Proc. Nat!. Acad. Sci. USAVol. 85, pp. 4945-4952, July 1988Seminar Report

This paper is based on a presentation by C.K.N.P. at a special session organized to discuss "Breakthroughs inScience: Superconductors and Supernova" during the 124th Annual Meeting of the National Academy ofSciences, April 27, 1987, Washington, DC.

Toward room temperature superconductivity?C. K. N. PATEL AND R. C. DYNESAT & T Bell Laboratories, Murray Hill, NJ 07974

ABSTRACT The last 12 months have witnessed frenziedactivity in condensed matter physics, unmatched by any othersince the invention of the laser. In this article, we summarizethe status, promise, and problems in the field of high-temper-ature superconductivity. We also comment on the mechanisand policies needed for the United States to economicallybenefit from the recent discoveries in the face of what can bebest described as an international race to win the battle.

Many, if not all, of the preconceived notions about super-conducting transition temperatures have fallen in the past 12months. This time period, relatively miniscule compared tothe time from the first observation of superconductivity inmercury by Kammerlingh-Onnes (1) in 1911 [or short evencompared to that from the next significant advance inhigh-critical-temperature (Tc) superconductors that led to theA15 compounds] has seen the T, of superconductors go from23 K for Nb3Ge (2, 3) to 94.5 K for YBa2Cu307 (4). There arepersistent rumors/reports (5-7) of "glitches" seen in theresistivity and susceptibility measurements on materials ofrelated composition at temperatures as high as room temper-ature, which may be due to a minor phase that becomessuperconducting at these high temperatures. Even with thepresent confirmed results, raising of the T. to above liquidnitrogen temperature is likely to remove the economicbarriers for many of the applications of superconductivitythat were precluded by the relatively high cost of liquidhelium. The economic importance of raising the Tc forsuperconductors is shown in Table 1, which summarizes thepractical cryogens, their latent heat of evaporation, and anapproximate dollar per calorie of cooling capacity. It is easyto recognize that from practical considerations there are fourimportant temperature regions: those accessible using liquidhelium (4.2 K), those accessible using liquid nitrogen (77 K),those accessible using dry ice (193 K), and those accessibleusing water as the cryogen. However, we should point outthat raising the T, above the presently reached value of -94.5K will also be useful in practice (even though one may notreach 193 K) because a higher T, will result in higher criticalcurrent densities (J, values), critical magnetic fields (H,,values), and margins of safety at the operating temperatureof 77 K (see Note Added in Proof).As an aside it is interesting to note the scale of research

activity that led to these advances and resulted in the NobelPrize in Physics to Bednorz and Muller this year in a remark-ably short period of time. These activities, typical of othercondensed matter science activities, are table-top science. Aneducated guess of the research expenditure leading to thepresent advances is not more than a few tens of millions ofdollars per year over the last dozen years. This is to becompared with the projected cost of the superconductingsupercollider (SSC) of approximately $5,000,000,000. Theanticipated yearly operating expense of the SSC overshadowsthe entire investment in superconducting materials researchover the last dozen years.

Table 1. Important parameters of various cryogens

Boiling point, Latent heat, Cooling cost,Cryogen K cal'cm- 3 $/cal*4He 4.2 0.65 6 x 10-3H2 20.4 7.56Ne 27.2 25N2 77 38.6 5 x 10-6Ar 87.4 53.502 90.1 58.1CO2 194.6 223 5 x 10-7H20 373 550*1cal = 4.18J.

In this paper we review some ofthe materials advances thathave occurred in the past 12 months and discuss the activitiesthat are pushing the engineering of these materials forpractical superconducting applications. Finally, we willbriefly describe the potential applications of the new super-conducting materials and give suggestions regarding a needednational policy that will facilitate economic exploitation ofthe advances by the United States.

MATERIALSThe most important property of a superconducting materialis the temperature at which it loses all electrical resistance.Since the discovery of superconductivity in mercury at 4.2 Kby Kammerlingh-Onnes in 1911, materials scientists havepursued the discovery and/or synthesis ofnew materials thatwould exhibit superconductivity at higher and higher tem-peratures. Of course, for practical applications many otherparameters such as the J, values and HC2 values together withmaterials mechanical properties are also important. None-theless, for an economic application of superconductingmaterials the ability to use cryogens other than liquid heliumhas been seen to be most important. In the first 40 yearsfollowing the discovery of superconductivity, the Tc valueshad been raised to about 15 K with NbN. The discovery ofsuperconductivity in the A15 compound V3Si by Hulm andHardy (8) in 1951 opened up an era of practical usage ofsuperconductivity. Kunzler's discovery (9) of the high-field,high-current capabilities of Nb3Sn (Tc = 19 K), anothermember of the A15 class, was a very important step in thisprocess. By 1975, the Tc had reached 23.4 K in Nb3Ge, whereit stayed until about a year ago (Fig. 1). The critical step thattriggered the current activities was the publication by Bed-norz and Muller (10) of the possible observation of super-conductivity in the Ba-La-Cu-O system. The (La,Ba)2CuO4phase had been originally synthesized and reported byShaplygin et al. (11) in the Soviet Union and by Nguyen et al.(12) in France. Takagi et al. (13) in Japan were able to confirm

Abbreviations: Tc, critical temperature; Jc, critical current; H,2,critical magnetic field; BCS, Bardeen-Cooper-Schrieffer; SSC, su-perconducting supercollider.

4945

4946 Seminar Report: Patel and Dynes

w

Ir

w

a.

w

F-

z

w

x(D

100

90 * (Bo2Y) Cu307

80

70

60 _

50

l(LaSr)2 CuO440 - (BaLa)2 CuO4|30 - Nb3 Al0.75 GeO025Nb3Sn7 Nb3 Ge20 NbN VSi15 -

10 - NbC5 Hg SrTi 030 I

1910 1950 1990 2030 2070 2110YEAR

FIG. 1. Highest transition temperature achieved vs. time begin-ning with the discovery of superconductivity in 1911. Until recently,the maximum T, changed linearly with time. The perovskite super-conducting oxides are boxed.

superconductivity in the Ba-La-Cu-O system and identify(La,Ba)2CuO4 as the superconducting phase at =30 K. Cavaet al. (14) and Kishio et al. (15) synthesized an analog of thismaterial, (La,Sr)2CuO4, which showed zero resistivity and100% Meissner effect at 40 K. Chu et al. (16) reported raisingthe Tc of the Ba-La-Cu-O compound to =40 K by subjectingthe material to a compressive stress. The next event and theone that initiated the present activities was the report ofsuperconductivity above 90 K in the Ba-Y-Cu-O ceramicmixture by Wu et al. (4). Cava et al. (17) isolated the puresingle phase responsible for superconductivity at 93 K andidentified the compound as YBa2Cu307. Siegrist et al. (18,19) established the crystal structure of YBa2Cu307 to be aperovskite relative. Neutron diffraction (20) has furthershown the oxygen ordering as depicted in Fig. 2. Thestructure is highly anisotropic and may be thought of as beingcomprised of two-dimensional layers and one-dimensionalchains as shown on the right-hand side of Fig. 2. Thisstructure leads to anisotropy in the properties of the materialas will be discussed below. The identification ofthe phase ledto the synthesis of as many as 13 new compounds (21, 22) thatshow superconductivity at a temperature above 77 K (Table2). The general class has the formula AB2Cu3O, where A isa rare earth element (Y, Sc, . . .), and B is an alkaline earthelement (Ba, Ca, . . .). These compounds are ceramics and a

general recipe for their synthesis is given in Fig. 3.

BARIUM

YTTRIUME

* COPPER O 0°o0 0

0iQ OXYGEN

VAC~

FIG. 2. Crystal structure of YBa2Cu307.

A remarkable feature of the chemistry of the AB2Cu3O0class ofcompounds is the wide range ofoxygen stoichiometry.Gallagher et al. (23) have found, by thermogravimetry, a stablerange of 6.0 c x c 7.0, and a fast oxygen absorption-desorp-tion for powdered samples in pure oxygen (Fig. 4). Thestructure of the compound at x = 6.0 was determined bySantoro et al. (24). It has the same cation framework as thatfor x = 7.0, but the oxygen sites along the chains are vacantresulting in a tetragonal rather than an orthorhombic crystalstructure. The oxygen stoichiometry is crucial to supercon-ductivity. At x = 6.0 the compound is a semiconductor. Cavaet al. (25) have found that T, depends on the conditions ofoxygen removal and on ordering. The Tc dependence on x isshown in Fig. 5 for samples prepared at low temperatures.

MATERIALS: ENGINEERINGFor scientific and practical applications, the needed"shapes" are thin films, thick films, single crystals, tapes,bulk ceramics, and wires. In each of these categories, in theshort time from early 1987 to the present, enormous progresshas been made.Thin Films. Chaudhari et al. (26) were the first to report

results for a near-single-crystal oriented film epitaxiallygrown on SrTiO3 substrates. These films showed J, values of3 x 10' A-cm2at 77 K and 2 x 106A-cm-2at 4.2 K. Recentdata of Enomoto et al. (27) and Mankiewich et al. (28) showimproved epitaxial YBa2Cu307 films grown on single-crystal-oriented SrTiO3 that exhibit J, values of up to 2 x 106A-cm-2 at 77 K. Data for Jc as a function of temperature forthin films (28) are shown in Fig. 6 (see Table 3, whichsummarizes the properties of YBa2Cu307).

Single Crystals. To understand the details of physics andchemistry underlying these high T, superconductors, singlecrystals are necessary. These allow one to study the aniso-tropic behavior of Jc, the H 2, and the macroscopic excita-tions ultimately responsible for superconductivity. A numberof individuals have succeeded in growing YBa2Cu307-8 insizes necessary for these studies and for neutron diffraction,as well as for optical and x-ray scattering measurements.Typically, these crystals are grown by slowly cooling a meltcontaining all the constituents, Schneemeyer et al. (29) havesucceeded in obtaining single crystals of _1-cm2 dimensions.These have to be annealed in an oxygen atmosphere to get theproper oxidation state necessary for the 93 K superconduc-tor.

Tapes. For many possible applications, forming the ce-ramic superconductors in the form of flexible tapes appearsdesirable. Johnson et al. (30) have fabricated tapes usingflexible polymers as the substrate. The superconductingmaterial is "squeegied" (tape cast) in the form of a slurry onthe tape. In this form ("green") the tape is very flexible. Thetape is subsequently sintered in an oxygen atmosphere todrive off the binder and to obtain the correct oxidation state,at which point the tape loses most but by no means all of itsflexibility. The measured Tc for the tapes is the same as thatfor the bulk (93 K) but Jc values are low (=103 Acm-2)compared to those for epitaxial films.

Wires. For applications involving high-current capabilitysuch as electrical power transmission and magnets, super-conducting wires would be required. Jin et al. (31) havefabricated superconducting wires by drawing (cold working)a silver tube containing pulverized YBa2Cu3O7 material.Subsequent to drawing to the desired diameter, the wires areshaped in coils, etc., and then sintered and annealed in anoxygen atmosphere to obtain the proper oxidation state. Suchannealed wires have a Tc of =93 K. The Jc in zero magneticfield is -1000 A-cm2; however, it drops to <100 A-cm-2when a field of 100 G is applied (32) (Fig. 7). This is acharacteristic that is clearly undesirable and will be discussed

Proc. Natl. Acad. Sci. USA 85 (1988)

Proc. Natl. Acad. Sci. USA 85 (1988) 4947

Table 2. Crystallographic and superconduction data for M3CU3O0Tonset mCid , R=o sp onset)

M3 a, A b, A c, A x K K K PnQcMm P300P(T onse)Ba2Y 3.87 3.86 11.67 6.9 93.5 92.5 91 260 2.3Ba2Eu 3.88 3.85 11.77 7.1 96 94.5 93.5 740 2.5Ba2.1Eu 96 94.5 92 8500 1.9Ba2La 3.934 6.6 77 60 48 3900 1.04BaCaLa 3.877 6.7 81 79.3 77.8 2100 2.5BaCaY NSP 83 80 77Ba2Y0.75SC0.25 3.86 3.84 11.74 92 91 87 1860 2.15Ba2Eu0.75SC0.25 3.89 3.87 11.77 96 93 91 3000 1.73Ba2Y0.75La0.25 3.88 3.86 11.69 92 87 82 4200 1.91Ba1.4Sr0.5Y 3.83 3.82 11.67 91 87 86 1260 2.60Ba2Eu0.9Pr0.1 3.88 3.86 11.76 85 82 80 1860 1.93Ba2EuO.9YO.1 3.87 3.85 11.77 96 94.5 93.7 320 3.0Ba2Eu0.75Y0.25 3.87 3.85 11.75 96 95 94 800 2.6

p, Resistivity in normal phase; NSP, not single phase.

below. It is clear that much work needs to be done to improvethe current-carrying capability-i.e., to bring Jc in wiresclose to the maximum values seen for thin films.

PHYSICS ISSUESOne of the crucial questions is the elucidation of the mech-anism of superconductivity in these new cuprates. Thetraditional model, which is generally applicable to lower-temperature (i.e., Tc < 23 K) superconductors, which areelements or intermetailic compounds, is that electron pairing(Cooper pairs) is caused by a phonon-mediated mechanism(33, 34) (Bardeen-Cooper-Schrieffer model; BCS). We willnot go through the details of the model, but will address anumber of salient features. Flux quantization measurementshave established that Cooper pairs are still involved in thesuperconducting state of these materials, but there arequestions regarding the mediating mechanism.

T,. Following the BCS model, calculations predict the T,for phonon-mediated superconductivity

Tc - wexp[ -1/VN(0)], [1]

where ( is a characteristic phonon energy, V is the strengthof the electron-phonon interaction, and N(0) is the electrondensity of states at the Fermi level. From these and moredetailed studies it can be concluded that in principle there isno explicit upper limit to the Tc for electron-phonon-mediated superconductivity. Tc can be increased by increas-ing any or a combination of (i) the electron-phonon interac-tion, (ii) the electron density of states, or (iii) the character-istic phonon frequency. Phonon frequencies can be increasedby using lighter elements, so the presence of oxygen and itsimportance in electrical conduction as seen from the band-structure calculations could indicate that the BCS model

applies. The phonon frequencies, however, cannot be arbi-trarily increased because (a) the lightest element that can beused is hydrogen and (b) structural phase transitions, theformation of charge density waves, etc., could and indeedoften truncate the strong electron-pairing mechanism.There are at present two observations that point toward a

mechanism other than phonon-mediated pairing. The first isthe variation of T, with N(0) for which the electronic specificheat coefficient, -y, provides a good surrogate measure. Acompendium of data on Tc vs. y is presented in Fig. 8. Threeclasses of materials are seen. The elemental metals andintermetallic compounds with T, < 23 K fall in one band, thehigh T, oxides fall in a second band that is higher than that forintermetallic compounds, and the third group are thosematerials that have very low Tc values and high specific heatsoccupying the far right of the figure. The last categoryincludes all of the heavy Fermion systems (35), whichprobably involve a different symmetry for pairing (d-wavepairing). The clustering of the oxide superconductors (36-39)in a band different from elemental and intermetallic com-pounds suggests that a mechanism other than the phonon-mediated one may be instrumental for superconductivity inthe oxides.The second observation is that in a BCS superconductor,

a change in T, is seen to result from changing the phonon

68

0 66

caN 64amz

x 62

_ _ _ _ __ _ _____

___ _ ____ 'I

> ''I

_ \

_ \'\\,

60 _

|MIX Y203,BaC03,CuOl

|CALCINE 900-950°C IN 02|

|BALL MILL 4 |

FIG. 3. Typical procedure for synthesizing YBa2CU307_s. This isby no means the only way to fabricate these compounds butillustrates the relative simplicity of the synthesis.

I I_) V

0 100 200 300 400 500 600 700 800 900 1000TEMPERATURE (0C)

FIG. 4. Sequential thermogravitic data for Ba2YCU30 showingthe rapid oxygen absorption-desorption. Data were obtained first in02, then in air, and finally in N2. The heating rate was 10C/min. Thesamples were cooled in the same atmosphere at 100'C/min and thencharged for the next cycle [reproduced with permission from ref. 23(copyright, Pergamon Press)].

Seminar Report: Patel and Dynes

70r

11

N2

4948 Seminar Report: Patel and Dynes

wI-

4

w

a.w

I-z0

z

W.

olI7.0 6.8 6.6 6.4

OXYGEN CONTENT x

Table 3. Physical parameters of YBa2Cu307-8Parameter Value

CeramicsTC 93 KHC2 >106Gat4.2K

>250 kG at 77 KJc >104A-cm-2 at 4.2 K

>103 A-cm-2 at 77 KEpitaxial films (c axis l film)

TC 93 KH,2 (estimated) -4 x 106 G, H film, at 4.2 K

-1 x 106 G,H film, at 4.2 K-7 x 105G,H I film, at 77 K

-1.5 x 105G,H II film, at 77 Kic >4 x 106 Acm-2 at 77 K

6.2 6.0

FIG. 5. TC vs. oxygen content. The lower curve is the room-temperature resistivity at the same oxygen concentration [repro-duced with permission from ref. 25 (copyright, American PhysicalSociety)].

frequency through isotopic substitution. This variation,called the isotope effect, has been extensively studied formetals and intermetallic compounds and is generally seen tobe satisfied (40). Within the framework of the BCS model, Tcshould vary as M-", where M is thC reduced mass of thephonon mode important in the pair- g mechanism. In thelimit of no Coulomb interactions, a = 0.5. For elementalmetals and intermetallic compounds, a varies from 0.1 to 0.5.Recently, Batlogg et al. (41) and Bourne et al. (42) havecarried out experiments to observe the isotope effect inYBa2Cu307. Since there are good reasons to believe thatoxygen atoms play an important role in superconductivity, asshown by the dependence of Tc on oxidation state (43) and byband-structure calculations (44), 160 was replaced by 180.The Raman scattering spectra of the 160 and 180 compoundsindicate that phonon frequencies have changed as expectedfrom the mass ratios (Fig. 9). Fig. 10 shows magnetization asa function of temperature; for a full isotope effect, theexpected change in Tc should be -3.7 ± 0.5 K. However,within measurement error, the change in Tc is <0.2 K. This

* SAMPLE I

* SAMPLE 2

* o

* 0

III I* o

78 80 82 84 86

TEMPERATURE (K)88 90

FIG. 6. Jc for epitaxial thin films of YBa2Cu307 as a function oftemperature.

implies a small isotope effect but by itself does not rule outphonon-mediated mechanism. Recent measurements byLeary et al. (45) on YBa2Cu307 samples with z90% 180exchange for 160 indicate a decrease of T, of about 0.3 + 0.1K, corresponding to a 0.04 and indicating a very weakisotope effect (Fig. 11). These results are to be contrastedwith those of Batlogg et al. (41) and Bourne et al. (42) on=80% 80-O60-exchanged samples described above. Onemight conjecture that different oxygen sites in the crystallinelattice (18) of YBa2Cu307 are exchanged preferentially. Theoxygen isotope effect in a related superconducting com-pound, La1 85SrO 15Cu04, which has a T, of 40 K, has beenmeasured (46, 47) to be 0.3 ± 0.1 K, corresponding to a =0.16 ± 0.02. Further, we can plot the measured a for threecompounds, La185SrO15Cu04, YBa2Cu307, and BaPbo.75BiEJ25O3, as a function of Tc. As shown in Fig. 12, theelectron-phonon coupling strength decreases as T, increasesfor the oxide superconductors, indicating a weakening role of

EU

r--rU

APPLIED FIELD (GAUSS)

FIG. 7. JC as a function of applied magnetic field, H, forYBa2Cu307 _. The lower band represents most bulk sinteredpolycrystalline ceramics and illustrates a major practical hurdle toovercome-i.e., at 10 kG (1 Tesla), Jc is reduced to 1 A-cm 2. Recentdevelopments using a bulk melt technique show substantial improve-ments.

t10a.

4L-u0 8

z

w4cr-20

u

Proc. Natl. Acad. Sci. USA 85 (1988)

Seminar Report: Patel and Dynes

v1.35 *Pb * *Nb

aHg L- 1-Hg. Sn Ta aLa

1n

L / *TR/ A *Re Th

1 *Zn M HEAVYOs Zr FERMION-* SUPERCONDUCTORS*Cd Rue*

Ti

l I l l

1 10 100)(mJ/mole f.u.)

FIG. 8. Compilation of T, vs. y, where y represents the electroniccontribution to the heat capacity and is a measure of the electronicdensity of states, N(O), which is the electronic parameter in Eq. 1.f.u., Formula unit.

electron-phonon interaction in the superconducting pairformation for these materials. Evidence of the inadequacy ofthe electron-phonon-mediated BCS model is mounting.

Anisotropic Properties. It is now becoming clear that thehigh-Ta oxide superconductors, especially YBa2Cu3078,,have very anisotropic electrical and magnetic properties, asexpected from the anisotropic structure. Measurements ofconductivity and upper critical fields on single crystals ofYBa2Cu307 -, have shown that the electrical coupling alongthe c axis is quite weak, with conductivity anisotropiesapproaching =100 being observed (48). Measured ratios ofHc, for magnetic fields parallel and perpendicular to the c axisare =5. These measurements imply superconducting coher-ence lengths that are remarkably short and also are aniso-tropic. For example, the coherence length between the Cu-Oplanes is -7 A, while that in the Cu-O planes is 34 A. Thesevery short lengths are to be compared with simple metallicsuperconductors with coherence lengths of several hundredangstroms. This value of 7 A along the c axis is shorter thanthe unit-cell dimension and calls into question many of theusual "mean field" descriptions of superconductors. Therehave been several reports of measurements of the supercon-ducting gap (2A) in these high-Tc materials but considerablespread in the values obtained, which have ranged from 0 to10 kTc (3.5 kTc would be expected from a BCS model).Nuclear quadrupole resonance data (49) have shown that twogaps may exist, one in the Cu-O planes and the other in thechains. This observation suggests that, at x 6.7 where Tc67 K, superconductivity occurs in the layers and that thechains are responsible for the Tc 2 93 K at x 7.0.

Alternative Models. The various models proposed to ex-plain these new superconductors fall into three generalcategories. The first is an extension of the electron-phononcoupling picture taken to an extreme limit where the phononfrequencies are high and the coupling is extreme. The usualperturbative approach does not work in this limit and varioustechniques for describing this strong coupling limit are beinginvestigated. The second category considers an electronicmechanism to replace the phonon as a coupling medium.Electron-exciton coupling had been suggested several years

Proc. Natl. Acad. Sci. USA 85 (1988) 4949

0

T =77K qt B

0

I-~~~~~~~~~~~~~~~~~~I

Co)zw 0z AAz

CY)

I700 600 500 400

RAMAN FREQUENCY (Cm-1)

FIG. 9. Raman spectra of superconducting EuBa2Cu307 at 77 K.Trace A, sample containing 160; trace B, sample containing 180 (74± 7% exchange) [reproduced with permission from ref. 41 (copy-right, American Physical Society)].

earlier by several authors (50-52) and is being seriouslyconsidered in addition to models incorporating electron-induced charge exchange in the bonding orbitals (53). Thethird category incorporates electron coupling to spin fluctu-ations (54) and results in radically different thermodynamicproperties. At the moment, both theorists and experimental-ists are working very hard trying to sort out these variousmodels (55).

o

E0ro

-J

z

nC/)

w

zU)

C)w

-2k

-4

-6k

-8

-10

86 87 88 89TEMPERATURE (K)

90 91

FIG. 10. Meissner signal of YBa2Cu307. For a full isotope effect,a &T, of 3.7 + 0.5 K is estimated [reproduced with permission fromref. 41 (copyright, American Physical Society)].

000e

.5

580

s° Ba2.YCU3070

0000: - 018°°" . 016

4950 Seminar Report: Patel and Dynes

0oh

N.2.E -0.5000

-1 .0x

CPx

-1.5

-2.088 89 90 91 92

TEMPERATURE (K)93 94

FIG. 11. Magnetic susceptibility of YBa2Cu307 as a function oftemperature for samples containing 160 (r) and 180 (v) [reproducedwith permission from ref. 45 (copyright, American Physical Society)].

POTENTIAL APPLICATIONSIt is a rare breakthrough that has many potential applicationsalready extant (see for example, ref. 56). The synthesis ofsuperconducting materials with T, above liquid N2 temper-ature has created a unique situation in which many applica-tions that previously used Nb3Sn-type superconductors at 4.2K can be replaced by the YBa2Cu307 class of superconduc-tors operating at >77 K. We have already enunciated theeconomic impact of liquid N2 over liquid He for providing thecryogenic temperatures for appropriate superconductors. Inprinciple, all of the present applications of superconductorsat 4.2 K can be extended to superconductors at 77 K, whichwill lead to substantial economic benefits if the needed Jc canbe reached in the presence of magnetic fields appropriate forspecific applications (see Table 4 for a compilation of data fora number of applications). We summarize some of thesepotential applications below.

Electronic Applications. At the integrated circuit level thereare two networks, the power distribution network and thesignal network. A superconducting power grid would resultin very stable voltage levels and tighter control on voltagethresholds. On the signal network, those few interconnec-tions that are longest and hence result in the greatest lossesand pulse dispersion could be replaced by superconductingstrip lines, resulting in faster clock frequencies. A similarargument can be applied in integrating up to interchipinterconnections, intra- and interboard communications, and

0.6sTI_FROM BCS TCcM-05(l-C)

0.4 Zn O5

a

0.2

0

0 20 40 60 80 100

FIG. 12. Oxygen isotope effect for three oxide superconductorsas a function of their superconduction temperatures.

Table 4. J, for superconductor applications

Application Jc, A-cm -2 Magnetic field, tesla

Electrical powerTransmission

ac 105 Nodc 104 0

Energy storage 106 3Generators 104 5-7MotorsLarge 104 5-7Small 103 0.1

Accelerators 2 x 105 2-10Fusion magnets 105 10-15MR1 104-_105 0.4-2High-field magnets 105 30MRI, magnetic resonance imaging.

finally back-plane connections. The success of these poten-tial applications will depend on many as-yet unansweredquestions concerning processing, contacts, stability, inter-diffusion, economics, etc.

Superconducting switches using Josephson devices fabri-cated of these oxides have not yet been demonstrated butquite probably will be soon. Some of the earlier reasons whyJosephson circuits did not find practical and widespreadapplications are no longer valid but others remain. At thisstage it is not clear whether Josephson devices have enoughadvantage over high-speed semiconducting devices. It ispossible that a hybrid semiconducting-superconducting cir-cuit is the most advantageous. Finally, discrete devices suchas infrared detectors and magnetometers using these high-Tasuperconductors have already been demonstrated. Theirapplicability will be determined by sensitivity, economics,and reliability issues.A potential application related to electronics is that of

low-frequency shielding. A room or Faraday cage made ofsuperconducting walls will serve as an electromagnetically"clean" room down to dc. This could find applications insensitive areas.

Electrical Power Transmission. One of the issues studiedextensively in the mid- and late 1970s following the oilembargo was that of superconducting power transmissionlines because, depending on the distance between the powergeneration site and the user site, as much as 5-25% (de-pending on the source of information) of the generatedelectrical power is wasted in resistive losses even in thehighest voltage conventional electrical transmission lines.Superconducting power transmission could make a sizableimpact on national energy usage but both the complexity ofliquid He-cooled superconducting transmission lines and theassociated cryogen costs precluded its deployment. With thenew superconductors many of these limitations could beameliorated. Typical current densities for dc superconduct-ing power transmission lines are system designed to be at-104 A-cm2 in the presence of a self-generated magneticfield of 100-200 G. At present, YBa2Cu307 bulk materialcapable of carrying -15,000 A cm-2 (in zero magnetic field)is available (Fig. 7) and, thus, we are almost at the stagewhere exploration of this application should be possible. Fora sound design, however, the wires should have a currentcapacity of >104 Acm -2, which is likely to be reached in thenot too distant future. Apart from the impact on electricaltransmission losses, the ability to use electrical power gen-erated at far-away sites could change the picture of genera-tion-distribution optimization. For example, hydroelectricpower generated far away from population centers could beeconomical. Another benefit is the possibility of siting fossilfuel as well as nuclear power plants far from populationcenters, reducing the local effects of pollution on such

a m

*nO-_ 0

-a

a0

U0

* 0

0 I I I

MoCd

B7Pb07Bi0.3O7zo5Os La15Sr0.15CuO4 TREND FOROs ~~~~~~~OXIDES?; -

° Nb3Sn- Ru

B0 Zr Bo2YCuI a I I , I, ,,, I I I I,,a,,,, . r

Proc. Natl. Acad. Sci. USA 85 (1988)

Proc. Natl. Acad. Sci. USA 85 (1988) 4951

centers. Finally, photovoltaically generated electrical power,which is economical in desert areas, could be transportedover long distances.Magnet Applications. High-field superconducting magnets

at 4.2 K are extensively used in scientific applications and inmagnetic resonance imaging machines. Other applications ofhigh magnetic fields include magnetic levitation, magneticseparation of materials, and energy storage for electrical loadleveling. The potential for superconducting magnets at 77 Kcould lead to replacement of existing magnets and create newapplications. For high-field magnet applications, the super-conducting wires need to have a current capability of >105A-cm -2 in the presence of magnetic fields in excess of 105 Gand considerable amount of strain (Table 4). As shown in Fig.7, however, considerable work needs to be done in improvingthe value OfJc in bulk YBa2Cu307 for this application to takehold. Results on epitaxial thin films already have shown thatsuch material is capable of sustaining large Jc values even inmagnetic fields in excess 105 G. In addition, techniques forfabricating the bulk material into long wires and cables mustbe developed.

CONCLUSIONSEnormous progress has been made in the year or so since thefirst announcement of copper-based oxide superconductors,yet with every measurement the mystery of the physicsunderlying the pairing mechanism leading to the high Tc ofthese materials has deepened. Excitement among scientistsand engineers runs high and the popular press has allowed ourimaginations to run amok. Graphic demonstrations of high-Tcsuperconductors abound. These include the levitation of apermanent magnet above a superconducting disc ofYBa2Cu307 (Fig. 13) and persistent current in a ceramic loopas detected by the magnetic field. Much remains to be done,however, both in science and in engineering the materials forpractical applications. Some technological challenges arelisted below (in no particular order):1. The search for ever-higher Tc materials and elucidation of

the "glitchite,"2. Understanding the science,3. Increasing the Jc of ceramic materials especially in the

presence of large magnetic fields,4. Engineering fabrication of wires and cables, and5. Engineering fabrication of films on substrates for elec-

tronic applications (e.g., silicon and gallium arsenide).Challenges that face our nation in exploiting technical

advances for economic benefit fall into two primary areas.The first area is the need to support fundamental research and

FIG. 13. Cube of a permanent magnet levitated above a disc ofsuperconducting YBa2Cu307-8 held at 77 K.

development activities. The former are being carried outprimarily by universities, a few industrial laboratories, andthe national laboratories. The latter is most beneficiallycarried out by industrial laboratories where the transfer oftechnology to engineering demonstration and eventual man-ufacturing is needed. The second area is in engineeringapplications and demonstrations. Both of these will need tobe supported over the long term because the economic payoffis likely to be several years away. At the research anddevelopment end, maximum progress is possible by individ-uals and small teams because the needed advances are likelyto occur primarily from individual insight. Engineering ap-plications and demonstrations are a different matter alto-gether; large teams are often necessary.Our national response to the advances in superconducting

materials and the resulting challenge for reaping economicbenefit from engineering applications has been far too narrowand appears to be designed to make newspaper headlinesrather than come up with a systematic and strategic plan. Asone of our colleagues states (P. A. Fleury, private commu-nication), proposals calling for the establishment of super-conductivity centers are analogous to setting up rocketryschools following the launch of Sputnik in 1957 by the SovietUnion. A comprehensive plan needs to have certain mini-mum components, including a broadly based initiative inmaterials science and materials-processing activities in ouruniversities and industrial laboratories. The lack of a strongsolid-state chemistry and related synthesis activity in oureducational institutions over the last half century has led toa decline in the number of outstanding materials scientiststrained in the United States, as described in a NationalResearch Council report (57). A concerted effort to rectifythis shortcoming is one of the absolute musts in preparing theUnited States for the competition that surely will exist in thecommercialization of high-T, superconductors. Further,commercialization by definition implies a strong commitmentby our industrial institutions, which are responsible fortechnology transfer to manufacturing. It is well establishedthat such technology transfer is most efficient when thedevelopment work leading to engineering applications iscarried out by the institutions that will carry out manufac-turing. Therefore, our natural response should recognize, inthe present case where all nations are at the starting gate atnearly the same time, that expanding development andengineering applications activities on university campuses orat the national laboratories could lead to suboptimal deploy-ment of national resources and ultimately contribute to ourinability to compete internationally. There is a need for acoherent plan that facilitates development of processingtechnologies in the industrial sector. Therefore, a nationalpolicy should be to commit our resources for materialsprocessing science, development, and engineering of super-conducting materials in industrial institutions with a strongpartnership with universities (rather than the other wayaround).We as a nation have traditionally been very successful at

the front end-i.e., the research and development (R&D)phase-because these activities can be sustained over longperiods with only modest infusion of funds. Engineeringactivities, on the other hand, require substantially largerfunding over a period of time of 5-10 years. A rule of thumbis that each step from research to development to engineeringfor manufacturing is accompanied by an order of magnitudeincrease in its cost. These considerations require a nationaldetermination and staying power. It is much too fashionablefor various agencies tojump on the bandwagon of increasingnational R&D expenditures in superconductivity becausethese represent a small fraction of the resources needed forengineering and resultant economic exploitation of the ad-vances. There is a need to combine the apparent long-term

Seminar Report: Patel and Dynes

4952 Seminar Report: Patel and Dynes

stability of national institutions and the technological prow-ess of industrial institutions. A policy that brings these twoinstitutions into partnership is certainly one that has areasonable probability of handsome payoff in the end. Andyet this policy is only rarely mentioned in the discussions atwhich the focus is still at the R&D end, which by all meansshould be strengthened. Technological problems will cer-tainly be solved given sufficient resolve. Economic benefitwill be derived only when we recognize that science requirescommitment of relatively smaller amounts of resourcescompared to development and manufacturing-i.e., smallscience but big engineering and bigger manufacturing.

Note Added in Proof. Since this paper was written, two additionalcopper-oxide compounds have been reported that drive the maxi-mum T, upward. The present record is held by Tl2Ba2Ca2Cu3010with T- 125 K.

We thank Drs. B. Batlogg, R. J. Cava, G. Y. Chin, P. A. Fleury,R. A. Laudise, and D. W. Murphy for their critical reading of themanuscript and comments. We also thank Dr. S. Jin for help incompiling Table 4.

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