The fields of condensed-matter physics and materials
science entered
a new era with the discovery of high-temperature
superconductors more than two
decades ago. However, despite the
large volume of work published on their properties,
and despite
their many uses, there is surprisingly little
agreement in the
condensed matter community regarding one of their
most fundamental
behaviors: the normal-superconducting phase
transition. This book describes investigations of
the superconducting
phase transition of YBCO thin films in zero magnetic
field. Simple
voltage vs. current curves at temperatures close
to the transition temperature show that the
standard scaling analysis
fails at low currents, contrary to what is widely
accepted -- but that there is
evidence for the phase transition at higher currents.
At lower currents, there are significant finite-size
effects due to
the thickness of the films. The crossover to
from three-dimensional to two-dimensional
behavior occurs even in thick films, and
obscures the transition in zero field. This
effect would also occur in a magnetic field, and may
explain the
wide range of critical exponents found in the literature.