4PO9-40 Chemomagnetism: Characterization of Chemical Reactions Using High-T_c SQUIDs

James R. Claycomb 1, Mikael Nersessian 2, J.T. Ritchie 2, John H. Miller, Jr. 1, and Dan Luss 2. 1 Department of Physics and Texas Center for Superconductivity, University of Houston, Houston TX 77204-5932. 2 Department of Chemical Engineering, University of Houston, Houston TX 77204-5932.

Presenting Author: J.R. Claycomb

We have recently used high-T_c superconducting quantum interference devices (SQUIDs) to characterize a new phenomenon we call chemomagnetism. High-T_c SQUIDs can detect magnetic, or chemomagnetic, fields produced by a variety of chemical reactions, including solid-gas reactions (metal oxidation during solid combustion), solid-liquid reactions (between metals and acids or bases), liquid-liquid (acid-base reactions), and replacement reactions (liquid-solid and liquid-liquid). Slowly varying magnetic signals, which become permanent upon cooling, have also been recorded during the synthesis of ferromagnetic materials.

In all of the reactions studied, convection and diffusion of ions appear to contribute to the observed complex magnetic behavior. Additional contributing factors include: (1) the formation of precipitate layers on solid reactants (during liquid-solid reactions), (2) the thermal emission of electrons and ions (during solid state reactions), and (3) the formation of magnetic domains upon cooling (during synthesis of ferromagnetic materials). The relation between electric and magnetic fields produced in liquid-liquid and liquid-solid reactions may be described by the Nernst-Planck equation. The observed magnetic time series and frequency spectra exhibit qualitatively different behaviors, depending on the specific chemical reactions and reactant concentrations. SQUIDs may thus distinguish between different chemical reactions, and may prove to be important tools for studying the underlying dynamics of chemical reactions.

4PO9-41 High-T_c SQUID-Based Impedance Spectroscopy of Living Cell Suspensions

Camelia Prodan 1, Emil Prodan 2, James R. Claycomb 1, and John H. Miller, Jr. 1. 1 Department of Physics and Texas Center for Superconductivity, University of Houston, 3201 Cullen Blvd., Houston TX, 77204-5932. 2 Department of Physics, Rice University, P.O. Box 1892, Houston TX, 77251-1892.

Presenting Author: C. Prodan

We have recently demonstrated the use of a high-T_c SQUID to measure the ac dielectric response of living cell suspensions. The development of quantitative methods of characterizing living matter is of considerable interest in biophysics, pharmacology, and medicine. It has recently been proposed that the membrane potential of living cells can be determined by measuring the ac response of living cell suspensions at low frequencies (0-10 kHz) and low electric field amplitudes. Characterization of living matter via dielectric response, or impedance spectroscopy, measurements is a well-known problem in biophysics. However, despite all attempts, no such measurements have been carried out with sufficient accuracy at low frequencies. SQUIDs provide a solution, since their extraordinary sensitivity enables one to work in the limit of weak electric field amplitudes. This noninvasive feature allows the study the properties of living cells without disturbing the underlying biological processes.

The potential applications are numerous, and can be divided into two classes. The first is the study of living cells' responses to various stress factors (great value to industry). The second lies in medicine and pharmacology. These are concerned with the detection of pathologies in human or animal tissues, and the study of cell reactions to various drugs.

4PO9-42 The nonlinearity of the High-T_c rf SQUID System

D.F. He 1, Y. Zhang 2, and Y.D. Dai 1. 1 Department of Physics, Peking University, Beijing 100871. 2 Institut fur Schicht-und Ionentechnik (ISI), Forschungszentrum Juelich, D-52415 Juelich, Germany.

Presenting Author: D.F. He

The nonlinearity of SQUID system can be described by the total harmonic distortion (THD) of the output signal for an applied sinusoidal signal. For our YBCO High-T_c rf SQUID system operated in a flux-locked loop without modulation, we measured the THD by analyzing the error signals between input and output, by this way, the influence caused by the harmonics of input signal could be decreased. The result of our measurements showed that the THD had strong relation with the flux-locked point and decreased with the signal amplitude and frequency. For input signals at frequency up to 250Hz and amplitude up to 100F ₀, the THD was below 10ppm. We also observed some anomalous behaviors of the THD, such as, the abnormal curve of THD versus frequency for the input signal at amplitude of about 30F ₀. A qualitative explanation for these phenomena was given in this paper.

4PO9-43 High-T_c Bicrystal rf SQUID

F.X. Xie, J.C. Wang, D.F. He, R.J. Nie, T. Yang, P. Ma, L.Y. Liu, Y.D. Dai, S.G. Wang, and S.Z. Wang, Department of Physics, Peking University, Beijing 100871, P.R. China

Presenting Author: D.F. He

We fabricated rf superconducting quantum interferometer devices (SQUIDs) using YBCO bicrystal junction, which was made on a chip of 10 x 12mm² YSZ bicritical substrates with a 24 degree angle of in-plane misorientation. We characterized our rf SQUIDs immersed in liquid N₂ and the magnetic field sensitivity was 84fT/Hz^1/2. The tank frequency at which rf SQUID operated was near 800MHz. According to our result, the boundary across the superconducting loop did not show significant effect on the 1/f noise level.

4PO9-44 Temperature Dependent Characteristics of Step-edge YBCO SQUIDs

H.E. Horng 1, S.Y. Yang 1, Chun-Hui Chen 1, J.T. Jeng 1, and H.C. Yang 2. 1 Department of Physics, National Taiwan Normal University, Taipei 117, Taiwan. 2 Department of Physics, National Taiwan University, Taipei 106, Taiwan.

Presenting Author: H.-.E Horng

In this report, the temperature dependencies of the characterizations such as the critical current (I_c-T curve) and the magnetically modulated voltage oscillation, are investigated for the step-edge YBCO SQUID. According to the experimental results, a linear relationship between the I_c and T was found. This reveals a SIS-like tunneling behavior for the step-edge SQUID. On the other hand, the peak-to-peak voltage V_pp in the V-F curve of the SQUID was found to decrease almost linearly when the temperature was raised from 10 K to 60 K. By keeping increasing the temperature close to the critical temperature of the SQUID, the V_pp was reduced abruptly. This abrupt reduction in V_pp is believed to be dominated by the variation of the inductance of the SQUID at various temperatures. All the details will be discussed.

4PO9-45 Voltage Modulation of The High-T_c SQUIDs With Step-Edge Junctions

H.C. Yang 1, J.T. Jeng 2, Y.C. Liu 2, and H.E. Horng 2. 1 Department of Physics, National Taiwan University, Taipei 106, Taiwan. 2 Department of Physics, National Taiwan Normal University, Taipei 117, Taiwan.

Presenting Author: H.C. Yang

We studied the voltage modulation of YBa₂C₃uO₇ SQUIDs by using single-layered YBa₂C₃uO₇ films deposited on step-edged substrates. The V-I characteristic curves of the fabricated SQUIDs exhibited RSJ like behavior with an asymptotic SQUID resistance of 1~5 W . It was found that the voltage modulation amplitude V_pp of the SQUID correlated to the dynamic resistance R_d = dV/dI at various bias currents. In addition, there were two peaks in the R_d-I curve, which corresponded to the observed kinks (or peaks) in the V_pp-I curve. Similar kinks were also found in the relation of voltage noise density versus the bias current for the step-edge junctions. These kinks may be attributed to the intrinsic two grain boundaries in the step-edge junctions of the SQUIDs.

4PO9-46 Advancing on the Yield-rate and Performance of High-T_c rf SQUID

P. Ma, T. Yang, S.Z. Wang, L.Y. Liu, R.J. Nie, F.X. Xie, D.F. He, S.G. Wang, and Y.D. Dai, Department of Physics, Peking University, Beijing 100871

Presenting Author: P. Ma

Successful implementation of high temperature superconducting electronics based on Josephson junctions requires the reproducible fabrication of junctions with appropriate and identical electrical properties. Other important requirements are the ability to operate in a fix temperature. It has been shown in our recent work, the step-edge junctions can be used for rf SQUIDs, which being with promising performance at liquid nitrogen temperature. But for rf SQUIDs with an I_c of several m A and optimum performance at 77K, to get a high yield-rate is very difficult. Special for gradiometer, the requirement for rf SQUIDs with the same performance is not easy to be fulfilled. In order to optimize the high T_c rf SQUID performance, a method to control and determine the critical current I_c of a step-edge junction has been developed. The process not only pay attention on the creation of sharp and straight step edges on a substrate, but also emphasizes on quality and thickness of superconducting materials at step edges. Using this method, critical current of the step-edge junctions are easily adjusted within a range which is useful for obtaining high performance rf SQUID magnetometer or gradiometer. The magnetic field noise of our rf SQUID can easily reach the level of less than 100fT/Hz^1/2.

4PO9-47 Evaluating the Entry of Vortices into a Slotted High-T_c SQUID Magnetometer with a Double-Pickup-Coil Configuration

Koichi Yokosawa 1, Hiroshi Oyama 2, Shinya Kuriki 2, Daisuke Suzuki 1, and Keiji Tsukada 1. 1 Central Research Lab., Hitachi Ltd., P.O. Box 2, Kokubunji, Tokyo 185-8601, Japan. 2 Research Institute for Electronic Sciences, Hokkaido Univ., Sapporo 060-0812, Japan.

Presenting Author: K. Yokosawa

For various applications of a high-T_c SQUID magnetometer, especially for measuring biomagnetic fields, low noise at low frequencies is needed. One source of the noise is thermal motion of vortices trapped in the superconducting films of the device composed of the SQUID and pickup coil. To eliminate the entry of vortices into the films, a device design with narrow line-width superconducting films has been proposed recently.

We previously proposed a double-pickup-coil configuration for direct-coupling high-T_c SQUID magnetometers. In this configuration, two pickup coils are connected directly to one SQUID on a substrate. We have since fabricated a device with this configuration whose superconducting films (YBCO) have 5 x 5 microns holes and 20 x 5 microns slits with a separation distance of 5 microns. We evaluated the entry of flux vortices into the device under different magnetic fields, observing the voltage-flux characteristics of the SQUID. The entry of vortices was detected from the continuous entry of flux into the SQUID while the bias current and the magnetic field were held constant. The vortex entry was observed when the absolute value of the magnetic-field increase exceeded a threshold of 7 micro-tesla, regardless of whether the device was previously zero-field-cooled or field-cooled.

4PO9-48 The Excess Current Existing in High-T_c Superconducting Step-edge Josephson Junction

T. Yang, P. Ma, S.Z. Wang, L.Y. Liu, R.J. Nie, F.X. Xie, D.F. He, S.G. Wang, and Y.D. Dai, Department of Physics, Peking University, Beijing 100871

Presenting Author: T. Yang

Due to a remarkable simplicity of fabrication and low 1/f junction noise, step-edge junctions are currently the most widely used as Josephson devices to make SQUIDs. However, the excess current comparable with I_c is often observed on the current-voltage characteristics even that width of junction is smaller than the Josephson penetration depth. The other hand, the yield-rate of rf SQUID fabricated by step-edge junction technology is very lower. It is well known, the RSJ model is a traditional model to describe a superconducting device made of Josephson junction, in which, practical Josephson junction is expressed as I_csinf and a shunted resistance R_n. In this paper, we discuss the affection of excess current on performance of SQUID, and point out the probably reason causing excess current in the step-edge junctions.

4PO9-49 Long baseline high-T_c first-order planar SQUID gradiometers fabricated on large LaAlO₃ substrates

K. Chen, Y.J. Tian, L. Chen, T. Yang, P.J. Wu, D.N. Zheng, and Z.X. Zhao, National Laboratory for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China

Presenting Author: D.N. Zheng

High-T_c planar dc SQUID gradiometers with integrated antenna have been designed and investigated. The dc SQUID is based on step-edge junctions. The whole gradiometer structure is made on LaAlO₃ substrates. The length of the substrates ranges from 10 to 40 mm and all has a width of 10 mm. The longest baseline obtained is 35 mm. Critical current, voltage response to magnetic field, and noise characteristics was investigated at 77 K. Magnetic field gradient sensitivity of 100 fT_cm^-1Hz^-1/2 at 1 kHz was obtained. All gradiometers could work stable in unshielded and disturbed environments. The gradiometers were tested for non-destructive evaluation and magnetocardiograms measurements. Results obtained by gradiometers with different baseline length are compared.