# Charged particle on a sphere with magnetic dipole at its center

1. Aug 4, 2014

### ShayanJ

I just wanted something to play with so I thought of this problem and solved it but now I have questions about it.
Consider a charged particle with charge q and mass $\mu$ which is constrained to move on the surface of a sphere of radius R. There is a magnetic dipole with moment $\vec m=m \hat z$ at the centre of the sphere.
The vector potential produced by the magnetic dipole is $\vec A= \frac {\mu_0 m \sin\theta} {4\pi R^2}\hat \varphi$ and so the Hamiltonian is:

$\hat H=\frac{1}{2\mu}\left( -\hbar^2 \nabla^2-\frac{\mu_0 q m}{4\pi R^3}\frac{\partial}{\partial \varphi}+\frac{\mu_0^2q^2m^2\sin^2\theta}{16\pi^2R^4}\right)$.

I assumed $q^2 m^2$ to be small and so the $A^2$ term to be negligible.
So the time independent Schrodinger equation is:

$\left( \frac{\partial^2}{\partial \theta^2}+\cot\theta \frac{\partial}{\partial \theta}+\frac{1}{\sin^2\theta}\frac{\partial^2}{\partial\varphi^2}+ \frac {\mu_0 q m}{4 \pi R \hbar^2}\frac{\partial}{\partial \varphi} \right) \psi=-\frac{2\mu R^2}{\hbar^2}E \psi$

Then I assumed $\psi=T(\theta) P(\varphi)$. It turns out that $P(\varphi)=const$ otherwise the equation is not separable.This is also supported by symmetry considerations.
So the equation becomes:

$T''+\cot\theta T'+\frac{2\mu R^2 E}{\hbar^2} T=0$

Which is Legendre's differential equation. But the coefficient of T should be of the form n(n+1) so that the solutions are well-behaved which gives us the energy spectrum:

$\frac{2\mu R^2 E}{\hbar^2}=n(n+1) \Rightarrow E=\frac{\hbar^2}{2\mu R^2}n(n+1)$

Now the questions:

1- Can it be an example of Aharonov-Bohm effect. Because we can somehow isolate the magnetic dipole so that its magnetic field is zero on the surface of the sphere. How is that isolation?

2-As you can see, the energies are independent of the charge of the particle. I know that by adding the $A^2$ term as a perturbation, we'll have q dependence but it is still strange to me. Can it be somehow explained?

I have a candidate answer to these solution but I don't know how much its correct. Looks like the geometry of the problem is pushing Aharonov-Bohm effect away, at least pushing it to the correction terms. Interesting.

Thanks

P.S.
Looks like I know the answer to the questions. So there is no question!!!
Consider it as my first publication so !!!

Last edited: Aug 4, 2014