Noise and conversion properties of Y-Ba-Cu-O Josephson mixers at operating temperatures above 20 K

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 -10 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 Omega and the fluctuation parameter Gamma 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. (C) 2000 American Institute of Physics. [S0003-6951(00)00113-3]

Experimentelle Untersuchungen zur Wechselwirkung von HTSL-Josephson-Kontakten in kleinen Netzwerken

The mutual interactions of high-T$_{c}$ grain boundary Josephson junctions in different types of small arrays have been investigated experimentally. Among the different possible dynamical states in Josephson arrays, the totally phase locked state is the most interesting and, for high frequency oscillator applications, most important state. The approach to use simple one-dimensional, antenna-coupled series arrays of up to 100 junctions was only partially successfull, because of deficiensies of state-of-the-art high-T$_{c}$ technology. Two dimensional (2d) arrays have been shown theoretically to be most tolerant against spread in junction parameters. Therefare small 2d arrays of 4x4 junctions were fabricated and characterised via dc-measurements. The synchronisation of these arrays was extremely sensitive on external magnetic field. This might be a consequence of vortex motion in the arrays. Nevertheless, phase locking of up to 16 high-T$_{c}$ junctions was demonstrated by dc-measurements for the first time. Stimulated by previous numerical results, a new array type, i.e., the multijunction superconducting loop (MSL), has been investigated as oscillator for the first time. In comparison to usual types of arrays, e.g. series arrays, the advantages of this type of circuit are the relativily high tolerable parameter spread and the possibility of stable in-phase locking. The simplest MSL consisting of 4 junctions was realised using different types of high-T$_{c}$ grain boundary junctions. For full dc-characterisation of the circuits, a special setup was developed. Using this approach, the phase locking of 4 high-T$_{c}$ junctions was observed directly via a dc-measurement for the first time. Phase locking was achieved up to 180 GHz on SrTiO$_{3}$ substrates and up to nearly 1 THz with the low-Ioss substrate LaAlO$_{3}$. The experimental investigation confirmed various theoretical predictions about the proposed locking mechanism and the enhanced stability of the phase locked state. To relate the observed phase locking to in-phase or anti-phase synchronisation, the arrays were coupled to Josephson radiation detectors. The radiation coupled to the detectors (e.g. 1,3 nW at 60 GHz) was sufficient to investigate the radiation properties in detail. The unique measurement setup allowed to investigate the phase-coherence using both, dc measurement and on-chip radiation detection, at the same time. This way it was possible to compare between fully synchronised as well as unlocked states of the oscillator. The experiments showed clearly the in-phase nature of synchronisation in this type of array, i.e., that MSL are very promising far oscillator applications. In conclusion, a clear picture of a new type of Josephson oscillator was developed in this work. Although the oscillator is not really application-ready, steps towards potential applications, e.g. integrated receivers, were made

Coherent states in a multi-junction superconducting loop

The mutual coherent interaction of Josephson junctions in a multi-junction superconducting loop (MSL) in the limit of zero-loop inductance is theoretically investigated. For the loop containing 2M junctions there are M coherent states corresponding to different values of effective flux $\phi_{\rm eff}$ in the loop. The stability analysis using the method of Floquet exponents shows the highest stability for the state with $\phi_{\rm eff}=1/2$. In this state, the MSL behaves like an oscillating magnetic dipole. The optimum stability is reached for the normalised bias current $i_{\rm b}\simeq 2.5$ and the McCumber parameter $\beta_{\rm C}\simeq 0.5$

Experimentelle Untersuchungen zur Wechselwirkung von HTSL-Josephson-Kontakten in kleinen Netzwerken

The mutual interactions of high-T_c grain boundary Josephson junctions in different types of small arrays have been investigated experimentally. Among the different possible dynamical states in Josephson arrays, the totally phase locked state is the most interesting and, for high frequency oscillator applications, most important state. The approach to use simple one-dimensional, antenna-coupled series arrays of up to 100 junctions was only partially successfull, because of deficiensies of state-of-the-art high-T_c technology. Two dimensional (2d) arrays have been shown theoretically to be most tolerant against spread in junction parameters. Therefore small 2d arrays of 4 x 4 junctions were fabricated and characterised via dc-measurements. The synchronisation of these arrays was extremely sensitive on external magnetic field. This might be a consequence of vortex motion in the arrays. Nevertheless, phase locking of up to 16 high-T_c junctions was demonstrated by dc-measurements for the first time. (orig.)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman