Spin populations have
lifetimes which depend greatly on the materials and
conditions in which they exist. The amount of time
for which a spin population exists is called the
coherence time. We can also define the length that a
net spin population can travel in the presence of an
electric field, called the spin transport length. In
Gallium Arsenide at 4 Kelvin
(-269 Celsius), the coherence length is about 20
micro meters. That’s one meter divided by 50,000,
this can start to give you an idea about that scale
of these devices. (FYI: A CMOS transistor in your
computer is at least 20 times smaller). The
decoherence time under the same conditions is about
200 nanoseconds. Temperature greatly effects the
time and distance a spin population can endure. At
greater temperatures, particles move faster, and
collisions with the atoms in the structure of the
semiconductor and other electrons, are more
frequent. These collisions cause an electron spin to
loose its preferred orientation, and destroy the net
spin population. We can generate a net spin
population in a semiconductor that is characterized
by a decoherence length and a relaxation time, but
we must be very careful if we try to infer to
understand the behavior of a single electron from
this information. The times and lengths measured are
the statistical averages over many electrons. WE CAN
ASSIGN NO DECOHERENCE LENGTH OR RELAXATION TIME TO A
SINGLE ELECTRON, it simply makes no sense. If we do
an experiment to measure these properties of single
electrons we would get a large range of values who’s
average would correspond to the large scale
decoherence length we speak of. The same concept
applies to the relaxation times. Even if all of the
microscopic parameters of the experiment were kept
exactly the same, it is the understanding of modern
quantum physics that we would continually get
different results, for the single electron
measurements, which would appear in an unpredictable
sequence, but would in the end, fall into the
statistical spread which is mathematically predicted
for the relaxation time.
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