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Submitted on 1 Jan 1978
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ON CONDUCTION MECHANISM OF
HIGH-QUALITY SUPERCONDUCTING POINT
CONTACTS
Yu.Ya. Divin, F.Ya. Nad’
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au ,no 8, Tome 39, aotit 1978, page ~ 6 - 5 9 9
ON COYDUCTION
MECHANISM
OF HIGH-QUALITY SUPERCONDUCTINGPOINT
CONTACTSInstitute of Radio Engeneering and Electronics of the Academy of Sciences of the USSR, M m x Avenue 18, Moscow, 103907, USSR.
RQsum6.- On a fait l'ltude expsrimentale des caractsristiques IV de contacts ponctuels supraconduc- teurs dans la ganrme de resistances de contacts 10'
-
lo-'
Ohm entre 4,2 et 10 K. On propose pour modsle celui des contacts ponctuels supraconducteurs de haute qualitd ayant une grande valeur du produit I R.Abstract.- Experimental study of I
-
V characteristics of niobium point contacts has been carried out in contact resistance rangelo5
-
lo-'
Ohm and temperature range 4.2-
10 K. The model of high- quality superconducting point contacts with large IcR product is suggested.Superconducting point contacts (SPC) have shows an excess current above the gap voltage and been more widely used in practice than any other simultaneously with this excess current the struc- type of weak links. However, the physical nature of ture at V = Ale appears (see curve 2). Noticeable SPC remains uncertain and conduction mechanism is critical current I was observed at R < 4 kil and not yet understood /I/. In this work the modifica- the shape of dc I-V characteristics did not change tion of dc I
-
V characteristics of SPC resulted until the values of R of the order of 1 il (curve 3 from contact adjustement has been studied and tem- and 4).perature dependences of I
-
V characteristics have been measured. High-quality SPC have been obtained with large IcR product (2.2-
2.5 mV) and other pro-perties being similar to the "clean"
111
or "ideal" 4 - /2,3/ contacts. The model of this high-quality SPCbased on the data obtained is suggested.
We have studied niobium point contacts with electrodes being fabricated with the help of elec- tropolishing techniques. Electrodes were brought into contact in liquid helium. With increasing of
contact load a series of stable contacts with nor- 7 2 3
ma1 state resistances R from 10' il to
lo-'
R
was re-Fie. 1
-
: The modification of reduced I-V characte- producibly obtained. The I-
V curves and voltage ristics of Nb-Nbpoint contacts by successive adjus-tments. Contact resistances R, Ohm : I
-
45 x lo3, dependences of derivative curves dVIdI, d2v/d~' we--
15.5 103.-
530.-
Ss4 (T=4.2 K). Forre measured in the temperature range 4.2
-
10 K. curve 5 R=8.4 Q and T=6.5 K. Dashed line is normal state I-V characteristic.The accuracy of electrical measurements was better than 1 %. Absolute accuracy of temperature. measure- ments was 0.05 K.
A typical series of dc I-V characteristics is shown in figure 1 (curves 1
-
4). In the resistance range 10'-
2 x 10' Q I-V characteristics are simi- lar to that of for quasiparticle tunnelling (curve 1). For these contacts the voltage VZA, at which the minimum of dV/dI was observed, was 2.8 f 0.1 mV. This value is close enough to the energy gap volta- ge 2AIe for pure Nb. With decreasing values of R from 16.7 152 to 15.5 kR, the I-V characteristicContacts of this type have an excess current and pronounced structures at V r 2Ale and V Ale. The I R product for these contacts increases from 0.6 mV at R 2 1 kQ to 2.5 mV at R 1 Q. Thus our con-
tacts in the resistance range from 1 kQ to % 1 Q
show all characteristic features of "clean"/l/ or "ideal" I31 contacts.
We have also studied temperature dependences of I-V characteristics of SPC in the resistance range from 1 152 to 1
R.
In figure 1 I-V characteris- tic of the contact with R = 8.4R
is shown for tem-peratures 4.2 K (curve 4) and 6.5 K (curve 5). The decrease of Ic, 61 and V at higher tempera-
2A
ture is clearly seen. For contacts with R from 50
52 to 1 R a linear region at I > I was observed at temperature range 5-9 K, its differential resistan- ce increasing with the temperature increase (curve 5).
The temperature dependences Ic(T). 6I(T) and V (T), which were typical of our high-quality con-
2A
tacts, are shown in figure 2. The dependence V 2A (T) coincides with BCS dependence 2A(T), the value 2A(0) being 3-02
*
d.06 mV and critical temperature being 9.48 2 0.05 K. The dependence 6I(T) turned out to be proportional to 2A(T) and their ratio 6I(T)/2A(T) was constant with accuracy at 5 %.Experimental dependence I (T) was compared with theoretical curves for tunnel junctions 141 and microbridges 151. The shape of experimental depen- dence I (T) does not fit the theoretical Ic(T) for tunnel junctions /4/, but is in agreement with that for microbridges 151. Quantitative agreement of experimental data and theory 151 was obtained with normal state resistance of microbridge RN = 1 1
a ,
while normal state resistance of contact being 8.4R.
Fig. 2 : Temperature dependences of critical cur- rent I, (.), excess current 61 ( 0 ) and gap struc- ture V,A
(A)
for contact with R=8.4R.
Solid line shows BCS dependence ~A(T), dashed line is the theoretical dependence Ic(T) for microbridges 151.To interpret the modification of dc I-V charac- teristics (figure 1 ) it is useful to compare them with the changes of point contact structure /6/.
During the first slight contact of electrodes, elas- tic deformation takes place, oxide film remains on the electrodes and tunnel junction (or few junctions)
with small cross section area is formed (curve 1 in figure I). With increasing contact pressure plastic deformation starts, metal is pressed through the oxide film and conducting channels (microshorts) are formed. This process seems to correspond to the appearance of excess current and subharmonic gap structure at V I Ale (curve 2, figure 1). An excess
current is known to be observed only in weak links with direct channels for current flow (bridges) rather than in tunnel junctions. To account for the structure at V 2A/en (n = 2,3
...
) in all types of weak links Josephson oscillations are required 171.Simultaneous appearance of 61 and the structure at V = Ale can be explained only by the formation of mi- croshort with Josephson properties. Because of the noise no critical current is observed for these contacts with R % 10 kR. With further increasing of contact load the total area of contact spots incraa- ses, with all stages of microstructure change taking place at each spot 161. I increases but on I-V characteristics the structure at V = 2A/e caused by the tunnelling conduction mechanism is observed.
According to the above-mentioned data the fol- lowing model of high-quality SPC can be suggested. This contact is considered as tunnel junction and microshort connected in parallel. Quasiparticle tu- nnelling is likely to be the only conduction mecha- nism in this tunnel junction and all Josephson pro- perties are due to microshort. As the contact load is increased the microshort part of the total cur- rent increases and the tunnel part decreases. Expe- rimental temperature dependence I (T) is also likely to confirm the model. On the one hand, functional agreement of this experimental curve with theory /5/ for microbridges shows that it is the microshort that determines 2osephson properties of SPC. On the other hand the fact that qualitative agreement was obtained for
%
> R is the consequence of shunting of microshort by tunnel junction.A linear region of I-V characteristic at I >
R e f e r e n c e s
/I/ Zimmerman, J . E . , Proc. Appl. S u p e r c o n d u c t i v i t y Conf., IEEE Conf. Rec, (1972) 544.
/2/ Weitz, D.A., Skocpol, W . J . and Tinkham M., Appl. Phys. L e t t .
2
(1977) 227./3/ Weitz, D.A., Skocpol, W . J . and Tinkham, M . , Phys. Rev. L e t t . (1978) 253.
/4/ Ambegaokar V . and B a r a t o f f , A., Phys. Rev. L e t t . 10 (1963) 216.
-
/5/ K u l i k , 1 . 0 . and Omelyanchuck, A.N., Pisma Zh. Eksp. & Teor. F i z . ,
2
(1975) 216./6/ Holm, R., E l e c t r i c C o n t a c t s (Hugo Gebers F o r l a g , Stockholm, 1946).
/7/ Solymar, L., S u p e r c o n d u c t i v e t u n n e l l i n g and a p p l i - c a t i o n s (Chapman and H a l l L t d . , London, 1972). 181 Aslamazov, L.G. and L a r k i n , A . I . , Zh. Eksp. &
Theor. F i z .
