Floquet theorem In the periodic potential,we may find: 出1(x+a)=平(x) Ψ2(x+a)=2Ψ2(x) 出(x+aΨ2(x+a Define Dx+a)d+a 出(x)平(x) then Dx+a)=12Ψ()Ψ（✉ So,22=1→=ea,2=ea for simiplity,we choose-π≤Ka≤πFloquet theorem In the periodic potential, we may find: ( ) ( ) ( ) ( ) 2 2 2 1 1 1 x a x x a x                        for simiplity, we choose - Ka So, 1 , , 1 2 1 2 iKa iKa e e                 x x x x x a x a x a x a ' ' 1 2 1 2 ' ' 1 2 1 2 1 2 then D x a Define D x a                  