Condensation energy of a superconductor ($F_s-F_n=-a^2/(2b)$)

The condensation energy of a superconductor is the free-energy lowering on entering the superconducting state, with sign convention specified. For a local Landau density $a|\Delta {|}^2+b|\Delta {|}^4/2$ and $a<0$, its minimum is $-a^2/(2b)$ relative to the normal-state value. The positive energy required to destroy the condensate is the opposite sign.