Influence of 4 f hybridization on the structural and electronic properties of CeM
2
Si
2
„ M Ä Ru, Rh, and Pd…
V. Vildosola and A. M. Llois
Departamento de Fı ´sica, Comisio ´n Nacional de Energı ´a Ato ´mica, Av. Gral. Paz 1499, 1650 San Martı ´n, Argentina
and Departamento de Fı ´sica, J. J. Giambiagi, FCEyN, University of Buenos Aires, Buenos Aires, Argentina
J. G. Sereni
Centro Ato ´mico Bariloche and Instituto Balseiro, 8400 S.C. de Bariloche, Argentina
Received 10 June 2003; revised manuscript received 29 September 2003; published 30 March 2004
We present full potential ab initio calculations to study structural and electronic properties of CeM
2
Si
2
with
M =Ru, Rh, and Pd. Two kind of calculations are performed: one considering the 4 f states as hybridized ones
and another simulating Ce systems with nonhybridized 4 f states. We calculate the hybridization function and
analyze the influence of the 4 f states on structural properties such as equilibrium volume, c / a ratio, and bulk
modulus and study their evolution together with that of the spin magnetic moment under applied hydrostatic
pressure. The results are compared with experimental data.
DOI: 10.1103/PhysRevB.69.125116 PACS numbers: 71.20.Eh, 71.28.+d
I. INTRODUCTION
Cerium compounds present several unusual physical
properties, which have their origin in the behavior of the
highly correlated 4 f electrons. The hybridization of the 4 f
states with the conduction band is considered as the param-
eter that controls the wide variety of ground states observed
in these systems. Hybridization can be tuned in two ways: by
applying hydrostatic pressure or by changing the chemical
environment of Ce. In particular, the family of CeM
2
X
2
in-
termetallics with M a transition metal and X a p-type met-
alloid which crystallizes in the ThCr
2
Si
2
-type structure
shows a wide variety of physical behaviors depending on M
and X, such as ambient or pressure induced superconductiv-
ity, long-range magnetic order, heavy fermion character,
paramagnetism, and nonmagnetic order with magnetic corre-
lations, among others.
1
The competition between the Ruderman-Kittel-Kasuya-
Yosida RKKY interaction and the Kondo effect determines
the ground state of these systems. In a regime of weak hy-
bridization the dominant RKKY interaction leads to long-
range magnetic order, while in the case of strong hybridiza-
tion the Kondo interaction prevails and spin compensation
favors a nonmagnetic valence fluctuating state at low
temperatures.
2
Within the above-mentioned family of compounds, our
interest lies on the study of the influence of 4 f and
conduction-band hybridization on the structural and elec-
tronic properties of the CeM
2
Si
2
series, being M a4 d tran-
sition metal with increasing d-occupation number ( M =Ru,
Rh, and Pd.
The low-temperature properties of these systems can be
briefly summarized as follows. CeRu
2
Si
2
is a heavy fermion
and its low-temperature behavior is well described as a para-
magnetic Fermi liquid, in which no long-range magnetic or-
der has been observed down to 20 mK. The low-energy ex-
citations are strongly renormalized heavy quasiparticles, as
inferred from the large value of its specific-heat coefficient
.
3
CePd
2
Si
2
and CeRh
2
Si
2
show antiferromagnetic order
AF being their respective Ne
´
el temperatures T
N
10 K and
36 K. Both systems show a rather fast decrease in the value
of T
N
with applied pressure, which is the prelude of pressure
induced superconductivity in both cases. In CePd
2
Si
2
the
critical pressure at which AF is suppressed was reported at
P
C
2.5 GPa, while for CeRh
2
Si
2
a lower P
C
0.9 GPa de-
stroys the AF order.
4
Among the peculiarities shown by the systems under
study, there is a controversy about the nature of the 4 f states
in CeRh
2
Si
2
, on the one hand, the fact that CeRh
2
Si
2
has a
large ordering temperature and that the value of the magnetic
moment of Ce is close to the free-ion one (
ord
=1.5
B
)
Ref. 5 suggests that the 4 f states are very localized in this
compound. On the other hand, the evolution of these quan-
tities with applied pressure and the large value of the Kondo
temperature ( T
K
T
N
) as compared to other compounds of
the CeM
2
Si
2
family, leads to the assumption that the 4 f
states are strongly hybridized and quite delocalized.
6
Another
peculiar feature of CeRh
2
Si
2
, which makes out of it a singu-
lar system in the series, is the anomalous value of its lattice
parameters, showing the highest c / a value within the series.
With the aim of getting deeper insight into the different
kinds of behaviors observed in this family of compounds,
several experiments have been recently done to follow the
evolution of the c / a ratio and of the cell volume as a func-
tion of chemical substitution and also to study their relation-
ship with magnetic, thermal, and transport properties.
7,8
For
instance, it has been reported that a decrease in the concen-
tration of Rh in the alloy Ce(Ru
1 -x
Rh
x
)
2
Si
2
induces the
same effect as the application of pressure for the compound
with x =0.05. That is, under these two control parameters a
reduction of the c / a value and in the Ce-Ru distances take
place. This seems to play a relevant role in the observed
spin-density wave transition shown by this system.
9
Several
experimental studies on the variation of the lattice param-
eters upon alloying have already been undertaken
8,10
how-
ever, as far as we are aware of, their evolution under hydro-
PHYSICAL REVIEW B 69, 125116 2004
0163-1829/2004/6912/1251167/$22.50 ©2004 The American Physical Society 69 125116-1