- #1
Threepwood
- 8
- 0
Homework Statement
I have been given the Hamiltonian
[tex]H = \sum_{k}\left(\epsilon_k - \mu\right) c_k^{\dag} c_k + \gamma \sum_{kp}c_k^{\dag} c_p[/tex]
and also that
[tex]c_p = \sum_{q} U_{pq} b_q[/tex]
I have to prove that this matrix [tex]U_{pq}[/tex] is unitary, and find an equation for [tex]U_{pq}[/tex].
Homework Equations
This is equivalent to proving that
[tex]\{b_q, b_p\} = 0[/tex]
and
[tex]\{b_q , b_p^{\dag}\} = \delta_{pq}[/tex]
where [tex]b[/tex] and [tex]c[/tex] are creation and annihiliation operators.
The Attempt at a Solution
Knowing that
[tex]c_p = \sum_{q} U_{pq} b_q[/tex]
then
[tex]c_q = \sum_{p} U_{pq} b_p[/tex]
and
[tex]\{b_q , b_p\} = b_q b_p + b_p b_q[/tex]
[tex]c_p b_p = \left(\sum_{q} U_{pq} b_q\right) b_p[/tex]
[tex]b_q c_q = b_q \left(\sum_{p} U_{pq} b_p\right)[/tex]
So that
[tex]c_p b_p + b_q c_q = \left(\sum_{q} U_{pq} b_q\right) b_p + b_q \left(\sum_{p} U_{pq} b_p\right)[/tex]
Hmm, now what?