# I How to diagonalize Hamiltonian with Zeeman field

1. Aug 30, 2016

### aaaa202

Recently I have been asked to solve the problem of an electron in a Zeeman-field that couples the spin of the electron to the magnetic field.
I am not sure how to correctly set up the problem. I think, however, that what I have done on the picture is correct. The usual p^2/2m + V term in the Hamiltonian is tensored with the identity matrix and the Zeeman term is added in the usual way.
I am however unsure how to solve this Hamiltonian. How do you solve something like this with both an eigenvalue equation in the matrix structure and in the spatial part of H?

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2. Aug 30, 2016

### Demystifier

This is usually solved perturbatively. Most QM textbooks have a chapter on time-independent perturbation theory.

3. Aug 30, 2016

### aaaa202

Okay but is it not possible to find the exact analytical expression for the eigenenergies?

4. Aug 31, 2016

### DrDu

Easy. Just find a unitary transformation which rotates B in the direction of the z axis. Then the hamiltonian will be diagonal as only the sigma_z component remains.

5. Aug 31, 2016

### Demystifier

This diagonalizes the interaction Hamiltonian, but not the full Hamiltonian.

6. Aug 31, 2016

### DrDu

If B is constant, it does, at least in spin space.