  DC voltage versus current measurements of superconductors in a magnetic field
are widely interpreted to imply that a phase transition occurs into a state of
zero resistance. We show that the widely-used scaling function approach has a
problem: Good data collapse occurs for a wide range of critical exponents and
temperatures. This strongly suggests that agreement with scaling alone does not
prove the existence of the phase transition. We discuss a criterion to
determine if the scaling analysis is valid, and find that all of the data in
the literature that we have analyzed fail to meet this criterion. Our data on
YBCO films, and other data that we have analyzed, are more consistent with the
occurrence of small but non-zero resistance at low temperature.
