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

摘要

A contribution to find optimum mutual-induction arrangements for investigating the linear AC response properties of the pinned vortices in the mixed state of HTSC is given. In particular, the complex calibration function (mutual-induction coefficient L(12) as a function of the complex AC penetration depth lambda) is analyzed, strictly axial symmetry and flat sample geometries provided (finite disk and its limiting case of infinite platelet) assuming the radius of at least one induction coil is smaller than the sample radius. Because the sample is positioned transverse to the AC induction B, the mutual inductance becomes, in general, strongly modified by distortions of B around the sample, which results in a strong dependence of L(12)(lambda) upon certain geometry parameters and an enhanced sensitivity to lambda for selected lambda intervals. The analysis is based upon powerful analytic approximation formulae for elementary arrangements derived in a separate paper. Also, formulae to estimate the errors from irregular (eccentric) sample shapes, from the field enhancement near the disk edges as well as from treating the finite-disk problem in the limit of an infinite platelet are given. Except for ultrathin films with thickness much smaller than lambda, arrangements with both the coils at the same sample side and with the loop radii sufficiently smaller than the disk radius are favored for measurements of small lambda with D/lambda greater than or equal to 1 (D is the sample thickness), which also includes the possibility of a lambda measurement in the bulk limit D/lambda> --> infinity [References: 45]