MUTUAL-INDUCTION MEASUREMENT OF THE AC PENETRATION DEPTH IN HTSCS - THEORY OF CALIBRATION FUNCTION FOR FLAT SAMPLES UNDER AXIAL SYMMETRY

A contribution to find optimum mutual-induction arrangements for inves tigating the linear AC response properties of the pinned vortices in t he mixed state of HTSC is given. In particular, the complex calibratio n function (mutual-induction coefficient L(12) as a function of the co mplex AC penetration depth lambda) is analyzed, strictly axial symmetr y and flat sample geometries provided (finite disk and its limiting ca se of infinite platelet) assuming the radius of at least one induction coil is smaller than the sample radius. Because the sample is positio ned transverse to the AC induction B, the mutual inductance becomes, i n general, strongly modified by distortions of B around the sample, wh ich results in a strong dependence of L(12)(lambda) upon certain geome try parameters and an enhanced sensitivity to lambda for selected \lam bda\ intervals. The analysis is based upon powerful analytic approxima tion formulae for elementary arrangements derived in a separate paper. Also, formulae to estimate the errors from irregular (eccentric) samp le shapes, from the field enhancement near the disk edges as well as f rom treating the finite-disk problem in the limit of an infinite plate let are given. Except for ultrathin films with thickness much smaller than \lambda\, arrangements with both the coils at the same sample sid e and with the loop radii sufficiently smaller than the disk radius ar e favored for measurements of small lambda with \D/lambda\ greater tha n or equal to 1 (D is the sample thickness), which also includes the p ossibility of a lambda measurement in the bulk limit \D/lambda>\ --> i nfinity