Condensed matter physics has seen its fair share of landmark experiments. In the field of high temperature superconductivity, there is one series that stands out among the rest, however. These are the Josephson Interference experiments conducted in the early 90s by van Harlingen and co-workers. These investigations were technically challenging, and some hard work no doubt went into the experimental design.
To understand the impact of the experiments, a little background and historical context is needed. At the time, the field of high temperature superconductivity was approximately seven years old and the symmetry of the superconducting order parameter was an open question. It was known that the order parameter had nodes, but whether the order parameter changed sign (i.e. was d-wave or extended s-wave) was unresolved.
The Josephson Interference experiments unambiguously settled this issue. The second iteration of the experiment (PDF link!), was particularly decisive. Essentially, the expected critical current as a function of flux would look like so for an s-wave and d-wave superconductor respectively:
The authors obtained the latter pattern and the order parameter symmetry problem was solved (the symmetry was d-wave). It is not often in condensed matter physics that experiments are this clean, unambiguous and illuminating. When they are, however, they deserve to be celebrated.