 Superconducting gap

The gap feature in a superconducting single-particle excitation spectrum. In a simple BCS model, excitations have $E_{\mathbf{k}}=\sqrt{{\xi }_{\mathbf{k}}^2+|\Delta (\mathbf{k}){|}^2}$. The superconducting gap function specifies the pairing amplitude and its phase; its magnitude need not be constant, and nodes can leave gapless excitations.