Non-Fermi-liquid behavior of YbCu
5 x
Al
x
E. Bauer, R. Hauser, A. Galatanu, H. Michor, and G. Hilscher
Institut fu ¨r Experimentalphysik, Technische Universita ¨t Wien, A-1040 Wien, Austria
J. Sereni, M. G. Berisso, and P. Pedrazzini
Centro Atomico Bariloche, 8400 San Carlos de Bariloche, Argentina
M. Galli and F. Marabelli
Istituto Nationale Nazionale per la Fisica della Materia, Dipartmento di Fisica ‘‘Alessandro Volta,’’ Universita ` di Pavia,
I-27100 Pavia, Italy
P. Bonville
CEA, CE Saclay, DRECAM, Service de Physique de l’Etat Condense ´, F-91191 Gif-sur-Yvette, France
Received 22 April 1998; revised manuscript received 27 October 1998
A valency change in YbCu
5-x
Al
x
from 2.2( x =0) to =3( x =2) causes a magnetic instability near a
critical concentration x
cr
1.5. Alloys in the vicinity of x
cr
exhibit typical non-Fermi-liquid properties like a
negative logarithmic contribution to the specific heat or substantial deviations of the electrical resistivity from
a T
2
behavior. Both pressure and alloying in this series bring about a strong reduction of the Kondo tempera-
ture T
K
; the crystal-field interaction and the interatomic exchange gain therefore importance and magnetic
order becomes possible beyond x
cr
. S0163-18299903825-4
I. INTRODUCTION
In recent investigations we have shown that a Cu/Al sub-
stitution in YbCu
5
causes a crossover from an almost non-
magnetic 4 f
14
state (YbCu
5
) to the magnetic 4 f
13
state in
YbCu
3
Al
2
.
1,2
This change in the ground-state configuration
is accompanied by the onset of long-range magnetic order
and by an increase of both the ordering temperature and the
magnitude of the ordered moment.
Such an evolution of the magnetic behavior, driven by a
change of the valence, is promising with respect to the ap-
pearance of a non-Fermi-liquid NFL behavior near to the
critical concentration x
cr
at which long-range magnetic order
sets in. There, a quantum critical point shall exist,
3
and mag-
netic correlations can be much higher in energy than
k
B
T
N,( C)
. As a consequence, the Fermi-liquid FL model is
insufficient to account for the physical behavior of such
systems.
4
Previously, we have shown
2
that the specific heat
of YbCu
3.5
Al
1.5
behaves logarithmically at low temperature,
indicating substantial deviations from a FL behavior. This
particular alloy is in the proximity of the onset of long-range
magnetic order in this series, which was confirmed by elastic
neutron scattering experiments for x =1.75 ( T
N
1 K,
ord
1.2
B
) and x =2( T
N
2 K,
ord
2
B
).
2
More recently,
a study of the magnetic susceptibility and of the temperature-
dependent Mo
¨
ssbauer spectra revealed magnetic order below
about 0.25 and 0.55 K for x =1.6 and 1.7, respectively.
5
Investigations of Yb systems, in general, provide the pos-
sibility to approach a magnetic instability point from the op-
posite side when compared to isomorphous Ce systems. In
particular, it was shown that the binary compound CeCu
5
,
which is isostructural to YbCu
5
, orders antiferromagnetically
below 4 K.
6
While a Cu/Al substitution in the former sup-
presses long-range magnetic order, it is responsible for the
occurrence of a magnetically ordered ground state in the lat-
ter. Moreover, we note that in both cases a simple antiferro-
magnetic structure was deduced from elastic neutron-
scattering experiments.
2,6
Another important aspect of Yb
systems is the fact that pressure can drive such systems from
a nonmagnetic to a magnetic state, while Ce systems behave
usually mirrorlike due to the electron-hole symmetry be-
tween the Ce
3 +
and the Yb
3 +
ions.
The aim of the present paper is to investigate in some
detail the low-temperature physical behavior of alloys in a
certain range around x x
cr
and to show that NFL properties
may occur also in Yb systems.
II. EXPERIMENTAL DETAILS
Samples with concentrations near to x
cr
( x =1.2, 1.3,
1.4, 1.5, 1.6, and 1.7 have been prepared from stoichio-
metric amounts of elements using high frequency melting.
To ensure phase purity and homogeneity, the ingots were
remelted several times and subsequently annealed for 10
days at 750 °C. X-ray diffraction measurements were carried
out applying CoK
radiation. The x-ray pattern proved phase
purity hexagonal CaCu
5
structure, P 6/mmm ), and revealed
a continuous increase of the lattice parameter a and a slight
decrease of c; hence the volume of the unit cell grows with
increasing Al content see Fig. 1.
The electrical resistivity and magnetoresistivity of bar-
shaped samples were measured using a four-probe dc method
in the temperature range from 300 mK to room temperature.
A liquid pressure cell with a 4:1 methanol-ethanol mixture as
pressure transmitter served to generate hydrostatic pressure
up to about 15 kbar. The absolute value of the pressure was
determined from the superconducting transition temperature
of lead.
7
A superconducting quantum interference device magneto-
meter served for the determination of the magnetization from
2 K up to 300 K in fields up to 6 T.
Specific-heat measurements on samples of about 2 g were
performed at temperatures ranging from 1.5 up to 60 K by
PHYSICAL REVIEW B 1 JULY 1999-II VOLUME 60, NUMBER 2
PRB 60 0163-1829/99/602/12389/$15.00 1238 ©1999 The American Physical Society