Possible suggestions for order parameter phase-sensitive experiments in the superconducting iron pnictides

The iron pnictide superconductors have undergone intensive study since the original discovery by Kamihara et al early in 2008, with maximum T$_{c}$'s exceeding 50 K. Despite this, the most basic questions such as pairing symmetry and mechanism have not been definitively settled. For the cuprates, the SQUID loop and tricrystal phase-sensitive experiments were instrumental in finally determining the d-wave gap symmetry; similar experiments were designed and implemented for triplet p-wave superconductivity. However, the main challenge in pnictides is to distinguish between two superconducting states, the sign-changing ``s$_\pm$" and single-sign ``s$_{++}$ states, which belong to the same point symmetry class. This means that while designing a Josephson loop one needs to invent a recipe to filter out different types of carriers at the two different contacts. By definition this is a {\it quantitative} rather than {\it qualitative} effect, and involves the relative amplitude of the order parameter, density-of-states and Fermi velocity, and the character of the electronic wavefunctions. Presently proposed methods either attempt to determine an optimal angle (i.e., non-90$^{\circ}$) for a SQUID junction, use different barrier characteristics in different directions, or exploit `sandwich' junctions employing two or more superconductors. In this talk, I discuss several recent proposals for phase-sensitive experiments which could help resolve the pairing symmetry controversy, as well as experimental work in this area.