Question 09/98
MISSING MOMENTUM
Consider the creation of a phonon by the scattering of a neutron
from a crystal. Let the initial and final momentum of the neutron be
p and p'. The neutron interacts only weakly with matter,
and is essentially a free particle. The phonon which is produced has a
pseudomomentum K=p-p'.
(a) Prove that momentum of a phonon vanishes.
(b) Where does the missing momentum go?
This is a reformulation of a question asked by R. Peierls.
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