We present an investigation of the superconducting properties of thin YBa2Cu3O7-x (YBCO) films grown on [001] single-crystalline quartz substrates by pulsed laser deposition (PLD) technique. The growth of YBCO on quartz is challenging due to large crystal lattice mismatch between YBCO and quartz. We obtained highly c-oriented textured films with superconducting transition at about 85 K, transition width of 1 K, and critical current density j(c) approximate to 10(4) A cm(-2) at 77 K. Analysis of the temperature dependences of the resistivity and current-voltage char- acteristics has shown the presence of the thermally-assisted flux flow at temperatures above 70 K. From the temperature and magnetic field dependences of critical current, vortex activation energy, U-0(H), was estimated for both parallel and perpendicular orientation of magnetic field. Second critical field, H-c2(0), was estimated from extrapolation of the H-c2(T) dependences. The obtained results are an important step towards realization of YBCO thin films on amorphous silica fibers for the third generation (3G) of the HTS long wire with low losses in alternative current applications.

Superconducting quantum interference devices (SQUIDs) based on high critical-temperature superconducting nanowire junctions were designed, fabricated, and characterized in terms of their potential as magnetometers for magnetoencephalography (MEG). In these devices, the high kinetic inductance of junctions and the thin film thickness (50 nm) pose special challenges in optimizing the field coupling. The high kinetic inductance also brings difficulties in reaching a low SQUID noise. To explore the technique for achieving a high field sensitivity, single-layer devices with a directly connected pickup loop and flip-chip devices with an inductively coupled flux transformer using a two-level coupling approach were fabricated and tested. Two-level coupling is an approach designed for flip-chip nanowire-based SQUIDs, in which a washer type SQUID pickup loop is introduced as an intermediate coupling level between the SQUID loop and the flux transformer input coil. The inductances and effective areas of all these devices were simulated. We found that at T = 77 K, flip-chip devices with the two-level coupling approach (coupling coefficient of 0.37) provided the best effective area of 0.46 mm(2) among all the tested devices. With a flux noise level of 55 mu Phi(0) Hz-1/2, the field sensitivity level was 240 fTHz-1/2. This sensitivity is not yet adequate for MEG applications but it is the best level ever reached for nanowire-based high-Tc SQUID magnetometers.

Development of ultra-sensitive magnetic sensor technologies for bioassays using magnetic multi-core nanoparticles and RCA coils

Practical applications of high-T$_{c}$ 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 Joule-Thomson microfluidic cooling system to operate our high-T$_{c}$ SQUIDs [1]. The micro-cooler system is based on a commercial desktop CryoLab unit from DEMCON kryoz [2]. It contains a two-stage MEMS micro-cooler with a base temperature of 75 K, gross cooling power of 75 mW@80 K, and temperature stability ± 50 mK. Our high-TC dc SQUID gradiometers were fabricated from YBa$_{2}$Cu$_{3}$O$_{7-x}$ thin films grown by pulsed laser deposition on 10 mm × 10 mm SrTiO$_{3}$ bicrystal substrates with 24° misorientation angle. The SQUID chip was glued onto a 0.3 mm thick silicon wafer chip carrier that was attached to the second stage of the cold head. The vacuum housing of the cold stage was made from non-magnetic material (polyethylene terephthalate, PET) and evacuated to a base pressure below 2x10$^{-3}$ mbar. The vacuum chamber features a 0.3 mm thick sapphire window that is placed above the sensor/cold stage. We demonstrated that the equivalent magnetic flux noise of the high-T$_{c}$ SQUID gradiometer is largely unaffected by the micro-cooler setup. The cut-off frequency of the 1/f noise in our SQUID measured on the micro-cooler was around 10 Hz. This indicates that the micro-cooler does not introduce significant magnetic fields in the vicinity of the cold stage. We thus demonstrate that such a microfluidic cooling system is a promising technology for cooling of high-T$_{c}$ SQUIDs in practical applications. We also used the micro-cooler system to build a prototype a magnetic ac susceptibility (ACS) system for detection of specific binding reactions between DNA target molecules and functionalized magnetic nanoparticles (fMNP) in liquid solution. The detection principle relies on changes in Brownian rotation dynamics of fMNPs. We present the results of experiments with various concentrations of magnetic nanoparticles and discuss further development of the portable magnetic bioassay system for detection of influenza virus using oligonucleotide-tagged magnetic nanoparticles with sub-picomolar sensitivity. [1] A. Kalabukhov et al., Supercond. Sci. Technol. 29 095014 (2016). [2] http://kryoz.nl/portfolio-item/cryolab-msg/

We analyzed the sensitivity of a separationless immunoassay scheme using functionalized magnetic nanoparticles (MNPs) and a sensitive HTS SQUID magnetometer. The signal of a 100 μL sample at a concentration of 1 mg/mL and field of 7.5 nT was 20 mΦ 0 . This makes it possible for the sensitivity to be within the range of 50 ng/mL at the required time of up to 100 s per a point in the frequency spectrum.

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.

We explore the potential that high critical-temperature (high-T c ) 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-T c SQUID-based) 306-channel MEG system (Elekta Neuromag TRIUX, courtesy of NatMEG) and a single channel high-T c 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-T c SQUID magnetometer consisted of a single layer YBa 2 Cu 3 O 7-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-T c SQUID demonstrated signal-to-noise ratios (SNRs) about 7-14 times lower than that of similarly-positioned low-T c SQUIDs in a commercial MEG system. Only considering single-channel SNR, high-T c SQUIDs with resolution better than fT/√Hz would be required to outperform the low-T c system for shallow dipole sources. This work demonstrates a proof of principle study for future multichannel high-T c MEG system development.

We have developed a multilayer flux-transformer-based high-T-C SQUID (flip-chip) magnetometer that improves signal-to-noise-ratios (SNR) in ultra-low field magnetic resonance (ulf-MR) recordings of protons in water. Direct ulf-MR-based benchmarking of the flip-chip versus a standard planar high-T-C SQUID magnetometer resulted in improvement of the SNR by a factor of 2. This gain is attributable to the improved transformation coefficient (1.9 vs 5.3 nT/Phi(0)) that increased the signal available to the flip-chip sensor and to the lower noise at the measurement frequency (15 vs 25 fT/Hz(1/2) at 4 kHz). The improved SNR can lead to better spectroscopic resolution, lower imaging times, and higher resolution in ulf-MR imaging systems based on high-TC SQUID technology. The experimental details of the sensors, calibration, and ulf-MR benchmarking are presented in this report.

The thesis describes the development of high T c superconducting quantum interference devices (SQUIDs) with multilayer thin film flux transformers. High- T c SQUID magnetometers are promising in various biomedical applications, including magnetoencephalography (MEG) and ultra-low field magnetic resonance imaging (ulf-MRI). Both MEG and ulf-MRI demand magnetic field sensitivity of less than 10 fT/Hz 1/2 at frequencies as low as 10 Hz. The magnetic field sensitivity of single-layer high- T c SQUID magnetometers is typically about 50 fT/Hz 1/2 . To improve the magnetic field sensitivity, superconducting flux transformer with a multiturn input coil should be used. The flux transformer requires multilayer superconducting structures, which is a significant challenge for high- T c superconducting materials due to the anisotropy of the material and high temperatures required for a deposition of thin films. Chemical-mechanical polishing (CMP) of high- T c superconducting films has been developed in this work to fabricate multilayer structures. CMP improves surface smoothness of films, thereby reducing galvanic shorts between top and bottom superconducting electrodes. It has been shown that edge slope angles of about 2 degree can be fabricated using CMP, meaning that crossovers with very high critical current densities 2*10$ 6 A/cm 2 can be obtained. These results were important for successful fabrication of high-quality multilayer structures with high yield. The CMP technique was used to fabricate multiturn magnetic flux transformers on 10*10 mm 2 STO substrates. A flip-chip magnetometer based on the developed multilayer flux transformer and a bicrystal SQUID was designed, fabricated and characterized. Magnetic field sensitivity of 8 fT/Hz 1/2 at 2 kHz and 80 fT/Hz 1/2 at 10 Hz has been demonstrated. Low-frequency magnetic flux noise was investigated and related with the microstructure of the flux transformer. The developed multilayer flip-chip flux transformer was used in ulf-NMR experiments and demonstrated the improvement in the signal-to-noise ratio (SNR) when compared to a planar SQUID magnetometer. The demonstrated gain in SNR indicates the new multilayer structures are a promising technology for high- T c SQUID-based ulf-MRI systems. Lastly, high- T c superconducting quantum interference filters (SQIFs) were designed and fabricated. The SQIF consisted of array of 50 SQUID loops connected in series along the bicrystal grain boundary. Electrical characterizations revealed a large spread of individual SQUID parameters that lead to the situation when only few SQUIDs from the whole array are operational at a certain bias current and voltage-to-field response demonstrates an absence of voltage dip at the zero external field. At the same time, it has been demonstrated that an array of identical SQUIDs benefit from higher voltage-to-flux transfer function as compared with a single SQUID.

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.

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.

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.

Magnetoencephalography (MEG) enables the study of brain activity by recording the magnetic fields generated by neural currents and has become an important technique for neuroscientists in research and clinical settings. Unlike the liquid-helium cooled low-Tc superconducting quantum interference devices (SQUIDs) that have been at the heart of modern MEG systems since their invention, high-Tc SQUIDs can operate with liquid nitrogen cooling. The relaxation of thermal insulation requirements allows for a reduction in the stand-off distance between the sensor and the room-temperature environment from a few centimeters to less than a millimeter, where MEG signal strength is significantly higher. Despite this advantage, high-Tc SQUIDs have only been used for proof-of-principle MEG recordings of well-understood evoked activity. Here we show high-Tc SQUID-based MEG may be capable of providing novel information about brain activity due to the close proximity of the sensor to the head. We have performed single- and two-channel high-Tc SQUID MEG recordings of spontaneous brain activity in two healthy human subjects. We demonstrate modulation of the occipital alpha rhythm and the mu rhythm found in the motor cortex. Furthermore, we have discovered uncharacteristically high-amplitude activity in the theta-band from the occipital region of the brain. Our results suggest high-Tc SQUIDs can provide a closer look into the brain.

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.