtef summary of recommendattons

Superconductivity has exploded into many areas of condensed matter physics and will have Impact nearly everywhere electricity and magnetism are used throughout the industrial world A workshop was convened to assess the research needs created by the recent breakthroughs m superconductlwty This report provides insight into the new research opportumtles In superconductivity which were highlighted by the Workshop members It provides a systematic approach to fundamental understanding and to the necessary research focussing on the new discoveries of high temperature superconductivity The workshop members were clear, however, in their recognition that research and development must continue to support the several successful applications that use the low temperature superconductors that are now in or near production There was also strong support for research on heavy fermion and organic superconductors and the continued search for new materials Sustained and sufficient funding was identified as the key issue

The recommendations that evolved from 1½ days of intensive discussion are based upon the need for fundamental understanding for all superconductors and the recognition that their properties are very sensitive to the details of structure, microstructure, and Stolchlometry These recommendations are more fully described in the subsequent sections and outlined in the closing summary, but the key elements are listed below

*A report on the Workshop on Problemsm Superconducttwty held at Copper Mountain, Colorado on 10 12 Apnl 1988 under the sponsorship of the National Smence Foundatmn through D H Liebenberg under agreementnumber DMR8813937 and the Office of NavalResearchthrough EA Edelsack Any opmton, findings and conclusions, or recommendationsexpressedm thin pubhcat=on are those of the authors and do not necessarilyreflectthe views of the Nattonal Science Foundation or the Office of Naval Research The Workshop was orgamzed underthe auspicesof the National Bureau of Standards Boulder, Colorado, USA and the Ames Laboratoryof the DoE, Ames, Iowa USA tAIso at NationalBureauof Standards,ElectromagneticTechnology Diwston, MS 724 05, Boulder, CO 80303, USA 0011-2275/88/110711-13 $03 00 (, 1988 Butterworth & Co (Pubhshers) Ltd

1 Funding is an Issue more important than issues of material characterization, accurate solutions of model Hamlltonlans, and the development of new processes It is the key barrier to progress at the present time Graduate students are eager to .loin in this research, excellent young faculty are interested in developing new activities, and scientists and engineers from greatly varied disciplines are excited to contribute However, suffictent and sustained funding ts required 2 Understanding the fundamental mechamsm is both an intellectual challenge and important to relating various superconducting, magnetic, insulating, and normal conductor properties in high temperature superconductors Basic theory and single crystal properties are essential for this development 3 Physical and ~hemwal properttes measurements are essential and must be obtained on well-characterized samples with controlled Stolchiometry, impurities, and microstructure Crystal structure and crystal chemistry must be included to provide systematic mformauon on phase equilibria, chemical compat~bdlty, and environmental stablhty Along with the fundamental theory, phenomenologIcal models are needed to compare, correlate, and guide these experimental measurements

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New research opportumtms m superconductlwty D J Scalapmo et al 4

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Technological progress is limited primarily by critical current and sensitivity of the supercurrent to magnetic field Investigations are reqmred to understand flux pmmng and the control and Influence of interfaces and gram boundaries m both thin film and bulk materials Understanding Interfaces is essentml to the behavlour and the development of junctions, contacts, and even stablhzation Development of non-tradtttonal processing wdl be expected m order to enhance the crmcal current and electrical properties and to develop mechanically strong and fracture resistant films, fibres, and wires Low temperature processes, must be found to provide compatlblhty with semiconductors Multllayer film processes provide a special opportunity to control properties by processing and to produce new metastable compounds Innovation in the development of devtces and device concepts is possible owing to the complexity of the new materials However, some replication of present device development at higher temperatures will enhance their utlhty m many commercial and scientific applications Continued development of conventional superconductor electronics systems is an essential technology base Investigations of new matertals are essential, both on matermls derived from known superconductors and those that reach out for new classes of materials At present the hmltatlons on superconductivity are completely unknown

The advent of the new high critical temperature superconductors opens new areas for the exploration of fundamental superconducting mechanisms and it may have significant influence on the commerce of the succeeding decades provided that support for research, student training, and transfer of technology to the commercial sector ~s sufficient and sustained

New high Tc superconductors The recent discovery of a whole new class of materials that show superconductlwty above 30 K has revolutionized the field In the normal state their properties are unlike any other metals, in the superconducting state the fundamental mechanism causing the pairing is qmte different from other superconductors These materials have unusual chemical bonding, they are ceramic in mechanical behavlour Indeed, it is clearly necessary to develop a whole new conceptual framework to understand these materials It all began in 1986 when Bednorz and Muller showed that a new class of matermls, now called copper oxide superconductors, had a transition temperature above 30 K The first material was a lanthanum-barium-copper-oxide w~th a perovsklte-hke crystal structure having copper atoms in the centre of the oxygen octahedra Within a few months, Chu's group found a different perovsklte-hke crystal structure containing yttrium in place of lanthanum that showed superconductwlty above 90 K Recently both the bismuth and thalhum modlficatmns have shown superconductivity well above 100 K A feature common to all of these materials is a highly amsotroplc crystal structure containing copperoxygen sheets, thus forming a rather unusual metal

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More recently, It has been found that the copper is not essential It is ~mportant to develop a thorough understanding of both the electrical and mechanical properties of these intricate materials

Context for the Workshop In 1983 a workshop was convened to assess progress in the field of superconductivity for the Office of Naval Research and the National Science Foundation The report from that workshop, Research Opportunities in Superconductivity, was pubhshed in Cryogenics1, and the findings of that workshop stdl apply Indeed, the first sentence of the Bednorz and Muller paper is a quote from that report Considerable progress has been made since then the magnetic Imaging technique has grown Into a bdhon dollar industry, SQUIDs are finding use in the study of functmns of the brain, for example, in focal epilepsy, a whole new range of high frequency (600 MHz) nuclear magnetic resonance spectroscopy has been built on nloblum-tln magnet technology, which is especially important for biology and the study of proton resonance in DNA molecules, finally, of course, there was the spectacular discovery of the copper oxide class of high transmon temperature superconductors With these discoveries, it was decided to convene a slmdar workshop to explore the present opportunities This was a 1½ day meeting of intensive discussion attended by 29 scientists active in superconductivity research The results of that discussion are outlined below in the general categories of fundamentals, materials, and applications although the discussion found them intimately intertwined

Fundamentals The discovery of the high transition temperature oxide superconductors has raised a broad set of fundamental questions regarding the basic electronic properties of metallic oxides Experimentally one has systems that can be insulating, semiconductlng, antlferromagnetic, chargedensity-wave, metallic or superconducting, depending upon subtle changes m StOlchiometry or structure Theoretically we have come to reahze that we have a great deal more to learn about electronic correlations in the intermediate regime separating localized and itinerant behavlour In particular, it ~s not s~mply that we cannot calculate with confidence the transition temperature of a particular model, we do not even know with certainty whether ~ts ground state is that of normal metal, antlferromagnetic insulator, charge densIty-Pe~erls insulator, or a superconductor In some ways the proximity of various phases and the Interrelationship of superconductivity, antlferromagnetlsm and charge density waves in the oxide superconductors is reminiscent of the heavy fermion and organic superconductors Thus, it is an exciting time, where there are a number of ~mportant problems, whose solutions promise to both deepen our basic scientific understanding and also impact technology In this section we wdl review some of the current questzons associated with the superconducting oxides which provide basic research opportunities We wdl also &scuss some of the central issues which remain to be addressed

New research opportumttes tn superconducttwty D J Scalapmo et al Mechamsms One of the key problems, and an active area of research, concerns the question of the mechanism responsible for the high Tc values of the new superconducting oxides From the materials side we have the various famlhes of oxide superconductors ranging from BaPbl - x Blx O3 and the recently discovered Bal _xKxBIO 3 to the well known La2 -xMxCuO4, YBa2Cu307 -x and the newer BI and T1 (CuO2)-n-layer compounds These matermls and others obtained by various substitutions contain the code we are to decipher In addition, a wide variety of physical measurements are providing valuable information These range from thermodynamic properties such as specific heat, cntlcal fields, penetraUon depth, and susceptibility to transport measurements, such as frequency dependent infrared and microwave conductivity, thermopower, ultrasonic propagation, electron tunneling, and nuclear quadrapole resonance (NQR) In addition, X-ray, neutron, muon spin rotation (/~SR), hght scattering, and positron annihilation along with UPS, EELS, and other surface probes have provided key insights Here we should recognize the important role that the national neutron and #SR facilities have played As the ability to characterize adequately the materials used m various experiments has improved, measurements from different groups are convergmg The problem and need for careful sample characterization remains The availability of well characterized oriented thin films and umform smgle crystals are particularly Important because of the anlsotropy of the CuO2 materials A natural area for further extension ~s the measurement of a variety of physical properties on the same well characterized sample Also, the discovery of superconductivity in Bao 6Ko 4BIO3 clearly suggests the need for further studies of other metal oxides The new materials and the variety of measurements have raised a number of questions which either suggest mechanisms or must be addressed by proposed mechanIsms From the structural and chemical side, why are the metal-oxide perovsklte-hke structures so special9 Is it that they provide two subsystems or is it the special role of oxygen 9 What is the relationship of the insulating state of the metallic and superconducting states9 Are we to think of an underlying charge-density-wave Pelerls mechanism for the Ba1_~,KxBIO 3 system and an antlferromagnetlc based mechanism for the CuO2 systems9 When the antfferromagnetlsm is suppressed by doping, does it mediate the pairing on the CuO 2 sheets or as ~t s~mply reduced, allowing another mechanism such as C u - O charge fluctuations to drive the system superconducting9 How does T~ depend on the number of CuO2 layers m the new T1 and BI compounds 9 What can we say about the symmetry of the pair wave function9 Finally, hngerlng questions Involved with ferroelectric-hke strong electron phonon couphng remain Clearly there are many more questions, marking th~s as a unique area for research In response to these questions and others, a wide variety of theoretical models have been proposed They range from BCS-hke approaches in which the pairing is mediated by the exchange of charge or spin fluctuations to magnetic bag or charge distortion mechanisms In some very strong coupling theories, pairs are preformed above Tc One reason that there are still so many possible mechanisms is the lack of detailed physically observable,

predictions We need to know the physical consequences of a given model We need to know the nature of the strong couphng and weak couphng limits of a model Here it is essential to make sure that one learns about the properties of a given model and not simply the artifacts of a particular approximation In addition, we must recognize that the short coherence length and the low carrier density may give rise to basic changes in the usual mean field BCS theory

Phenomeno~gy The problem of the interpretation of measurements remains a central issue since at present we have no complete microscopic theory of the 'normal' or the 'superconducting' state Even with careful measurements of a physical property on a well characterized sample, one is faced with the fact that neither the normal state nor the superconducting state are presently understood Thus the interpretation of the results remains a challenge One solution ~s to present more raw data as opposed to data processed through theories which may have little to do with the basic underlying physics Alternatively, there is a clear need for phenomenologlcal theories which address in a more general manner important physical features w~thout necessarily focussing directly on the microscopic mechanisms For example, the analysis of the temperature dependence of the spin correlations in a 2D spin-l/2 antlferromagneUc and the effect of quantum fluctuations on the formation of a NSel ground state has led to important insights Work on phenomenologlcal models which take into account the short superconducting coherence length and the low carrier concentration will be of great interest What happens when one rejects a quasiparticle Into a system with such a short coherence length and where there are a relatively dilute number of carriers9 What is the microwave absorption in such a system, particularly ff it is a highly anisotroplc 2D system like the CuO 2 materials Phenomenologlcal theories which treat the vortex lattice, problems of pinning, and interface beha~lour are needed The problem of granular materials and random 3D arrays of JosephsonjuncUons represent a clear area for further research

InnovaUon As with all basic science, innovative new ideas are likely to hold the key to a full understanding of the high Tc phenomena Clearly, the discovery of new materials such as the recently reported 3, 4, and 5-layer Tl and B1 compounds and the Ba 06Ko 4B103 30 K material provides important new information and suggests new directions It is equally clear that it is important to continue to pursue detailed studies of the earlier materials Just as electron tunneling provided the most detailed probe of the phonon exchange pairing mechanism in the low temperature superconductors, perhaps some innovative technique will allow tunneling Into the oxide superconductors to be more effectively used or possibly more effectively interpreted Beyond this, novel ideas for local space time probes of these materials should be encouraged Photo-induced infrared absorption measurements have demonstrated that photo-excited carriers

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New research opportunities in superconductlwty D J Scalaptno et al produce relatwely long-hved polarons or blpolarons Measurements of the dynamic response of these &stortmns, as well as possible magnetic or charge density &stortmns could provide ~mportant insight into the pamng mechanism

Apphca~ons The new oxide materials raise important fundamental questions closely related to apphcatlons These revolve, for example, mterfacml phenomena, flux pmnlng, electron tunnehng, and microwave loss phenomena Here the high critical temperature, large amsotropy, (of the CuO2 materials), short coherence lengths (~ab ~ 15--30 A and ¢c ~< the c axis cell size) and the relatively low density of carriers can lead to new physical properties The interface problem is particularly important, smce it may become possible to make microscopically layered systems mvolwng superconductmg-msulatmg, superconductmg-antlferromagnetlc, or superconductmgsemlconductmg mterfaces The fact that these structures may occur on an atomic super lattice scale raises excmng possibilities for new types of collective effects The structure of vortices, pinning and the intermediate states are another area that merit specml attention For example, new types of mtermedmte flux states for H shghtly greater than H¢x have been proposed The tunnehng conductance exhibits a hnear voltage variation over a large regime and clearly contains new information on the structure of the quasi-particle states The properties of granular systems, particularly granular systems of highly anlsotroplc matermls, prowdes a number of interesting problems, important for composite oxide materials For example, one may v~ew a granular system as providing a 3D x y model and study ~ts phase dmgram One also needs theoretical work on the conduction, microwave and optical properties of granular systems It may also be that art~ficmlly created layered structures offer useful analogues or models to explore

Exotic superconductors Whde we have chosen to focus our &scusslon on the oxide superconductors, ~t~sessential to remain aware of both the recent advances in our understanding of the heavyfermlon materials and the organic superconductors, and the opportunmes for new work in these important fields Neutron dlffractmn studies on UPt 3 have shown that ~t can have antfferromagnet~c order m its superconducting state Furthermore, the development of the antlferromagnetlc order which, begms at TN = 5 K, terminates at 0 5 K when the system undergoes a superconducting transmon This prowdes strong ewdence that the superconductw~ty of the heavy fermmn system arises from magnetic fluctuatmns Thin as but one example of the striking features of the heavy fermmn systems Prior to the &scovery of the high Tc oxides, a large number of mvestlgators were making slgmficant progress in investigating the umque properties of these matermls arising from the strong couphng between the conductmn electrons and the local f-electron moment fluctuations The h~ghly correlated states m these materials continue to provide a rich area for condensed matter physics The problems ofalternatwe ground states,

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multiple order parameters, mterrelatmnshlp of superconductivity, magnetism, and insulating states need further investigation The interrelationship of band structure and many-body effects appears to occupy a central role For example, band structure results provide remarkably accurate Fermi surfaces as determmed by positron anmhflatmn stu&es and yet the effective masses are some 100 times larger than found m most matermls Both the heavy fermmn systems and the organic superconductors provide a rich arena for new mslghts into strongly correlated systems Whether some of the phenomena m these systems are in fact related to the physical properties of the metal oxides remains to be seen What is clear is that these areas have a number of unique problems and many s~gnlficant research opportunmes

Barrier to progress and recommendations The excmng &scovenes of the high Tc oxide superconducting materials has focussed renewed attentmn on the problems of strongly correlated electromc systems Superconductlwty, w~th ~ts recent act~vmes m heavy fermlon materials and organic superconductors offers specml fundamental research opportunmes m th~s area Furthermore, these problems are central to both science and technology At this t~me, outstanding physics graduate students are eager to join in th~s research which offers an excellent training ground for scientists In addition, young faculty are interested m developing new research actwmes in superconductwlty The Umted States has an opportunity to build a national strength m this area, provided sufficient sustained funding for basic science is made available This funding issue, more than ~ssues such as materml charactenzatmn, accurate solutmns of model Hamfltomans, development of phenomenologlcal theories, and even lnnovatmn ~s the key barrier to progress at the present t~me We must not lose a generation of the best young people for lack of funds Thus, the first recommendatmn ~s for adequate and stable long-term funding of basle research With respect to materials we again need basle support for lndlwduals and small groups In addition we need costsharing arrangements m order for mStltutmns to obtain the basic eqmpment necessary to control and monitor the very dehcate materml preparahon and charactenzatmn reqmred This extends to the need for special computational equipment for the numencal s~mulatlon of model systems

Materials Unquestionably the most significant change since the previous workshop has been the discovery of matermls exhibiting high temperature superconductivity The rapid discovery of three, and possibly four, distinct and hitherto unrecognized crystal structures opens up the posslblhty of manipulation of the crystal structure and lomc substitution to tailor the materials to provide a wide range of electrical properties For instance, it has already been shown that minor doping can change the superconducting material into one that is metallic or semlconductmg or indeed insulating This ~s in marked contrast to the conventional superconductors, which w~th the exception

New research opportumttes tn superconducttwty D J Scalapmo et al of the Chevrel phases, exhibited httle structural or substitutional flexlbdlty The prospect of being able to tailor the electrical properties is one of the most scientifically excltmg challenges offered by these new matermls It also holds substantial potentml for apphcatlons but, as wdl be discussed later, requires considerable systematic, exploratory research into basic crystal chemistry, phase equdlbrla and crystal structure determination - areas m which the US has tradmonally not placed much research emphasis The other unique characteristics of the cuprate superconductors, namely their strong crystallographic (and consequentml electromc and magnetic) anlsotropy and their unusually short coherence lengths also offer opportunmes, not only in exploring the physics of superconductivity as detailed m an earlier secuon, but also to devise new properties and dewces It is probably not an exaggeration to say that the understanding and processing of the new matermls offers an opportunity to deepen our understandmg of the materials sciences m general not just those &rectly related to superconductmg materials The broad challenges to the materials community, some of the techmcal barriers to progress, and the consequential opportunities may be grouped as below

Crystal structures and crystal chemtstry The unanticipated &scovenes of the new cuprate superconductors, argue that an empirical search for ad&tlonal new matenals must be continued w~th the highest priority However, in addition there must also be systematic studies of both metalhc, non-superconducting oxides and superconducting oxides Investlgat~on of the metalhc systems, with structural arrangements s~mdar to their superconducting counterparts, could lead to ~mportant mechamstlc and structural insights mto the behavlour of superconducting materials For instance, La2SrCu206 is structurally slmdar to the B1BaCuO material and yet (at the time of writing) ~s stubbornly metalhc but not superconducting Invest~gatlons in th~s area should not be restricted to the cuprate matermls, but should also include the study of other transition metal, variable oxidation state, systems Stmdarly, some consideration should be given to non-oxides, for instance the sulphldes, bondes and mtndes An Important part of the investigation of the cuprate superconductors is that of the underlying thermodynamic stablhty of these materials Although not particularly glamorous systematic stu&es of the phase eqmhbrla, chemical compatlbdlty and enwronmental stabdlty are essentml to both further developments and possible apphcatlons Comprehensive data for the eqmhbrm in many 'simple' binary and ternary cuprate systems are lacking, let alone for the multi-component matermls now being discovered Similarly the effects of oxygen pressure and the important gaseous speoes (for example, CO2) upon the chemical stabdlty of the cuprates need to be thoroughly lnveshgated Finally, very httle data IS available on the bulk thermodynamic and kmetlc stabdlty of the new copper oxide systems Determination of the stability is of importance to the processing of the known high Tc oxide, its compatlblhty with materials in which it may find use ($1, GaAs, Ag, etc ), and to the development of new matermls

Smgle crystals The most immediate need is for good quality single crystals in both bulk and thin film morphologies so as to be able to measure their superconducting and normal state properties, especially as a function of crystallographic orientation Although some early success has been achieved tn growing such single crystals they have, in general, been poorly characterized and of indeterminate purity and mlcrostructure Formidable problems have been encountered in the growth of single crystals from the melt for a number of reasons including the corrosive nature of copper oxide rich melts, the lack of suitable crucibles, and lack of appropriate fluxes Nevertheless the need for them is a general one affecting the ability to make preose superconducting property measurements at one extreme, to determine diffusion coefficients and to develop electrical contact technology at the other extreme

Interfaces and gram boundaries The role of interfaces and gram boundaries, although significant in earlier superconductors, is of particular Importance in the new cuprate materials given their very short coherence lengths relative to their lattice parameters Whether or not grain boundaries and interfaces do indeed act as 'weak links', as a growing body of evidence appears to suggest, it is clear that there is an opportunity to change their properties by structural and chemical manipulation While it may be premature to speak of the 'crystal chemistry' of gram boundaries and Interfaces, the development of techniques for the manufacture of structurally controlled interfaces (see the following section) by thin film techniques will be an essential step to study the electrical properties (Including tunneling characteristics) of interfaces A detaded understanding of the behavlour of such interfaces will be necessary for subsequent development of devices, tunnel juncuons and for hybrid mlcroelectronlcs that may take advantage of the complementary properties of superconductors and semiconductors at low temperatures (see the next section) It might also be expected that such controlled interfaces may have pecuhar interface states, amenable to chemical doping, that may provide a tool for the study of interfaces in general and could form the basis for a new class of devices As any application of the cuprate materials in polycrystalline form, whether it be as polycrystalhne films, thick films, or bulk ceramics, requires the transport of current across grain boundaries, the electrical properties of the grain boundaries loom large The low critical currents reported for polycrystalhne materials have been attributed to a variety of properties of the grain boundaries, for example, to their mlsorlentatlon, segregation of impurities, stress effects, impurities, and remnant carbonate A systematic investigation of the transport critical currents for individual grain boundaries (and interfaces) as a function of the crystallographic plane of the boundary and the mlsorlentatlon (tilt and twist) is reqmred for each of the new materials Then the variation w~th magnetic field and with doping needs to be established Only then can a comprehensive picture of the relationship between the critical current and the character grain boundaries be determined There is a well established body of knowledge in the materials community for other grain boundary related behavlour Such knowledge ~s essential before a rational deoslon can be

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New research opportumtms m superconductlwty D J Scalapmo et al made on the constraints on processing for bulk apphcat~ons, such as fibres or w~res Is ahgnment ofgralns m one dimension adequate or wdl grains need to be ahgned in two dimensions for high critical current apphcauons9 The investigation of the role of gram boundaries is not only of importance for understanding the importance of microstructure m processing It also is an opportumty for some of those m the condensed matter physics community and those m the materials commumty to collaborate m studying gram boundaries With the possible exception of Schottky barriers at semiconductor Interfaces, there has been httle convergence between the two communities Yet there are many examples of gram boundaries in real matermls of commercial interest (boundary layer capacitors and varistors) where the unique electrical properties derive from doping of the gram boundaries and there exists no fundamental understanding of the phenomena

Artlhclal multdayers The layered nature of the cuprate superconductors discovered to date suggests that they might also be built up atom layer by atom layer much as MBE is presently used to grow artificial mult]layers of compound semiconductors and galhum arsemde The abd~ty to grow crystal structures, such as those of yttrium barium cuprate, bismuth barium strontmm cuprate and thalhum caloum barium cuprate, would open up the opportumty to investigate the effects ofmtergrowths, alter the spacing of the Cu-O2 layers, and introduce composmonal modulations, all in a systematic and controlled manner There is also the posslbdlty to grow new metastable compounds m this way, and already there are indications that the compound Y2Ba4CusO x has been formed m such exploratory expenments whereas all attempts to grow it by more conventional slntenng routes have been unsuccessful Ftbres a n d w f f e s At the present t~me it appears unlikely

Processing sc/ence for superconductors The importance of processing m the development of high performance superconductors ISgaming recognmon but ~s underscored by two quite distract experiences The first is the difficultythat many encountered when learning how to make the cuprate superconductors and discovering the sensmvlty to process variable such as oxygen partml pressure, heating schedules, lmpunty gases, remnant carbonate, etc The second is the success m increasing the crmcal current capabdmes of Nb-T~ by careful and systematic control of the m~crostructure through manipulation of the drawmg and heating process The low transport crmcal currents of bulk cuprate superconductors and their attendant low fracture resistance (brittleness) suggests that traditional ceramic processes wdl be unsuitable for preparation of these materials for other than lnmal, exploratory investigations Rather, it is likely that non-tradmonal processes will be required, especmlly for the preparation of thin films, multllayer films and devices, and for fibres The need for low processmg temperature, for compatlblhty with semiconductors and easily reduced metals and for the relatwely low melting temperatures (relatwe to most ceramics and many metals) will probably also drive towards non-ceramic processes The sensmvlty to processlng atmospheres, ~mpurmes and composites already experienced m the synthesis and fabrication of the cuprates discovered to date, further suggests that process control will be an essentml feature of any new process

Thmfdms One of the particularly encouraging features to date has been the variety of techmques with which high quahty thin films of yttrium barium cuprate have been grown Since the first films were made by electron beam evaporation from metal targets and subsequent oxidation, thm films have been reported to have been grown by reactive sputtermg, RF sputtering, laser ablation and molecular beam epltaxy, etc Thin films, admittedly not of such high quahty, have also been grown by essentml chemical routes, such as decomposmon of alkoxldes and metalorganlcs spun onto substrates These successes point to the possibility of the development of addmonal preparative techmques, ones stated to particular applications

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that any traditional metallurgical process, without extensive modification, will be able to form long wires of the cuprate materials for high current capabilities Considerable progress has been made in both the USA and Japan m demonstrating that short lengths of conductor can be fabricated by oxidation of a drawn wire of yttriumbarium-copper alloy and also by drawing down silver tubes filled with yttrium barium cuprate powder While such approaches have considerable merit and appeal, alternative approaches utlhzlng chemical and vapour processing promise greater process flexibdlty and, at the same time, have wider applicability to the manufacture of fibres for other areas of materials development, such as fibre composites Although the larger specific heat of the cuprate superconductors (compared with their earlier counterparts) suggests that correspondingly larger diameter filaments (possibly 01-2 mm) can be used in high field magnets and still withstand going normal, the substantially lower fracture resistance would imply that slgmficantly smaller diameter filaments wdl be reqmred* to withstand filament forming and conductor fabrication stresses As with glass fibres (which have a similar fracture resistance, 0 8 MPam 1/2 compared to 13 MPam 1/2 for yttrium barium cuprate) the demands of flexibility and strength alone will probably dictate that fibres of the cuprate materials be grown with a diameter of only a few microns This may be an achievable goal, since fibres of other ceramic materials, such as A1203 and SiC and ZrO2 with diameters of tens of microns are already commercially available They have been successfully grown by a variety of chemical routes, including pulling from sol gel and by the thermal decomposition of a polymeric fibre Whale the scientific understanding of the processes involved is still at a rather rudimentary stage and the general technology is relatively undeveloped, the general area has attracted a lot of interest recently as a number of umversmes and companies m the US, Japan, and Europe have started research programmes to grow ceramic fibres from chemical precursors

*The strength of bnttle matenals increases as the reverse of the square root of the flaw s=ze Assuming that the character)stlc flaw sezescales w=th the fibre dmmeter the fibre strength should increase quadratically w)th decreas)ng fibre diameter

New research opportumttes tn superconductivity D J Scalaptno et al An alternative approach, and one not restricted to the same small fibre dmmeter by the brittleness of the material, is to deposit the superconductor onto an existing fibre A particularly exciting development in this direction is the recent development (in FRG) in which NbN is deposited onto carbon fibres The flexibility of the carbon fibres and their electrical conductivity, enables a conductor to be made directly by winding a bundle of such fibres together The anlsotroplc conductivity of the cupratesuperconductors discovered to date necessitates that the deposited material be crystallographically ahgned just as wdl be required for thin films The challenge is clear to develop epItaxial growth on a substrate having cyhndncal morphology as distinct from the more usual planar geometry The successful development of process sciences and techniques for producing superconducting fibres will probably be pivotal for many anticipated large-scale apphcatlons with the new superconductors However, the biggest pay-off may well be in advancing the processing sciences for making fibres of hitherto uncontemplated materials for composites, for optoelectronlc apphcat~ons, and for other advanced technology applications

Existing superconductors The processing of Nb-T1 to fabricate filamentary conductors has become a well developed technology and In many respects is now a mature process However, although the importance of the microstructural scale of the ~-TI is estabhshed, little progress has been made in identifyingthe pinning mechanism since the last report Despite intensive efforts, in the development of superconducting magnets for large-scale applications, the fabrication of multlfilamentary Nb3Sn remains limited by its low ductility (low fracture resistance) Two promising developments from the field of brittle ceramics might be usefully employed to increase the fracture toughness of Nb3Sn The first is to attempt to utilize the concept of transformation toughening by incorporating tetragonal zirconium oxide as a second phase into Nb3Sn* This appears to be feasible since ZrO z is known to be chemically compatible with Nb3Sn The second development that may be fruitful to explore to Increase the formability of Nb 3Sn is that of nanocrystalline processing recently demonstrated in FRG The approach would be to synthesize the material Nb3Sn as 2-10 nm particles and densffy the material during deposition As the resultant material has a very fine grain size (about 10 nm) it would be expected to deform by diffUSlOnalflowt thereby imparting some ductility at small strain rates even at low temperatures Successful demonstration of this behaviour in

*The fracture resistance ~s increased when tetragonal zlrcoma part)cles are transformed to the=r monochmc polymorph m the stress field of a propagating crack The energy dissipation accompanying the irreversible transformation of the zlrcoma leads to an increase m the fracture toughness of the materml tThe strata that can be accommodated by dfffusional f l o w =s proportmnal to either the reverse th=rd power of the gram s=ze (Coble Creep) or to the reverse second power of the gram raze (NabarroHerring Creep) depending on whether the f l o w ts along gram boundaries or through the lattice

nanocrystalllne Nb3Sn would add considerable impetus to this radical development Overshadowed by the discovery of the cuprate superconductors have been the steady developments of alternate superconductors, such as the Chevrel phases, rare earth borldes, and organic superconductors Their synthesis and processing remain very much a matter of art rather than science, and once again pose a challenge to our abilities In processing science Microstructural control One of the unifying features of the different processing approaches outlined above, whether for thin film or magnet conductor configurations, is the necessity for control of the mlcrostructure Fortunately, considerable progress has been achieved in the last decade in developing techniques for characterizing mlcrostructures and in building relationships between mlcrostructure, processing and mechanical properties in a variety of metal alloys and ceramics In addition, there recently has been a renewed emphasis on estabhshlng such relationships for composite materials In working with the new cuprate superconductors as a material, the materials community ISfaced with the challenge of extending the mlcrostructural relationships to the design of materials in which the electrical (de) and magnetic properties are the principal desired properties and the others (mechanical and environmental) have then to be optimized At this point In time, perhaps, the greatest need is a set of phenomenological models, incorporating the single crystal properties of the materials, together with the presumed properties of grain boundaries that provides guidance as to the variation of Jc of a polycrystalhne superconductor as a function of these parameters Such microstructural models are essential to the rational development of processmg schedules for the optimization of properties through the design of mlcrostructures Barriers to progress There exists at the present time no single identifiable barrier preventing an intellectual or technological advance that would lead on the one hand to the discovery of new classes of superconducting crystal structures or on the other to a method of making fibres having high (105-106 A cm-2) critical currents in magnetic fields Rather, as pointed out in the above sections, numerous barriers must be overcome before we know how to process the materials Into forms statable for experimentation and possible application Many of the barriers will be overcome by a sustained materials research programme, but others will undoubtedly require considerable innovation This requires funding support for basic materials science studies of superconducting ceramics, for novel processing approaches and for investigations into the role of interfaces in superconducting materials Recommendabons The Office of Naval Research and the National Science Foundation are encouraged to continue to do what they have traditionally done best, namely provide support for

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New research opportun/t/es m superconductlwty D J Scalapmo et al

basic science by funding ln&vlduals, or small groups, whilst also helping, through appropriate cost-sharing arrangements with other agencies and Institutions, to provide support for large-scale equipment This is particularly important as many of the other US Government agencies appear to have focussed their support towards speofic areas of appllcatmn Specifically, support is needed for research in universities into the structural crystal chemistry of oxides, nltrldes, hahdes and borldes with the express purpose of building up, through research and teaching, expertise in this enabling soence There is a particular need for the support of young investigators - new blood - whde the field remains glamorous and is particularly attractive to the very bright graduate students, post-does, and young faculty The present scientific excitement and spontaneous collaboration between scientists in the physics, matermls, and chemistry departments make it timely to support such collaboration to further encourage joint Investigations in superconductivity as well as more traditional areas of condensed matter physics and materials for the long term enrichment and benefit of these soences Science and Technology Centres may be an ~mportant step m fundmg these multldlsclpllnary collaborations

Superconducting electronics Superconducting electronics, which was developed mostly in the 1970s and 1980s, embraces a rich variety of devices ranging from ultrasensltlve detectors of electromagnetic radmtlon to fast analog processors and high speed logic and memory elements for digital computers Of these devices, which operate at hqmd 4He temperatures, one may identify three in particular that have been outstandingly successful The first is the superconducting quantum interference device (SQUID) which finds a broad range of apphcatlons ranging from NMR to blomagnetlsm, and which has been commercmlly available for many years In many areas of ultrasensmve measurement SQUIDs have no competition The second device is the superconductorinsulator-superconductor (SIS) quaslpartlcle mixer, which is the quietest available for the detection of electromagnetic radlatmn at mllhmetre wavelengths Such detectors are used successfully on a number of radm telescopes, working mostly at frequencies around 100 GHz The third device involves large arrays of Josephson tunnel junctions to maintain the standard volt m many national standards laboratories In the US, this standard maintains an accuracy of a few parts in 108 It is vital to maintain a strong research effort in liquid 4He superconducting electronic devices, based mostly on Nb-technology, for several reasons 1 some of the devices are capable of makmg unique ultrasensmve measurements that would otherwise be out of the question, 2 after a long period of gestation others of these devices can now be exploited relatively qmckly into state of the art technologies, and 3 the low temperature devices form the basis for many potential dewces based on high T¢ matermls These issues are addressed briefly below

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Low Tc opportumtles

The continued use of sensors for novel, ultrasensmve measurements is to be strongly encouraged Examples of on-going research made posslble by SQUID magnetometers are bmmagnetlsm - the detectmn of spontaneous or evoked magnetic fields from the human brain, the investigation of spin-dynamics with 1 ps resolutmn, and novel kinds of nuclear magnetic resonance A quite different example is the use of a SQUID parametric amplifier to demonstrate squeezed states at microwave frequencies One potential lmpe&ment to the broadening of this research, however, is the lack of statable thin-film sensors on a commercial basis Extended development of high speed processors based on demonstrated technology ISan Important opportumty One example is the development of high-speed microprocessors using known orcmts for logic and memory In this context, it is noted that a four-bit processor w~th a 780 MHz clock-rate has already been demonstrated in Japan, this performance is an order of magnitude faster than exlstmg semiconductor competmon A second example ~s the use of superconducting delay hnes to perform such functmns as matched filtering, convolwng and Founer transforming, bandwldths of 10 GHz and computational rates equivalent to 10 ~2 arithmetic operatmns per second are projected To hnk analog and &gltal processors one reqmres an ultrafast A to D converter, conventional superconducting devices are expected to prowde conversion rates at least several times faster than present or projected semiconductor orcmts The development of these various components would make possible an integrated superconducting system lncorporatmg a detector (for example, a SIS mixer), a fast analog processor, an A/D converter and a &gital processor Exploration for novel superconducting dewces should be continued For example, the Invention of a threeterminal device with gain would be very important New ideas for high Tc devices could well be more easdy tested using low T¢ materials for which there are well estabhshed processing techmques

High Tc opportumtles

The advent of the high Tc oxide superconductors not only offers the opportunity to extend the operation of devices developed with low T¢ materials to high temperatures, but may lead to entirely new device concepts Several specific examples are outlined below Of devices already developed with low T¢ matenals, only SQUIDs have been fabncated with high T¢ materials These devices have been operated successfully at temperatures up to 77 K, and although they have so far exhibited relatwely high levels of low frequency noise, there is every reason to beheve these noise levels will &romish as the quahty of thin films is improved The avadabdlty of high T¢ SQUIDs of reasonable sensmvlty would have a major impact on instrumentation - for example, voltmeters and magnetometers, and on apphcatlons m remote areas - for example, geophysics Other high T¢ devices that could be available on a relatively short time scale include strlpllnes for optoelectromc applications such as high speed samphng and the standard volt for use as an internal reference for voltmeters

New research opportumtms tn superconducttwty D J Scalapmo et al A rich area ofapphcatlon is likely to be the combination of high T~ superconductors with semiconductors, for example, CMOS or GaAs devices The two classes of material are complementary in many ways, semiconductors provide amplifiers and rectifiers while superconductors provide low loss transmission lines, quantum interference devices, and the a c Josephson effect One possible integration of these technologies is in the fast analog processor - superconducting delay lines combined with semiconductor multipliers and amplifiers to make microwave filters or convolvers The structure and physical properties of the new materials lend themselves to novel processing techniques and the possibility of entirely new device concepts For example, one could envision multilayered structures in which successive layers of the same material (for example, YBCO) were processed to yield a superconductor, a normal metal, a semiconductor and an insulator The intriguing (and presently ill-understood) magnetic and optical properties could perhaps be combined with the superconducting properties to produce new physical phenomena processes and eventually new devices based on principles one cannot possibly foresee Thus, it is vitally important to seek novel devices that are not merely an extension of concepts developed with the low T~ devices

Bamers to progress on htgh Tc dewces In contrast to low T, films, such as Nb and NbN, where there is a very developed technology, a great deal of research is required to develop the processing of high T~ films to the point where they can be used in device technologies Some key areas are given below The most obvious need is for the reproducible fabrication of high quality films, that is, with high transition temperature (well above 77 K), high critical current density, a smooth surface, and low RF and microwave losses It IS essential that the films do not deteriorate with time Epltaxlally grown, oriented films are likely to be essential for most although not necessarily all applications It will be necessary to develop processing for integrated circuits, for example, one will need materials to insulate multllayers of high T~ superconductors To achieve these goals, further investigation of existing deposition techniques such as co-evaporation, sputtering from single or multiple targets, and laser ablation as well as the development of new techniques are required In sttu, low temperature processing particularly is to be encouraged Substrate materials are of major concern Most epItaxlally-grown YBCO films have been on SrT10 s, which IS undesirable not only because of its high cost but also because it has unacceptably high dielectric losses for many RF and microwave applications Recent progress In the epltaxial growth of YBCO on MgO is most encouraging In searching for alternative substrates, one should seek materials with low dielectric loss that have minimal interaction with the film (perhaps with the aid of a passlvation layer) and that (for some applications) are compatible with semiconductor processing Also, the development of reliable means of making metals is necessary Finally, to produce active devices it is crucial to develop junctions between high T¢ superconductors

These include Josephson or quasipartlcle tunnel junctions, in which the barrier is an insulating layer, and 'weak link' Josephson junctions, in which the barrier is (for example) a normal metal layer High T¢ superconducting electronics will be severely restricted unless a reproducible means is developed for the production of junctions with long-term stability

Impact of operatton at hqutd nttrogen temperatures The difficulty perceived by many potential users in employing superconducting electronics at liquid helium temperatures should be greatly alleviated by the advent of high Tc superconducting devices Many devices, such as microprocessors or analog signal processors, could be cooled to 7 7 K or below by relatively simple and inexpensive refrigerators capable of maintaining the low temperature Indefinitely Other systems, for example, those operating in remote areas where electrical power is not available, or those that are particularly sensitive to magnetic noise or mechanical vibration, will require liquid nitrogen as a refrigerant However, since the latent heat of vaporization of hquid nitrogen is roughly 60 times that of liquid helium, a cryostat that requires replenishing with liquid helium once a week could run unattended for a year when filled with liquid nitrogen The very long hold time of a liquid nitrogen cryostat would enormously simplify the logistics of supplying the refrigerant to systems in remote areas Thus, for example, SQUID magnetometers operating at 77 K cold find much more widespread applications in geophysics than have those at 4 2 K An alternative strategy when liquid nitrogen is readily available is to operate a device for relatively short times in an inexpensive dewar This approach would enable one to operate superconducting devices, for example, a SQUID-based voltmeter, very inexpensively, thereby greatly expanding the range of potential applications

Long-term fundmg pohctes It should be emphasized that to maintain successful programmes and technologies in both low Tc and high Tc devices it will be essential to provide long-term, stable funding Even though the processing of low T~ films (for example, Nb) is well-established, there are remarkably few groups in the USA with the capability of producing devices or circuits The high Tc materials are much more complex, and a considerable effort will be necessary to fund groups large enough to maintain the sustained efforts necessary to produce new device technologies

Large-scale appl,catlons Since the preparation of the last report 1 considerable progress has been made in large-scale applications of superconductivity The Tevatron has been operating successfully at the Fermi Laboratory since 1983, demonstrating the reliability and ruggedness of a large, superconductive system incorporating over 1000 magnets, a multi-station helium refrigeration system, and a complex

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New research opportumtms tn superconductlwty D J Scalapmo et a l control system Over 1000 medical magnetic resonance imaging (MRI) systems have been mstalled in hospitals and clinics world-wide An additional 500 plus were to be added in 1988 These systems have been used to perform over a mflhon dmgnostlc procedures, and MRI has become the &agnostic modahty of choice for pathologies ranging from Multiple Sclerosis to torn knee hgaments The number of systems installed and the variety of diagnostic procedures employed continues to grow at an ever-increasing rate In vtvo magnetic resonance spectroscopy, although not yet a clinical procedure, offers the posslbdlty of non-lnvaslve, real-time characterization of the basic biochemical processes, providing another powerful tool for the investigation and dmgnosls of disease A magnetic separator incorporating a superconductwe magnet has been installed m a processing plant in Georgia This system is used for the beneficatlon (purification) of kaolin clay for the paper industry Operational experience with the system has been sufficiently successful that a second system is currently on order A large-scale magnet and several smaller magnets of Nb3Sn have been successfully tested in plasma fusion prototype machines Compact electron storage rings have been Identified as a source of high Intensity X-rays for use in submlcron scale lithography for the production of integrated c~rcmts Storage rings incorporating superconductive magnets are currently under construction in the FRG, UK, and Japan A ship propulsion system consisting of a superconductive generator, superconductive homopolar motor and closed cycle refrigerator has been installed on a 60 ft vessel by the US Navy and extensively tested Coincident with the growth in apphcat~ons has come encouraging progress m supporting subsystems, such as cryogenics and electronics, and in the superconductive matermls themselves

Supporting subsystems The development of supporting subsystems is perhaps best dlustrated by the MRI systems Current production MRI magnets - horizontal bore solenoids with 1 m diameter bore, 2 m outside diameter, and 3 m length have a helium consumption of approximately 0 25 dm 3 h - 1 and a nitrogen consumption of less than 1 dm 3 h - 1 Hehum is refilled once per month and mtrogen every two weeks The addition of a small closed cycle refrigerator can ehmlnate the need for mtrogen and cut the hehum consumption by a factor of 2 to 4 Several hundred MRI systems are operatmg as 'Mobile Imaging Centers' These systems are mounted on motor=zed vans and moved, on a regular schedule, between several medical lnstltuUons each too small to support a dedicated system Automated control systems allow medical technicians with no experience m cryogenics or superconductwlty to discharge the magnet for movement, energize It at the destination, and adjust the field homogeneity to compensate for the local magnetic environment Rehabfllty of the magnetic systems have exceeded specifications with up-time well In excess of 99% In fact, the superconducting magnet has proven to be the most reliable major system component m both stationary and mobile MRI systems

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Materials In reference 1, it was stated 'A good understanding of the metallurgical and flUxold mlcrostructure will undoubtedly lead to significant increases in J¢ (the production J~ at 5 T of Nb-TI is about half that of the best model alloys, as is the case for the J~ at 12 T of Nb3Sn)' A comprehensive investigation of the mlcrostructure of Nb-TI and its relationship to critical current was undertaken subsequent to the last report, and the results have been qmte dramatic The critical current density avadable in commercial N ~ T I is now twice that of 5 years ago, thereby significantly reducing the cost per ampere-metre of conductor While the major effect of this cost reduction would be seen in very large projects such as the SSC or fusion reactors, the immediate benefits to applications such as MRI are already being realized It should be noted that this step-function increase m materials performance was obtained only by bringing to bear on the materials, m a umverslty environment, more basic and detailed techniques of mlcrostructural characterization than those available to the materials manufacturers In the absence of this externally funded, long-term, academically-based, investigation it is doubtful that the precipitate structure central to the critical current enhancement would have been identified In the following section we will attempt to identify similar situations where the apphcatlon of sustained basic investigations can yield technological progress beyond that possible within the hmltatlons of the small US superconductivity industry

Research opportumttes There exist research opportunities of technological import in the 'low T~' alloy and compound superconductors and in the high T~ oxide materials In addition, successful utlhzatlon of the oxide materials, will, in many cases, depend on the technology previously developed for the metallic-based conductors One must, therefore, resist the temptation to immediately apply all resources to oxide based devices and seek rather an optimum allocation of resources to establish a sound techmcal base supporting a variety of apphcatlons Study of the basic flux pruning mechamsms in metalhc superconductors - the elementary interaction, the summation problem, and the detailed dynamics of the flux hne lattice - remains an important opportunity The previously mentioned mlcrostructural characterizations have allowed better process control of the defect lattice in Nb-T1 and hence higher critical current There remains, however, a lack of knowledge of the basic flux pinning mechanisms Flux pinning studies remain at a very low level in the US and the advent of the new materials only increases the need for more work In this area With the increased knowledge and control of the metallurgical mlcrostructure now avadable in the alloy materials, and the obvious rapid technological rewards for success, this remains an area of significant opportunity Mlcrostructural characterization of pinning in compound superconductors, particularly Nb3Sn, is an area with potentially high return Our knowledge of pinning in

New research opportuntttes tn superconducttvtty D J Scalapmo et al the metallic compounds - particularly the commercial material Nb3Sn - has not changed dramatically since 1983 There is a need to apply the mlcrostructural characterization which was so successful in Nb-T1 to Nb3Sn Such a characterization, combined with the basic work on flux pinning described above, could significantly increase the critical current density available for the design of magnets above 8 T Development of fabrication methods for advanced conductors (Nb3A1, NbN, etc) provides another opportunity At the present t~me, most development on high field superconductors is taking place in Japan Given the uncertainty of successful development of high T~ superconductors, or the time frame for such success, ~t appears important to encourage work on 'convenUonal' h~gh field conductors Basic work on fabrication methods (rapid quenching, powder metallurgy, etc ) and diffusion remains as valuable today as it was m 1983, particularly given the more general apphcablhty of the techniques to areas outside superconductwlty Development of high T¢ materials for large-scale apphcatmns should continue Speofic classes of applications include 1 magnetic shielding, 2 low field (e g power transmission lines), and 3 high field (e g magnets and motors) Reahzatlon of d c magnetic shielding, assuming a proper device design, requires primarily a material w~th a suffioently high crmcal current at low field, and environmental stablhty The material must also be bonded to a substrate supplying structural mtegrlty and thermal/ electrical stablhzatlon Low field applications are those which require fabrication of the conductor in a somewhat flexible form (presumably a tape) with moderate low frequency a c losses In addmon, methods of making terminations and joints are required High field apphcations would require, in ad&tlon, crmcal current at high field, good working strain, insensltiwty of J¢ to strain, approprmte conductor subdwislon to achieve adiabatic stablhty, and probably mcreased flexiblhty for more complex device fabrication Bamers to progress Moving from a new conductor material to any high field conductor on a commercial scale is clearly a large task, comparable to going from the germanium junction transistor to an early generation integrated circuit In what follows, we discuss some areas where initial efforts might be concentrated for high Tc superconductor applications Given the infant state of the materials, it should not be surprising that the efforts needed to realize large-scale apphcatlons are identical in many cases to those needed by small-scale applications and for understanding of the materials themselves Crtttcalcurrent denstty Overall winding current densities of at least 104 A c m - 2 are necessary to make practical high field superconductive magnets Since the winding area Includes not only the superconductor but also the stabdizer, structure, insulation, and perhaps some cryogen, the critical current density required in the super-

conductor itself is on the order of 105 A cm -2 In the interest of simplified device design, it is preferable to have the critical current independent of magnetic field orientatlon This clearly IS comphcated by the inherent anlsotropy of the oxide materials and by weak coupling between grains of these materials The contlnmty of critical current density or absence of faults must also be addressed For an ordinary (nonpersistent) superconductive magnet, the conductor resistivity constant over many kilometres must be < 10- ~2 cm For persistent mode NMR magnets the requirement is several orders of magnitude more stringent A c losses For use in power transmission lines and transformers, the energy dissipation in the conductor due to flux motion in the 6 0 H z alternating field must be minimized This will require knowledge of flux penetration into the surface of the material, 1e, surface pinning, as well as the control of surface Irregularities Conductor stram The winding hoop stress in a high field magnet typically will be at least 50 M Pa This stress must be supported either by the conductor itself or reacted onto some other structure In either case, the conductor must function in a strain state that is the sum of the thermal and magnetic strains This requires not only that the conductor retain physical integrity but that the critical current be maintained under strain Thermal and electrtcal stabthty Methods of effectively stablhzlng high T~ superconductors must be developed Present generation alloy and compound superconductors usually consist of a multitude of fine filaments in a matrix of normal metal - typically copper The matrix serves to provide thermal conduction between the superconductor and cryogen, and to slow the entry of magnetic flux into the composite in the event of a flux jump or other local field excursion To be effective, the stabilizer must be in Intimate thermal and electrical contact with the filaments Hence techniques for achieving an intimate bond to the new materials must be realized

Impact of operation at 77 K It is difficult to predict the ultimate impact that a high Tc superconductor could have on large-scale devices This analysis in each case IS dependent on numerous details, such as conductor properties, about which there is currently little or no information One can, however, draw some qualitative conclusions about the impact of a material operating at 77 K versus one operating at 4 2 K 1 the capital cost ofcryostat fabrication can be reduced, perhaps by a factor of 2, 2 the power required for refrigeration will decrease by over a factor of 20, and 3 the refrigerator cost will decline by about a factor of 10 In numerous apphcatlons, such as magnetic energy storage, magnetic separation, and rotating machinery, these cost reductions could significantly enhance the economic advantage of the superconducting devices It IS highly likely, however, that the major uses of superconductivity in 1996 are as unknown to us today as MRI

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New research opportuntttes /n superconducttwty D J Scalapmo et al was in 1980 The basic scientific investigations underway now on high T¢ superconductors are the analogue of the alloy superconductor studies of the 1960s and 1970s Without this early work we would have no MRI Without similar, sustained, work on the oxides we close the door on an unknown variety of new techniques and devices

Summary Based on the detailed analyses described above, we can recommend specific areas that are unique opportunities in superconductivity research First, there are some strong common problems that arise in all of the subjects discussed and not hmlted to the high T~ superconductivity The potential value of any proposed superconductivity research programme could be assessed against meeting one or more of the following basic needs •

• • • • • •

long-term, consistent funding, understanding the fundamental mechanism, well characterized single crystals or oriented films, understandmg flux lattice mlcrostructure interactions, characterization of interfaces, new superconducting materials, and new or advanced devices and apphcations

Matertals 1 Studies on well characterized single crystals, oriented thin films, known grain boundaries, 2 studies on controlled mlcrostructure in the new high Tc materials, 3 new superconductors in any of the oxides, heavy fermlon, organic, Chevrel or other categories, 4 new methods of material processing (e g multilayers) with the basle science as the primary goal, 5 materials processing leading to thin film synthesis, cool substrates, ceramic fibres, strain tolerance, flexible wires, or other useful objectives, and 6 phenomenologlcal models for comparison with materials properties, especially for granular, multilayer, or junction-dominated materials

Superconductmg electronics 1 Commercially reproducible patterned thln films of the new high T¢ materials on appropriate substrates, 2 new devices and circuit designs with liquid helium superconducting electronics, 3 high Tc counterparts to known devices and circuits, e g SQUIDs and mixers, 4 hybrid super-semiconductor devices and circuits, and 5 package superconducting systems, instruments, or measuring devices, e g self-calibrating voltmeters

Specthc problem areas The workshop members also identified specific problem areas that meet one or more of the above cntena, and that were felt to be ripe for further advances These are hsted below in the same general categories as discussed and approximately in priority order within each one

Fundmg 1 Stable funding of individuals or small groups to work on fundamental problems, 2 starting young or new soentlsts into superconductivity, 3 large-scale, state of the art equipment for general characterization available to many, and knowledgeably operated and well maintained, and 4 mterdlsciphnary teams to address some of the more difficult problems and train students with an lnterdlsclphnary approach (Some examples being the crystal-physical chemistry of the oxides, nitrldes, hahdes, and borldes or the physical properties-processing interactions in ceramic or metallic wire processing )

Fundamental studtes 1 The pairing mechanism in the new oxide superconductors, 2 heavy fermlon and organic superconductors, 3 metal-,semlconductor-,andsuperconductor-superconductor interfaces (including tunneling), 4 flux pinning, especially in the high temperature, short coherence length regime, and 5 metals in the high correlation energy regime

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Large-scale applications l 2 3 4

Fundamental understanding of supercurrent transfer across grain boundaries and interfaces, enhanced critical current conductors, both high Tc and conventional, multlfilamentary nltrlde or Chevrel conductors, and strain tolerant and fracture tough conductors both conventional and high T¢

These areas are general in definition in order to leave open innovative approaches More detailed discussions of each are in the general text which includes specific model analyses as suggestions

Concluding remarks The unique environment created by the breakthrough in high Tc superconductors has created an opportunity for not only superconductivity research but in condensed matter physics, structural chemistry, and materials research as well The new enthusiasm in young scientists, the focus of attention by pubhc officials, and the widespread exposure to the general public all provide a basis for developing innovative approaches to the public support of research This must begin by addressing the opportunities at hand and should be considered as an example and a healthy starting point Pay-offs will be in areas both expected and unexpected Expected returns will come in applications based on conventional superconducting technology and on extrapolatlons of this technology using the new materials Innovative design and reliable production will yield returns in further development of the base technology,

New research opportumttes such as three-terminal devices or integrated superconductm~semlconductmg systems It wdl also permit the logical extrapolations to higher fields and higher temperatures such as self-cahbratmg instruments or medical applications using new techniques m an area of the world where hqmd hehum is rare The biggest pay-off`s, however, wdl come in the unexpected areas, such as superconducting MRI, which was unexpected from convenUonal superconductors These wdl come from the dramatically enhanced range of properties and the

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superconductlvtty D J Scalapmo et al

creative input from new dlsophnes The best approach to enhance this probabdlty Is to draw m good people and encourage the m~xmg of ideas It is an opportumty we must eagerly pursue

Reference Tinkham, M, Beasley, M R, Larbalestler, D C, Clark, A F and Fmnemore, D K Research opportumtles m superconductivity Crvogemt~ (1984)24 378-388

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