This paper reviews the present status and future perspectives of discrete Josephson junction arrays for applications as sub-mm wavelength radiation sources. It is intended to cover the whole field, i.e. theory, fabrication and experimental results. The theoretical part reviews the fundamental aspects of Josephson junctions for oscillator applications and introduces the different possible array types. The recent results of analytical as well as numerical investigations are discussed. After the description of the fabrication of both low- as well as high- superconductor Josephson junctions and arrays, methods to investigate the array dynamics experimentally are mentioned. Finally, the recent experimental results are reviewed. This topic is divided into two parts, the first dealing with low- arrays, the second with high- arrays. The different possibilities to design arrays and to include them in practical applications are discussed and compared, with special emphasis on those experiments where radiation was generated successfully. The article is completed with a discussion of the most important experimental results.
Mutual phase locking of Josephson junctions in a multijunction superconducting loop (MSL) was investigated both theoretically and experimentally. The theoretical analysis predicts the existence of phase-locked states, where the circulating current in the loop serves for phase locking between junctions. The basic operating principles of a MSL were studied experimentally using high-TC bicrystal Josephson junctions. The enhancement of phase locking stability with respect to series arrays is reported.
We have measured the noise performance and conversion efficiency of Y-Ba-Cu-O bicrystal Josephson mixers at operating temperatures between 20 and 60 K and at operating frequencies around 90 GHz. A double-sideband mixer noise temperature of about 1600 K and a conversion efficiency of Ϫ10 dB at 20 K operating temperature has been measured using the Y-factor method. The absorbed local oscillator power was in the range of 10 nW. The dependence of the mixer performance on the normalized frequency ⍀ and the fluctuation parameter ⌫ has been studied. In accordance with the resistively shunted junction model, the experimental data show the presence of excess noise. The temperature dependence of the mixer noise temperature can be explained by the variation of the linewidth of the Josephson oscillations with the operating temperature. © 2000 American Institute of Physics. ͓S0003-6951͑00͒00113-3͔Mixers based on a single high-temperature superconductor ͑HTS͒ Josephson junction recently became attractive devices for receiver applications in long-term remote-sensing satellite missions. Low-noise operation utilizing the ac Josephson effect in the THz frequency range is expected at temperatures well above 10 K because of the large band gap of HTS superconductor materials. Additionally, the Josephson mixer requires only low local oscillator ͑LO͒ power levels on the order of several nanowatts, which can be considered to be a major advantage over Schottky diode mixers. Josephson mixers based on several different HTS junction technologies were reported for millimeter and submillimeter wave mixing. 1-3 Grossman et al. reported on mixing experiments at 30 THz using YBa 2 Cu 3 O 7Ϫ␦ ͑YBCO͒ superconductor-normal-superconductor Josephson junctions. 1 The mixer noise temperature T M , the figure of merit of any mixer device, of 1200 K at Tϭ4.2 K has been measured using a YBCO step-edge Josephson junction at 345 GHz. 2 Tarasov et al. have measured mixer noise temperatures of 1200 K at 430 GHz and 1100 K at 546 GHz and Tϭ4.2 K using a YBCO bicrystal junction ͑BCJ͒. 3 However, the mixer noise performance at operating temperatures higher than 4.2 K is still unclear. The determination of the temperature dependence of the device noise is important for the identification of noise mechanisms that limit the mixer performance. The aim of our work was to measure the mixer noise performance of BCJs at operating temperatures above 20 K and to compare the experimental data with predictions by the resistively shunted junction ͑RSJ͒ model.As shown in earlier studies, the Josephson mixer suffers from excess noise, which is believed to be self-generated due to the ac Josephson effect. 4,5 The mixer noise temperature is expected to exceed the thermal noise limit by several factors. After the RSJ model, the mixer noise depends on the fluctuation parameter, which is defined by ⌫ϭ2ek B T/បI C , the ratio of the thermal energy k B T to the Josephson coupling energy I C ប/2e, where I C is the critical current of the Josephson junction. Although the mixer noise c...
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