Josephson Effect in Low-Capacitance Tunnel Junctions
In a broad discussion of Josephson junctions, many regimes exist, distinguished by the relative magnitudes of various parameters. Characteristic energies are the Josephson energy E j , the charging energy E c = e 2/2C, and the thermal energy k B T. Equally important are the characteristic resistances: the normal state resistance R n , the quantum resistance1 R q = h/4e 2 = 6453 Ω, and the impedance of free space Z o = 377 Ω. The fact that Z o ≪ Rq implies that quantum effects are hard to observe, even in high resistance junctions, unless the junction is buffered from the low electromagnetic impedance of its leads. Recently, several groups have accomplished this to a considerable extent by inserting physically small isolating resistors of high resistance in series with the leads in the immediate vicinity of the junction. Because of stray capacitance effects, however, effective high impedance filters at microwave frequencies are not simple to achieve, and the extensive and systematic early experiments of the Harvard group were made with essentially direct connection to leads, which presumably acted as transmission lines characterized by characteristic impedances of order Z o . These data could be, and were, explained in terms of quantum effects related to delocalization of phase by single electron charging energies. Now, with the general recognition of the importance of transmission line effects and the development of more comprehensive theories, the interpretation of these data needs to be reexamined.
KeywordsVersus Curve Josephson Junction Cooper Pair Tunnel Junction Quantum Tunneling
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