Yunseok Hwang

DIRECT-COUPLED SECOND-ORDER SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE GRADIOMETER FROM SINGLE LAYER OF HIGH TEMPERATURE SUPERCONDUCTOR

We developed a noble design of the planar-type single-layer second-order superconducting quantum interference device (SQUID) gradiometer, and demonstrated that the gradiometer did not respond to a uniform field or the first-order gradient of the field but responded sensitively to the second-order gradient. The device consisted of three parallel-connected pickup loops, each of which is directly coupled to the step-edge junction SQUID. The entire structure was made from a single layer of YBa2Cu3O7 patterned by photolithography with ion milling technique. Response of the device to the field was tested with three identical wire-wound coils coupled to each loop and balancing was achieved by using a small piece of superconductor placed in the center loop. Measured off balance was about 0.6% for the uniform field and 1.4% for the first-order gradient, which were believed not due to intrinsic imbalance but mostly due to slight difference in alignment of the test coils.

Fabrication of MgB2 thin film by rf magnetron sputtering

This work was supported by Korea Research Foundation Grant (KRF-2001-041-D00073) and also by KOSEF research project no. R01-2001-00038.

Improvement of the balance of the single-layer second-order high Tc SQUID gradiometer

Abstract We have studied detailed balancing of the directly coupled second-order YBa 2 Cu 3 O 7 superconducting quantum interference device gradiometer by adjusting the line width of the center pickup loop with all other parameters fixed. We obtained a balancing factor of 1.7×10 3 for the best-balanced device. Measured gradient noise level was 2 pT/cm 2 /√Hz at 10 Hz and the corresponding field noise was 200 fT/√Hz.

Balancing of the single-layer second-order high-T/sub c/ SQUID gradiometer

We have designed a prototype single-layer second-order high-T/sub c/ SQUID gradiometer that detects the transverse field gradient, and balanced the device by adopting a step-down method. The device consists of symmetrically designed 3 parallel-connected square pickup loops that are directly coupled to the SQUID. The dimensions of side loops are 3 mm outside and 1.8 mm inside. The inner dimension of the center loop is 1 mm. Step-down balancing was achieved by varying the outer dimension of the center loop until a null response was detected under uniform fields. The uniform field was generated by a 250 mm/spl times/25 mm square coil wound on a grooved garolite tube. Deviation of the coil field along the abscissa was calculated to be less than 1/spl times/10/sup -5/ within the sample range. We have investigated response of the device to field gradients along the transverse direction. Details of the design, fabrication, and results will be discussed.

Gap symmetry and critical current of YBa2Cu3O7 step-edge Josephson junctions

Effects of d-wave symmetry on the critical current of high Tc step-edge Josephson junctions were studied theoretically by using the concept of grain-boundary junctions, and the theory was compared with experimental results of YBa2Cu3O7 step-edge junctions on SrTiO3 (100) substrates. A step-edge junction contains two grain boundaries, and each grain boundary has both in-plane and out-of-plane misorientation angles at the same time. The critical current of the step-edge junction as a function of the in-plane misorientation angle has a periodic modulation with maxima when the step line is parallel to one of the major axes of the SrTiO3 (100) substrate. Experimental results of c-oriented YBa2Cu3O7 step-edge junctions with various in-plane misorientation angles were qualitatively in good agreement with the theory. The out-of-plane misorientation angle is formed between two grains usually with the c axes perpendicular to each other and is normally not controllable.

Effects of step conditions on the properties of YBCO step-edge Josephson junctions

We have studied transition properties of YBa/sub 2/Cu/sub 3/O/sub 7/ step-edge junctions as a function of the angle of the step-line with respect to the major axes of the SrTiO/sub 3/ substrate. Substrate steps were prepared by Ar ion milling with photoresist mask, and the YBa/sub 2/Cu/sub 3/O/sub 7/ film was deposited by pulsed laser ablation and patterned by ion milling to form junctions. The critical temperature of the junctions was independent of the angle. However, the critical current of the junctions showed a modulation with the angle. A maximum was observed for the step-line oriented parallel to the major axes of the substrate and a minimum for the step-line oriented 45/spl deg/ against the axes. The modulation of the critical current is believed to be caused by the difference in the microscopic structure of the epitaxially grown YBa/sub 2/Cu/sub 3/O/sub 7/ film at the step and also symmetry of the high Tc superconductor.

The effects of annealing step-edges on SrTiO/sub 3/ and MgO single-crystal substrates on YBa/sub 2/Cu/sub 3/O/sub 7/ Josephson junctions

The effect of annealing step-edges on SrTiO/sub 3/ and MgO single-crystal substrates on Josephson junctions of YBa/sub 2/Cu/sub 3/O/sub 7/, has been studied. The step-edge was fabricated by argon-ion milling technique and was annealed at 1050/spl deg/C in oxygen atmosphere. YBa/sub 2/Cu/sub 3/O/sub 7/ thin film was deposited on the annealed step-edge by a standard pulsed laser deposition. The effect of annealing the step-edge on the junction was characterized by AFM and current-voltage (I-V)characteristic. The annealed step-edge on SrTiO/sub 3/ and MgO substrates showed that the surface of the substrates was smoother and I-V characteristic of Josephson junction improved.

Effects of d-wave symmetry in high Tc step-edge Josephson junctions

Abstract We have studied critical currents of YBa2Cu3O7 step-edge junctions with different step-line angles with respect to the major axes of SrTiO3 (100) substrates. Measurements of the junction critical current indicated a periodic modulation with the step-line angle: maxima near 0° and 90° degree and minima near 45° and 135°. These results are in qualitative agreement with d-wave coupling superconductivity at the step-edge grain-boundary of the junctions.

Characteristics of the planar second-order high-Tc SQUID gradiometer

We have studied the planar-type single-layer second-order high-Tc SQUID gradiometer that detects the transverse field gradient, d2Bz/dx2. The gradiometer consists of three parallel-connected rectangular pickup loops that are directly coupled to a SQUID. The pickup loops are designed in such a way that the SQUID measures the difference in the differential screening currents of neighbouring pickup loops. The SQUID has either step-edge or bicrystal Josephson junctions. The gradiometer was made from a single layer of YBa2Cu3O7 film on a 1 cm × 1 cm chip. The response of the gradiometer was tested in various flux distributions generated by three identical multiturn film coils patterned on the same chip in parallel with each pickup loop. The balanced gradiometer responded sensitively only to the second-order field gradient. Reduction of the inductive coupling between loops improved the intrinsic balance.

Second-order dc SQUID gradiometer from single layer of high temperature superconducting film

We developed a new design of the planar-type single-layer second-order SQUID gradiometer. The device consisted of 3 parallel-connected pickup loops, each of which is directly coupled to the step-edge junction dc SQUID. The device was made from a single layer of YBa/sub 2/Cu/sub 3/O/sub 7/ film patterned by photolithography with ion milling technique. Response of the device to various field distributions was tested, with 3 identical wire-wound coils coupled to each loop. Balancing was achieved by using a small piece of superconductor placed in the center loop. The gradiometer responded sensitively to the second-order field gradient, but did not respond to a uniform field or the first-order gradient. Measured off-balance was about 0.6% for the uniform field and 1.4% for the first order gradient.