- #1
vputz
- 11
- 0
Homework Statement
Okay, this would be easy if it hadn't been 15 years since undergrad quantum. Here goes.
I'm finding the energy spectrum of a Heisenberg "two-electron ferromagnet", if you will, with a Hamiltonian described by
[tex]H=-J\hat{S_1}\cdot\hat{S_2}-h(\hat{S_{z1}}+\hat{S_{z2}})[/tex]
Homework Equations
The Attempt at a Solution
Well, after a while of dusting off my brain and groveling to fellow students, I figured out that
[tex](\hat{S_1}+\hat{S_2})^2 = \hat{S_1}^2 + \hat{S_2}^2 + 2\hat{S_1}\cdot\hat{S_2} \rightarrow \hat{S_1}\cdot\hat{S_2} = \frac{1}{2}( (\hat{S_1}+\hat{S_2})^2 -\hat{S_1}^2 -\hat{S_2}^2 ) [/tex]
So my Hamiltonian is now
[tex]H = -\frac{1}{2}J((\hat{S_1}+\hat{S_2})^2 - \hat{S_1}^2 -\hat{S_2}^2 ) - h(\hat{S_{z1}}+\hat{S_{z2}})[/tex]
Okay. Now, the eigenvalues of [tex]\hat{S}^2[/tex] are [tex]s(s+1)[/tex] (we're doing the usual [tex]\hbar=1[/tex] trick). And the eigenvalues of [tex]\hat{S_z}[/tex] are [tex]m[/tex]. And I know that electrons have [tex]s=\frac{1}{2}[/tex] and [tex]m=-s...s[/tex] in integer steps.
So... it should just be a matter of plugging in possible values for, er, s&m, so to speak. But the [tex](\hat{S_1}+\hat{S_2})^2[/tex] term confuses me. My gut feeling is to treat that as an [tex]\hat{S}^2[/tex] term but use values [tex]-1,0,1[/tex] as possible values of [tex]\hat{S_1}+\hat{S_2}[/tex]. Is that the right way to handle it?