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Applied Physics Letters

Observation of the intrinsic noise of a thin−film microbridge Josephson junction

S. S. Pei, James E. Lukens

Published Apr 15, 1975• 2 citations
Publisher

Abstract

We have measured the intrinsic noise of a thin−film microbridge Josephson junction included as part of a closed superconducting ring. This microbridge, which was fabricated using electron beam lithography and has submicron dimensions, exhibited a noise magnitude which was in excellent agreement with that predicted for an ideal Josephson junction.

DOI
10.1063/1.88218

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We have measured the intrinsic noise of a thin−film microbridge Josephson junction included as part of a closed superconducting ring. This microbridge, which was fabricated using electron beam lithography and has submicron dimensions, exhibited a noise magnitude which was in excellent agreement with that predicted for an ideal Josephson junction.
An experimental study of small superconducting microbridges in order to examine parameter conditions (i.e., junction resistance and junction inductance) necessary for these junctions to exhibit the Josephson effect is described. Nb variable thickness junctions with very narrow width (<50 nm) and short length (<100 nm) were fabricated using electron beam lithography. The junctions which showed the Josephson effect were thinned by ion beam etching, and consequent changes of junction characteristics were measured. The experimental results indicate that the Josephson effect disappears at some critical resistance. These parameter limits of the Josephson junction are compared with theory.
We report on the characteristics of thin film microbridge Josephson junctions fabricated using electron beam lithography and discuss the techniques used to fabricate these junctions, which are typically 0.3μ square. Extensive measurements have been made on both individual junctions of tin and indium and on small arrays of these junctions. Synchronization of current biased series arrays by an applied microwave field has been achieved. This synchronization appears as a zero differential resistance across the entire array at a voltage of MVJwhere M is the number of elements in series and VJis the Josephson voltage hv/2e. Furthermore we have observed a systematic variation with microbridge dimension of the microwave enhancement of the critical current.
We measured the low-frequency-voltage noise properties of a small-sized stack of intrinsic Josephson junctions formed on the surface of a cleaved Bi2Sr2CaCu2Oy (Bi-2212) single crystal, containing a few intrinsic Josephson junctions. A stack with an area of 2×2μm was fabricated by electron-beam lithography and argon-ion-milling. The current–voltage (I–V) characteristics along the c-axis direction of the stack were measured by a three-terminal method. The stack showed (I–V) and dV∕dI-V curves without backbending due to heating effects. Five discrete resistive branches with hysteresis were observed in the low-voltage region of these curves. The measured noise-voltage spectral density SV(f) had a 1∕f dependence on frequency. We estimated the magnitude of the 1∕f noise parameter η from SV(f) in a Bi-2212 stack and found that above the sum-gap voltage in a small-sized stack η was almost the same as that for low-Tc Nb∕Al+AlOx∕Nb high-quality tunnel junctions.
Sub-micron superconducting electronic devices may play a role in the realisation of proposed architectures for quantum computing and metrology. Further miniaturisation of superconducting electronics relies on the ability to fabricate the basic building block, the Josephson junction, on the scale of the superconducting coherence length, with sufficient accuracy that the junction characteristics may be controlled. Making use of focused ion beam technology to fabricate thin-film Nb microbridge devices, we examine the evolution of device characteristics with dimension, focusing on control of the critical current and suppression of superconductivity due to phase-slip behaviour.
Measurements at 3 GHz of the microwave properties of a superconducting thin-film microbridge coupled to a narrowly tuned strip-line resonator (coupled Q approximately 10 3 ) indicate that such a device displays the same variations in small-signal impedance as predicted by the resistively shunted junction model of a Josephson weak-link. Whenever this impedance becomes negative the microbridge is found to generate microwave radiation whose frequency is determined not by the junction bias voltage but by the natural resonance of the strip-line.
Measurements are reported of the Josephson radiation from microbridges coupled to an X-band receiver via a nonresonant microwave transformer. Although closely coupled to the bridge, the transformer does not modify the bridge properties. We find that near the transition temperature the radiation linewidth is proportional to the square of the dynamic resistance, which is also predicted by the resistively shunted junction model. The effective noise temperature determined by the linewidth is 19±3 K and the measured maximum integral power is 5×10−12 W.