Anomalies in the Josephson radiation linewidth

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Physica 108B (1981) 1297-1298 North-Holland Publishing Company

ANOMALIES IN THE JOSEPHSON RADIATION LINEWIDTH D.B. Schwartz and J.E. Lukens* Department of Physics, State University of New York at Stony Brook Stony Brook, New York 11794 G.J. Dolan Bell Laboratories Murray Hill, New Jersey

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Radiation linewidths have been measured at microwave frequencies for resistively shunted submicron tunnel junctions and variable thickness microbridges. While the tunnel junction data were in reasonable agreement with theory, anomalously wide linewidths were observed for the microbridges indicating that presently accepted theories do not account for the dominant noise effects in these Josephson junctions. There has been renewed interest in the intrinsic noise of Josephson junctions due in part to the desire to understand the ultimate limit to the sensitivity of devices using the Josephson effect. The present analyses of this noise are based on the resistively shunted junction (RSJ) model which has ~e~n extended to include quantum effects. ' In particular, these models predict the spectral density of the low frequency voltage noise S (0) and thu~ also the linewidth thru the relation A~ = (~/~o)Sv(0). Measurements on microbridges where nonequilibrium effects are important revealed an intrinsic noise which is dramatically ~ f ~ r e n t than that predicted by the RSJ model.~'-'~ This suggests that for a large class of Josephson junctions, the intrinsic noise is dominated by some processes other than Johnson noise in the junction resistance. In this paper we present the results of measurements of the radiation linewidth of resistively shunted small area tunnel junctions. These data are compared with the anomalous linewidth data which were obtained in an identical manner from variable thickness microbridges (VTBs) of similar resistance and critical current. ~ The Pb alloy tunnel junctions studied were fabricated using an oblique evaporation technique, allowing the junction and the resistor to be fabricated in a single vacuum step. 6 The junction had an a r e a ~ .5 ~ and a critical current density ~ I0- A-cm--. The ~ .I~ re2 sistive shunt gave B = IcR C/~ o < 10 -3 making the IV curve non-hysteric over a wide range of temperatures. The LC resonance due to the inductance in the shunt is estimated to be at 1012 Hz and does not appear as structure in the differential resistance. The differential resistance of the shunted tunnel junctions as sho~m in Fig. 1 is constant for V > I R, but c the peak at I falls off more rapidly than the c RSJ prediction with no capacitance. The llnewidth was taken to be the full width at half maximum. Since the line shape was Lorentzian, this linewidth determination was unambigious.

T= 5 . 2 5 K RSJ experiment

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i gc

z6o

~

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5bo

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t, A] Fig. 1--The differential resistance of the shunted tunnel junction at 5.25 K. The dotted line is RSJ. The measured linewidth vs temperature (and thus ) at fixed frequency is plotted in Fig. and Fig. 2b for a VTB and a resistively shunted tunnel junction respectively. The data are normalized to the value

~

AVo=

~R~S_(O)/~ 2 (i) ui o where SI(0) is the spectral density of the Johnson noise in the resistive shunt, i.e., ST(0) = 4kT/R where R is obtained from the n n dlfferentlal reslstance measured at I >> I and the dynamic resistance RD is the measured ~alue at the operating ~oJnt. The contribution to SI(O) due to down conversion has been neglected slnce it is small compared to the effects of interest (less than 25% for the tunnel junction). To take accurate account of it, the conversion efficiencies should be measured directly. 1

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As can be seen in Fig. 2, the tunnel junction data are reasonably well described by Eq. i, while there is a large anomaly in the micro-

*The work at Stony Brook was supported by the Office of Naval Research

0378-4363/81/0000-0000/$02.50

© North-Holland Publishing Company

1297

1298

bridge data, particularly for higher I . We can not yet rule out external noise as theCcause of the slight excess linewidth for the tunnel junctions at 12 GHz. In any case, this linewidth sets a limit on the effects of external noise far below the observed linewidth for the VTB and supports our belief that the excess tunnel junction line width at 6.5 GHz is intrinsic. Figure 3 shows the frequency dependence of the VTB linewidth at contant I . The large increase c in A~ as V decreases is unexplained by theory. While such eff~c~s might be expected in the quantum limit, ' these data are well inside the classical region.

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Figure 3--The normalized linewidth vs frequency for a VTB at I = 705 ~A. c

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We conclude from these results that the dominate noise source in VTBs is not accounted for by current theories. The data also indicate possible additional noise contributions in high current density tunnel junctions.

2 zoo

~oi= ~mO~ soo

Figure 2a--The normalized linewidth (see text) vs I for a VTB. The lines are for visual clarity only.

We would like to thank Paul M. Mankiewich for the fabrication of the VTB junction and Aloke Jain for his assistance with the VTB measurements. REFERENCES:

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5

G

i.

K.K. Likharev and V.K. Semenov, Pis'ma Zh. Eskp. Teor. Fiz. 15, 625 (1972) [JETP Lett 15, 442 (1972)].

2.

R.H. Koch, D.J. Van Harlingen, and J. Clarke, Phys. Rev. Lett., 45, 2132 (1980).

3.

T.W. Lee, Ph.D. dissertation (unpublished), State University of New York at Stony Brook, i977.

~

12G

Tk]

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J. Mygind, W.F. Pederson, O.H. Soerensen, B. Oreholm, T.F. Finnegan, J. Bindslev-Hansen M.T. Levinsen, P.E. Lindelof, I.C. SQUID 1980, Conference Digest, 139 (1980).

5.

D.B. Schwartz, P.M. Mankiewich, A.K. Jain and J.E. Lukens, IEEE Trans. Mag., MAG-17 92

(1981). Figure 2b--The normalized linewidth vs temperature for a shunted tunnel Junction. The measured linewidth is A~ = (2.6 MHz/K)(A~/AUo)T.

"6. G.J. Oolan, Appl Phys Lett. 31, 337 (1977).