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Applied Physics Letters

Josephson oscillations and noise temperatures in YBa2Cu3O7−x grain-boundary junctions

Yu. Ya. Divin, J. Mygind, N. F. Pedersen, P. K. Chaudhari

Published Dec 21, 1992• 36 citations
Publisher

Abstract

The ac Josephson effect was studied in YBa2Cu3O7−x grain-boundary junctions (GBJ) in the temperature range from 4 to 90 K. The temperature dependence of the linewidth of millimeter-wave Josephson oscillations was measured and it is shown that the derived effective noise temperatures may be as low as the physical temperature in the temperature range investigated. In the millimeter-wave range, linewidths as low as 380 MHz were found at liquid-nitrogen temperatures.

DOI
10.1063/1.108007

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Applied Physics Letters•Mar 27, 2000•14 citations
The ac Josephson effect was studied in YBa2Cu3O7−x grain-boundary junctions (GBJ) in the temperature range from 4 to 90 K. The temperature dependence of the linewidth of millimeter-wave Josephson oscillations was measured and it is shown that the derived effective noise temperatures may be as low as the physical temperature in the temperature range investigated. In the millimeter-wave range, linewidths as low as 380 MHz were found at liquid-nitrogen temperatures.
The AC Josephson effect in YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary junctions (GBJs) was studied in the temperature range from 4 K to 90 K. The temperature dependence of the linewidth of millimeter-wave Josephson oscillations was measured, and it is shown that the derived effective noise temperature of GBJ might be as low as the physical temperature in the temperature range investigated. This makes it possible to use the resistively shunted junction (RSJ) model with thermal fluctuations to get a limiting performance of high-T/sub c/ devices utilizing the AC Josephson effect. The lowest value of the linewidth of 72 GHz Josephson oscillations observed at 77 K was equal to 380 MHz, which demonstrates the applicability of GBJ, particularly in the field of radiation spectroscopy, even at liquid nitrogen temperatures.>
We have studied the response of YBa2Cu3O7 step-edge junctions to low-intensity millimeter-wave radiation in the temperature range from 4 to 80 K. The linewidth of the Josephson oscillations derived from the resonant part of the response at voltages V≂(h/2e)f is shown to be determined by thermal fluctuations at liquid nitrogen temperatures. At lower temperatures the observed linewidth increases indicating that low-frequency fluctuations become dominant in the junction as the temperature is reduced. Due to an inhomogeneous spatial distribution of the current the step-edge junction might be considered as a multijunction multiloop interferometer and the excess noise can be discussed in terms of spontaneous transitions between the different states of these interferometers.
The IV curves and the direct detector response of HTS bicrystal grain-boundary Josephson junctions have been measured under submm-wave irradiation at frequencies 350-750 GHz. The Josephson oscillation spectrum has been measured using the Hilbert transformation of detection response dependence. The Josephson oscillation linewidth and corresponding value of effective noise is compared to the estimates from the dc measurements of noise rounding of IV curve. To separate sources of noise the direct measurements of junction's output noise has been performed at 1.5 GHz and low frequencies 0.5-20 kHz.
YBCO grain boundary Josephson junctions (GBJJ's), with the normal-state resistance (R/sub N/) around 1 /spl Omega/ and I/sub C/R/sub N/ product of less than 2 mV at 45 K, where I/sub C/ is the critical current of the junction, are used to study the dynamic range of high temperature superconducting (HTS) Josephson detecting systems at millimeter-wave band. A compact pulse tube cryocooler (PTC), or a liquid helium cryostat (LHC), or a liquid nitrogen cryostat (LNC) provides the necessary low temperature environment for the operation, while Gunn oscillators together with doubler generate signals at the frequencies of 50, 98, 196 GHz whose amplitude can be adjusted by a series of calibrated attenuators. By changing the junction parameters, operating frequencies, and the cryocooler or cryostat, we have carefully looked at the dynamic range of the system. For a measuring channel bandwidth of 1 Hz, the dynamic range is around 38 dB which is determined basically by the preamplifier. This indicates that the PTC does not introduce excess noise compared with other kinds of cryostats such as LHC's or LNC's. A measuring channel bandwidth up to 1 MHz can be helpful if fast measurements are needed.
We have demonstrated the first applications of Josephson-effect-based Hilbert-transform spectral analysis using high-T/sub c/ Josephson junctions. YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary Josephson junctions have been fabricated on [110] NdGaO/sub 3/ bicrystal substrates for these measurements. At first, spectra of Josephson radiation have been measured by the Hilbert-transform technique in the range 60-800 GHz. A Lorentzian shape of the Josephson radiation and the possibility to obtain a spectral resolution as low as 0.1 cm/sup -1/ in the submillimeter-wave range has been demonstrated at liquid nitrogen temperature. Moreover, spectra of high-harmonic content of the radiation coming from commercial millimeter-wave oscillators have been measured in the spectral range from 60 to 450 GHz. A comparison of Hilbert-transform spectral analysis with conventional techniques is given.
High-temperature-superconducting (HTS) resistively shunted superconducting quantum interference devices, consisting of two yttrium–barium–copper–oxide (YBCO) grain boundary junctions shunted by gold-film resistors of ∼20 μΩ, have been prepared and characterized at temperatures from 17 to 74 K. The small value and stability of the resistor allows narrow-band heterodyne oscillations to be generated between the Josephson oscillations in the individual junctions. The frequency can be precisely controlled in accordance with the ac Josephson effect by adjustment of an external current applied to the YBCO–Au–YBCO shunt resistor, and the linewidth is determined in principle only by Johnson noise in the resistor. Off-chip detection and spectral analysis of the heterodyne signal generated in this way has not been previously reported. Measurements of the signal power (∼nW), the tunability of the center frequency from 5 to 50 MHz, the frequency stability and the linewidth of the heterodyne Josephson oscillation are described. Although linewidth broadening due to nonthermal fluctuations is observed, the linewidth of ∼30 kHz at T=17 K, at a center frequency of 24 MHz, is believed to be the narrowest reported for Josephson oscillations in an HTS device of this type.