Charged Particle in a Magnetic Field
Michael Fowler 1/16/08
Introduction
Classically, the force on a charged particle in electric and magnetic fields is given by the Lorentz force law:
This velocity-dependent force is quite different from the conservative forces from potentials that we have dealt with so far, and the recipe for going from classical to quantum mechanics—replacing momenta with the appropriate derivative operators—has to be carried out with more care. We begin by demonstrating how the Lorentz force law arises classically in the Lagrangian and Hamiltonian formulations.
Laws of Classical Mechanics
Recall first (or look it up in Shankar, Chapter 2) that the Principle of Least Action leads to the Euler-Lagrange equations for the Lagrangian L:
The canonical momentum pi is defined by the equation
and the Hamiltonian is defined by performing a Legendre transformation of the Lagrangian:
It is straightforward to check that the equations of motion can be written:
These are known as Hamilton’s Equations. Note that if the Hamiltonian is independent of a particular coordinate qi, the corresponding momentum pi remains constant. (Such a coordinate is termed cyclic, because the most common example is an angular coordinate in a spherically symmetric Hamiltonian, where angular momentum remains constant.)
For the conservative forces we have been considering so far, L = T - V, H = T + V, with T the kinetic energy, V the potential energy.
Poisson Brackets
Any dynamical variable f in the system is some function of the qi’s and pi’s and (assuming it does not depend explicitly on time) its development is given by:
The curly brackets are called Poisson Brackets, and are defined for any dynamical variables as:
We have shown from 
It is easy to check that for the coordinates and canonical momenta,
This was the classical mathematical structure that led Dirac to link up classical and quantum mechanics: he realized that the Poisson brackets were the classical version of the commutators, so a classical canonical momentum must correspond to the quantum differential operator in the corresponding coordinate.
Particle in a Magnetic Field
The Lorentz force is velocity dependent, so cannot be just the gradient of some potential. Nevertheless, the classical particle path is still given by the Principle of Least Action. The electric and magnetic fields can be written in terms of a scalar and a vector potential:
The right Lagrangian turns out to be:
(Note: if you’re
familiar with Relativity, the interaction term here looks less arbitrary: the
relativistic version would have the relativistically invariant 
added to the action
integral, where the four-potential 
and 
. This is the simplest
possible invariant interaction between the electromagnetic field and the
particle’s four-velocity. Then in the
nonrelativistic limit, just becomes 
)
The derivation of the Lorentz force from this Lagrangian is
given by Shankar on page 84. We give the
(equivalent) derivation from the
Note that for zero vector potential, the Lagrangian has the usual T - V form.
For this one-particle problem, the general coordinates qi are just the Cartesian
co-ordinates 
, the position of the particle, and the 
are the three
components 
of the particle’s
velocity.
The important new point is that the canonical momentum
is no longer mass 
velocity—there is an
extra term!
The Hamiltonian is
Reassuringly, the Hamiltonian just has the familiar form of kinetic
energy plus potential energy. However,
to get
where we have noted explicitly that the potentials mean
those at the position 
of the particle
at time t.
Let us now consider
It is easy to see how the first equation
comes out, bearing in mind that
The second equation yields the Lorentz force law, but is a little more tricky. The first point to bear in mind is that dp/dt is not the acceleration, the A term also varies in time, and in a quite complicated way, since it is the field at a point moving with the particle. That is,
The
right-hand side of the second 
is
Putting
the two sides together, the
and the expressions
for the electric and magnetic fields in terms of the potentials, the Lorentz
force law emerges:
Quantum Mechanics of a Particle in a Magnetic Field
We make the standard substitution:
This leads to the novel situation that the velocities in different directions do not commute. From
it is easy to check that
To actually solve Schrödinger’s equation for an electron confined to a plane in a uniform perpendicular magnetic field, it is convenient to use the Landau gauge,
giving a constant field B in the z direction. The equation is
Note that x does
not appear in this Hamiltonian, so it is a cyclic coordinate, and px
is conserved. In other words, this H
commutes with px, so H and
px have a common
set of eigenstates. We know the
eigenstates of px are just
the plane waves 
, so the common eigenstates must have the form:
Operating on this wavefunction with the Hamiltonian, the
operator px appearing in H simply gives its eigenvalue. That is,
the px in H
just becomes a number! Therefore,
writing 
the y-component 
of the wavefunction satisfies:
Where
We now see that the conserved canonical momentum px in the x-direction is actually the coordinate of the center of a simple harmonic oscillator potential in the y-direction! This simple harmonic oscillator has frequency w = |q|B/mc, so the allowed values of energy for a particle in a plane in a perpendicular magnetic field are:
The frequency is of course the cyclotron frequency—that of
the classical electron in a circular orbit in the field (given by 
) .
Let us confine our attention to states corresponding to the
lowest oscillator state, 
. How many such states
are there? Consider a square of
conductor, area 
, and, for simplicity, take periodic boundary
conditions. The center of the oscillator
wave function y0 must lie between 0 and Ly. But remember that 
, and with periodic boundary conditions 
This means that y0
takes a series of evenly-spaced discrete values, separated by
So the total number of states 
where 
is called the “flux
quantum”. So the total number of states
in the lowest energy level (usually referred to
as the lowest Landau level, or LLL)
is exactly equal to the total number of flux quanta making up the field B
penetrating the area A.
It is instructive to find y0 from a purely classical analysis.
These equations integrate trivially to give:
Here (x0, y0) are the
coordinates of the center of the classical circular motion (the velocity vector
is always perpendicular to 
) , and 
is given by
(Recall that we are using the gauge , and 
, etc.)
Just as y0 is a conserved quantity, so is x0: it commutes with the Hamiltonian since
However, x0 and y0 do not commute with each other:
This is why, when we chose a gauge in which y0 was sharply defined, x0 was spread over the sample. If we attempt to localize the point (x0, y0) as well as possible, it is fuzzed out over an area essentially that occupied by one flux quantum. The natural length scale of the problem is therefore the magnetic length defined by
References: the classical mechanics at the beginning is similar to Shankar’s presentation, the quantum mechanics is closer to that in Landau.