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Japanese Journal of Applied Physics

Effects of Kinetic Inductance on the Performance of a Flux-Flow-Type Josephson Amplifier

Tatsunori Hashimoto, Keiji Enpuku, Keiji Yoshida, Kaoru Yamafuji

Published Nov 1, 1987• 4 citations
Publisher

Abstract

The effects of the kinetic inductance of superconducting thin electrodes on the basic parameters of a flux-flow-type Josephson amplifier (FFA) are studied theoretically. It is shown that performance of the FFA can be improved considerably if the kinetic inductance is utilized. A preliminary experiment confirms the theoretical result.
DOI
10.1143/jjap.26.l1874

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IEEE Transactions on Magnetics•Mar 1, 1991•5 citations
The authors have been developing an analog amplifier utilizing a flux flow phenomena in a Josephson line, i.e. the flux-flow-type Josephson amplifier (FFA). The FFA converts a signal current I/sub s/ to a voltage V/sub dc/ across the Josephson line, and the I/sub s/-V/sub dc/ characteristic determines the performance of the FFA, e.g. the gain and saturation level. Studies have been made to improve the I/sub s/-V/sub dc/ relation. It is shown that the transresistance can be improved significantly by reducing the width of the Josephson line and increasing the kinetic inductance of the line. The kinetic inductance can be increased by thinning the line electrode and by using an electrode with large London penetration depth, such as NbN. The transresistance obtained is as large as 6.8 Omega for NbN/Pb-alloy Josephson lines with a thickness of 5 nm and a width of 4 mu m. It is also shown that the linear operating range of the amplifier is as large as 1 mV.>
A theoretical and experimental study of the coupling of the flux-flow-type Josephson oscillator (FFO) to an external stripline has been carried out to overcome the large impedance mismatch between them. For efficient coupling and impedance matching at millimeter and submillimeter wavelengths, the authors have proposed a scheme where the Josephson oscillator is inductively coupled to the stripline by a thin superconducting electrode. Theoretical modeling of the coupling section and the design of the stripline impedance-transformer have been carried out. In a preliminary experiment, the authors obtained a power of about 10/sup -7/ W emitted from the FFO into the stripline in the frequency range from 184 GHz to 193 GHz.>
In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and an external stripline, the authors carried out experiments to investigate the inductive coupling scheme in a strong coupling regime. By adopting NbN films with large penetration depths as electrodes for strong coupling, they were able to realize coupling sections with a coupling coefficient alpha =M/ square root L/sub 1/L/sub 2/ as large as 0.7, with M, L/sub 1/, and L/sub 2/ being mutual inductance, self-inductances of, the FFO, and the stripline, respectively. Using this design for the coupling, the authors succeeded in detecting AC fields at frequencies around 150 GHz, which were emitted from FFO to the stripline via the coupling section.
Thin electrode flux penetration effects in Josephson tunneling gates are analyzed and experimental results exhibiting these effects are reported for the first time. Deep flux penetration accompanied by deep current penetration contribute added inductance not coupled to the external field, reducing sensitivity to control current. In addition, the field within the lower electrode reduces the end-to-end asymmetry introduced by the superconducting ground plane and makes the threshold characteristic more symmetrical. The thin film effects are embodied in an equivalent circuit. Experimental results for devices with thick and thin films are compared showing the expected increased symmetry and reduced sensitivity for the thin film case. Experimental results agree well with calculations based on the equivalent circuit.
Experimental studies to enlarge kinetic inductance of NbN thin films using the effects of quality and thickness of the film are made by measuring Fiske current steps in the current-voltage (I-V) characteristics of NbN/Pb-In Josephson junctions. Experiments demonstrate that the specific kinetic inductance L K can be enlarged over a wide range by changing stoichiometry of the film, as well as by making the film thickness thin, and that this enlargement becomes larger by both effects when the film thickness is in an ultra thin regime. This inductance is shown to be well fitted to L K=µ0λ2/d, w h e r e µ_0 i s t h e v a c u u m p e r m e a b i l i t y, d i s t h e t h i c k n e s s, a n d λ i s t h e m a g n e t i c p e n e t r a t i o n d e p t h i n t h e d i r t y l i m i t. I n t h e p r e s e n t e x p e r i m e n t w e a t t a i n e d t h e i n d u c t a n c e a s h i g h a s L_K=130 p H b y t h e c o m b i n e d e f f e c t s o f i m p u r i t y a n d t h i c k n e s s f o r a f i l m w i t h
The properties and applications of the inductive coupling of a superconducting stripline system, which takes advantage of kinetic inductance of thin superconducting films, are studied. The system, composed of two striplines inductively coupled through a common electrode as a four-port circuit, was formulated by obtaining its scattering coefficients in a general form. The system is characterized with the coupled-mode theory. Based on the analysis, it is proposed that a novel type of kinetic inductance devices for microwave analog components, such as directional couplers, microwave switches, variable filters, and efficient coupling elements for striplines. Experiments on stripline coupling with this scheme are also discussed.