  Superconducting niobium film cavities deposited on copper substrates (Nb/Cu)
have suffered from strong field-dependent surface resistance, often referred to
as the Q-slope problem, since their invention. We argue that the Q-slope may
not be an intrinsic problem, but rather originates from a combination of
factors which can be revealed in appropriate environmental conditions. In this
study, extrinsic effects were carefully minimized in a series of experiments on
a seamless cavity. The origin of the Q-slope in low frequency cavities is
traced back to two contributions with different temperature and magnetic field
dependences. The first component of Q-slope, affecting the residual resistance,
is caused by trapped magnetic flux which is normally suppressed by a magnetic
shield for bulk niobium cavities. The second, temperature dependent component
of Q-slope, is similar to the medium-field Q-slope which is well known in bulk
niobium cavities. These results are compared with theoretical models and
possible future studies are proposed.
