Y. Gim

One‐dimensional magnetic flux microscope based on the dc superconducting quantum interference device

We have designed and operated a magnetic flux microscope which measures the magnetic field above a sample surface by scanning a 4.2 K thin‐film dc superconducting quantum interference device (SQUID) along one direction. With the SQUID and sample separated by about 160 μm, the system can image features with a spatial resolution of about 220 μm and a magnetic field resolution of 1.8×10−13 THz−1/2. We have used the system to view trapped magnetic flux, to image thin‐film strips of superconducting Pb in field strengths of 0–750 nT, and to monitor the position of a sample with a resolution of approximately 0.5 nm Hz−1/2 at a frequency of 4 kHz.

Magnetic microscopy using SQUIDs

SQUID-based magnetic microscopy involves scanning a sample closely past a low-noise SQUID. With the SQUID held in a flux-locked loop, a computer records the feedback output as a function of sample position and converts the resulting data into a false color image of magnetic field strength. Present systems have achieved spatial resolution down to about 5 /spl mu/m and flux resolution down to about 1 /spl mu//spl Phi//sub 0//Hz/sup 1/2/. They have been used to study the pairing symmetry of the high-T/sub c/ superconductors, for high-frequency imaging, and for a variety of applications related studies. Recently, microscopes have also been developed for high resolution magnetic imaging of room-temperature samples. We briefly describe the design, operation, and capabilities of these systems.

High resolution magnetic microscopy using a DC SQUID

Using a 4.2 K Nb-PbIn DC superconducting quantum interference device (SQUID) with a 60- mu m inner hole side length, the authors have constructed a novel 1-D magnetic flux microscope with an unprecedented combination of spatial and magnetic field resolutions. During imaging, the sample is moved past the SQUID at a separation of about 38 mu m, and the output from the SQUID is recorded as a function of the sample position. The system achieves a spatial resolution of about 66 mu m and a magnetic field resolution of about 5.2 pTHz/sup -1/2/ at a frequency of 6 kHz. The microscope has been used to obtain susceptibility images of patterned superconducting samples in low fields, and a simple method for measuring static magnetic fields has been devised.<<ETX>>

Electric field effect control of a superconducting YBa2Cu3O7 inductor

We discuss the design, fabrication, and testing of a thin‐film superconducting voltage‐controlled inductor which is made from a YBa2Cu3O7 (YBCO) superconducting field effect transistor. Applying voltage to an Au gate layer alters the areal carrier density, and hence the kinetic inductance, of an underlying 100‐nm‐thick YBCO channel layer. The channel is connected in series with an input coil to form a closed superconducting loop. We use a dc superconducting quantum interference device at 4.2 K to measure changes in loop inductance and find a fractional change in the kinetic inductance of about +2.6×10−4/V of applied gate voltage, close to the expected value.

Angular dependence of the symmetry of the order parameter in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//

Using a 4.2 K scanning SQUID microscope, we examined twinned thin-film YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO)-Ag-PbIn SQUIDs and measured the phase of the order parameter in the YBCO for 13 different tunneling angles. We have found that the order parameter in YBCO is time-reversal symmetric and shows a d(x/sup 2/-y/sup 2/)-pairing symmetry, provided the junctions are properly made. Detailed analysis of our data reveals that any time-reversal breaking component is less than 5%, which rules out states such as d(x/sup 2/-y/sup 2/)+ is or d(x/sup 2/-y/sup 2/)+ id/sub xy/.

Experimental determination of the symmetry of the order parameter in YBCO

At present, the symmetry of the order parameter in the high temperature superconductor YBCO is quite controversial. Recent experiments using SQUIDs and Josephson junctions appear to support competing theories, with some experiments supporting a dx2−y2 pairing symmetry for the order parameter and others a s-wave pairing symmetry. We note that a number of factors such as trapped flux, magnetic field gradients and SQUID asymmetries could lead such measurements astray. We use a Scanning SQUID Microscope and a time-reversal invariance test to resolve these experimental problems. We find the order parameter in YBCO has a time-reversal invariant dx2−y2 symmetric component. We estimate the amplitude of anyimaginary s-wave symmetric component to be less than 4% and anyreal s-wave component to be less than 82%.

Using a scanning SQUID to determine the symmetry of the order parameter in YBCO

We have determined the time and spatial symmetry of the superconducting order parameter in YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) by using a Scanning SQUID Microscope to perform a time-reversal invariance test. We find the order parameter in YBCO has a time-reversal invariant d(x/sup 2/-y/sup 2/) symmetric component. In addition, we find that the amplitude of any imaginary isotropic s-wave symmetric component must be less than 6% and any real isotropic s-wave component less than 89%.<<ETX>>