Solid StateCommunications, vol. 7, PP.771—774, 1969. PergamonPress. Printedin Great Britain

COHERENCELENGTH EFFECTSIN ZERO BIAS TUNNELING ANOMALIES

F. Mezei

Central ResearchInstitutefor Physics,Budapest,and Institutefor ExperimentalPhysics,

EbtvosUniversity, Budapest

(Received18 March 1969 by P.G. de Gennes)

Experimentswereperformedwith Al—A1203—Al tunnel junctionsfabricatedby a new proceduresupposedto producea slight impuritylayerinside theAl electrode.Impurities displacedat distanceslessthan about 100A from thejunction surfacewere found to be effectivein causingzerobias anomalies.

ZERO bias tunneling anomaliesare believedto half-madejunctionstayeda ‘waiting time’ t ofbe due to somekind of interactionbetween the order 10 mm at a pressure2.10~torr in theconductanceelectronsand impuritiesat the evaporationchamber.In this way we havegotbarrier. The questionariseswhetherimpurities junctionsexhibiting giant zerobias resistancein the immediatevicinity of the junctionsurface maximum, which is most probablythe effectonly areeffective or thoseplaced insidethe recentlyfound in severaljunctions preparedbyelectrodemetal as well. The effectsof the othertechniques.~

3Thus with small D thelatter type impuritiescanreasonablyberegarded junctions show the samequasilogarithmicas local disturbancesin the conductanceelectron anomalyin R(V) — R

0(V) = L~~R(V)curvewave functions, ratherthan modification of the {R0(V) is the backgroundresistance]asfoundtunnelingmatrix elementssuchas impurity by Shen andRowell,

4 and exactly the sametern-assistedchannels.This would give an oppor- peraturedependenceas foundin earlier experi-turiity to investigatebulk impurity effectsvia ments.The similarity is pronouncedby the facttunneling. In the presentwork new types of that in somecaseswe havefound eventhe wellexperimentswere startedto investigatethis known36 conductancepeakat zerobiasproblem. superposedon the broadresistivity maximum,

howeverat the presentwe cannotreproducethisThe dynamicalresistanceof Al—Al

2 03—Al effectat will.tunnel junctionsR(V) = dV/di vs. the voltageV wasmeasuredby standardmodulation We haveinvestigatedthe dependenceof thetèchnique.The junctions werefabricatedin the anomalyon the waiting time I and on the thick-following way. An aluminium strip evaporated nessof the additionalAl layer D, showninat first wasoxidized at atmosphericpressure Fig. l.a. and1.b. respectively.We mention thatin air. The secondaluminium electrodewas the changeof the evaporationrateof the addition-preparedin two steps.First a thin Al layer of al layerbetween5—50A/sechasno seriousthicknessD 20—200 A (hereafterreferredto effect, so we might think that this layeris ratheras ‘additional layer’) was depositedonto the uniform. The junctionshaveshown goodgapoxide surface(Fig. 2). The thicknessD was characteristicsunderthe critical temperaturemeasuredby optical absorption.Beforeevap- closeto the bulk value for Al 1.20K°.Thus noorating the rest of the secondAl electrodethis indication of granular~or sandwich

8structureoccuredconcerningthe valueof T~.

771

772 LENGTH EFFECTSIN ZERO BIAS ANOMALIES Vol. 7, No. 10

interactionof the additional layerand the lowpressureatmospherewithin the evaporationchamber.Whenthe pressurewasincreasedduring the waiting time to 10~tori, the anomalyoccuredafter much shortertime I, i.e. aftera few

0seconds.On theother hand,without the addition-al layer no anomalywasfound at all. In thepresentcircumstances(room temperature,2.10-stori) this interactioncannotbe expectedFe,. ~

to displayany dramaticeffecton the structure

of the additional layer, such as formation of asecondinsulatingoxidelayer asin the experi-

haveshown no insulationbeyondthe experimental

resolution 10~~l/mm2,and no decreasein the~ ment of Giaeveret al.9 Control measurementsoptical absorptionof the additional layeroccuredbeyondthe error 0.5 percentindicating anoxidationrate lessthanone atomic layer.

layer, thereis a pronouncedchangein the shape0° of the anomalyplottedas G(V) = dV/dI vs. V,c.~J Fig. ic. In the conductancethe contributionsof

~On increasingthe thicknessof the additionaldifferent macroscopicregionsof the junctionare simply summedup. This fact rulesout the1..~ assumptionthat the very thin portionsof thehighly non-uniformadditional layerwere

— 4 effectiveonly in causingthe anomaly,and the- v(”v) decreaseof its amplitude with increasingD

weredueto the decreasein areaof such thinFIG. 1(a) Normalized resistanceR (V )/R (0)vs. the voltage V characteristicsfor = 27, macroscopicregions of the additional layer.9,3 and 1 miii curvesa b, c andd respectively;D = 60A, T = 2K°~(The curvesb, c, dare Thus we concludethat we havean effectshifted upwards.) due to some kind of impuritiesdisplaced

(b) Normalized resistanceR (V )/R(0) definitely insidethe secondelectrodeat aboutvs. the voltage V characteristicsfor D = 25,60, 100 and150A, curvesa, b, c end d, a distanceD from the oxide surface.In therespectively; I = Smin, T 2K°.(Thecurves formation of theseimpuritiesan oxidationprocessb, c, d areshifted upwards). may havean importantrole. It is probablethat a

(c) The characteristicsshown in Fig.l(a) considerableamountof tungstencoming from theplotted as normalizedconductance sourcefilament is containediii our evaporatedG(V)/G(—200mV)vs. the voltage V. (The Al films. The oxidation of W atomson thecurvesb, c, d are shifted upwards).

surfaceof theadditional layercould give theeffective impurities. This ideais supportedby

We believethat the anomalyis due to the experimentsof Klein andLeger.10

impurities formedduringthe waiting time on thesurfaceof the additional layer, which in turn The presentresultscanbe explainedgot insidethe secondAl electrodeby the corn- supposinglocal scatteringof conductionpletion of the junction. This is establishedby electronson the impuritiesas in the theoryofthe following. Sólyom andZawadowskiconsideringKondo

scattering11(W-oxidesare in fact magnetic).The anomalywasfound to be due to the From this approachthe imaginarypart of the

Vol. 7, No. 10 LENGTH EFFECTSIN ZERO BIAS ANOMALIES 773

scatteringamplitude leadsto a decreasein the experimentthe inplantationof impuritiescauseslocal valueof the effectivetunnelingdensityof presumablynegligible additionalchangesin thestatesin the vicinity of the impurity layer. The structureof thejunctions. Becauseof this it isspatial spreadout of this suppressionof the possibleto comparequalitatively the absolutedensityof statesis characterizedby the co- valueof the resistanceof the ‘normal’ andherencelength ~ .~‘.. n/Ak,, wherei~k~is ‘anomalous’junctions. In spiteof the fact thatthe cut-off momentumof the scattering.II This the valueof the zero bias resistanceis not ais supposedto be relatedto the energyof the well reproducibleparameterof tunnel junctions,localizedmagneticd level measuredfrom the a generaltendencycould be observedthat ano-Fermi energy,and may be estimatedto a few malousjunctions give considerablyhigherR (0)percent of k~’~By this ~ has to be of the valuesthan the normal onesmadeon the sameorder 10—100A asa typical valuebeing in substrate.If so, this rulesout the assistedreasonableagreementwith the observeddepen- tunnelingmechanism’4for the presentcase,denceof the conductanceanomalyon D shown while it is in agreementwith the Sólyom—in Fig. 2. Zawadowskitheory.

In the presentwork giant zero bias anomalywasfound to occurunderentirely new preparationconditions. The junctionsusedin previousexperimentswerepreparedby ratherunrefined

methodslike evaporationof impurities, oxidationat atmosphericpressures,which cancauseun-

Al controllableadditional changesin the junctionD structure.Beside the coherencelength effects,

• we seethe major importanceof the presentexperimentin finding suchan anomalyclearlydue to extremelyslight modification in the

o io preparation.This suggeststhat giant zerobias

(A anomaliesare causedby a dramaticeffect of aD few atomsor molecules.If this is not the Kondo

FIG. 2. The conductanceanomaly~ G(0)/G0(0) scatteringwhich seemsto be the only known

[where ~G(V) = G(V) — G (V) and G0(V) is effect strongenough,it may not be much lessthe backgroundconductance~as a function of interesting. In agreementwith our conclusionD for I = Smin., T = 1.7K and the schematic . 15

structureof the junctions. recentmeasurementsby %~iyattandLythallhavequantitativelyestablishedthat the evapor-ation of impurities in a total quantityamounting

The changein shapeof the G(t’) curves only to a fraction of a uniform nionatorniclayershown in Fig. ic can be explainedby supposing may causelargeanomaliesas well.that the local disturbancesof the conductionelectronGreen’s function die off with increasing Acknowledgements— The author is indebtedtodistancethe more rapidly the further the energy ProfessorL, Pal andProf. E. Nagy for theirconcernedlies from the Fermi level. The continuousinterest in this work, and to Dr.

A.F.G. Wyatt for making it possibleto performnarrowingof the anomalywith increasing1 somecontrol measurementsandmaking newi.e. impurity concentration,shown up in Fig. la experimentaldataavailableprior to publication.is explainedby taking into accountin a self- He wishesalso to thank Drs. A. Zawadowski,consistentmannerthe suppressionof the density A.F.G. Wyatt, C. Hargitai, J. Sólyom and D.of statesin the Kondo scatteringitself.

12 This Lythall for many stimulatingdiscussions.effect was observedin other experimentsongiant zerobias anomalies.35

It is worth mentioning that in the present

774 LENGTH EFFECTSIN ZERO BIAS ANOMALIES Vol. 7, No. 10

REFERENCES

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St. Andrews(1968).13. This rules out oncemore the capacitancemechanism~ in the presentcase,since a growing

of oxide layerreducesthe capacityof grains, whichwould lead to a broadeningof theanomaly.

14. DUKE C.B., SILVERSTEIN S.D. andBENNETT A.J., Phys.Rev. Left. 19, 315 (1967).

15. WYATT A.F.G. and LYTHALL D.J., private communication.

On a confectionnèdesjonctionstunnel Al/Al2 03/Al par un procadénouveau,destinea produireune mincecouched’impuretCsa l’inté-rieur de l’une des electrodes(Al). On trouve que les anomalies~voltagenul apparaissentlorsqueles impuretéssonta moms de iooAde la surfacede la jonction.