Page 270 - Semiconductor For Micro- and Nanotechnology An Introduction For Engineers

Electron-Photon
• Righi-Leduc effect. When heat flows across lines of magnetic force,
we observe another heat current perpendicular to both the original
heat current and the magnetic field.
• Hall effect, discussed in 6.2.3.
• Ettinghauser-Nernst effect. When an electric current flows across the
lines of force of a magnetic field, we observe a voltage (called the
Hall voltage) which is perpendicular to both the electric current and
the magnetic field. In addition, we observe a temperature gradient in
the opposite direction to the Hall voltage.

As mentioned before, in formal transport theory, the Onsager coefficients
are anti-symmetric w.r.t. the presence of the field, or

T
B
L () = L ( – B) (7.79)
kl kl
For vector quantities, this anti-symmetry expresses itself through the
cross product and through the rotation operator, and is again the reason
for the recurring “perpendicular” fields.

7.4 Electron-Photon

There exists an optically induced transition between electronic states.
The energy of absorbed photons is transferred to the electronic system,
such that the electron goes from the state of lower energy to the one with
higher energy. The inverse process exists as well, delivering the energy
from the electronic system to the photon system. These two processes we
call absorption and emission. Therefore, the basics of electron-photon
interactions, as they enter the time dependent perturbation given in 3.2.7,
are important for the description of arising phenomena.

As in the preceding sections we ask how the interaction operator looks
like. Suppose there is an external harmonic electric wave
(
,
Er t) = E cos ( kr ωt) that acts on an electron. Assume that the
–
0
Semiconductors for Micro and Nanosystem Technology 267