Half-Integer Flux Quantization in Unconventional Superconductors

In this article, we wish to give a historical and personal account of the advent of our tricrystal phase-sensitive symmetry experiments using scanning SQUID microscope for establishing d-wave pairing symmetry in the cuprate superconductors. On the occasion of commemorating the 100th anniversary of the discovery of superconductivity by Kamerlingh Onnes, we are honored to present this article as part of the Jubilee Celebration.