Superconducting Gold AlloysA REVIEW OF THE STATE OF THE ART

H.R. Khan 1

Forschungsinstitute für Edelmetalle und Metallchemie, D-7070 Schwabisch Gmünd, Federal Republic of Germany

Although the discovery of a gold alloy with a superconducting state at relatively hightemperature wouldbesignificantand exating, ithas been afeature oftbepastdecadethattechnology has advanced to the stage of producing applications for gold and other alloyswhich achieve superconductivity at low criticaltemperatures. A significant research efforthas gone into the search for superconducting gold alloys, study of their properties, andapplications for them.

In 1975 the author and Ch. J. Raub reviewed the field ofsuperconductivity of gold alloys for this journal (1). At that time itwas concluded that although no intermetallic compound of goldhad yet been found to exhibit superconductivity at temperaturesthat would make it industrially useful, the investigation of goldcompounds had assisted solid state physicists in their understandingof the phenomenon of superconductivity. It must be borne in mind,of course, that in the mid-1970's `industrially useful' implied acritical temperature (T,) of greater than about 20 K. Thetechnological and engineering advances which allow readyapplication of alloys with very much lower T, values were still atthat time on the drawing board.

In the review of the state of the art of superconductivity in goldalloys which follows, some material from the author's earlier reviewin this journal is included and more recent work is discussed indetail. This approach has been selected in view of the changingparameters of technology which are allowing practical considerationof gold alloys which become superconducting at levels oftemperatures too low to have been envisaged a decade ago, and inorder to present a more complete and comprehensive review.

General Superconducting Behaviour of GoldGold belongs to a group of elements in the periodic system

which do not become superconductors even down to the lowestavailable temperature, although most of the elements aresuperconducting under normal conditions or high pressures. Somesolid solutions and intermetallic compounds of gold with thetransition metals show peculiar superconducting behaviour. Ingeneral gold lowers the superconducting transition temperature(T,) of the host transition metal in solid solutions, whereas anenhancement of T, is observed for some intermetallic compounds.Study of a gold-lanthanum alloy led to the discovery of metallic glasssuperconductors. These glassy superconductors are of great interestbecause of their superior mechanical and superconductingproperties, and the investigations of the superconducting behaviourof gold-containing intermetallic compounds, solid solutions andmetallic glasses have enhanced our knowledge of the electronicstructure of materials. At the same time some superconducting goldalloys are useful as low temperature reference materials, and aselectrode and counter electrode materials in Metal/Oxide/Metalsandwich systems (Josephson junctions).

After the discovery of superconductivity in mercury byKammerlingh-Onnes in 1908 following his success in liquifyinghelium, the first superconducting intermetallic compounddiscovered by de Haas in 1929, was the CIS-type phase Au 2Bi.Since then most of the elements have been shown to becomesuperconductors under normal conditions or under high pressures.Figure 1 shows the distribution of superconducting elements in theperiodic system.

Present Understanding of SuperconductivityThe author discussed briefly in his earlier review (1) theoretical

developments in the understanding of superconductivity at thattime, and in particular why superconductivity occurred in elementswhich were poor metallic conductors rather easier than in thosewhich exhibited good conductivity at room temperature.

Thus in relating critical temperatures to other solid stateproperties it was found that the parameter of greatest importancefor the critical temperature was the electronic structure of thematerial giving a high electron density of states at the Fermi level,the most favourable being the A-15 or (3 W structure. The empiricalTule discovered by B.T. Matthias which predicted that alloys withaverage numbers of valence electrons per atom on the lower side ofvalences 5 and 7 would have high critical temperatures was animportant step forward, and the experimental confirmation of theFröhlich equation describing the inverse proportionality of thecritical temperature to the square root of the atomic mass

T, aM -12

led to the discovery that superconductivity is caused by a kind ofelectron-lattice interaction.

Understanding of superconductivity is largely based on theBardeen, Cooper and Schrieffer (BCS) model, and its subsequentmodifications, which showed that in the superconducting statecondensation of pairs of conduction electrons, bonded by virtualphonons, into a lower energy state occurs (2). The acceptability ofthis model was in no small way due to the wanner in which it readilyexplained the observed trend that superconductivity is more readilyachievable in poorer metallic conductors than in good ones.

In this model the critical temperature was related to the Debyetemperature 8D , the electron density of states N(0) and theelectron-phonon interaction parameter V by:

94 GoldBull., 1984, 17, (3)

IANT.1-IANIDÉS{HARE GAPT SJ

i,y pr ., ^.pEh - Sm 4 psd d Tb ©K: Ho Er TrW b.

7FP+15Fr!CïN ELEáS1E1JT;£+UF'ËiiC^9i^llt!`Ti]R3., ^uFERGaiFCHJ^TFi^(a3 ._

..

^iJ .Y liti6^Eï rE3ESs.'li rsé.!

7R91V ! fIÓtJ. ELE^^ÉnI:T. SUR FicoN Duo"roF5 'm {ONLY {1.1 DE-R"PRE SURF}c : r .. i íIN TRANSiTIp^ E12MAFM,1 -1 PËR=._ _.

C.pNf)LiC:TOFts{!lL1'IJNUER:F?RESSl1R}

,̂ ^f y hl ( 7, y^. qry afTIC3N;El..ER9FIdT :tipl'. aFF EA..G"il j ^UG.TRàUfGG ".:.

Tig.i. I3istrii^r m^fi esupercvnducérag;e ^ènts3gtfië S±élresvstéár.,

T,a0Dexp.— (N()V) Superconducting Gold CompoundsCompounds of gold with other elements which exhibit

Further modification by Eliashberg (3) and McMillan (4) gave superconductivity have been described below in relation to thethe more refined equation for the superconducting transition Periodic Table group in which the alloying element is found.temperature of:

Compounds with Group land II ElementsT = 9 exp. — ( 1.04 (1+ ^)

62 The compounds of gold with the elements of groups IA and IB1.45 \^ — µ (1 +0. )I) do not become superconductors, typical examples being

Where 2, is the electron-phonon interaction constant and µ* is the compounds like Au 2Na, AuNa2 , Au 5K, AuCu and Cu,Au.coulomb repulsion parameter. In weak coupling where k « 1 this However the compounds of gold with the elements of groups IIAexpression reduces to the BCS model with N(0) Vequal to a, — µ *. and IIB do become superconductors with T, values rangingThus superconductors are classified as weak or intermediate coupled between 0.34 and 2.79 K as listed in Table 1.(k < 1), and strongly coupled (k > 1). Barium is superconducting only under pressure but an

As examples, the values of 2, for V3Au and Nb 3Au alloys with intermetallic compound with gold becomes superconducting atA15 structure are 0.54 and 0.87 respectively (5). temperatures as high as 2.79 K. The intermetallic compound of

GoldBull., 1984, 17, (3) 95

up ncopdu t ámpound§,0f-Au.,1N 4lemenis n. `. ups liFÍ anc lil

composition Au,Ba (D2d structure) exists in the homogeneityrange of compositions BaAu 3 5 to BaAu6 5 . The variation of T, andlattice parameters with composition is shown in Figure 2, fromwhich it can be seen that Tc decreases with increasing goldconcentration.

Compounds with Group IH ElementsTable II shows the listing of the superconducting compounds of

gold formed with the elements of IIIA and IIIB groups along withtheir crystal structures and T, values. The highest Tc value isobserved for the compound AuT1 (T, = 1.92 K). Hamilton et. al.

(6) reported on the existence of superconductivity in the gold-gallium system. The compound AuGa becomes superconductingat 1.24 K whereas AuGa, is not superconducting above 0.34 K.Later Wernick et al. (7) reported that AuGa 2 with a crystal structureof the CaF2 -cubic type became superconducting at 1.1 K. Thediscovery of superconductivity in AuGa 2 and the ternarycompound (Au(,_ x)Pd3Ga2 (x <0.15) led Schirber (8) to redict andsubsequently verify (9) that the parameter (ST /SP) of AuGa 2

shows a sharp increase for applied pressures between 5 and 6 kbar,that is to say T, increases from 1.3 Kat 5 kbar to 1.9 Kat 6 kbar. Theauthor and his colleagues investigated a number of binary andternary compounds of gold, gallium and palladium of differentcompositions (10). The T, values, as well as the gradient (dH/dT) TC

of the plot of the critical field versus temperature at the zero appliedmagnetic field are listed in Table III. The transition widths as wellas the X-ray and metallographic data are also listed in the sametable, and these indicate whether a compound is single phase orconsists of two or more phases. T, as well as (dH,/dT)TC increaseswith the increase of gallium concentration in Au(,_, ) Ga,compounds.

The temperature variation of the critical magnetic field ofdifferent samples is also shown in Figure 3. The investigations fromwhich these results are derived have shown that thesuperconductivity of the gold-gallium system is not as simple tounderstand as was thought earlier. The T, values of the compoundAuGa, (Cl structure) depend upon the metallurgical history ofthe sample. The homogeneity range of the Cl structure lies between67.2 to 71.9 gallium atomic per cent with Tc values significantlyhigher than the reported values of 1.1 K for the single crystalsamples. Hyot and Mota (11) studied the T, variation of vanadium-gallium dilute solid solutions and predicted that gold wouldbecome superconducting at a temperature lower than 0.2 mK.

Compounds with Group IV ElementsNone of the transition metals titanium, zirconium and hafnium

of group IVB form superconducting compounds with gold. Due tothe stabilization of the b.c.c. (3 phase, gold enhances the T c value ofzirconium to a value of 2.8 K (12). Superconducting compounds ofgermanium (13), tin (14) and lead (15) are formed with gold,although there is a possibility that in the case of gold-tin and gold-lead the presence of free tin or lead might have caused thesuperconductivity rather than the intermetallic compounds. Asuperconducting gold-germanium compound (T, = 2.7 K) isformed only by rapid quenching, hence the structure andcomposition are not completely established. Compounds like AuSnand AuSn4 have hexagonal and primitive cubic structures, and

96 GoidBull., 1984, 17, (3)

TableIIIThe Superconducting, Chemical andMetallurgical Datafor

lim Au~Ga Compound SamplesandThreePd-doped AuGa.Sampies

Composition Sample T, ftT (dH,ldl)", RemarksOa/K

AU.",.Ga",.1 1 1.17 -106 Single phase,homogeneous,AuGa.B31AU,,,Ga,,,, 2 1.378 -123 Two phase,AuGa+AuGa,

AU""Ga,,, 4 1.387 0.14 Two phase,AUGa+AuGa,

AU",.Ga"" 3 1.355 -112 Twophase,AuGa+AuGa,

AU".Ga"", 8 1.27 Two phase,AuGa,+Ga

AU"'BGa"" 7 1.668 0.23 -412 Singlephase,homogeneous,AuGa, -:C1

AU""Ga"" 5- 1.630 0.26 -252 Singlephase,homogeneous,AuGa,":"C1AU,.,Ge". 6" 1.587 0.12 -241 Singlephase,homogeneous,AuGa, -C1AumGa".> 9 1.615 0.13 -243 Two phase,AuGa,+GaAU"'DGa...O 10 1.587 0.13 -243 Two phase,AuGa,+Ga

(Au",Pd,)Ga,(Au"""Pd=)Ga, - 1.510 Singlephase.homogeneous(Au,",Pd,,,,jGa, - 1.773 0.02 Singlephase.homogeneous(AU,.,Pd, ,jGe, - 1.790 0.10 Two phase

'By X-rayand metallographs

"Small (3% 01 total signal) transition atT -1.40 K, (dH,kH)" -140 OelK

their T, values are 2.0 and 3.0 K, respectively.

Fig 3 Dependence ofcritical magnencfields 00 temperarureforselectedAu-Gasamples

Compounds with Group V ElementsOnly antimony (15) and bismuth ofgroup VAreact to form

superconducting compounds such as Au,Sb (T, =0.58 K) andAu,Bi (T, =1.84 K), whereas Au,P 3 is not superconducting abovelOmK.

Other Transition Element Compounds ofGoldGold forms A15-phase compounds with the elements

vanadium, niobium and tantalum which are superconducting.Nb3Au (A15 -phase) has the highest T, value of 10.5K (16)whereasTa3Au has a T, value of O. 5 K (17). Addition of a third transitionmetal like rhodium and platinum to Nb3Au further raises the T,value. Thus the criticaltemperature ofNb3Auo98Rh" o, is 10.9K (18)and that of Nb3Auo 7Pt O 3 has a value of 12.8 K (16). Thesuperconducting as well as the electronic properties ofNb3Au(1_X)Ptx are dependent on the annealing conditions, whichaffect the long range atomic order parameter. For exampleNb3Auolto.3 , in as-cast condition, has a T, value of9.3 K whereasthesamesample-afteraheattreatmentof4h/1450°C + 7d/750°Cachievesan increased T, value of 12.45K asshown in Table IV (19).The critical magnetic fields ofcompositions within this system areshown as a function of temperature in Figure 4. A systematicvariation of T, with composition in the ternary Nb3Au-Nb3Pt

system is shown for both the as-cast and annealed conditions inFigure 5 (20).

V3Au possesses the A15-structure and has the same electron

GoldBull., 1984,17, (3)

100

0 75 0 00

i:Cl 0..JWi:LU 0FwZ 50 0(!)

""::;:." ~<CUF 05

025 -

0

0.5 1,0TEMPERATURE, K

o SAMPLE ,

• TA~~E m 2~ 3... 5

97

w.5 (6ototn) riacrop- o [hèsupérco u tig t nsitrot; feiispei pure„ bf 1^Tb3 Aïi11oys d clie:.áá-cast aii1 ámreái d,.sufes sa u cpC 1$ [i{ tlie Cup i1 öf 1Vh Pt

Température: {T J, and,Pta ,and Nb Pt'after

ompountl= Heat Lattic?trëatt^i nt: constant aí:: Tx, y; Referentie

K mJ/Kzát 9.

Nb Pt 10h/1450 5155 9 555 14(a)f±lb3Pt 10 14 0 ,5155 1d6 55 144(a)

+7dl0D.1lob Au 24ht1Q5a 5.202 105. 93 14(h)

í b Au0. Pt as cast: 5196 93 5.8. IjP bA Ptt, . 1O.h/1'455Q . Po 5196. 12:45 , ;854 14(a)

H.

k A^ï

PÍtit j

El II.

O A .-ÁS.G uI)IT"NNEAL£k7 14 0A P ?cao G

concentration as Nb 3Au but a critical temperature value of only0.74 K. A low temperature annealing raises the T, value to 2.8 Kdue to an increase of the order parameter which subsequently raisesthe electron density of states and hence T, (21). RecentlyJarlbordet al. (22) have derived the T, value of this compound from theelectronic structure data. The calculated value of T, is around 10.2K. Such a big differente between the experimental and calculatedvalues of Tc suggests the presence of spin fluctuations, the presenceof which would bring a compound closer to a ferromagnetic regime.In any event V,Au is a good candidate for the study of the physicsof spin fluctuations since the compound has an observable T,value. A similar effect was observed in the case of the solid solutionsof vanadium with the 3d; 4d and 5 cd transition metals. It was foundthat in the solid solutions of the series, V0 90X010 (X= Cr, Mn, Nb,Mo, Pd, Ta , W, Re, Pt and Au), the lowest critical temperature valueof 0.30 K is observed for the composition V090Au0 ,0 (23) (Figure 6)whereas the calculated value from the electronic structure is 0.9 K(24). The experimental and calculated values of T, differ by a factorof 3, similar to the compound V,Au, which also suggests thepresence of spin fluctuations in the V 090Au010 solid solution. Inaddition to this it should be noted that Au 4V has the interestingproperty of being one of the few ferromagnets with noferromagnetic constituent elements.

Among the group VIA elements, only the gold-telluriumcompound Au,Te, is superconducting (25) and none of the goldcompounds with the elements of groups VIB and VIIIB aresuperconductors. Some other superconductors worth mentioningare AuLa (T, =0.35 K) (26) and the pseudobinary carbideAuG, 3Y09 (T, = 10.1 K) (27).

Gold forms a low temperature eutectic witti lanthanum and thislow eutectic condition was pointed out as a favourable condition forobtaining an amorphous alloy by liquid quenching (28). In 1975an amorphous superconducting alloy was obtained by rapidquenching from the liquid state (29), which had the compositionof La80Au20 . The critical temperature value of this metallic glass is3.5 K and decreases with an increase in gold concentration, forexample the T, value of La76Au24 is 3.3 K. Since then many metallic

98 GoldBull., 1984, 17, (3)

glass superconductors based on transition metals and metalloidshave been discovered. These metallic glass superconductors havepotential applications due to good mechanical properties, highmagnetic field and critical current in the pseudoamorphous state,as well as resistance to neutron radiation damage.

Epitaxial Metal Film Sandwiches (EMFS)Recent work by Brodsky and his colleagues (30a, 30b) has shown

that by epitaxial growth of thin films of metal on layers of gold toform Au-M-Au sandwich structures, a new class of materials can beproduced in which novel electronic and magnetic properties areevident. Among these is enhanced superconductivity which occursparticularly in the Au-Cr-Au system. The highest measured T c forthis material was 3.0 K which is thought to occur from the formation

Au {.73nmi,.:Cr.{1.ynrny. 4.-, AFTER- 348':RAl S AT RO0Jp1 TÉMPERATURE

^s z

4:

p ,.Au.{73 n}n}^ Er í11^7m}.

Au[39nm1,:.0 t9írr]

TËRËRATURE. 'EC

?ig, 7 R afdifferen Xa Cr-Auepi sip meEà1 Sandw ch

of metastable fcc chromium on the fcc gold. Measurements of T c

for sputtered bcc chromium have yielded a maximum value of only1.52K. Figure 7 shows the resistivity of two Au-Cr-Au compositesas a function of temperature, and in Figure 8 the relationshipbetween critical magnetic field and critical temperature is shownfor a sample of Au-Cr-Au in which the gold layers are 73 nm and thechromium film 1.1 nm thick. The latter figure shows the effect ofapplying the magnetic field both perpendicular and parallel to theplane of the metal film sandwich.

Such materials are thought to have possible applications in anumber of electronic devices, although further research is requiredin some areas, in particular an observed tendency for the enhancedsuperconductivity effect of the EMFS to decrease when the samplehas been standing at room temperature for a time.

Applications of Superconducting Gold AlloysSome superconducting alloys of gold find applications as

reference temperature material below 0.5 K. For example AuIn2 issuperconducting at 0.204 K and has a good reproducibility between0.1 and 0.4 mK (31). Figure 9 shows a fixed point superconductivedevice based on Auln, and AuAl2 as two of the five materials whichprovide the reference temperatures for the device. The other

GolcdBull., 1984, 17, (3) 99

Fig. S;< Ctit caimag et ë iiel l asá fiinctionofcr tical reropsrarorefooaAu CrÀu ítetal£rljnsandwrcliwherethethieknessèsoffiególdandchromiumIi ware. 73and11un:sespecnvely;Theupjerplateiresennthe apliccionofthemagneticetdinthepiarieperpendicul ír'to:thatof ches2ndwich,'andhe:to wwer plat` s'deiiyedttom a;parailci magneticField.

•PigK 9' viel point :dë !icë-for pr6iding.lve.re enge temperaturesbël v0:5.Ï withexc u t`zépto. n91. i Sahp1ex fAS illz,.ÂuI ,lT(!BeandTrace uncliedmmpperwrreswhi lia1so enaose2 setsfcoil$[P ?cY f^d'samá ány) xhew1ii51eassesnlilyue t aplated.w^ithputegold to a thidcnéss.oEa^prtlxInatel}; 05,µir1

ZD Á ÉR$ RIi iIA^fti

` ... .:. .. .. .:COP^ER'SECONDARV....... aao4.TURUs

`..GD:.PLÁTED.:

I ^::PHËriáLiG.C^IL;é"3o FORMER

100

applications of gold-based superconducting alloys are inJosephsontunnel junctions for logic and memory circuits (32, 33). A type ofdevice based on these junctions, SQUID (SuperconductingQuantum Interference Device), is able to detectmagnetic fields ofabout 10 femtotesla (the earth's magnetic field is - 60 microtesla).There is a possibility of producing extremely fast, large scaleintegrated circuit chips with this technology (34). Failure of theseJosephson junctions takes place due to thermal cycling. The use oflead-gold and lead-gold-indium alloys as electrodes and counterelectrodes in these devices has improved the workability, and theyhave been reported to withstand several hundred thermal cyclesbetween room temperature and 4.2 K (35, 36).

References

1 H.R. Khan and Ch. J. Raub, GoldBull., 1975, 8, (4), 114-1182 J. Bardeen, L.N. Cooper andJ.R. Schrieffer, Phys. Reu, 1957, 108, 11753 G.M. Eliashberg, Zh. eksper. teor. Fix., 1960, 38, 966 and 1960, 39, 14374 W.L. McMillan, Phys. Reu, 1968, 167, 3315 F. Heiniger, E. Bucher and J. Muller, Phys. Kodens. Materie, 1966, 5, 2436 D.C. Hamilton, Ch J. Raub, B.T. Matthias, E. Corenzwit and G.W. Hull, Jr.,

J Phys. Chem. Solids, 1965, 26, 6657 J.H. Wernick, A. Menth, T.H. Geballe, G. Hull andJ.E Maitaj.. Phys. Chem.

Solids, 1969, 30, 19498 J.E. Schirber, Phys. Rev. B, 1972, 6, 3339 J.E. Schirber, Phys. Rev. Lett., 1972, 28, 1127

10 R.A. Hein, J.E. Cox, J. Wills, H.R. Khan and Ch J. Raub,J. Less-CommonMetals, 1978, 62, 197-209

11 R.F. Hoyt and A.C. Mota, Solid State Commun., 1976, 18, 13912 B.W. Roberts, in 'Progress in Cryogenics, Vol. 4', Heywood, London, 1964,

p. 16013 B. StritzkerandH. Wuhi, Z. Phys., 1971, 243; H.L. Luo, M.F. Merriamand

D.C. Hamilton, Science, 1964, 145, 58114 E. Klokholm and C. Chion, Acta Metall., 1966, 14, 56515 B.T. Matthias, T.H. Geballe and V.B. Compton, Rev. Mod Phys., 1963, 35, 116 P. Spitzli, R. Fhlkiger, P Heiniger and j. Muller, Phys. Lett., 1969, 30A, 17017 H.L. Luo, E. Vielhaber and E. Corenzwit, Z. Phys., 1970, 230, 44318 S.T. Zegler, Phys. Rev., 1965, 137A, 143819 R. Flükiger, S. Foner, Ej. McNiff, Jr. and E. Fischer, Solid State Commun.,

1979,30,723-72620 H.R. Khan, E. Röschel and Chj. Raub, Z. Phys., 1973, 262, 27921 E.C. van Reuth, R.M. Waterstraat, R.D. Blaugher, R.A. Hein andJ.E. Cox,

in'Proc. loth Internat. Conf. InwTemp. Phys. Vol. II B', p.137; E.C. vanReuthand R.M. Waterstraat, Acta Crystallogx, 1968,24, 187; R.A. Hein, J.E. Cox,R.D. Blaugher, R.M. WaterstraatandE.C. vanReuth, Physica, 1971, 55, 523

22 T Jarlborg, A. Junod and M. Peter, Phys. Reu B, 1983, 27, (3), 1558-156723 H.R. Khan, Ch J. Raub, W. Daumer, J. Goebbels, K. Lüders and G. Roth,

in 'Superconductivity in d- and f-band Metals', edited byW. Buckel and W.Werner, Kernforschungszentrum Karlsruhe, 1982, p. 381

24 F. Brouers, H.R. Khan andJ. Van der Rest, ibis, p. 38525 B.W. Roberts, in 'Progress in Cryogenics. Vol. 4', Heywood, London, 1964,

p. 16026 T.F. Smith and H.L. Luo, J. Phys. Chem. Solids, 1967, 28, 56927 M.C. Krupka, A.L. Giorgi, N.H. Krikorian andE.G. Szklarzj.. Less-Common

Metals, 1969, 19, 11328 M.H. Cohen and D. Turnbull, Nature (London), 1961, 189, 13129 W.L. Johnson, S.J. Poon and P. Duwez, Phys. Ree.. B, 1975, 11, 15030(a) M.B. Brodsky,J. Appl. Phys., 1981, 52, (3, Pt.2), 1665-166930(b) M.B. Brodsky, D. Marikar, R J. Friddle, L. Singer and C.H. Sowers, SolidState

Commun., 1982, 42, (9), 675-67831 R J. Soulen, Jr., Journal de Physique, Cofloque C6, Supplement No. 8, Vol.

39, August, 1978, p. C6-116632 J.H. Greiner, S. Basavaiah, and I.J. Ames, J. Vac. Sci. Technol., 1974,11, 8133 Dj. Herrell, I,E.E.E. J. SolidState Circuits, 1975, SC-10, 360; H.H. Zappe,

I.E.E.E. J. SolidState Circuits, 1975, SC-10, 1234 W. Anacker etal., Josephson Computer Technology', IBMJ.Res. Develop.,

1980, 24, 107-26435 S. Basavaiah andJ.H. Greiner, J. Appl. Phys., 1977, 48, 463036 M. Murakarm. H.C.W. HuangandCJ. Kircher,J. Appl. Phys., 1983, 54,743

GoldBull., 1984, 17, (3)