CONTROLLABLE PROXIMITY-EFFECT WEAK-LINKS

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Submitted on 1 Jan 1978

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CONTROLLABLE PROXIMITY-EFFECT

WEAK-LINKS

G. Dharmadurai, B. Ratnam

To cite this version:

CONTROLLABLE PROXIMITY-EFFECT WEAK-LINKS+

G. Dharmadurai and B.A. Ratnam

Solid State Physios Section, Bhabha Atomic Research Centre, Bombay 400 085, India Tata Institute of Fundamental Research, Homi Bhabha road, Bombay 400 005 * India

Abstract.- We have observed that dc injection through the normal-metal strip across a long proximity -effect weak-link causes a systematic reduction in the critical current of the weak-link. We also observe that a simple well-defined relationship exists between the injection current and the critical current. This suggests that the characteristics (e.g. critical currents) of these types of weak-links can be electrically and continuously tuned at any temperature below the superconducting transition temperature.

INTRODUCTION.- A wide range of devices incorporating

weakly coupled superconductors (known as weak-links)

exhibit Josephson effects/1,5/ and are used in a

va-riety of applications/1/ such as SQUIDS/6/, computer elements etc. A common feature of all these

weak-links is a localised region of depressed

supercon-ductivity separating two superconductors/4/. In

mi-crobridges, this is achieved by a local narrowing

of the thin-film to a width comparable to the co-herence length/2/ and in proximity-effect bridges

by evaporation of a normal metal strip across the

film/3/.

The condition that the size of the localised

weak region must be at the micrometer level imposes

rather severe constraints on the fabrication and

often, the desired operating characteristics are mostly achieved only in a very restricted

tempera-ture interval. To obviate these difficulties and

constraints, recently a new class of weak-links

known as quasiparticle injection induced weak-links

/4/ and magnetically weafened superconductors/5/ ha-ving an externally controllable critical current

have been introduced. In this paper, we present

ex-perimental evidence for the controllability of

con-ventional proximity-effect weak-links by injecting

a direct current through the normal metal strip transverse to the weak-link.

EXPERIMENTAL.- The structures used in our

experi-ments involve crossed thin-film strips of silver

and tin (see inset figure 1) prepared by sequential

vacuum evaporation of these materials onto clean glass substrates. The transition temperature of the

overlay region is depressed relative to the

main-film by the proximity-effect of the normal metal on

the superconductor/3/. The lengths of the overlay

region range between 200 ym and 500 urn and the width is approximately 200 ym with the thicknesses

of the tin and silver strips varying between 0.1 ym and 0.3 ym. The sample is vertically mounted in a

liquid helium bath inside a glass cryostat which can be pumped down to about 1.6 K and temperature

36 - T.2

Fig. 1 : The I-V characteristics of a typical pro-ximity-effect weak-link (TD 259) for different in-jector currents Ij. Ic o - The critical current of the weak-link for zero injection. The inset shows the sample geometry.

Research carried out at the Tata Institute of Fun-damental Research

JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-1243

Résumé.- On remarque que l'injection d'un courant continu dans la couche de métal normal d'une jonc-tion faible avec effet de proximité, a pour résultat une diminujonc-tion systématique du courant critique de la jonction. On observe aussi qu'un rapport simple mais bien défini existe entre le courant d'in-jection et le courant critique. Ceci suggère que les caractéristiques (c'est-à-dire les courants cri-tiques) de ce type de jonction pourraient être ajustés électriquement et continuement à n'importe quelle température au-dessous de la température de transition supraconductrice.

controlled with a manostat to better than f 1 mK.

A superconducting niobium shield is used to minimi- se stray fields.

The current-voltage (I-V) characteristics of the weak-links are experimentally obtained by ram- ping the voltage across a 25 ohm shunt resistor in parallel with the series combination of the weak- link and a 10ohm current detecting resistor. Vol- tages and currents through the weak-link are simul- taneously recorded on an x-y plotter. A direct cur- rent IJ can also be injected through the silver strip (see inset figure 1)using a separate battery.

RESULTS AND DISCUSSION.- The observed I-V characte- ristics of the weak-links (see figure 1) exhibit the general features observed by previous workers/3/, viz. an excess supercurrent I at all voltages which approaches Ic/2 for V>>I R(1 = Is + V/R) where Ic is the critical current and R the normal state re- sistance of the weak-link. The protruding normal me- tal strip on either side of the weak-link (about 3

mm) has not affected the general features of the I-V characteristics.

Now, when a direct current IJ is injected through the transverse normal metal (silver) strip (see inset figure 1) nonequilibrium quasiparticles are generated in the overlay region (viz. the pro- ximity-effect weak-link) by pair breaking by ther- mal phonons from this heater strip/4/ (which are divided between the superconductor on one side of the injector and the bath on the other/7/) and de- press the BCS energy gap parameter of the overlay region causing a systematic decrease in the criti- cal current as observed in figure 1, which is remi- niscent of an effect observed/3/ when the bath temperature is raised.

Because of uncertainties about the fraction of the total dissipated injection energy which appears as the energy of pair-breaking phonons in the brid- ge region of the superconductor and subsequently as excited particle energy/7/, it is not straightfor- ward to write down a relationship between the quasi- particle injection rate and the injection current. Nevertheless, the data collected for several sam- ples a.t different bath temperatures could be fitted to an equation of the form

(1) where I is the critical current of the weak-link

CO

for zero injection current and I the critical in- Jo

jector current that drives normal the weak-link.

Fig. 2 : Injector current dependence of weak-link critical current for different samples at T = 2.21K The error bars are typical for the indicated regions of (Ic/IcO)

Figure 2 displays the variation of (I /I )"

C co against (I /I )2 for few typical weak links at a

J Jo

fixed bath temperature. The data fit equation (1)

to within f 5 % which clearly indicates that once Ico and IJo at a given bath temperature are determi- ned, the critical current of the weak-link can be tuned to any desired value I by suitably adjusting I according to equation (1) (or by using figure 2

J

as a calibration curve).

CONCLUSIONS.- The above experimental results demons- trate that the characteristics (e.g. critical cur- rents) of proximity-effect weak-links can be elec- trically and continuously tuned at any temperature below the bulk transition temperature of the over- lay region where in a way they then behave like mi- crobridges.

References

/I/ Waldram,J.R., Rep. Prog. Phys.

2

13 (1964) 195

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/3/ Notarys ,H.A. and Mercereau, J.E., J. Appl

.

.

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/4/ Ting-Wah Wong, Yeh, J.T .C. and Langenberg,D .N., Phys. Rev. Lett. 37 (1976) 50 and IEEE Trans. Mag.

13

/5/ Dolan,G.J. and Lukens,J.E., IEEE Trans. Mag. 13 (1977) 581

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