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Submitted on 1 Jan 1978
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CURRENT-STEPS IN LARGE JOSEPHSON TUNNEL
JUNCTIONS
F. Irie, K. Hamasaki, K. Yoshida
To cite this version:
JOURNAL DE PHYSIQUE
Colloque
C6, supplgment au
no
8,Tome
39,aozit
1978,page
C6-1236
CURRENT-STEPS I N LARGE JOSEPHSON TUNNEL J U N C T I O N S F. Irie, K. Hamasaki and K. Yoshida
Department o f EZectronics, Kyushu University, Fukuoka 812, Japan
Rdsum6.- Un nouveau type de marches de courant a dt6 observe dans la caractdristique courant-tension d'une jonction tunnel Josephson large comportant un r6seau de vortex en mouvement. Ces marches cor- respondent 2. la relation awo/aBo = r/n (n = 2,3 et 4) oii wo et
Bo
sont la frsquence et la constante de phaseassocides au rdseau de vortex et Z la v6locit6 des ondes 6lectromagn6tiques dans la jonction. Abstract.- A new type of current-steps corresponding to a relation awo/aBo = E/n (n = 2, 3 and 4) has been observed in dc current-voltage characteristics of a large Josephson tunnel junction with a mo- ving vortex array, where wo andPo
are fundamental components of the frequency and the phase constant associated with the moving vortex array, respectively, and c is the velocity of an electromagnetic wave in the junction.In dc current-voltage (I-V) characteristics of a Josephson tunnel junction which is in a flux-flow state, Eck et al./l/ observed current steps which satisfy the so-called velocity-matching condition wo/Bo =
c ,
where wo = (2r/Q0)Vdc is the frequency of self oscillation andBo
= ( 2 ~ i d ~ ~ / @ ~ ) H ~ is the phase constant of the vortex array. Here Vdc is a dc voltage, H an applied magnetic field, Q o the flux flux quantum, po permeability of vacuum, and d the penetration depth of a magnetic field in the junc- tion. In this report we shall present experimental results on current steps in I-V characteristics of a large Josephson tunnel junction, which are diffe- rent from existing ones.The Josephson junction used in our experiment is an in-line type Pb-PbOx-Pb tunnel junction, as shown in the inset of figure 2. Sample dimensions are L x W = 1.45 x 0.13 m2, corresponding to L/XJ = 29 and W/A = 2.6. Other junction parameters
J
are ; normal tunneling resistance %T = 8.0 nil, cri- tical current density j = 1.0 x
lo6
~ / m * , the Jo- sephson penetration depthXJ
= 50 pm, critical ma- gnetic field H = 0.83 Oe and the velocityc 1
i5 = 1.54 x
lo7
m/s. We have measured I-V characte- ristics by increasing the magnitude of an applied magnetic field 0.6 Oe at a time.The observed magnetic field dependence of I-V characteristics is shown in figure 1. In a low vol- tage region the "displaced linear brancht'/2,3/ was observed, its slope strongly depending on the ma- gnetic field/4/. Series of current-steps can be al- so seen on this figure, which are denoted by n = l, 2, 3 and 4. In figure 2 the magnetic field depen- dences of the voltages corresponding to these cur-
1 ; ; g - 0 a s .
-
0 0.4 0.6 0.8
Vdc(rnV )
Fig. 1 : I-V characteristics of a large Josephson tunnel junction for different magnetic fields.
0.9
-
0.8 8- 0.7-
0.6-
-
3
as-
Oh-
-
Fig. 2 : Magnetic field dependence of voltages cor- responding to current steps.
rent steps are shown. The solid lines in this figu- re are determined as follows. Their slopes are cal- culated from the conditions
awo =
E
-
a60
n (n = 1, 2, 3 and 4) (1)and their positions are determined so as to give
the best visual fit of the experimental points- to References- these lines. A reasonable agreement between almost
all experimental points and the lines calculated / I / Eck,R.E., Scalapin0,D.J. and Taylor,B.N., Phys. from the above equation is obtained. In the above Rev. Lett.
13
(1964) 15equation the case of n = 1 corresponds to the velo- /2/ Scott,A.C. and Johnson,W.J., Appl. Phys. Lett. 14 (1969) 316
-
city-matching current steps observed by Eck et al./3/ Barone,A., J. Appl. Phys.
42
(1971) 2747 Other cases (n = 2, 3and 4), however, indicate that141 Yoshida,K., Lrie,F. and Hamasaki,K., Phys. Lett the velocities of moving vortices corresponding to
-
63A (1977) 376these current steps are submultiples of E . These 151 Lebwoh1,P. and Stephen,M.J., Phys. Rev.
163
current steps seem to be due to the coupling bet- (1967) 376ween electromagnetic waves induced by the moving 161 Yoshida,K. and Irie,F., Appl. Phys. Lett.
2
(1975) 469vortex array.
Lebwohl and Stephen151 have ever predicted that current steps might arise in the case of wo
180
= S/(2R+I) ( 8 = 0 , 1 , 2 , 3,...)
(2) Our experimental results differ from their predic- tion in two points ; i) the straight lines connec- ting each series of current-steps do not meet with the origin, i.e., the point of wo=8o=O, except for the case n = I, and ii) current-steps corresponding to even numbers for n are also observed.The reason for ii) may be understood as fol- lows. According to our previous paper/6/, two pro- pagation modes with frequencies wl=(p+l)wo+w and W~=PW,J+W (p=O, 21, f2,
...
) have an active coupling via a moving vortex array, leading to current steps when the following condition for phase constants is satisfied :81+82 = 80 (3)
