= Pairing gap
{title2=$\Delta$}
A <pairing gap> is an <energy> scale for fermionic excitations above a paired <superfluid> or <superconductor> <ground state>. In a simple isotropic paired system the <quasiparticle> dispersion has the form $E_{\mathbf k}=\sqrt{\xi_{\mathbf k}^2+|\Delta|^2}$, where $\xi_{\mathbf k}$ measures the unpaired single-particle <energy> relative to the <chemical potential>. A nonzero minimum gap gives exponentially small fermionic thermal occupations when $k_BT\ll|\Delta|$. If the gap varies with direction and has nodes, gapless directions can instead produce power-law thermodynamics. Collective excitations must be counted separately from the gapped fermionic <quasiparticles>.
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