Electrical and thermal transport in CeNi and LaNi

Article abstract of DOI:10.1016/j.physb.2006.01.277

We have measured the electrical resistivity and thermal conductivity of CeNi, LaNi and La _0.15Ce _0.85Ni in the temperature range 4-400 K simultaneously on the same specimen using the TTO option in PPMS (Quantum Design) facility. Anom

Full text of DOI:10.1016/j.physb.2006.01.277

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Physica B 378–380 (2006) 758–759
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Electrical and thermal transport in CeNi and LaNi
Ã
Alexandra Rudajevova
´
, Denis Vasylyev, Ondr
ˇ
ej Musil
Faculty of Mathematics and Physics, Department of Electronic Structures, Charles University, CZ-121 16 Prague 2, The Czech Republic
Abstract
We have measured the electrical resistivity and thermal conductivity of CeNi, LaNi and La0:15Ce0:85Ni in the temperature range
4–400 K simultaneously on the same specimen using the TTO option in PPMS (Quantum Design) facility. Anomalous behaviour of the
resistivity and the Lorenz number for CeNi and La0:15Ce0:85Ni can be attributed to valence fluctuations.
r 2006 Elsevier B.V. All rights reserved.
PACS: 44.10.+i; 65.40.ꢀb; 72.15.Qm; 72.15.ꢀv
Keywords: Valence fluctuation; Thermal and electrical conductivity
CeNi belongs to the intermetallic compounds, in which
the existence of valence fluctuations was confirmed
experimentally by measurement of the thermal expansion
characteristics, electrical resistivity, specific heat or by the
magnetic measurements [1–3]. Clementyev et al. [4] showed
by neutron scattering experiments anomalous electron–
phonon interaction. The scattering of the conduction
electrons and phonons in the systems with instabilities
causes the anomalies in the transport properties. The aim
of this work is the determination of the temperature
dependences of the thermal conductivity and electrical
resistivity in CeNi, La0:15Ce0:85Ni and LaNi intermetallic
compounds for a better understanding of their relation if
the Ce-4f states are strongly hybridized with conduction–
electrons states.
Polycrystalline samples of CeNi, La0:15Ce0:85Ni and
LaNi were prepared by arc-melting of stoichiometric
amount of the pure elements (Ce–3N, La and Ni–4N) in
Ar atmosphere. Thermal conductivity and electrical
resistivity were measured using the standard PPMS 14 T
device. The electrical resistivity and thermal conductivity
were measured on the same sample simultaneously (Fig. 1).
Obtained values of the electrical resistivity r of a LaNi
are higher than the published values at T ¼ 300 K.
For example, the value r ¼ 2:0 ꢁ 10ꢀ7 Om is presented in
Ref. [3]. r300 K for monocrystalline LaNi measured along
the a- and c-axis was found to be 4.0 and 3:0 ꢁ 10ꢀ7 O m,
respectively [1]. r of CeNi measured by us is in a good
agreement with published values. r of CeNi monocrystal
measured along the a- and c-axis was found to be 7.8 and
9:7 ꢁ 10ꢀ7Om in Ref. [1]. But the character of the
temperature dependence of the electrical resistivity is the
same as in our work for both compounds. La0:15Ce0:85Ni
has much higher r0 (residual electrical resistivity) than
CeNi, while it is bit smaller at high temperature than CeNi.
The Matthiessen’s rule does not work, which is a quite
general characteristic of impurity scattering in valence
fluctuations [5]. The flattening makes the knee around T ¼
100 K in CeNi less apparent than in La0:15Ce0:85Ni, but we
may assume that its position, which represents the energy
of fluctuation is not dramatically shifted. The thermal
conductivity of CeNi and La0:15Ce0:85Ni is practically the
same in temperature range 50–400 K. The pronounced
maximum in the temperature dependence of the thermal
conductivity of CeNi is obviously due to small residual
thermal resistivity [4]. The thermal conductivity of LaNi is
the highest from all measured alloys in the temperature
range 20–400 K.
The simultaneous measurement of the electrical resistiv-
ity and thermal conductivity allows us to use the
Wiedemann–Franz law (WFL) for the analysis of the
obtained results (residual electrical and thermal resistivities
are not reproducible quantities). By the Lorenz rule, the
Ã
Corresponding author. Tel.: +420 2 2191 1356; fax: +420 2 2491 1061.
E-mail address: rud@mag.mff.cuni.cz (A. Rudajevova).
´
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doi:10.1016/j.physb.2006.01.277

ARTICLE IN PRESS
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A. Rudajevova et al. / Physica B 378–380 (2006) 758–759
759
Fig. 1. (a) Temperature dependence of the electrical resistivity for LaNi,
CeNi and La0:15Ce0:85Ni. (b) Temperature dependence of the thermal
conductivity for LaNi, CeNi and La0:15Ce0:85Ni.
Fig. 2. Temperature dependence of the Lorenz number for LaNi, CeNi
and La0:15Ce0:85Ni. Inset: temperature dependence of the differences of the
Lorenz number between CeNi, La0:15Ce0:85Ni and LaNi.
ratio of the conductivities for heat and electrical current of
pure metals is proportional to the absolute temperature.
The proportionality constant is called the Lorenz number
L which would be equal to L0 ¼ 2:44 ꢁ 10ꢀ8 WOKꢀ2
(Sommerfeld value) if the electron gas is highly degenerate
and also if the electron mean free path (l) is the same for
electrical and thermal conductivities. The values of the l for
electrical and thermal conductivities, however, depend on
the nature of the scattering process. Deviations from the
Sommerfeld value of the Lorenz number are due to various
reasons. In metals at low temperatures, the deviations are
due to the inelastic nature of electron–phonon interactions.
The higher Lorenz number in alloys is due to phonon
contribution of the thermal conductivity. The deviations in
Lorenz number occur also due to the changes in band
structure. Other variations of the Lorenz number are due
to magnetic field or phase transitions.
The temperature dependence of the Lorenz number is
seen in Fig. 2. We notice that L is nearly identical for CeNi
and LaNi in the temperature range of 120–200 K. The
shallow minimum of L below 70K for LaNi is probably
connected with the fact that in this temperature range the l
for electrons is not the same in electrical and thermal
conductivity (WF rule does not work). The higher values
than L0 of the Lorenz number are due to existence of a
phonon thermal conductivity in all compounds studied. We
can express the total Lorenz number as sum of several
contributions: L ¼ L0 þ DLanom þ DLphon þ DLfluct. Where
DLanom is a deviation due to difference of the l between
electron–phonon scattering in electrical and thermal con-
ductivity, DLphon is the contribution from the phonon
thermal conductivity and DLfluct is the contribution from the
hybridization of the 4f electrons with conduction electrons.
The last term is zero for LaNi. If we assume that the phonon
contribution is the same for all compounds studied and
anomaly due to the invalidity of the WF rule is also the same
for all compounds, then by subtracting of L for CeNi from
L for LaNi and LaNi from La0:15Ce0:85Ni compounds, we
can estimate the temperature dependences of the DLfluct
shown in Fig. 2—Inset. The sharp bend on the temperature
dependence of the DLfluct defines a characteristic tempera-
ture of the hybridization of the 4f electrons with conduction
electrons. This temperature is ꢂ 100 K for CeNi and 80K
for La0:15Ce0:85Ni. These temperatures are nearly the same
as temperatures when the maximal deviation on the thermal
expansion coefficient was found for these compounds [2]. At
the same temperatures, the deviation of the electrical
resistivity from linear dependence occurs in the temperature
dependence of this parameter (Fig. 1a). As a conclusion, we
can say that the valence fluctuations in CeNi and
La0:15Ce0:85Ni are responsible for detected anomalies in
the temperature dependences of electrical resistivity and
thermal conductivity.
This work is a part of the research program MSM
0021620834 that is financed by the Ministry of education of
the Czech Republic.
References
[1] D. Gignoux, et al., J. Less-Common Met. 94 (1983) 165.
[2] Y. Uwatoko, et al., J. Alloys Comp. 192 (1993) 242.
[3] E. Gratz, et al., J. Magn. Magn. Mater. 29 (1982) 181.
[4] E.S. Clementyev, et al., Phys. Rev. B 57 (1998) R8099.
[5] P.S. Riseborough, Solid State Commun. 38 (1981) 79.