BCS quasiparticle
= BCS quasiparticle
{c}
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.