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Submitted on 1 Jan 1978
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INCREASED RESISTANCE BELOW THE
SUPERCONDUCTING TRANSITION IN
DISCONTINUOUS FILMS OF (SN)x AND INDIUM
M. Young, J. Thomas, C. Adkins, J. Tate
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-448
INCREASED RESISTANCE BELOW THE SUPERCONDUCTING TRANSITION IN DISCONTINUOUS FILMS OF ( S N )X AND INDIUM
M.W. Young, J.M.D. Thomas, C . J . Adkins and J.W. T a t e
Cavendish Laboratory, Madingley Road, Cambridge, U.K.
Résumé.- La résistance en courant continu de films discontinus et non-orientés de (SN) a été mesu-rée sous divers champs électriques et à plusieurs températures. On a trouvé que la transition supra-conductrice se traduisait par une augmentation de la résistance. On propose une explication simple basée sur le modèle de conduction qui rend compte des données acquises à haute température. Des résultats semblables ont été obtenus avec des films discontinus d'indium.
Abstract.- The DC resistance of discontinuous ndh-oriented films of (SN)X has been measured at va-rious electric fields and temperatures. The transition to the superconducting state is found to result in an increased resistance and a simple explanation is put forward in terms of the cond-uction model which fits the high temperature data. Similar results have been obtained in disconti-nuous indium films.
The (SN) films were grown from S„N„ vapour on sapphire and quartz substrates using the original technique III. Transmission electron micrographs of ultra thin films grown directly onto 10 ran films of carbon indicate that the grains are about 0.1 ym in size and are oriented randomly in the plane of the substrate.
DC low field measurements have been made in the range 100 K to 100 mK. To counter the high sample resistances which occur at low temperatures gold electrodes in the form of an interlocking comb structure were evaporated onto each sample to give a configuration of favourable aspect ratio. Measure-ments on 25 samples in the range 20 K to 400 mK
(fig. 2) all show an excellent fit to the relation
lg R = A + (C/kT)1''2 (1)
where C is a constant and k is the Boltzman constant. Poor fits are obtained for other fractional powers. Similar results were obtained by Soulen /2/ on ori-ented films. Both indicate the granular metallic nature of the material, as do high field measurements of conductivity which have been made on 50 ym by
1 cm strips of (SN) film between gold electrodes. These show a limited regime at 1.2 K which is consis-tent with a relationship of the form
lg a K C/eue (2)
where e is the electric field, C is the constant of equation (1), e the electronic charge and u the dis-tance between grain centres. This behaviour is des-cribed for cermets by Abeles'theory /3/. If this
theory holds for discontinuous (SN) films then the grain size can be determined by combining low and high field data. Values obtained, around 0.1 \xra, are consistent with the electron microscope pictures (fig. 1 ) .
(A) (B)
I I I I I I I " ~ 1 I I I I
0 0.5 PM 0 1.0 PM
Fig. 1 : Electron micrographs. Fig. 1(A) transmis-sion micrograph of a (SN)X film; Fig. 1(B) scanning micrograph of an Indium'film.
The high and low field measuraments together indicate that the transport properties of disconti-nuous (SN) films can be viewed in terms of existing granular metal theories / 4 / according to which there is an equilibrium number of charged grains
nc = nQ exp (-E°/2kT) (3)
where E is the capacitive charging energy needed to create a pair of oppositely charged grains, and the charges subsequently tunnel from grain to-grain.
When measurements are extended to lower tern- temperature in discontinuous indium films (fig. 3)
peratures, it is found that there is a sharp kink in by us, and in lead films/5/ by others. The behaviour the lg R versus (I/T)'/~ plot at around 300 mK, the in (SN) hence provides the first positive indica-
X
bulk superconducting transition temperature in crys- tion that at least some of the crystallites in dis- talline (SN)x (fig. 2). The same effect has also continuous (SN)x films go superconducting 121. been observed near the superconducting transition
Fig. 2 : Resistance as a function of temperature for two typical (SN)x films. The solid lines below the arrowed discontinuities are theoretical predictions.
It is suggested that an explanation for the discontinuity in (SN)x is as follows : the charging energy, ,:E will be of the order of 2 x eV, which is that of an isolated sphere of 0.1 um radius.
Fig. 3 : Resistance as a function of temperature for the discontinuous Indium film of Fig. 1 ( B ) .
This is much larger than the maximum superconducting pairing energy of 1.75 kTc per electron, which is about 5 X
lo-'
eV. Hence the superconducting state does not significantly affect the energy of a singlycharged grain and hence the number present. However, at T = 0, electrons in a superconductor are paired and cannot tunnel singly to a normal conductor or superconductor unless a voltage drop of
V
or 20 is present across the junction. In our granular (SN)x samples, a maximum of 10 mV is present across about- I mm, or l o 4 V per grain, which is too small to promote single particle tunnelling by pair breaking: Near the transition temperature, however, the number of "superelectronsl' in the two fluid model decre- ases proportionally to (I-(T/T~)'), where Tc is the superconducting transition temperature. For reasons explained above, these electrons cannot contribute to transfer of charge. Even if Josephson tunnelling between grains were possible, it would be very unfa- vourable energetically because of the extremelylar- ge charging energies involved. Hence the number of unpaired electrons available to tunnel decreases below Tc as (TITc)',
and the conductivity should be proportionately reduced. Two typical fits tothis theory, in which the high temperature gradient.is measured and the value of T taken the discontinui-agreement is good in that the experimental points are always on the line or slightly below it. The latter can be explained if not all the grains become superconducting.
Similar results have been obtained in discon- tinuous indium films (fi. 3), though here the change in gradient at the discontinuity is only half the size that would be predicted by the above simple theory. Clearly the situation here is much more complex. The pairing energy in indium is larger than that in (SN)x and, in our samples, the indium grain charging energy is smaller, making the two energies of comparable magnitude : furthermore the temperature dependence of resistivity in the normal state does not fit well any particular fractional power of temperature, and the transport mechanism is not well understood.
References
/I/ Hsu, C. and Labes, M.M., J. Chem.Phys.61 (1974) 4540
/2/ Soulen, R. and Ulton, D.B., Solid State Commun. 21 (1977) 105
-
/ 3 / Abeles, B., Ping Sheng and Arie, Y., Adv.Phys. 24 (1975)' 407
-
/4/ Morris, J.E. and Coutts, T.J., Thin Solid Films 47 (1977) 1
