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We fabricated an anode-type ion source driven by a charge repulsion mechanism and investigated its beam shape controlled by a Whenelt mask integrated at the front face of the source. The ion beam shape was observed to vary by changing the geometry of the Whenelt mask. As the angle of inclination of ...

We developed a transverse second-order SQUID gradiometer from a single layer of high T/sub c/ film, which can measure the off-diagonal element d/sup 2/B/sub z//dxdy of the second-order field gradient. In this design, four-way 'clover-leaf' pick-up loops are coupled directly to a 4-junction dc SQUID in such a way that the coupling polarity of the two diagonal loops is opposite to that of the other two loops. The pickup loops are intrinsically balanced for both uniform field and the 1st-order gradient. We made the device by using a pulsed-laser deposition and Ar ion-milling with photolithography. The entire structure was made on a single crystalline SrTiO/sub 3/ substrate. The gradiometer responded sensitively to the 2nd-order gradient. Induced field focused on the SQUID loop by the screening current flowing along the device perimeter is believed the major off-balancing factor, which can be balanced by adding a closed superconducting loop just outside the device perimeter or merging facing sides of neighboring loops.

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.

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.

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.

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.

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.

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.

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.

We have studied hysteresis of the grain boundary YBa/sub 2/Cu/sub 3/O/sub 7/ Josephson junctions with Au film shunt capacitors. The capacitive shunt clearly developed hysteresis in the current-voltage curves of the junctions. However, the McCumber parameter /spl beta//sub c/, obtained from data was much smaller than the calculated values. The reason is believed to be due to the fact that the capacitive reactance was dominated by the kinetic inductance of the bridge and the series resistance of the Au film connecting the two capacitors structured on both sides of the electrodes. Using a superconducting capacitor built directly on the bridge will enhance the hysteresis. More study is needed on the inductive shunt in the junction.

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.

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

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.

The effect of photolithographic treatments on the surface of a YBa2Cu3O7-δ thin film has been investigated by measuring the electrical contact resistance and performing X-ray photoemission spectroscopy (XPS) experiment. Water plays an important role in increasing the electrical contact resistance of the YBa2Cu3O7-δ thin film, as compared with the developer and photoresist (PR); water removes oxygen from the surface and increases the contact resistance. The chemical interaction with the PR or developer is not a key element in increasing the contact resistance. XPS spectra show that the oxygen concentration of the surface of a YBa2Cu3O7-δ thin film is reduced after the photolithographic process.