Marko Banzet

Substrate resonator for HTS rf SQUID operation

Abstract We describe the use of single substrates as resonators for radio-frequency (rf) superconducting quantum interference devices (SQUID) operation. A standard strontium-titanate substrate with dimensions 10×10×1 mm 3 serves as a tank circuit (resonator), a YBCO thin film SQUID washer structure is patterned on it. On the resonator substrate, a small rf washer SQUID with a step-edge junction is positioned in flip-chip geometry, thus forming a magnetometer sensor. This resonator is inductively coupled to the readout electronics. At 77 K, the field sensitivity of this SQUID magnetometer achieved 24 fT/ Hz in the white noise range, 38 fT/ Hz at 10 Hz, and 83 fT/ Hz at 1 Hz.

Planar HTS gradiometers with large baseline

We have designed an HTS single-layer planar rf double-hole gradiometer with long and well defined baseline up to 5 mm and small washer areas. We measured a gradient field resolution of about 1 pT/(cm/spl middot//spl radic/Hz) above 10 Hz in an unshielded environment. The gradiometer could be operated placed on a swinging pendulum in the geomagnetic field. This permitted a simple demonstration of a movable SQUID system. We placed a SQUID system on a scanning table and performed two-dimensional eddy-current nondestructive evaluation measurements on realistic samples.

Multi-channel HTS rf SQUID gradiometer system recording fetal and adult magnetocardiograms

We report on experiments with a multi-channel HTS radio-frequency (rf) SQUID gradiometer for recording fetal and adult magnetocardiograms. Four sensing SQUID magnetometers and two common reference SQUID magnetometers form a 4 channel electronic gradiometer system of either first or second order. The magnetometers consist of HTS step edge SQUIDs and flux concentrators fabricated from YBaCuO thin films, with dielectric substrate resonators serving as tank circuits. With a washer area of 18 mm in diameter, all six magnetometers reached a field sensitivity of 20-30 fT//spl radic/Hz. Each gradiometer channel is formed using two or three such magnetometers with individual readouts in electronic difference. The dc and rf crosstalk between any channel pair was measured. In ordinary operation we did not find any noise contribution from neighboring channels, even though the resonant frequencies of the resonators are closely spaced. In a standard magnetically shielded room, using a first-order gradiometer configuration with an ultra-long baseline of about 20 cm, we demonstrated 4 channel real-time heart signal recordings of a fetus in the 33rd week of gestation.

A high-temperature rf SQUID system for magnetocardiography

A first-order axial electronic gradiometer having a baseline of 10 cm was constructed by assembling two rf SQUID magnetometers with coplanar tank resonators, each having a white magnetic field resolution of about at 77 K. The gradiometers near-field resolution was about , including the Dewar flasks noise. A peak-to-peak noise level of 3 pT was obtained in the bandwidth 0.016-250 Hz. Magnetocardiographic (MCG) measurements were performed using this bandwidth. Measurements on human subjects have been conducted in a magnetically shielded room of moderate shielding factor. Using the signal either of the lower magnetometer or of the gradiometer, high-quality heart signal traces could be collected, which were suitable for diagnostic use. A team of physicians, assisted by two of the authors, used the equipment over 10 months to perform MCG measurements in a medical study of about 80 clinical patients with cardiac arrhythmia problems and healthy persons. The systems performance was stable over that whole period.

HTS SQUID gradiometer using substrate resonators operating in an unshielded environment - a portable MCG system

We have demonstrated and verified the basic feasibility of performing magnetocardiographic (MCG) measurements without magnetic shielding when using a first-order electronic gradiometer with our novel dielectric substrate resonator rf SQUIDs. The setup at the operation site involved adjustment of the gradiometers baseline length and adaptive balancing. Our experimental portable system was tested in three environments differing in the level of electromagnetic interference.

Effects of the superconductivity transition on the response of YBCO edge transition bolometers

Dependence of the phase and magnitude of the response of Y–Ba–Cu–O edge transition bolometers on the superconducting transition is studied. The responses of both large and small area devices were investigated and several anomalies are observed. The response of small area LaAlO3 devices considerably differed from that expected based on the dR/dT curve. This discrepancy is observed to be strongly dependent on the superconducting transition. Both the phase and magnitude/(dR/dT) of the response of the devices showed abrupt changes for below the Tc-onset when measured versus temperature, while the phase variation also showed strong dependence on the modulation frequency. We present the analysis and propose mechanisms responsible for the modulation frequency dependence of the response characteristics versus temperature, within the superconductivity transition region of the devices.

HTS rf SQUIDs with fully integrated planar tank circuits

For the optimization of the operation of rf-SQUIDs, a high pumping frequency f/sub 0/ and a high quality factor Q of the tank circuit are desirable. For this purpose we used a washer rf SQUID or a current-injection loop SQUID integrated with a modified planar hairpin resonator around the SQUID. On LaAlO/sub 3/ substrates, we measured f/sub 0/ at resonance in the range of about 550 MHz to 1.1 GHz and unloaded quality factors Q/sub 0/ from 4000 to about 6000 using several layouts with different diameters. We found that the coupling coefficient k between the SQUID and the resonator can be estimated from the geometrical parameters of the layout. The requirement k/sup 2/Q>1 can easily be met with our design. Our layouts are very flexible, as they make possible the implementation of various single and multilayer antennas. Using a single layer flux transformer, 13 mm in outer diameter, together with a SQUID in the flip chip technique, we obtained a white flux noise of 50 fT//spl radic/Hz for a 150 pH SQUID.

The set-up of a high temperature superconductor radio-frequency SQUID microscope for magnetic nanoparticle detection

SQUID (superconducting quantum interference device) microscopes are versatile instruments for biosensing applications, in particular for magnetic nanoparticle detection in immunoassay experiments. We are developing a SQUID microscope based on an HTS rf SQUID magnetometer sensor with a substrate resonator. For the cryogenic set-up, a configuration was realized in which the cryostat is continuously refilled and kept at a constant liquid nitrogen level by an isolated tube connection to a large liquid nitrogen reservoir. The SQUID is mounted on top of a sapphire finger, connected to the inner vessel of the stainless steel cryostat. The vacuum gap between the cold SQUID and room temperature sample is adjusted by the precise approach of a 50??m thin sapphire window using a single fine thread wheel. We investigated possible sensing tip configurations and different sensor integration techniques in order to achieve an optimized design. A new scheme of coupling the rf SQUID from its back to a SrTiO3 substrate resonator was adopted for the purpose of minimization of the sensor-to-sample spacing. By SQUID substrate thinning and washer size reduction, the optimum coupling conditions for back coupling were determined for different rf SQUID magnetometers prepared on LaAlO3 and SrTiO3 substrates. The SQUID microscope system is characterized with respect to its spatial resolution and its magnetic field noise. The SQUID microscope instrument will be used for magnetic nanoparticle marker detection.

1/f noise characteristics of SEJ Y-Ba-Cu-O rf-SQUIDs on LaAlO/sub 3/ substrate and the step structure, film, and temperature dependence

Step edge junction (SEJ) rf-SQUID magnetometers and gradiometers were fabricated using PLD Y-Ba-Cu-O films on LaAlO/sub 3/(100) and SrTiO/sub 3/(100) substrates. Effects of different step structure and the film properties on the yield, optimal operating temperature, and the 1/f noise of the SQUIDs were investigated. The step structure was controlled using various IBE processes. The devices on LaAlO/sub 3/ showed higher sensitivity to the step structure compared to those on SrTiO/sub 3/. This was due to re-deposition of substrate material at the steps prepared using the conventional IBE process resulting in a very low yield of unstable SQUIDs. High yield of low 1/f noise stable SQUIDs was obtained on LaAlO/sub 3/ substrates with sharp steps prepared using an optimized IBE process. A typical 1/f noise corner frequency of about 10 Hz at 77 K with two major temperature dependencies was obtained. The temperature dependencies of the 1/f noise could be correlated to the junction and the film of washer area of the SQUIDs. The white noise of our devices showed a dependency mainly on the amplitude of the flux to voltage transfer function signal. The operating temperature range of the SQUIDs could be controlled by the step structure and narrowed when the optimal operating temperature range was increased. All the measured junctions of our devices on the modified steps showed RSJ type behavior with a moderate decrease of the R/sub N/ versus temperature.

Nanocavity crossbar arrays for parallel electrochemical sensing on a chip

Summary We introduce a novel device for the mapping of redox-active compounds at high spatial resolution based on a crossbar electrode architecture. The sensor array is formed by two sets of 16 parallel band electrodes that are arranged perpendicular to each other on the wafer surface. At each intersection, the crossing bars are separated by a ca. 65 nm high nanocavity, which is stabilized by the surrounding passivation layer. During operation, perpendicular bar electrodes are biased to potentials above and below the redox potential of species under investigation, thus, enabling repeated subsequent reactions at the two electrodes. By this means, a redox cycling current is formed across the gap that can be measured externally. As the nanocavity devices feature a very high current amplification in redox cycling mode, individual sensing spots can be addressed in parallel, enabling high-throughput electrochemical imaging. This paper introduces the design of the device, discusses the fabrication process and demonstrates its capabilities in sequential and parallel data acquisition mode by using a hexacyanoferrate probe.