Environmental Engineering Reference
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Fig. 7.6.
Neutron-scattering spectra from the acoustic branch of the
magnetic excitations propagating along the
c
-axis on the hexagonal sites
of Pr at 6 K. The observed values of Γ, the full width at half maximum,
increase rapidly as q decreases, due to scattering by the conduction elec-
trons, and at
q
= 0 it is dicult to distinguish the peak from the back-
ground. The experimental energy resolution is about 0.35 meV.
here dominate at most temperatures, but it is clear that this cannot hold
true in the high-temperature limit, where Γ
1
increases proportionally
to
T
,whereas
K
(0)
/β
approaches a constant value. So, in the high-
temperature limit, (7.3.19) leads to the
Korringa law
(Korringa 1950)
for the linewidth:
i
Γ
1
hω
+
i
Γ
1
G
(
q
,ω
)
G
(
ω
)
G
(0)
,
with
2
2
(
ε
F
)
k
B
T,
Γ
1
=2
ζ
(0)
k
B
T
=4
π
|
j
(
q
)
|
N
(7
.
3
.
20)
since
could
be neglected, in comparison with the intrinsic effects, at relatively low
ζ
(0)
=
ζ
0
in this limit.
We argued above that
ζ
(
ω
)
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