  c^\dagger_{0k\uparrow} c^{\phantom\dagger}_{0k\uparrow}  -c^\dagger_{0k\downarrow} c^{\phantom\dagger}_{0k\downarrow}  -c^\dagger_{0k\uparrow} c^{\phantom\dagger}_{0k\uparrow}  +c^\dagger_{0k\downarrow} c^{\phantom\dagger}_{0k\downarrow}