Specific heat and electrical resistivity of CeCu6 below 1 k

Abstract Results of measurements of the electrical resistivity ϱ between 0.04 and 1 K and the specific heat cp between 0.06 and 1 K of annealed polycrystalline CeCu6 are reported. ϱ varies proportional to T2 below 0.1 K but is linear in T above 0.6 K. The specific heat is proportional to T below 0.5 K and the electronic specific-heat parameter γ = 1.53 J mole K 2 .

Results of measurements of the electrical resistivity p between 0.04 and I K and the specific heat cn between 0.06 and I K of annealed polycrystalline CeCu 6 are reported, p varies proportional to T 2 below 0.1K but is linear in T above 0.6 K.The specific heat is proportional to T below 0.5 K and the electronic specific-heat parameter y = 1.53 3/mole K 2.
R~c~nt work reported by two different groups" " has shown that CeCu 6 has many of the properties believed to be characteristic of materials which are now often denoted 8s dense Kondo-or heavy-electron systems.The reported properties of this compound ate remarkably similar to those of CeAI 3.Among them we mention the electrical resistivity which in both cases increases with decreasing temperature below room temperature, reaches a maximum below 50 K and then decreases at still lower temperatures down to I K and below 3 5.In specific-heat experiments, a strong increase of the cp/T ratio is observed for both substances below 8 K, reaching a value of about 0.85 3/mole K 2 st 1.8 K with a still steep and negative slope at this temperature 2 6.Since it seemed quite natural to check For more similarities of these two materials at even lower temperatures, in this letter we report on measurements of the electrical resistivity p and the specific heat Cp of CeCu 6 below I K.
For these experiments, polycrystalline CeCu 6 was prepared by arc-melting the pure elements together in a water-cooled arc Furnace under argon atmosphere.The sample was subsequently annealed For four days at 750°C.This annealing procedure had no apparent effect on the magnitude or temperature dependence of the specific heat above I K.It proved, however, to be of paramount importance For the low-temperature behaviour of the electrical resistivity.To illustrate this effect, we show p(T) measured on a small piece cut From the annealed sample between I and 300 K in 300 same pattern with line broadening at large angles, probably due in pert to cold working on the sample when preparing the X-ray specimen.
The results of our resistivity measurements below 1K are shown in Fig. 2. With decreasing temperature, p First decreases linearly with T, amazingly extrapolating to zero resistivity at T = 0 K. Below 0.6 K, however, p(T) deviates From this behaviour and approaches s residual resistivity of 7.2 pOcm at zero temperature.It is not possible to express p(T) as a simple power law in temperature below 0.6 K.It is only below O.I K that there is any 2 suggestion of s T dependence of the resistivity as it is, or course, expected for quasiparticle scattering in a Fermi liquid for temperatures well below the Fermi temperature T F end was clearly observed in CeAI 3 below 0.3 K 4. In the mentioned limite~ temperature range, the coefficient of the T" term For our sample of CeCu 6 is 111 #0cm/K 2, roughly a factor of three larger than that observed in CeA] 3 4. Apart from a clearly higher residual resistivity in our case as compared to that observed in CeA13, we only have a weak case for a T 2 law of p(T) because the temperature interval for its approximate validity is obviously too narrow to come to a definite conc]usion.It is, however, clear that in both cases of CeCu 6 and CeAI 3 the e]ectrical resistivity is still varying considerably below I K, very atypical for ordinary metals and pointing to efficient scattering processes in that temperature range.As shown in Fig. I, the peak resistivity of CeCu 6 at 1) K is about 100 ~Qcm, considerably less than that of CeA] 3 at its peak temperature of )5 K 5 Interesting]y, this reduction of the peak temperature for CeCu 6 as compared to that of CeA] 3 is approximately the same as that observed for the respective temperature intervals where a T 2 dependence of the electrical resistivity is established.The same factor of three is recognized in the ratio of the T 2 coefficients for p, the scaling between the two compounds being inversed in this case, however.
The specific-heat data obtained for temperatures less than 1 K are shown in Fig. 3.It may readily be seen that the large electronic specific-heat parameter y suggested by earlier measurements at temperatures above 1.5 K ~ is indeed borne out.The limiting value as T approaches 0 K is 1.53 J/mole K , ver~ close to that reported previously for CeAI 3 .It is also clear from Fig. ) that the Cp/f ratio is temperature dependent above 0.5 K. as it increases above the mentioned value for T ÷ 0 K and, when taking into account the data published in ref. 2, must pass through a maximum in the vicinity of I K.A temperature dependence of this kind for co/T has recently been reported for CeAI 3 with ~ peak of cp/T near 0.5 K 7. The existence of this peak, which had already been observed in earlier work 8 has been interpreted in ref. 7 as evidence for a coherence effect in the electronic energy spectrum of a dense Kondo system.From our data this evidence is much less well developed in CeCu 6.Moreover, the scaling behaviour that may be recognized when comparing the electrical resisti- vities of CeCu 6 and CeAI 3 does not seem to be reflected in the low-temperature specific heats of the two compounds.Taking all the presented data together, CeCu 6 seems to fi E comfortably into the framework provided by current thinking of dense Kondo-or heavy-electron Ce-based systems 9.The lack of any phase transition, magnetic or superconducting, is another analogy with CeAI 3.However, the limiting Fermi-liquid behaviour appears to occur at lower temperatures for CeCu6, at least as it is indicated by p(f).It might be of interest to investigate, how much of the qualitative differences in the properties of CeCu6 and CeAl 3 may be ascribed to crysta]-e]ectric-field effects.In both compounds, the 4f e]ectron J = 5/2 Hund's ru]e ground state of the Ce 3+ ions is split into three doublets because of the symmetries of the respective crystal-lattice structures, hexagonal in the case of CeAI 3 I0 and orthorhombic for CeCu 6 ii.Differences in the separation of these energy ]evels might, in part, account for the differences observed in the p(T) data~ Since the published specific-heat results 2 and the apparent dependence of p(T) upon annealing in CeCu 6 rather contradict than support this conjecture, inelastic neutron-scattering experiments are clearly called for and the availability of single crystals I will certainly greatly facilitate such measurements in comparison with CeAI 3 12 where still only polycrystalline material is available.The same holds for other important experiments such as investigations of possible anisotropies in the physical properties which may now be peformed on CeCu6, a system so much alike the prototype metallic compound CeAI3, showing Fermi-liquid behaviour of its electronic subsystem at very low temperatures.
Acknowledgements -We wish to thank R.B. Roof for the X-ray measurements.Financial support from the Schweizerische Nationalfonds zur F6rderung der wissenschaftlichen Forschung is gratefully acknowledged.Work at Los Alamos was performed under the auspices of th U.5.Department of Energy.
SPECIFIC HEAT AND ELECTRICAL RESISTIVITY OF CeCu 6 BELOW I K H.R. Ott and H. Rudigier Laboratorium fdr FestkSrperphysik, ETH-HSnggerberg, 8093 Zdrich, Switzerland and Z. Fisk, J.O. Willis and G.R. Stewart Materials-Science and Technology Division, Los A1smos National Laboratory, Los Alamos, New Mexico 87545 (Received 10 October 1984 by P. Wachter)

Fig. 1 .Fig
Fig. I: Temperature dependence of the electrical resistivity of annealed CeCu 6 between 1.3 and 300 K.

Fig. 2 :
Fig. 2: Temperature dependence of the electrical resistivity of annealed CeCu 6 between 0.04 and I K.