Maxim Chukharkin

High-T-c superconducting quantum interference device recordings of spontaneous brain activity: Towards high-T-c magnetoencephalography

We have performed single-and two-channel high transition temperature (high-T-c) superconducting quantum interference device (SQUID) magnetoencephalography (MEG) recordings of spontaneous brain activity in two healthy human subjects. We demonstrate modulation of two well-known brain rhythms: the occipital alpha rhythm and the mu rhythm found in the motor cortex. We further show that despite higher noise-levels compared to their low-T-c counterparts, high-T-c SQUIDs can be used to detect and record physiologically relevant brain rhythms with comparable signal-to-noise ratios. These results indicate the utility of high-T-c technology in MEG recordings of a broader range of brain activity.

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.

Analysis of the possibility to amplify an RF signal with a superconducting quantum interference filter

A laboratory prototype RF amplifier for the frequency range 1–10 GHz is designed, created, and tested. The device is based on high-temperature superconducting quantum interference filters (SQUIFs) and the technology of bicrystalline substrates. The main characteristics of the prototype SQUIF amplifier are numerically simulated and measured.

Noise properties of high-T-c superconducting flux transformers fabricated using chemical-mechanical polishing

Reproducible high-temperature superconducting multilayer flux transformers were fabricated using chemical mechanical polishing. The measured magnetic field noise of the flip-chip magnetometer based on one such flux transformer with a 9 x 9 mm(2) pickup loop coupled to a bicrystal dc SQUID was 15 fT/Hz(1/2) above 2 kHz. We present an investigation of excess 1/f noise observed at low frequencies and its relationship with the microstructure of the interlayer connections within the flux transformer. The developed high-T-c SQUID magnetometers may be advantageous in ultra-low field magnetic resonance imaging and, with improved low frequency noise, magnetoencephalography applications.

High-Tc SQUID vs. low-Tc SQUID-based recordings on a head phantom: Benchmarking for magnetoencephalography

We explore the potential that high critical-temperature (high-Tc) superconducting quantum interference device (SQUID) technology has for magnetic recordings of brain activity, i.e., magnetoencephalography (MEG). To this end, we performed series of benchmarking experiments to directly compare recordings with a commercial (low-Tc SQUID-based) 306-channel MEG system (Elekta Neuromag TRIUX, courtesy of NatMEG) and a single channel high-Tc SQUID system. The source on which we recorded is a head phantom including 32 artificial current dipoles housed inside a half-spherical shell (courtesy Elekta Oy) for calibrating MEG systems. The high-Tc SQUID magnetometer consisted of a single layer YBa2Cu3O7-x (YBCO) film on a 10 mm × 10 mm bicrystal substrate with a magnetic field sensitivity of ~40 fT/√Hz down to 10 Hz. We recorded serial activations eight tangential current dipoles located at different depths from the surface of the head phantom. Results indicate that our individual high-Tc SQUID demonstrated signal-to-noise ratios (SNRs) about 7-14 times lower than that of similarly-positioned low-Tc SQUIDs in a commercial MEG system. Only considering single-channel SNR, high-Tc SQUIDs with resolution better than fT/√Hz would be required to outperform the low-Tc system for shallow dipole sources. This work demonstrates a proof of principle study for future multichannel high-Tc MEG system development.

High-temperature superconducting films on flexible substrates for flux transformers

The growth of the high-temperature superconductors (HTSCs) on flexible films for the fabrication of nanoelectronic devices is considered. A method for the creation of the HTSC films on the nickeltungsten substrates is proposed. The deposition of the CeO2 and YBCO epitaxial films that are grown on short substrates made of flexible nickel-tungsten ribbon manufactured using the RABiTS technology is comprehensively analyzed, and the properties of the resulting HTSC films are studied. The electrophysical properties of the HTSC films on flexible substrates are demonstrated.

HTS SQUID microscopy for measuring the magnetization relaxation of magnetic nanoparticles

HTS SQUID microscopy is applied for measuring the magnetization relaxation of the ensembles of noninteracting Fe/sub 3/O/sub 4/ nanoparticles dispersed in rigid polymer matrix preventing nanoparticles from agglomeration. Transmission electron microscopy (TEM) is used to determine the size distribution of magnetic nanoparticles and to control the homogeneity of their spatial distribution in the matrix. High sensitivity of SQUID microscope allows us to study samples at low contents of the magnetic component (0.1-0.5 vol%) magnetized in low magnetic field (/spl sim/10/sup -4/ T) produced by a low-inductance coil with short switching time (20 /spl mu/s). A low content of the magnetic component provides the absence of interparticle dipolar interactions, thus simplifying significantly the theoretical description of the magnetization relaxation process. The detection of magnetization relaxation starts after the external magnetizing field is switched off and proceeds for several minutes. A series of samples with a mean size of nanoparticles of about 7 nm possessing relaxation times less than 5 min was measured at a temperature of 77 K. The calculation formula based on the Stoner-Wohlfarth model for single-domain particles with a size distribution as determined from the TEM images will enable to obtain reliable data on the anisotropy constant and the saturation magnetization of Fe/sub 3/O/sub 4/ nanoparticles studied.

Operation of a high-T C SQUID gradiometer with a two-stage MEMS-based Joule–Thomson micro-cooler

Practical applications of high-T-C superconducting quantum interference devices (SQUIDs) require cheap, simple in operation, and cryogen-free cooling. Mechanical cryo-coolers are generally not suitable for operation with SQUIDs due to their inherent magnetic and vibrational noise. In this work, we utilized a commercial Joule-Thomson microfluidic two-stage cooling system with base temperature of 75 K. We achieved successful operation of a bicrystal high-T-C SQUID gradiometer in shielded magnetic environment. The micro-cooler head contains neither moving nor magnetic parts, and thus does not affect magnetic flux noise of the SQUID even at low frequencies. Our results demonstrate that such a microfluidic cooling system is a promising technology for cooling of high-T-C SQUIDs in practical applications such as magnetic bioassays.

Pulsed laser deposition of thin YBCO films on faceted YSZ single crystal fibers

Flexible rods of single crystals of 9% Y2O 3-stabilized ZrO2 (YSZ) were used as substrates for deposition of high-critical temperature superconducting (HTS) thin films. YSZ fibers were prepared by mini-pedestal method with laser heating and had average diameter of 300 micrometers and 30 mm length. X-ray diffraction analysis demonstrated high crystalline quality of obtained fibers and also indicated the presence of 15° deviation of the fiber axis from the [001] YSZ direction. Thin YBa2Cu3O7-x films were grown by pulsed laser deposition on YSZ rods using CeO2 buffer layer. Films have shown high critical temperature of 90 K with sharp superconducting transition. Critical current density was estimated to about 3×104 A/cm 2 at 80 K. Temperature dependence of critical current density suggests granular structure of films with grain size about several microns. Our results demonstrate feasibility of flexible YSZ fibers coated by HTS thin films for practical use.

The physical basis of the fabrication of the third generation of high-temperature superconducting wires on quartz substrates

The problem of the fabrication of third-generation high-temperature superconductors (HTSCs) that are designed for the transmission of electric energy and the creation of nanoelectronic devices is studied in this work. The issues of the fabrication of dielectric substrates for the third generation wires are considered. The technology of HTSC film deposition on the quartz substrates is presented. Complex studies of sputtering of the buffer and superconducting YBa2Cu3O7 − δ (YBCO) layers were performed. The results of studies of electrophysical properties of the HTSC films on the quartz substrates are discussed.