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Soviet Journal of Low Temperature Physics

Origin of the radio-frequency noise and synchronization of Josephson oscillators in YBaCuO films

S. A. Peskovatskiǐ, V.V. Poladich, L. P. Strizhko

Published May 1, 1990• 0 citations
Publisher

Abstract

The YBaCuO films investigated possess high-speed microwave response and have Josephson properties. The external signal at each of the three frequenices 35, 82, and 126 GHz initiates a rf noise, whose power increases with microwave frequency and periodically depends on dc voltage bias. The rf noise is caused by the synchronization breakdown processes in the system of large numbers of Josephson oscillators and limits the use of the films low-noise heterodyne receivers.

DOI
10.1063/10.0032610

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The YBaCuO films investigated possess high-speed microwave response and have Josephson properties. The external signal at each of the three frequenices 35, 82, and 126 GHz initiates a rf noise, whose power increases with microwave frequency and periodically depends on dc voltage bias. The rf noise is caused by the synchronization breakdown processes in the system of large numbers of Josephson oscillators and limits the use of the films low-noise heterodyne receivers.
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For Josephson bicrystal YBaCuO junctions 3–1.5 µm in size, the response to an external signal with a frequency of 165 GHz was measured. Receivers with similar ICRN values at helium temperatures and different RN values were chosen for the study. The amplitudes of the Shapiro steps were measured for various external signal powers and sample temperatures. The dependence of the output voltage noise on the operating point, temperature, and external signal power was measured. It is shown that fluctuations of the critical current have the greatest influence on the output noise of the receiver. The volt-watt responsivity and noise-equivalent power were evaluated for a 165 GHz external signal. The experimental results are compared with numerical calculations based on the model of a resistive-capacitive shunted Josephson junction. Optimal temperature operating ranges for the use of each structure were shown.
We present new heterodyne receiver results obtained at 100 GHz using resistively-shunted Nb and NbN tunnel junctions. In addition, we have carried out accurate measurements of the available noise power of these devices at the L-band (1.5 GHz) IF frequency. Both the heterodyne and the output noise measurements show that the noise of these devices can be a factor of five or more higher than that predicted by the simple current-biased RSJ model. The noise approaches the appropriate thermal or thermal and shot noise limits for bias voltages where the nonlinearity is not strong (i.e., V>I/sub C/R/sub N/), but as expected from the RSJ model, can be significantly higher at the low voltages where the mixers are typically biased. The bias voltage dependence of the noise shows structure which is associated with resonances in the RF embedding circuit. Surprisingly, we find that changes in the high-frequency (100 GHz) impedance presented to the junction can dramatically affect the magnitude and voltage dependence of the low-frequency (1.5 GHz) noise. This emphasizes the necessity of very closely matching the junction to free space over a wide frequency range.>
A Josephson quasioptical detector comprising YBaCuO junction on bicrystal MgO substrate, integrated log-periodic Au antenna, MgO extended hyperhemisphere lens attached to the substrate and 1.4 GHz cold amplifier with cold circulator has been designed, fabricated and experimentally studied. We fabricated several integrated Josephson junctions 1-2 /spl mu/m wide which at 4.2 K demonstrate RSJ IV curves with critical currents 100-300 /spl mu/A and normal resistance of 10-20 /spl Omega/. The noise temperature (T/sub N/) at 4.2 K and 330 GHz measured by a hot/cold load method in fundamental mixing mode with external BWO local oscillator brings T/sub N/=1100 K for the junction shunted by low-inductance resistive shunt and T/sub N/=1400 K without shunting. At bath temperatures of 20-40 K the noise temperature was in the range 1500-3000 K, at 50 K the noise temperature was below 2000 K, and at 60-70 K increased over 3000 K in the frequency range 310-586 GHz. The noise temperatures measured in the self-pumping mode vary between 580-2300 K with dc bias in the range 0.1-0.5 mV. The linewidth of Josephson oscillations has been extracted from a selective detector response and IF noise voltage dependence. The measured excess noise can be attributed to shot noise and multiple Andreev reflection in pinholes with Ss'S structure. We observed small subharmonic steps in IV curve and noise dependencies, which are typical for SNS junctions.
A detailed study has been made of the low-frequency excess noise of rf superconducting quantum interference devices (SQUIDs), fabricated from thin niobium films and operated at 4.2 K, with rf bias frequencies of 0.15, 1.7, and 3 GHz. When the SQUIDs were operated in an open-loop configuration in the absence of low-frequency flux modulation, the demodulated rf voltage exhibited a substantial level 1/f noise, which was essentially independent of the rf bias frequency. As the rf bias frequency was increased, the crossover frequency at which the 1/f noise power was equal to the white noise power moved to higher frequencies, because of the reduction in white noise. On the other hand, when the SQUID was flux modulated at 50 kHz and operated in a flux locked loop, no 1/f noise was observed at frequencies above 0.5 Hz. A detailed description of how the combination of rf bias and flux modulation removes 1/f noise due to critical current fluctuations is given. Thus, the results demonstrate that the 1/f noise observed in these SQUIDs is generated by critical current fluctuations, rather than by the hopping of flux vortices in the niobium films.
For Josephson bicrystal YBaCuO junctions 3-1.5 μ\muμm in size, the response to an external signal was measured. Receivers with similar IcRNI_{c}R_{N}Ic​RN​ values at helium temperatures and different RNvalues were considered. The amplitudes of the Shapiro steps were measured for various external signal powers. The dependence of the output voltage noise on structure temperature was measured. It is shown that fluctuations of the critical current have the greatest influence on the output noise of the receiver. The volt-watt sensitivity and noise-equivalent power were evaluated for a 165 GHz external signal. Broadband classical detection of THz radiation by a YBaCuO Josephson junction (JJ) was studied based on resistively and capacitively shunted Josephson Junction model. Optimal temperature operating ranges for the use of each structure were shown.