OBSERVATION OF AN ELECTROMAGNETIC ABSORPTION PEAK IN THE MILLIMETER-WAVE RANGE IN LIQUID-HELIUM AT THE SUPERFLUID LAMBDA-TRANSITION

An observation of an electromagnetic absorption peak in the millimeter wave range in liquid helium at the λ transition is reported. The value of the helium-loss tangent at the temperature Tλ of the λ transition is about 10-5. The peak shape is slightly asymmetric under temperature T. There is a sharper drop of absorption at T<Tλ, with a dependence close to t-1, where t=T-Tλ/Tλ, and a flatter one at T>Tλ. The occurrence of this microwave absorption peak at the λ point indicates directly that the superfluid fraction affects kinetic electrical properties. The mechanism of dielectric losses at the λ transition is discussed. © 1995 The American Physical Society.