M. L. Chuharkin¹⁾, O. V. Snigirev¹⁾, and S. A. Gudoshnikov²⁾

¹⁾ Physics Department, Moscow State University, Leninskie Gory 1, 119992, Moscow, Russia
²⁾ Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, IZMIRAN, Troitsk, Moscow Region 142190, Russia

Abstract – Thin film dc SQUID gradiometers using a single YBa₂Cu₃O_7-δ layer and grain boundary Josephson junctions on 30° SrTiO₃ bicrystal substrates have been prepared. The planar gradiometer consists of two pickup loops galvanically coupled to the two small area SQUIDs with inductances of about of 30 pH and 45 pH. The gradiometric balance close to 500 has been obtained with base 2.3 mm. White noise level about 7 μΦ₀/Hz^1/2 or 2 pT/cm∙Hz^1/2 in unshielded environment at 77 K.

S. V. Antipov, Yu. B. Vachtomin, S. N. Maslennikov, K. V. Smirnov, N. S. Kaurova, Е. V. Grishina, B. M. Voronov, and G. N. Goltsman

Radio-Physic Laboratory, Dept. of Physics, Moscow State Pedagogical University, 29 M.Pirogovskaia st., 119992, Moscow, Russia

Introduction
There are several atmospheric and astronomical international research projects such as SOFIA [1], TELIS [2], TREND [3], HERSCHEL-FIRST [4] aimed to the terahertz highresolution spectroscopy (up to 5 THz). To put the space-based and air-borne heterodyne instruments into operation at frequencies above 1 THz the superconducting NbN hot-electron bolometer (HEB) [5] will be incorporated into heterodyne receiver as a mixer. At the frequencies above 1.3 THz the sensitivity of the NbN HEB mixers outperform the one of the Schottky diodes and SIS-mixers, and the receiver noise temperature of the NbN HEB mixers increase with frequency following closely 10 - 20 hν/kB law. Besides, the NbN HEBMs have wide intermediate frequency (IF) band and require small local oscillator (LO) power (maximum 1µW). The IF gain bandwidth of such a mixer achieves 5 GHz [6,7]. At the 2.5 THz LO frequency, the best measured value of the double side band (DSB) noise temperature is 1400 K at T=4.2 K [7]. In this paper we present the results of the noise temperature measurements within one batch of NbN HEB mixers based on 3.5 nm thick superconducting NbN film grown on Si substrate with MgO buffer layer at the LO frequencies 2.5 THz and 3.8 THz. Such films show much better superconducting properties than NbN films grown on either Si or MgO substrates [8]. We also discuss the effect of the mixer dimensions on its noise temperature.

A.N. Vasiliyev, B.V. Kalinin, N.Yu. Medvedeva

Dept. of Electronics, St.-Petersburg Electrotechnical University, St.-Petersburg, 197376, Russia

Abstract – The front end of a microwave SQUID amplifier in the form of an opened spiral microstrip line is considered. The microstrip line is coupled with the SQUID, which is current and flux biased. The full-wave analysis of the structure consisting of the opened spiral microstrip line and the SQUID coil has been curried out. In the considered model two Josephson junctions were replaced by equivalent normal resistors. The analysis shows that matching the input circuit can be achieved when the electric length of the opened spiral microstrip line is approximately equal to a quarter wavelength.

M. A. Kucherenko, V. V. Kurin, Yu. N. Nozdrin, E. E. Pestov

Institute for Physics of Microstructures RAS, 603950, Nizhniy Novgorod, GSP-105

Abstract – A near-field microwave microscope was developed to study third-harmonic response of high temperature superconductors (HTS). The initial wave frequency was 472 MHz and the nonlinear response was measured at 1.42 GHz. The temperature was changed from 77 to 92 K. The sensitivity of this system is 10^-14 W/Hz at 1.42 GHz. The scan of the film with a resolution of 125 μm was realized using this technique. The critical temperature spatial distribution was obtained and the quality of HTS films was specified based of the map of the third-harmonic response.