  Creating complex flux configurations by superposing a dc current or magnetic
field onto the ac current in a type II superconducting tape should lead to a
variety of peculiar behaviors. An example is the appearance of the Clem valley,
a minimum in the ac losses as a function of the dc bias amplitude, which has
been theoretically studied by LeBlanc et al., in the continuation of Clem's
calculations. These situations have been investigated by applying a dc current
to a silver-gold sheathed Bi-2223 tape at 77 K (critical current 29 A), in
addition to the usual ac transport current. The ac losses were measured by the
null calorimetric method to ensure that the total losses were being accounted
for. These were recorded for different values of the ac and dc currents,
leading to the observation of two different behaviors depending on the ac
current. Our revelation of the Clem valley is, to our knowledge, the first
experimental validation of this phenomenon in high temperature superconductors,
and may provide a simple way of reducing the ac loss in industrial applications
of these materials.
