Optimization of large-area single-layer flux-transformers and concentrators coupled to RF-SQUIDs in flip-chip geometry

The effect of layout modifications of large-area single-layer YBa/sub 2/Cu/sub 3/O/sub 7/ flux transformers onto the effective area and system noise has been investigated systematically and the excellent performance of sputtered large-area flux transformers is demonstrated. First, the gain of sensitivity and noise contribution of the transformer (coupled to a 8/spl times/8 mm/sup 2/ washer rf-SQUID) was determined as a function of the line width of the YBa/sub 2/Cu/sub 3/O/sub 7/ pickup loop w/sub p/ ranging between 1 and 8.2 mm for 1" transformer (O=25.4 mm) and 1 and 19 mm for 2" transformer (O=50.8 mm), respectively. Leaving the diameter of the pickup coil (b/sub 1/=23 mm, b/sub 2/=46 mm) and the coupling coil (d=6 mm, width=1.2 mm) unchanged, the gain in sensitivity increases from 2.1 for w/sub p/=1 mm to 2.7 for w/sub p//spl ges/6 mm in the case of 1" transformers and from 3.3 for w/sub p/=1 mm to 4.6 for w/sub p/=19 mm for 2" transformers. No additional noise contribution of the transformer could be observed, i.e. a flux noise of 70 /spl mu//spl Phi//sub 0//spl radic/(Hz) was determined above 5 Hz and at 77 K. The best values for the field-to-flux conversion efficiency and the effective area were 0.74 nT//spl Phi//sub 0/ and 2.98 mm/sup 2/ for 1" and 0.43 nT//spl Phi//sub 0/ and 4.94 mm/sup 2/ for 2" transformers, respectively. The comparison of the experimental data with the theory indicates, that the expressions derived for low-T/sub c/ superconducting planar devices have to be modified in the case of high-T/sub c/ devices. Second, the performance of the sputter deposited large-area devices is demonstrated for the case of an axial gradiometer. The white noise level of the whole gradiometer system in an unshielded environment was <100 fT//spl radic/(Hz) for frequencies above 5 Hz and the common mode rejection of the homogeneous field was measured to be better than 10/sup 4/. Real-time magnetocardiographic signals obtained via this gradiometer in a magnetically unshielded environment show a large signal-to-noise ratio.