ELECTRON TUNNELING AND SUPERCONDUCTIVITY

Nobel Lect ure, Dece mber 12, 1973

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I V A R G I Æ V E R General Electric Research an d Develo p ment Center, Schenecta dy, N. Y., US A.

I n my laboratory notebook date d May 2, 1960 is t he e ntry: “Fri day, A pril 22, I perfor me d the follo wing ex peri ment ai me d at meas uring the forbi d den ga p i n a s u perco n d uctor.” T his was obvio usly a n extraor di nary eve nt not o nly be- ca use I rarely write i n my notebook, b ut beca use t he s uccess of t hat ex peri- me nt is t he reaso n I have t he great ho nor a n d pleas ure of a d dressi ng yo u t o d a y. I s h all tr y i n t his l e ct ur e, as b est I c a n, t o r e c oll e ct s o m e of t h e e v e nts a n d t h o u g hts t h at l e d t o t his n ot e b o o k e ntr y, t h o u g h it is diffi c ult t o d es cri b e w hat no w a p pears to me as fort uito us. I ho pe t hat t his perso nal a n d s ubjec- ti v e r e c oll e cti o n will b e m or e i nt er esti n g t o y o u t h a n a stri ctl y t e c h ni c al l e c- t ure, partic ularly si nce t here are no w so ma ny goo d revie w articles deali ng with s u percon d uctive t unneling. 1, 2 A rece nt hea dli ne i n a n Oslo pa per rea d a p proxi mately as follo ws: “ Mas- ter i n billiar ds a n d bri d ge, al most fl u n ke d p h ysics - gets Nobel Prize.” T he p a p er r ef ers t o m y st u d e nt d a ys i n Tr o n d h ei m. I h a v e t o a d mit t h at t h e r e p ort- i n g is reas o na bl y acc urate, t heref ore I s hall n ot atte m pt a “c o ver u p”, b ut co nfess t hat I al most fl u n ke d i n mat he matics as well. I n t hose da ys I was not ver y i ntereste d i n mec ha nical e n gi neeri n g a n d sc hool i n ge neral, b ut I di d ma nage to gra d uate wit h a n average degree i n 1952. Mai nly beca use of t he ho usi ng s hortage w hic h existe d i n Nor way, my wife a n d I fi nally deci de d to e migrate to Cana da where I soon foun d e mploy ment with Cana dian General Electric. A three year Co m pany co urse in engineering an d a p plie d mathe mat- ics k no w n as t he A, B a n d C co urse was offere d to me. I realize d t his ti me t hat sc hool was for real, a n d si nce it probably wo ul d be my last c ha nce, I really st u die d har d for a fe w years. W he n I was 28 years ol d I fo u n d myself i n Sc he necta dy, Ne w York w here I disco vere d t hat it was possible for so me peo ple to make a goo d li vi ng as physicists. I ha d worke d on vario us Co m pany assign ments in a p plie d mathe- matics, an d ha d develo pe d the feeling that the mathe matics was m uch more a dva nce d t ha n t he act ual k no wle dge of t he p hysical syste ms t hat we a p plie d it to. T h us, I t ho ug ht per ha ps I s ho ul d lear n so me p hysics a n d, e ve n t ho ug h I was still a n e n gi neer, I was gi ve n t he o p p ort u nit y t o tr y it at t he Ge neral Electric Researc h Laboratory. T he assig n me nt I was gi ve n was to work wit h t hi n fil ms a n d to me fil ms meant photography. Ho we ver I was fort u nate to be associate d wit h Jo h n Fis her w ho ob vio usly ha d ot her t hi ngs i n mi n d. Fis her ha d starte d o ut as a mec ha nical e n gi neer as well, b ut ha d latel y t ur ne d his atte n- 1 3 8

Fi g. 1. A. If a ma n t hro ws a ball agai nst a wall t he ball bo u nces bac k. T he la ws of p hysics allo w t he b all to pe netr ate or t u n nel t hro ug h t he w all b ut t he c h a nce is i nfi nitesi m ally s mall beca use t he ball is a macroscopic object. B. T wo metals separated by a vac u u m will a p proxi mate t he above sit uatio n. T he electro ns i n t he metals are t he “balls”, t he vac u u m represe nts t he wall. C. A pictorial e nergy diagra m of t he t wo metals. T he electro ns do not have e no ug h e nergy to escape i nto t he vac u u m. T he t wo metals ca n, ho wever, exc ha nge electro ns by t u n neli ng. If t he metals are s pace d close toget her t he probability for t u n neli ng is large beca use t he electro n is a microscopic particle.

tion to war ds theoretical physics. He ha d t he notio n t hat usef ul electro nic d e vi c es c o ul d b e m a d e usi n g t hi n fil m t e c h n ol o g y a n d b ef or e l o n g I w as w or k- i n g wit h m et al fil ms s e p ar at e d b y t hi n i ns ul ati n g l a y ers tr yi n g t o d o t u n n eli n g ex peri ments. I have no do ubt that Fisher kne w abo ut Leo Esaki’s t unneling ex peri me nts at t hat ti me, b ut I certai nl y di d not. T he co nce pt t hat a particle ca n go t hro ug h a barrier see me d sort of stra nge to me, j ust str uggli ng wit h q ua nt u m mec ha nics at Re nsselaer Polytec h nic I nstit ute i n Troy, w here I took f or mal c o urses i n P h ysics. F or a n e n gi neer it s o u n ds rat her stra n ge t hat if y o u I. Gi a e v er 1 3 9

Fi g. 2. A sc he matic dra wi ng of a vac u u m syste m for depositi ng metal fil ms. For exa mple, if al u mi n u m is heate d resistively i n a ta ntal u m boat, t he al u mi n u m first melts, t he n boils a nd evaporates. T he al u mi n u m vapor will solidify o n a ny cold s ubstrate placed i n t he vapor strea m. T he most co m mo n s ubstrates are ordi nary microscope glass slides. Pat- ter ns ca n be for med o n t he slides by s uitably s hieldi ng t he m wit h a metal mask.

t hr o w a t e n nis b all a g ai nst a w all e n o u g h ti m es it will e v e nt u all y g o t hr o u g h wit ho ut da ma gi n g eit her t he wall or itself. T hat m ust be t he har d wa y to a N o b el Pri z e! T h e tri c k, of c o urs e, is t o us e v er y ti n y b alls, a n d l ots of t h e m. T h us if we co ul d place t wo metals ver y close to get her wit ho ut ma ki n g a s hort, t he electr o ns i n t he metals ca n be c o nsi dere d as t he balls a n d t he wall is re- presente d by the s pacing bet ween the metals. These conce pts are sho wn in Fig ure 1. W hile classical mec ha nics correctly pre dicts t he be havior of large o bjects s uc h as te n nis balls, t o pre dict t he be ha vi or of s mall o bjects s uc h as electro ns we m ust use q ua nt u m mec ha nics. P hysical i nsig ht relates to every- day ex perie nces wit h large objects, t h us we s ho ul d not be too s ur prise d t hat electro ns so meti mes be have i n stra nge a n d u nex pecte d ways. Neither Fisher nor I ha d m uch backgro un d in ex peri mental physics, none t o be exact, a n d we ma de se veral false starts. T o be a ble t o meas ure a t u n nel- i ng c urre nt t he t wo metals m ust be s pace d no more t ha n abo ut 100 Å a part, a n d we deci de d early i n t he ga me not to atte m pt to use air or vac u u m bet wee n t he t wo metals beca use of proble ms wit h vibratio n. After all, we bot h h a d tr ai ni n g i n m e c h a ni c al e n gi n e eri n g ! We trie d i nstea d t o kee p t he t w o metals a part by usi ng a variety of t hi n i ns ulators ma de fro m La ng m uir fil ms a n d fro m For mvar. I nvariably, t hese fil ms ha d pi n holes a n d t he merc ury Physics 1973

Fi g. 3. A. A microscope glass slide with a vapor deposited alu minu m strip do wn the middle. As s o o n a s t h e al u mi n u m fil m is e x p o s e d t o ai r, a p r ot e cti v e i n s ul ati n g o xi d e f o r m s o n t h e s urface. T he t hic k ness of t he oxide depe nds upo n s uc h factors as ti me, te mperat ure a n d h u mi dity. B. After a s uitable oxi de has for me d, cross stri ps of al u mi n u m are eva- porated over the first fil m, sand wiching the oxide bet ween the t wo metal fil ms. Current is passed along one alu minu m fil m up through the oxide and out through the other fil m, while the voltage drop is monitored across the oxide. C. A sche matic circuit diagra m. We a r e meas uri ng t he c urre nt-voltage c haracteristics of t he capacitor-like arra nge me nt for med by the t wo alu minu m fil ms and the oxide. When the oxide thickness is less than 50 Å or so, a n appreciable dc c urre nt will flo w t hro ug h t he oxide.

co u nter electrode w hic h we used wo uld s hort t he fil ms. T h us we spe nt so me ti me measuring very interesting but al ways non-reproducible current-voltage c haracteristics w hic h we referred to as miracles si nce eac h occ urred o nly o nce. After a fe w mo nt hs we hit o n t he correct idea: to use evaporated metal fil ms a nd to separate t he m by a nat urally gro w n oxide layer. To carry o ut o ur ideas we needed a n evaporator, t h us I p urc hased my first piece of experi mental equip ment. While waiting for the evaporator to arrive I worrie d a lot-1 was afrai d I wo ul d get st uc k i n ex peri me ntal p hysics tie d 1 . 0

Fi g. 4. C urre nt-voltage c haracteristics of five differe nt t u n nel j u nctio ns all wit h t he sa me t hick- ness, b ut wit h five differe nt areas. T he c urre nt is pro portio nal to t he area of t he junction. This was one of the first clues that we were dealing with tunneling rather than s horts. I n t he early experi me nts we used a relatively t hick oxide, t h us very little c urre nt wo uld flo w at lo w voltages.

do wn to this ex pensive machine. M y pl a ns at t h e ti m e w er e t o s wit c h i nt o theory as soon as I ha d acquire d enough kno wle dge. The pre monition was correct; I di d get st uc k wit h t he e va porator, not beca use it was ex pe nsi ve b ut beca use it fasci nate d me. Fi g ure 2 s ho ws a sc he matic dia gra m of a n e va po- rator. To pre pare a t u n nel j u nctio n we first eva porate d a stri p of al u mi n u m o nt o a gl ass sli d e. T his fil m w as r e m o v e d fr o m t h e v a c u u m s yst e m a n d h e at e d 1 4 2 Physics 1973

CURRENT

Fi g. 5. A. A n e nergy diagra m of t wo metals se parate d by a barrier. T he Fer mi e nergies i n t he t wo metals are at differe nt levels beca use of t he voltage differe nce applied bet wee n t he metals. O nly t he left metal electro ns i n t he e nergy ra nge . ca n ma ke a tra nsitio n to t he metal o n t he rig ht, beca use o nly t hese electro ns face e m pty e nergy states. The Pauli Principle allo ws only one electron in each quantu m state. B. The right- ha nd metal is no w s uperco nd ucti ng, a nd a n e nergy gap h a s o p e n e d u p i n t h e el e c- tro n s pectr u m. No si ngle electro n i n a s u perco n d uctor ca n have a n e nergy s uc h t hat it will a p pear i nsi de t he ga p. T he electro ns fro m t he metal o n t he left ca n still t u n nel t hro ug h t he b arrier, b ut t hey c a n not e nter i nto t he met al o n t he rig ht as lo ng as t he a p plie d volt age is less t h a n bec a use t he electro ns eit her f ace a fille d st ate or a forbidde n e nergy ra nge. W he n t he applied voltage exceeds c urre nt will be gi n to flo w. C. A sc he matic c urre nt-voltage c haracteristic. W he n bot h metals are i n t he nor- mal state t he c urre nt is si m ply pro portio nal to t he voltage. W he n o ne metal is s u per- co n d ucti ng t he c urre nt-voltage c haracteristic is drastically altere d. T he exact s ha pe of the curve depends on the electronic energy spectru m in the superconductor.

to oxidize t he s urface rapidly. Several cross strips of al u mi n u m were t he n de- posited over t he first fil m ma ki ng several j u nctio ns at t he sa me ti me. T he steps i n t he sa mple preparatio n are ill ustrated i n Fig ure 3. T his proced ure solved t wo proble ms, first t here were no pi n holes i n t he oxide beca use it is I. Gi a e v er 1 4 3

Fi g. 6. A sta ndard experi me ntal arra nge me nt used for lo w te mperat ure experi me nts. It co n- sists of t wo de wars, the outer one contains liquid nitrogen, the inner one, liquid heliu m. Heliu m boils at 4.2” K at at mospheric pressure. The te mperature can be lo wered to about lo K by reducing the pressure. The sa mple si mply hangs into the liquid heliu m supported by t he meas uri ng leads.

self- heali ng, a n d seco n d we got ri d of mec ha nical proble ms t hat arose wit h t he merc ury co u nter electro de. By abo ut A pril, 1959, we ha d perfor me d several s uccessf ul t unneling ex- peri me nts. T he c urre nt-voltage c haracteristics of o ur sa m ples were reaso nably re pro d ucible, a n d co nfor me d well to t heory. A ty pical res ult is s ho w n i n Fig- ure 4. Se veral c hec ks were do ne, s uc h as var yi n g t he area a n d t he oxi de t hick ness of t he j u nctio n as well as c ha ngi ng t he te m perat ure. Everyt hi ng loo ke d O K, a n d I e ve n ga ve a se mi nar at t he La borator y. B y t his ti me, I ha d solve d Schro dinger's eq uation eno ugh ti mes to believe that electrons so me- ti mes be ha ve as wa ves, a n d I di d not worr y m uc h a bo ut t hat part a n y more. Ho we ver: t here were ma ny real p hysicists at t he Laboratory a n d t hey pro p- erly q uestione d my experi ment. Ho w di d I k no w I di d not ha ve metallic s horts? Io nic c urre nt? Se mico n d uctio n rat her t ha n t u n neli ng3 Of co urse, I di d not kno w, an d even tho ugh theory an d ex peri ments agree d well, do ubts abo ut t he vali dity were al ways i n my mi n d. I s pe nt a lot of ti me i n ve nti ng i m possible sc he mes s uc h as a t u n nel trio de or a col d cat ho de, bot h to tr y to prove co ncl usively t hat I dealt wit h t u n neli ng a n d to per ha ps make my work usef ul. It was rat her stra n ge f or me at t hat ti me t o get pai d f or d oi n g w hat I co nsi dere d havi ng f u n, a n d my co nscie nce bot here d me. B ut j ust like q ua n- t u m mec ha nics, yo u get use d to it, a n d no w I ofte n ar g ue t he o p posite poi nt; we should pay more people to do pure research. I co nti n ue d to try o ut my i deas o n Jo h n Fis her w ho was no w looki ng i nto the proble ms of f un da mental particles with his characteristic o pti mis m an d e nt h usias m; i n a d ditio n, I receive d more a n d more a dvice a n d g ui da nce fro m C harles Bea n a n d Walter Harriso n, bot h p hysicists wit h t he u nca n ny ability of ma ki n g t hi n gs clear as l o n g as a piece of c hal k a n d a blac k b oar d were a vaila ble. I co nti n ue d to ta ke for mal co urses at R PI, a n d o ne da y i n a soli d state p hysics co urse ta ug ht by Professor H u nti ngto n we got to s u perco n d uctiv- it y. Well, I di d n’t belie ve t hat t he resista nce dr o ps t o exactl y zer o- b ut w hat really ca ug ht my atte ntio n was t he me ntio n of t he e nergy ga p i n a s u perco n d uctor, ce ntral to t he ne w Bar dee n- Coo per-Sc hrieffer t heory. If t he t heory was a ny goo d a n d if my t u n neli ng ex peri me nts were a ny goo d, it was ob vio us to me t hat by co mbi ni ng t he t wo, so me pretty i nteresti ng t hi ngs s ho ul d ha p pe n, as ill ustrate d i n Fig ure 5. W he n I got back to t he G E Labo- r at or y I tri e d t his si m pl e i d e a o ut o n m y fri e n ds, a n d as I r e m e m b er, it di d n ot look as goo d to the m. The energy ga p was really a many bo dy effect an d co ul d not be i nter prete d literally t he way I ha d do ne. B ut e ve n t ho ug h t here was co nsi derable ske pticis m, everyo ne urge d me to go a hea d a n d make a try. T h e n I r e ali z e d t h at I di d n ot k n o w w h at t h e si z e of t h e g a p w as i n u nits I u n derstoo d-electro n volts. T his was easily solve d by my us ual met ho d: first aski ng Bea n a n d t he n Harriso n, a n d, w he n t hey agree d o n a fe w millielectro n v olts: I w as h a p p y b e c a us e t h at is i n a e asil y m e as ur e d v olt a g e r a n g e. I ha d never do ne a n ex peri me nt req uiri ng lo w te m perat ures a n d liq ui d heli u m-that see me d like co mplicate d b usiness. Ho wever one great a dvantage of bei n g ass ociate d wit h a lar ge la b orat or y li ke Ge neral Electric is t hat t here are al ways peo ple aro un d who are kno wle dgeable in al most any fiel d, an d b ett er still t h e y ar e willi n g t o l e n d y o u a h a n d. I n m y c as e, all I h a d t o d o w as go to t he e n d of t he hall w here Warre n DeSorbo was alrea dy doi ng ex peri- ments with s u percon d uctors. I no longer re me mber ho w long it took me to set u p t he heli u m de wars I borro we d, b ut probably no lo nger t ha n a day or t wo. Peo ple u nfa miliar wit h lo w te m perat ure work believe t hat t he w hole fiel d of l o w te m perat ure is prett y es oteric, b ut all it reall y re q uires is access Fi g. 7. The current-voltage characteristic of an alu minu m-alu minu m oxide-lead sa mple. As soon as the lead beco mes superconducting the current ceases to be proportional to the voltage. T he large c ha nge bet wee n 4.2” K a n d 1.6” K is d ue to t he c ha nge i n t he e nergy ga p with te mperature. So me current also flo ws at voltages less than because of ther mally excited electro ns i n t he co nd uctors.

to liq ui d heli u m, w hic h was rea dily available at t he Laboratory. T he ex peri- m e nt al s et u p is s h o w n i n Fi g ur e 6. T h e n I m a d e m y s a m pl es usi n g t h e f a mili ar al u mi n u m-al u mi n u m oxi de, b ut I p ut lea d stri ps o n to p. Bot h lea d a n d al u- 1 4 6 Physics 1973

0 . 5

0 . 4

Fi g. 8. T he c urre nt-volt age c h ar acteristic at 1. 6” K as a f u nctio n of t he a p plie d m ag netic fiel d. At 2 400 ga uss t he fil ms are nor mal, at 0 ga uss t he lea d fil m is s u perco n d ucti ng. T he r e a s o n f o r t h e c h a n g e i n t h e c h a r a ct e ri sti c s b et w e e n 8 0 0 g a u s s a n d 0 g a u s s is t h at t hi n fil ms have a n e nergy gap t hat is a f u nctio n of t he mag netic field. mi n u m are s u perco n d uctors, lea d is s u perco n d ucti ng at K a n d t h u s all yo u nee d to make it s u perco n d ucti ng is li q ui d heli u m w hic h boils at 4.2° K. Al u mi n u m beco mes s u perco n d ucti ng o nly belo w 1.2° K, a n d to reac h t his te m- perat ure a more co m plicate d ex peri me ntal set u p is req uire d. T he first t wo ex peri me nts I trie d were fail ures beca use I use d oxi de la yers w hic h were too t hick. I di d not get e no ug h c urre nt t hro ug h t he t hick oxi de to meas ure it reliably wit h t he i nstr u me nts I use d, w hic h were si m ply a sta n dar d volt meter a n d a sta n dar d a m meter. It is stra nge to t hi nk abo ut t hat I. Gi a e v er 1 4 7

Fi g. 9. Infor mal disc ussion over a c u p of coffee. Fro m left: Ivar Giaever, Walter Harrison, C h arl es B e a n, a n d J o h n Fis h er.

n o w, o nl y 13 years later, w he n t he La b orat or y is f ull of s o p histicate d x- y re- cor ders. Of co urse, we ha d ple nt y of oscillosco pes at t hat ti me b ut I was not ver y fa miliar wit h t heir use. I n t he t hir d atte m pt rat her t ha n deli beratel y o xi di zi n g t h e first al u mi n u m stri p, I si m pl y e x p os e d it t o air f or o nl y a f e w mi n utes, a n d p ut it bac k i n t he e va p orat or t o de p osit t he cr oss stri ps of lea d. T his way t he oxi de was no more t ha n abo ut 30 Å t hick, a n d I co ul d rea dily meas ure the c urrent-voltage characteristic with the available eq ui p ment. To me t he greatest m o me nt i n a n ex peri me nt is al wa ys j ust bef ore I lear n w het her t he partic ular i dea is a g o o d or a ba d o ne. T h us e ve n a fail ure is exciti n g, a n d most of my i deas have of co urse bee n wro ng. B ut t his ti me it worke d! The c urrent-voltage characteristic change d marke dly when the lea d change d fro m t he nor mal state to t he s u perco n d ucti n g state as s ho w n i n Fi g ure 7. T hat was exciti ng! I i m me diately re peate d t he ex peri me nt usi ng a differe nt sa m ple - everyt hi ng looke d goo d! B ut ho w to make certai n? It was well-k no w n t hat s u perco n d uctivity is destroye d by a mag netic fiel d, b ut my si m ple set u p of de wars ma de t hat ex peri me nt i m possible. T his ti me I ha d to go all t he way across the hall where Israel Jacobs st u die d magnetis m at lo w te m perat ures. Agai n I was l ucky e no ug h to go rig ht i nto a n ex peri me ntal rig w here bot h t he te m perat ure a n d t he mag netic fiel d co ul d be co ntrolle d a n d I co ul d q uick- l y d o all t h e pr o p er e x p eri m e nts. T h e b asi c r es ult is s h o w n i n Fi g ur e 8. E v er y- 1 4 8

CURRENT

Fi g. 1 0. T u n neli ng bet wee n t wo s uperco nd uctors wit h differe nt e nergy gaps at a te mperat ure l arger t h a n 0° K. A. No volt age is a p plie d bet wee n t he t wo co n d uctors. B. As a volt age is a p plie d it beco mes e nergetically possible for more a n d more of t he t her mally excite d electro ns to flo w fro m t he s uperco nd uctor wit h t he s maller gap i nto t he s uperco nd uc- tor wit h t he l arger g a p. At t he volt age s ho w n all t he excite d electro ns c a n fi n d e m pty st at e s o n t h e ri g ht. C. A s t h e v olt a g e is f u rt h e r i n c r e a s e d, n o m o r e el e ct r o n s c o m e i nt o play, and since the nu mber of states the electrons can tunnel into decreases, the current will decre ase as t he volt age is i ncre ase d. W he n t he volt age is i ncre ase d s ufficie ntly t he electrons belo w the gap in the superconductor on the left face e mpty states on the right, a n d a ra pi d i ncrease i n c urre nt will occ ur. D. A sc he matic pict ure of t he expected current-voltage characteristic.

t hi ng hel d toget her a n d t he w hole gro u p, as I re me mber it, was very excite d. In partic ular, I can re me mber Bean enth usiastically s prea ding the ne ws u p a n d d o w n t h e h alls i n o ur L a b or at or y, a n d als o p ati e ntl y e x pl ai ni n g t o m e t h e si g nifica nce of t he ex peri me nt. I was, of c o urse, n ot t he first pers o n t o meas ure t he e ner g y ga p i n a s u per- co n d uctor, a n d I soo n beca me a ware of t he nice ex peri me nts do ne by M. Ti nk ha m a n d his st u de nts usi ng i nfrare d tra ns missio n. I ca n re me mber t hat I was w orrie d t hat t he size of t he ga p t hat I meas ure d di d n ot q uite a gree wit h t hose pre vio us meas ure me nts. Bea n set me straig ht wit h wor ds to t he ef- fect t hat fro m t he n o n ot her peo ple wo ul d ha ve to agree wit h me; my ex peri- me nt w o ul d set t he sta n dar d, a n d I felt please d a n d li ke a p h ysicist f or t he first ti m e. T hat was a very exciti ng ti me i n my life; we ha d se veral great i deas to i m pro ve a n d exte n d t he ex peri me nt to all sorts of materials like nor mal met- als, magnetic materials an d se micon d uctors. I re me mber many infor mal dis- I. Giaever 1 4 9

MILLIVOLTS Fi g. 1 1. A negative resista nce c haracteristic obtai ned experi me ntally i n t u n neli ng bet wee n t wo differe nt s uperco nd uctors. c ussio ns over coffee abo ut w hat to tr y ne xt a n d o ne of t hese sessio ns is i n a p hotogra p h take n i n 1960 w hic h is s ho w n i n Fig ure 9. To be ho nest t he pic- t ure was stage d, we were n’t nor mally so dresse d u p, a n d rarely di d I fi n d myself i n c harge at t he blackboar d! Most of t he i deas we ha d di d not work very well a n d Harriso n soo n p ublis he d a t heory s ho wi ng t hat life is really co m plicate d after all. B ut the s u percon d ucting ex peri ment was char me d an d al ways worke d. It looke d like t he t u n neli ng probability was directly pro por- tio nal to t he de nsit y of states i n a s u perco n d uctor. No w if t his were strictl y tr ue, it di d not take m uc h i magi natio n to realize t hat t u n neli ng bet wee n t wo s u perco n d uctors s ho ul d dis play a negative resista nce c haracteristic as ill us- trate d i n Fi g ure 10. A ne gati ve resista nce c haracteristic mea nt, of co urse, a m plifiers, oscillators a n d ot her de vices. B ut nobo dy aro u n d me ha d facilities to p u m p on the heli u m s ufficiently to make al u min u m beco me s u percon d uct- i n g. T his ti m e I h a d t o l e a v e t h e b uil di n g a n d r e a cti v at e a n ol d l o w t e m p er- at ure set u p i n a n a djace nt b uil di ng. S ure e no ug h, as soo n as t he al u mi n u m we nt s u perco n d ucti ng a negative resista nce a p peare d, a n d, i n dee d, t he notio n t hat t he t u n neli n g pr o ba bilit y was directl y pr o p orti o nal t o t he de nsit y of states w as e x p eri m e nt all y c orr e ct. A t y pi c al c h ar a ct eristi c is s h o w n i n Fi g ur e 1 1. No w t hi ngs looke d very goo d beca use all sorts of electro nic devices co ul d be ma de usi n g t his effect, b ut, of c o urse, t he y w o ul d o nl y be o perati ve at lo w te m perat ures. We sho ul d re me mber that the se micon d ucting devices were not so a dvance d in 1960 an d we tho ught that the s u percon d ucting j unction 1 5 0 Physics 1973 4

I

Fi g. 1 2. A nor m alize d deriv ative of t he c urre nt wit h res pect to volt age of a le a d j u nctio n at lo w te mperat ure. T he si mple B C S-t heory predicts t hat t he derivative s ho uld approac h u nity asy mptotically as the energy increases. Instead several wiggles are observed in the range bet wee n a n d These wiggles are related to the phonon spectru m in lead.

wo ul d have a goo d c ha nce of co m peti ng wit h, for exa m ple, t he Esaki dio de. T he basic q uestio n I face d was w hic h way to go: e ngi neeri ng or scie nce? I deci de d t hat I s h o ul d d o t he scie nce first, a n d recei ve d f ull s u p p ort fr o m my i m me diate manager, Rolan d Sch mitt. I n retros pect I realize ho w te m pti ng it m ust ha ve bee n for Sc h mitt to e nco urage ot her peo ple to work i n t he ne w area, a n d for t he m uc h more ex perie nce d p h ysicists ar o u n d me t o d o s o as well. I nstea d, at t he ri g ht ti me, Sch mitt provi de d me with a co- worker, Karl Megerle, who joine d o ur Labo- ratory as a Research Training Fello w. Megerle an d I worke d well together a n d bef ore l o n g we p u blis he d a pa per deali n g wit h m ost of t he basic effects. I. Giaever 1 5 1

Fi g. 1 3. Effect of trapped mag netic field o n a t u n neli ng c haracteristic. C urve 1 is a virgi n c urve, w hile c urve 3 is i n a mo derate mag netic fiel d, a n d i n c urve 2 t he mag netic fiel d has bee n re move d. I n c urve 1 we also have a s mall resista nce-less c urre nt w hic h we i nter- prete d as c a use d by met allic s horts. I n retros pect, it w as act u ally d ue to t he Jose p hso n eff ect.

As al wa ys i n p h ysics, it is i m p orta nt t o exte n d ex peri me nts t o a hi g her e ner g y, a greater ma g netic fiel d, or, i n o ur case, to a lo wer te m perat ure. T herefore, we joi ne d forces wit h Ho war d Hart, w ho ha d j ust co m plete d a heli u m 3 refri gerator t hat was ca pa ble of getti n g do w n to a bo ut K. At t h e sa me ti me, Megerle fi nis he d a lock-i n a m plifier w hic h we co ul d use to mea- s ure directl y t he deri vati ve of t he c urre nt wit h res pect to t he volta ge. T hat was reall y a nice loo ki n g mac hi ne wit h a ma g net rotati n g past a pic k u p coil at ei g ht c ycles per sec o n d, b ut, of c o urse, vastl y i nferi or t o t he m o der n l oc k-i n a m plifier. We ha d k no w n for so me ti me t hat t here were a no malies i n t he c urre nt-voltage c haracteristics of lea d, a n d no w we fi nally pi n ne d t he m do w n b y fi n di n g s o m e e xtr a wi g gl es i n t h e d eri v ati v e c ur v e. T his is s h o w n i n Fi g ur e 12. T hat ma de us ha p py beca use all t hat t he t u n neli ng ex peri me nts ha d do ne u p till no w was to co nfir m t he B CS t heor y, a n d t hat is not w hat a n ex peri- m e nt alist w o ul d r e all y li k e t o d o. T h e dr e a m is t o s h o w t h at a f a m o us t h e or y is i ncorrect, a n d no w we ha d fi nally poke d a hole i n t he t heory. We s pec u- late d at t he ti me t hat t hese wiggles were so me ho w associate d wit h t he p ho no ns 1 5 2 Physics 1973 t h o u g ht t o be t he ca use of t he attracti ve electr o n-electr o n i nteracti o n i n a s u perco n d uctor. As ofte n ha p pe ns, t he t heorists t ur ne d t he tables o n us a n d cleverly use d t hese wiggles to pro perly exte n d t he t heory a n d to prove t hat t he B CS t heory i n dee d was correct. Professor Bar dee n gave a detaile d acco u nt of t his i n his m ost r e c e nt N o b el Pri z e le ct ur e. I ha ve, s o far, tal ke d mai nl y a b o ut w hat we nt o n at Ge neral Electric at t hat ti me; so meti mes it is diffic ult for me to realize t hat Sc he necta d y is not t he ce nter of t he worl d. Se veral ot her peo ple be ga n to do t u n neli n g wor k, a n d to me ntio n j ust a fe w: J. M. Ro well a n d W. L. Mc Milla n were reall y t he o nes w ho u nra vele d t he p ho no n str uct ure i n a s u perco n d uctor; W. J. To - masc h, of co urse, i nsiste d o n disco veri n g his o w n effect; S. S ha piro a n d col- leag ues di d t u n neli ng bet wee n t wo s u perco n d uctors at t he sa me ti me we di d; a n d J. B ar d e e n, a n d l at er M. H. C o h e n et al., t o o k c ar e of m ost of t h e t h e or y. Mea n w hile, back at R PI, I ha d fi nis he d my co urse work a n d deci de d to do a t heoretical t hesis o n or dere d- disor dere d alloys wit h Professor H u nti ngto n because tunneling in supercon ductors was mainly un derstoo d. Then so meone ma de me a ware of a s hort pa per b y Bria n Jose p hso n i n Physics Letters - w hat di d I t hi n k? Well, I di d n ot u n dersta n d t he pa per, b ut s h ortl y after I ha d the chance to meet Jose phson at Ca mbri dge an d I ca me a way i m presse d. O ne of t he effects Jose p hso n pre dicte d was t hat it s ho ul d be possi ble to pass a s u perc urrent with zero voltage dro p thro ugh the oxi de barrier when the metals o n bot h si des were s u perco n d ucti ng; t his is no w calle d t he dc Jose p h- so n effect. We ha d o bser ve d t his be ha vior ma n y ti mes; matter-of-fact, it is diffic ult not to see t his c urre nt w he n j u nctio ns are ma de of ti n-ti n oxi de-ti n or lea d-lea d oxi de-lea d. The early t unnel j unctions were us ually ma de with al u mi n u m oxi de w hic h ge nerally is t hicker a n d t herefore t her mal fl uct uatio ns s u p press t he dc c urre nt. I n o ur first pa per Megerle a n d I p ublis he d a c ur ve, w hic h is s ho w n i n Fig ure 13, de mo nstrati ng s uc h a s u perc urre nt a n d also t hat it de pe n de d stro ngly o n a mag netic fiel d. Ho wever, I ha d a rea dy- ma de ex- pla natio n for t his s u perc urre nt-it ca me fro m a metallic s hort or bri d ge. I w as p u z zl e d at t h e ti m e b e c a us e of t h e s e nsiti vit y t o t h e m a g n eti c fi el d w hi c h is u nex pecte d for a s mall bri dge, b ut no o ne k ne w ho w a 20 Å lo ng a n d 20 Å, wi de bri dge wo ul d be have a ny way. If I have lear ne d a nyt hi ng as a scie ntist it is t hat o ne s ho ul d not make t hi ngs co m plicate d w he n a si m ple ex pla natio n will do. Th us all the sa m ples we ma de sho wing the Jose phson effect were discar de d as ha vi ng s horts. T his ti me I was too si m ple- mi n de d! Later I ha ve bee n as ke d ma n y ti mes if I feel ba d f or missi n g t he effect? T he a ns wer is clearl y no, beca use to ma ke a n ex peri me ntal disco ver y it is not e no u g h to observe so met hi ng, o ne m ust also realize t he sig nifica nce of t he observatio n, a n d i n t his i nsta nce I was n ot e ve n cl ose. E ve n after I lear ne d a b o ut t he dc J ose p hs o n effect, I felt t hat it c o ul d n ot be disti n g uis he d fr o m real s h orts, t herefore I erro neo usl y belie ve d t hat o nl y t he obser vatio n of t he so-calle d ac effect wo ul d pro ve or dis pro ve Jose p hso n’s t heor y. In concl usion I ho pe that this rather personal acco unt may provi de so me sli g ht i nsi g ht i nto t he nat ure of scie ntific disco ver y. M y o w n beliefs are t hat t he r oa d t o a scie ntific disc o ver y is sel d o m direct, a n d t hat it d oes n ot neces- I. Giae ver 1 5 3 sarily re q uire great ex pertise. I n fact, I a m co n vi nce d t hat ofte n a ne wco mer t o a fiel d has a great a d va nta ge beca use he is i g n ora nt a n d d oes n ot k n o w all t he co m plicate d reaso ns w hy a partic ular ex peri me nt s ho ul d not be atte m pte d. H o w e v er, it is ess e nti al t o b e a bl e t o g et a d vi c e a n d h el p fr o m e x p erts i n t h e vario us sciences when yo u nee d it. For me the most i m portant ingre dients were t hat I was at t he ri g ht place at t he ri g ht ti me a n d t hat I f o u n d s o ma ny frie n ds bot h i nsi de a n d o utsi de Ge neral Electric w ho u nselfis hly s u p- p orte d me.

R EFERENCES

1. Tunneling Pheno mena in Solids edited by Burstein, E. and Lundquist, S. Plenu m Press, Ne w York, 1969. 2. Superconductivity edited by Parks, R. D. Marcel Dekker, Inc., Ne w York. 1969.