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The PAC technique
PAC measurements rely on a radioactive probe
atom like
111In
. The quantity measured is the electric field
gradient (efg), which arises from the immediate
lattice surrounding at the site of the probe atom.
The observed effect results from the hyperfine
interaction between the efg and the quadrupole
moment of the isotope 111Cd,
which which is a product of the radioactive decay
of the probe 111In.
A non zero efg, which is the result of a deviation
of the electronic charge distribution around the
probe from spherical symmetry, is observed in
case of a non-cubic lattice structure or a defect
in the immediate vicinity of the probe atom. The
efg is the second spatial derivative of the electrostatic
potential and, therefore, can be described by
a second rank, traceless tensor. In its principal
axis system this tensor is completely described
by two quantities, usually its largest component
Vzz, expressed via the quadrupole coupling
constant nQ
= eQVzz/h, and the asymmetry
parameter h =
(Vxx-Vyy)/Vzz
. By detecting
two g-quanta in coincidence, which are emitted following
the radioactive decay, a time
spectrum is obtained, which is described by
a function
 ,
which holds for the case of a single efg. The
frequencies wn
are extracted from the Fourier
transform F(w),
yielding nQ,
w1 and h,
which is determined by the ratio w2/w1 .
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