Fresnel aperture diffraction: a phase-sensitive probe for superconducting pairing symmetry

Fresnel single aperture diffraction (FSAD) is proposed as a phase-sensitive probe for pairing symmetry and Fermi surface of a superconductor. We consider electrons injected, through a small aperture, into a thin superconducting (SC) layer. It is shown that in case of SC gap symmetry $Delta(-k_x,mathbf{k}_parallel)=Delta(k_x,mathbf{k}_parallel)$ with $k_x$ and $mathbf{k}_parallel$ respectively the normal and parallel component of electron Fermi wavevector, quasiparticle FSAD pattern developed at the image plane is zeroth-order minimum if $k_x x=npi$ ($n$ is an integer and $x$ is SC layer thickness). In contrast, if $Delta(-k_x,mathbf{k}_parallel)=-Delta(k_x, mathbf{k}_parallel)$, the corresponding FSAD pattern is zeroth-order maximum. Observable consequences are discussed for iron-based superconductors of complex multi-band pairings.