
signatures of a magnetic instability around x
c
¼0.7
whence the NFL properties in this case might be due
to a QPT (Maple et al. 1999).
3.2 Distribution of Kondo Temperatures
Features of NFL behavior may arise from a quite
different single-ion scenario, i.e., a distribution of
Kondo temperatures (P(T
K
)) in disordered systems.
Fluctuations in J or the local N(E
F
) near a disorder-
induced metal–insulator transition may lead to a wide
distribution of P(T
K
). Indeed, the specific heat of
heavily doped Si:P on the metallic side of the tran-
sition shows a contribution DCBT
a
with aE0.2,
which is attributed to P(T
K
) (Lakner et al. 1994). Here
the local moments arise from the statistical distribu-
tion of P donor atoms in the single crystalline host,
with a distribution of hybridization strengths to the
conduction electrons and hence of J. In the case of
heavy-fermion solid solutions, the statistical distribu-
tion of atoms may lead to a distribution of T
K
, as first
put forward for the case of UCu
5x
Pd
x
(x ¼1 and 1.5)
on the basis of NMR measurements which directly
showed a distribution of Knight shifts, viz., Kondo
temperatures (Bernal et al. 1996). Even for a rather
narrow distribution of J and/or N(E
F
), a broad P(T
K
)
with a finite value for T
K
-0 may occur because of
the exponential dependence of T
K
on J and N(E
F
).
On the other hand, an interesting single-ion NFL
scaling was found in the dynamic structure factor
S(q, o) determined from INS (Aronson et al . 1995).
This system is also at the verge of a magnetic insta-
bility as evident from spin-glass behavior at the low-
est temperatures (Vollmer et al. 1997). A model
combining the proximity to a magnetic instability and
disorder, the so-called Griffiths phase model, has
been tested for a number of U systems (de Andrade
et al. 1998).
See also: Magnetic Excitations in Solids
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Non-Fermi Liquid Behavior: Quantum Phase Transitions