New current-voltage characteristics observed in superconducting thin film crossings

PITYSICS

Volume 50A, number 4

LETTERS

16 December 1974

NEW CURRENT-VOLTAGE CHARACTERISTICS OBSERVED IN SUPERCONDUCTING THIN FILM CROSSiNGS I. IGUCHI Department of Mathematical Engineering and Instrumentation, Physics, University of Tokyo, Tokyo, Japan Received 3 November 1974 We have observed new current-voltage characteristics in superconducting-normal (or superconducting) cross-type junctions. The observed characteristics are simple and general in spite of possible complicated physics involved due to an inhomogeneous current flow.

An interesting phenomenon was observed in currentvoltage characteristics of thin film cross-type junctions. The junction consists of two narrow microstrips, with one in normal or superconducting state (primary film), the other in superconducting state (secondary film). Typical dimensions of the films are 1—3 mm in length, 20—200 ji in width and 1500—3000 A in thickness. The samples were prepared by evaporating a primary film onto a glass substrate, then depositing a secondary film on it. The phenomenon was confirmed by testing nearly a hundred samples with various combinations

of primary (Al, Cu, Ni-Cr, Sn, In) and secondary (Pb, Sn) materials and is considered to arise from an inhomogeneous current flow at the intersection of two films. The basic characteristic of the junction is described as follows; if a current is fed from a primary film to a secondary film or vice versa, a dc voltage appears along the secondary film at a well-defined critical current (fig. 1). We define this critical current as ‘ic The magnitude of ~ is generally low and typically of order of ImA or lOmA at 4.2 K for the lead-normal crossings with the above dimensions, which is much

~ide~

I

~/

2C

J

-60

I

I

2O~~i~2~o I~(mA) ~

Pb—oxide-—Nj-Cr

-~

-‘5 V 2(mV)

-20_i 5

-1 0

-05

0~5

1 0

1 52.0

Fig. 1. Ii— V2 characteristic of Pb-oxide-Al junction. ‘2 c is the measured critical current of Pb film. The film width: 1OO~(Pb), 100 ~z(Al)

+8

--20

-~

‘~°

.12

.14

Fig. 2. Variation of 11—V2 characteristic with bias current‘ZBThe film width: 15M(Pb), 80~(Ni-Cr).

247

Volume 50A. number 4 p

1

PHYSICS 1FTTFRS

~1~J I

I

Pb—Oxide--Ni-Cr -

10

1~

16 Deccrnhcr 1974

It is important that the phenomenon seems to he almost independent of the type of the junction between two films whatever it may be. Therefore the

problem of four-probe method discussed in 1] and

-

enon is not directly related to a kind of heating effect or cryotron mechanism was confirmed by feeding a large amount of current through the primary film and

-

then observing no change in ‘2

-

V 2 characteristic of

—

101~—~__——.~ -

the ured clearly [21thin Itsecondary brings junction should simple betrouble resistance and film. emphasized qualitatively The here. simple also The that denies the fact thesame this thatpossibility. phenomenon using theany in phenomthe type measis of filmnocrossings in spite ofestimate possible complicated physics involved due to an inhomogeneous current

18 ~2 —-

,.-~‘

~/ji

~

-200

Fig. 3. Variation of (>1)

‘2--

V2P~V) ~

V2 characteristic with bias current JiB

lower than the critical current of the superconducting lead film itself. The same kind of behavior was seen in any type of crossings tested so far. It is noted that there is almost no hysterisis in the curve. If an additional bias current ‘TB is given along the secondary film, the I~--- 172 characteristic is remarkably affected. Fig. 2 shows such an example for Pb—Ni-Cr alloy junction. (In order to avoid possible heating by a Ni-Cr film, we put a copper film underneath it.) Very characteristically, for relatively large bias currents, it extends to a negative voltage region when I~is antiparallel to ‘2B’ As another interesting phenomenon associated with this, the current-voltage (j2— V2) characteristic of the secondary film remarkably changes in the presence of the bias current ‘iB’ In other words, we can easily control the large current flowing through the secondary film with the small bias current applied between two films. For ‘lB ~‘ic’ there appears that the critical current of the secondary film (‘2c) decreases with increasingI1~and completely vanishes at ‘lB = ‘ic For ‘lB >Ilc, a dc voltage develops for a finite sweep current

as in fig. 3. Note that fig. 2 and fig. 3 are obtained for the same sample.

248

flow. Therefore we consider that the phenomenon arises primarily from the properties of the superconducting secondary film. When a current flows from a primary film to a secondary film, the current should be bent into a circular path, producing a magnetic field perpendicular to the film surface. Because of its large demagnetizing factor, the film enters locally into the intermediate state in which a kind of flux tube is distributed. Then the observed characteristics can be cxplained in the same manner as flux flow resistivity in type II superconductor [3] and this idea is indirectly supported by the other experimental evidences. The detailed report including temperature dependence will be given subsequently [41The author wishes to thank Prof. K. Hara, Prof. F. Iwamoto, Prof. Y. Wada and Dr. A. Nakamura for helpful discussions and comments.

References [1] 1. Giaever, Tunneling phenomena in solids, eds. I-i. Burnstein and S. Lundqvist (Plenum Press, New York. 1969) P 19. 121 R.J. Pederson and F.L. Vernon, Appl. Phys. Lett. 1(1 (1967) 29. 13] YB. Kim, CF. Hempstead and AR. Strnad, Phys. Rev. 139 (1965) A1163.

[41 1. Iguchi, Proc. 6th Conf. Solid State Devices, to be published.