Probing the symmetry of the pairing state of unconventional superconductors by SQUID interferometer measurements

Phase-sensitive SQUID interferometer experiments have emerged as the most powerful technique for determining the symmetry of the pairing state of unconventional superconductors. This approach uses Josephson tunneling to probe specific directions in k-space and long-range phase coherence to measure the phase anisotropy of the order parameter, allowing a direct and definitive determination of the symmetry. Developed for testing the symmetry of the cuprates, corner SQUID and junction experiments indicate a sign change in the order parameter in orthogonal in-plane directions in YBCO, strong evidence for d(x)2-(y)2 pairing. Now, this technique is being applied to other classes of superconductors suspected to be unconventional.