Consider the scattering of an electron of energy and momentum by the electrostatic step-potential of the form (figure 5.4)
For a free electron, we have , whereas in the presence of the constant potential
where is the momentum of the particle inside the potential.
The Dirac equation for is
while for we have
where it is understood that and .
The incident wave in region I is
with . The reflected wave in region I is
The transmitted wave in region II is
For now, we consider the case of (strong field) for which the momentum is real and allows the free plane wave to propagate in region II. Continuity at the boundary requires and thus
The equations for and can only be satisfied if . There is thus no spin flip of the electron at the boundary. Also
We thus have
The particle current is given by
Since is real, the ratio of currents is
Since , we see that . This result corresponds to the fact that the flow of is in the -direction, ie. the electrons are leaving region II, but according to our assumptions up to now, there are no electrons in there anyways. A reinterpretation is thus necessary.
To prevent the transition of all electrons to states of negative energy one has to require that all electron states are occupied with electrons. The potential raises the electron energy in region II sufficiently for there to be an overlap between the negative continuum for and the positive continuum for , as shown in figure 5.5. In the case of the electrons striking the potential barrier from the left are able to knock additional electrons out of the vacuum on the right, leading to positron current flowing from left to right in the potential region. It is possible to understand the sign of by assuming that the electrons entering region I are coming from the negative continuum.
Since the holes remaining in region II are interpreted as positrons, the phenomena can be understood as electron-positron pair creation at the potential barrier and is related to the decay of the vacuum in the presence of a strong field.