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The s-f Model is a model who could describe the $\textbf{magnetic 4 $\textit{f}$ systems}$, i.e systems where we could identify localized electrons in $4\,\textit{f}$ orbitals and conductions electrons

The interaction between conduction electrons and $4\,\textit{f}$ electrons,could be described as an intra-atomic exchange interaction between the spins $\hat{{\sigma}}$ of the conduction electrons and the spin $\hat{S}$ of $4\,\textit{f}$ electrons:

\begin{equation} \hat{H}_{sf} = -g \sum_{i}\,\hat{\sigma}_{i}\,\cdot\,\hat{S}_{i} \end{equation}

Here the index $i$ indentify the lattice site in $\textbf{R}_{i}$, $g$ is the exchange coupling constant. For semplicity it is assumed to be identical for all pairs of conduction and $4\,\textit{f}$ electrons

Using the following relations for the spin operators $\hat{\sigma}_{i}$ and $\hat{S}_{i}$

\begin{align} \hat{S}_{j}^{x} &= \frac{1}{2} \left(\hat{S}_{j}^{+} + \hat{S}_{j}^{-}\right) \nonumber \\ \hat{S}_{j}^{y} &= \frac{1}{2} \left(\hat{S}_{j}^{+} - \hat{S}_{j}^{-}\right) \nonumber \\ \hat{S}_{j}^{z} &= \hat{S}_{j}^{z} \end{align}

and the second quantization rapresentation for the spin operators of conduction electrons

\begin{align} \hat{ \sigma}_{i}^{+} &= \hbar\,\hat{a}^{\dagger}_{i,\uparrow}\hat{a}_{i,\downarrow} \nonumber \\ \hat{ \sigma}_{i}^{-} &= \hbar\,\hat{a}^{\dagger}_{i,\downarrow}\hat{a}_{i,\uparrow} \nonumber \\ \hat{ \sigma}_{i}^{z} &= \frac{\hbar}{2}\, \left( \hat{a}^{\dagger}_{i,\uparrow}\hat{a}_{i,\uparrow} - \hat{a}^{\dagger}_{i,\downarrow}\hat{a}_{i,\downarrow} \right) \end{align}

The interaction term $\hat{H}_{sf}$ could be written in the following way

\begin{equation} \hat{H}_{sf} = - \frac{1}{2} \, g \, \hbar \sum_{i} \left[ \hat{a}^{\dagger}_{i,\uparrow}\hat{a}_{i,\downarrow} \hat{S}_{i}^{-} + \hat{a}^{\dagger}_{i,\downarrow}\hat{a}_{i,\uparrow} \hat{S}_{i}^{+} + \left( \hat{a}^{\dagger}_{i,\uparrow}\hat{a}_{i,\uparrow} - \hat{a}^{\dagger}_{i,\downarrow}\hat{a}_{i,\downarrow} \right) \, \hat{S}_{i}^{z}\right] \end{equation}

Since i have to compute commutators like $\left[\hat{a}_{i,\sigma}, \hat{H}\right]$, where $H$ is the s-f Hamiltonian,i have to know the commutation relations between the annihliation and creation operators of conduction electrons $\hat{a}^{\dagger}_{i,\sigma},\,\hat{a}_{i,\sigma}$ and the spin operator of localized electrons $\hat{S}_{i}^{z}, \hat{S}_{i}^{+}, \hat{S}_{i}^{-}$

I am thinking , since these are two different kind of electrons the commutator between them is 0 , right?

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You are correct. If this is not enough, these operators correspond to different degrees of freedom and you can safely assume that they are commuting.

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  • $\begingroup$ Can i formalize it ? $\endgroup$ – Giovanni Dec 12 '19 at 13:26
  • $\begingroup$ Yes, but it is trivial. Ladder operators act on occupation numbers that are a different degree of freedom with respect to spin. So, simply $[{\hat a}_{i,\sigma},{\hat S}^\alpha_j]=0$. $\endgroup$ – Jon Dec 12 '19 at 13:29

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