# Landau Levels in a Magnetic Field

This Demonstration considers the quantum-mechanical system of a free electron in a constant magnetic field, with definite values of the linear and angular momentum in the direction of the field. The wavefunction is plotted in a plane normal to the magnetic field. The corresponding energies are the equally spaced Landau levels, similar to the energies of a harmonic oscillator. These results find application in the theory of the quantum Hall effects.
You can select a 3D plot of the wavefunction, a plot of the radial function or an energy-level diagram. The first slider varies the magnetic field strength . You can then select and , the radial and angular quantum numbers, respectively.

### DETAILS

The nonrelativistic Hamiltonian for an electron in a magnetic field , where is vector potential, is given by ,
where and are the mass and charge of the electron, respectively. We also make use of the Coulomb gauge condition .
For a constant field in the direction, , it is convenient to work in cylindrical coordinates, . With a convenient choice of gauge, the vector potential can be represented by , .
This gives .
The Schrödinger equation for is given by .
The equation is separable in cylindrical coordinates, and we can write , for definite values of the angular and linear momenta. We consider only angular momentum anticlockwise about the axis. We set and consider only motion in a plane perpendicular to the magnetic field. Introducing atomic units , the radial equation reduces to .
The solution with the correct boundary conditions as is given by ,
where is an associated Laguerre polynomial. The corresponding energy eigenvalues are .
These are the well-known Landau levels, which are equivalent to the levels of a two-dimensional harmonic oscillator with .
Recall that is the cyclotron frequency for an electron in a magnetic field.
References
 L. D. Landau and E. M. Lifshitz, Quantum Mechanics: Non-relativistic Theory, 2nd ed., Oxford: Pergamon Press, 1965, pp. 424ff.
 D. ter Haar (ed. and tr.), Problems in Quantum Mechanics, 3rd ed., London: Pion, 1975 pp. 38, 254ff.

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