O.V. Snigirev

Josephson junction with lateral injection as a vortex transistor

Long narrow Josephson junctions with current injection to their long (lateral) sides are examined theoretically. Injection into a finite number of points as well as distributed injection are considered. The external magnetic field effect on the critical current and on the I-V curves is calculated. It is shown that the junction with current injection into many points in parallel is an almost complete analog of a conventional semiconductor transistor, the role of electric charge carriers being played by Josephson vortices carrying a single flux quanta. The use of a vortex transistor as a circuit element of both analog and digital devices is discussed.

Magnetic flux guide for high-resolution SQUID microscope

Magnetic properties of a permalloy needle designed for use in a high-spatial-resolution scanning SQUID magnetic microscope as a magnetic flux guide have been examined. The distributions of the magnetic field normal component over a needle were imaged at 77 K in various applied magnetic fields. Influences of the flux guide on the high temperature superconducting dc SQUID characteristics have been investigated.

Ultimate sensitivity of the dc SQUIDs using unshunted Josephson tunnel junctions

An analitical theory of dc SQUIDs using unshunted Josephson tunnel junctions is developed. An expression for the phase as a function of time has been found for the process of switching from zero-voltage state to gap-voltage state. Expressions are derived for the signal transfer function and for the voltage noise spectrum density in the SQUID. An ultimate sensitivity limit is found, and the comparison is performed between these devices and usual dc SQUIDs using shunted Josephson tunnel junctions.

Relaxation-oscillation-driven DC SQUIDs

Simple and reliable relaxation-oscillation (RO)-driven DC SQUIDs (superconducting quantum interference devices) have been fabricated with a flux-to-voltage factor H up to 1 mV/ Phi /sub 0/ and internal energy sensitivity E/sub v/ of (4+or-1) * 10/sup -31/ J/Hz at frequencies above 10 Hz and 1*10/sup -29/ (1 Hz/f) J/Hz at lower frequencies. Supplied with a 50- mu m-wire 20-turn spiral input coil and a 6-cm/sup 2/ pickup loop, such SQUIDs provided magnetic field sensitivity close to (6+or-3)*10/sup -15/ T/Hz/sup 1/2/ above 10 Hz. The dynamic range of the magnetometers was close to 1.5*10/sup 7/ Hz/sup 1/2/ above 10 Hz. The maximum slew rate was about 10 mu T/s at 0.1 kHz. Successful records of Earth-ionosphere magnetic resonances have been obtained with these magnetometers housed in a stainless steel transport dewar. A balanced version of the RO-driven DC SQUID is suggested with improved H and E/sub v/. >

Superconducting Quantum Interference Filters as RF Amplifiers

The superconducting quantum interference filter (SQIF) is a new type of superconducting device which has been recently proposed for highly sensitive magnetometers for absolute magnetic field measurements. It benefits of very high voltage-to-field response, which is, in contrast conventional dc SQUIDs, not periodical. The SQIF can also be used as a radiofrequency amplifier in a similar way as the dc SQUID that can operate in a gigahertz frequency range. We designed a series type of SQIF amplifier that is compatible to conventional YBa2Cu3O7 (YBCO) technology on bicrystal substrates. We present analytical, numerical and scale modeling as well as first electrical measurements results at frequencies up to 10 GHz. The SQIF array consists of 50 loops with randomly distributed areas from 0.5 to 1.5 times of . We also compared it to the regular array of conventional SQUIDs with the same loop areas. We have additional dc contacts to each 5-th SQUIDs and the SQIFs for control and comparison. Devices are fabricated using Josephson junctions with 3 mum width formed in YBCO over 24/24 and 12/12 degrees grain boundaries in yttrium-stabilized zirconia (YSZ) bicrystal substrates.

Liquid-State Nuclear Magnetic Resonance Measurements for Imaging Using HTS-rf-SQUID in Ultra-Low Field

In our study, we constructed a ultra-low field (ULF) nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI) system employing a high-temperature superconductor (HTS)-rf-SQUID and room-temperature coils. In the system, we employed a pulsed polarizing field <i>B</i>_p perpendicular to a measurement field <i>B</i>_m; a high-speed switching circuit with an optical FET was used to switch off <i>B</i>_p of approximately 30 mT. When a field gradient d<i>Bz</i>/d<i>z</i> of approximately 10 nT/cm was applied to a sample, which was designed to contain water in two compartments, a ¹H-NMR spectrum with two peaks was observed. The frequency difference between the two peaks roughly corresponded to the distance between each center of the water in the two parts. We measured the ¹H -NMR signals for water and mineral oil and found a clear difference in their signal amplitudes owing to their different molecular compositions. The longitudinal relaxation times T₁ of water and the oil were also estimated by changing the polarizing time of <i>B</i>_p. From these results, it was confirmed that ¹H- and T₁-weighted contrast imaging could be realized by using this system.

A double dc SQUID based magnetometer

We have developed a double dc SQUID based magnetometer with a direct read-out circuit and measured its signal and noise characteristics. The inductances of the input SQUID and SQUID-preamplifier were 120 and 250 pH. Thin-film flux transformers on the separate substrates had an inductance of the order of 1 /spl mu/H each and a current sensitivity of 0.54 and 0.32 /spl mu/A//spl Phi//sub 0/ for the first and the second SQUIDs, respectively. The intrinsic energy resolution of the fabricated magnetometer for an optimal gain of 58 for the first stage was close to 1.6/spl times/10/sup -32/ J/Hz. The dynamic range and the slew rate were about 126 dB and 4/spl times/10/sup 4/ /spl Phi//sub 0//s.<<ETX>>

Noise in relaxation-oscillation-driven DC SQUIDs

The noise properties of the simple relaxation-oscillation-driven DC SQUIDs (RO-SQUIDs) have been studied. A limitation of its internal energy sensitivity E/sub v/ at a level close to 4*10/sup -31/ J/Hz due to the influence of the Josephson junction plasma oscillation has been found for the 5- mu m-design-rule technology. The signal characteristics with the transfer flux-to-voltage factor up to 2 mV/ Phi /sub 0/ and equivalent noise flux of about 1.3*10/sup -6/ Phi /sub 0//Hz/sup 1/2/ in the new all-niobium version of the balanced RO-SQUID have been measured.

Magnetic microscope based on YBCO bicrystal thin film do SQUID operating at 77 K

Abstract We have developed two versions of magnetic flux microscope which use an YBCO thin film bicrystal do SQUID to image the two-dimensional distribution of magnetic field. With the size of the inner hole of the SQUID close to 30 μm a spatial resolution of about 200 μm and a field of view 3×4 mm 2 have been obtained. The equivalent magnetic field noise in the SQUID ranges from 100 pT/Hz −1/2 at 1 Hz to 10 pT/ Hz −1/2 at 100 Hz. We have employed the microscope to image 50 μA 110 Hz ac currents flowing in the thin parallel wires.

Properties of the YBCO thin film interferometers fabricated on ZrO/sub 2/ bicrystal substrates

The properties of DC SQUIDs made of YBCO thin film deposited on ZrO/sub 2/ bicrystal substrate and the nature of the fabricated Josephson junctions have been studied. It is found that the characteristics of the junctions are similar to those of the SNS structures with paramagnetic impurities in the N layer. A level of interferometer noise close to 5*10/sup -4/ Phi /sub 0//Hz/sup 1/2/ and a magnetic field sensitivity of 10/sup -10/ T/Hz/sup 1/2/ at frequencies higher than 25 Hz in the usual feedback mode of the SQUID operation have been achieved at 77 K.