Absolute measurement of penetration depth in a superconducting film by thetwo-coil technique

A two-coil mutual-inductance apparatus that is optimized to allow for the m easurement of the absolute value of penetration depth lambda in superconduc ting films is described. Nb films with thickness d ranging from 20 to 90 nm are used to illustrate the measurement. For a 70-nm-thick Nb film at 4.2 K , with d/lambda similar to 0.6, the uncertainty in the measured lambda is a bout +/- 2.3%. From the results on the Nb film series, we show that a satis factory determination of the absolute value of lambda is possible for these films with d/lambda < 0.95. (C) 1999 American Institute of Physics. [S0003 -6951(99)05146-3].