The 2016 Nobel Prize in Physics
I still remember my condensed matter professor saying "The Kosterlitz-Thouless transition is the most important discovery in physics in the second half of the 20th century". Surely partly motivated by professional bias, no doubt, but there's still merit to the claim. If you believe that "all physics is either harmonic oscillators or phase transitions", the Kosterlitz-Thouless is the first example (I believe) of a phase transition that does not contain a local order parameter. Because of that certain materials proved illusive to all mathematical models of the time.Related: Onsager received the 2000 Nobel Prize in Physics, also for 2D phase transitions. Trivia: Onsager was a Chemist! :DEDIT: I believe this is the original paper:
http://iopscience.iop.org/article/10.1088/0022-3719/6/7/010/...