- #1
Samuel Williams
- 20
- 3
Homework Statement
Consider a particle of spin 3/2. Find the matrix for the component of the spin along a unit vector
with arbitrary direction n. Find its eigenvalues and eigenvectors.
Homework Equations
I know that the general spin operator is
\begin{equation}
\widehat{S} = a\cdot \widehat{S}_x + b\cdot \widehat{S}_y + c\cdot \widehat{S}_z
\end{equation}
and that
\begin{equation}
\hat{n} = (\sin{\phi}\cos{\theta},\sin{\phi}\sin{\theta},\cos{\phi})
\end{equation}
The Attempt at a Solution
Now
\begin{equation}
\widehat{S}_x \stackrel{\cdot}{=} \frac{\hbar}{2}\begin{pmatrix}0 & \sqrt3 & 0 & 0 \\ \sqrt3 & 0 & 2 & 0 \\ 0 & 2 & 0 & \sqrt3 \\ 0 & 0 & \sqrt3 & 0\end{pmatrix}\, ,\
\widehat{S}_y \stackrel{\cdot}{=} \frac{\hbar}{2i}\begin{pmatrix}0 & \sqrt3 & 0 & 0 \\ -\sqrt3 & 0 & 2 & 0 \\ 0 & -2 & 0 & \sqrt3 \\ 0 & 0 & -\sqrt3 & 0\end{pmatrix}\, ,\
\widehat{S}_z \stackrel{\cdot}{=} \hbar\begin{pmatrix}3/2 & 0 & 0 & 0 \\ 0 & 1/2 & 0 & 0 \\ 0 & 0 & -1/2 & 0 \\ 0 & 0 & 0 & -3/2\end{pmatrix}\ .
\end{equation}
So then we get
\begin{equation}
\hat{n}\cdot \widehat{S} \stackrel{\cdot}{=} \frac{\hbar}{2}(\sin{\phi}\cos{\theta})\begin{pmatrix}0 & \sqrt3 & 0 & 0 \\ \sqrt3 & 0 & 2 & 0 \\ 0 & 2 & 0 & \sqrt3 \\ 0 & 0 & \sqrt3 & 0\end{pmatrix}\ + \frac{\hbar}{2i}(\sin{\phi}\sin{\theta})\begin{pmatrix}0 & \sqrt3 & 0 & 0 \\ -\sqrt3 & 0 & 2 & 0 \\ 0 & -2 & 0 & \sqrt3 \\ 0 & 0 & -\sqrt3 & 0\end{pmatrix}\ + \hbar(\cos{\phi})\begin{pmatrix}3/2 & 0 & 0 & 0 \\ 0 & 1/2 & 0 & 0 \\ 0 & 0 & -1/2 & 0 \\ 0 & 0 & 0 & -3/2\end{pmatrix}\ .
\end{equation}
Now, I know I should determine the secular equation (characteristic equation) after adding the matrices and find the eigenvectors and eigenvalues from there, but I keep getting long and complicated equations that don't seem to factor. Is there any shorter method or any other way to do it?