BCS quasiparticle

An elementary excitation of the paired mean-field state whose electron operator has mixed creation and annihilation components. Its positive excitation energy is $E_k=\sqrt{{\xi }_k^2+|{\Delta }_k{|}^2}$. At zero temperature the BCS ground state has zero occupation of these positive-energy modes, even though physical electrons have a smooth nontrivial momentum occupation.