Hall effect in the heavy fermion systems CeCu6 and UBe13

Abstract The Hall effect in both UBe13 and CeCu6 becomes very large at low temperature showing, with the resistivity and specific heat, the transition into the heavy fermion state. The Hall constant of CeCu6 changes sign, on cooling into the coherently scattering regime.

Heavy fermion systems [1,2] have a large density of states at the Fermi level, as indicated by their enormous low temperature specific heats. The origin of this narrow (of order 10 K) peak is a many-body Abriko-hov_Suhl resonance, as occurs for Kondo impurities and/or the hybridization of a very narrow f level with a d band. Surprisingly. these systems can be superconducting. as well as magnetically ordered, or unordered. Previous studies of the Hall effect in the heavy fermion systems, CeCu?Si ?. CeAI, and UB,3 have shown a positive Hall constant.
R,,. which increases to a very large value, without a sign change, with decreasing temperature.
[2-e]. For CePd,. which is not extremeI! heavy, R,, is large and positive above 10 K. but is negative beIon> [h]. single crystal of ref. [14] shou a small posltibe Hall constant at room temperature.
which rises monotonically to a large value at the lowest temperature measured. about 4 K. This behavior is similar to our:, aboLe 20 K. but there is no peak and ncj sign change.
0304-8853/86/$03.50 *t:) Elsevier Science Publishers The reason for this difference may be that our samples (and those of refs. [ll] and 121) h ave considerably lower resistivities at low temperature than those of ref. [9]. and may therefore be closer to the 'coherent regime. For comparison. R H of CeAl, increases positively to a large value at 2 K, where p is large, but was not measured in the very low temperature coherent regime [5].
The electronic (linear) contribution to the specific heat is normally determined from a plot of C/T vs. T'. For CeCu,, such a plot [8] is linear between about 10 K and 30 K with an intercept of about 250 mJ/mol K2. As the temperature decreases below 8 K, C/T (from ref. [8], replotted in fig. 1) shows a sharp rise. According to ref.
[Ill, C/T = 1530 mJ/mol K2 between 0.1 and 0.5 K and is slighly higher at 1.0 K. The C/T results of ref.
[15] are in general agreement with these. For comparison Cu has a C/T value of less than 1 mJ/mol K2. These results indicate that CeCu, is already very heavy at 30 K in the region where p and R, are increasing with decreasing T. showing increasing incoherent scattering. The transition from incoherent to coherent scatttering which occurs below 10 K, correlates with the strong increase in C/T. UBe,, makes an interesting comparison with CeCu,. The resistivity of UBe,3 also rises as T decreases from room temperature. (fig. 2). It reaches a shoulder around 20 K. then below 4 K rises slightly to a small peak at 2 K. Below this peak. the decrease in p may indicate the onset of coherent scattering, as in CeCu,. However, before a small p is reached, the sample becomes superconducting at 0.9 K [7]. The Hall constant is positive at room temperature T and increases with decreasing T. At 4 K. the rate of change increases sharply as the extreme heavy regime is entered. R H peaks at about 1.5 K, below the resistivity peak, then decreases by about 20% before superconductivity sets in. One wonders if R,, would go strongly negative and p would tend smoothly to a low value, as in CeCu,, had superconductivity not occurred first. Our Hall results are in agreement with the previous ones [4] at the temperatures reported (100. 4.2, 3, 1.9 K). The specific heat of UBe,3, is nearly linear between 12 K and 7 K, with C/T about 150 mJ/mol K2.
[7]. However, between 4 and 0.9 K. C/T increases to 800 mJ/mol K'. Like CeCu,, UBe,, goes from moderately heavy to extremely heavy, where C is not linear with T. Both C/T and x should be very large for T c TF if T, is very small. However. C/T shows the onset of the heavy fermion regime more clearly than x (figs. 1 and 3) because XT is already large at high temperature in the local moment regime.
In magnetic systems, the Hall resistivity [16] may be written as ph = ROB + R,477M. Here R, and R, are the ordinary and spontaneous Hall coefficients. In paramagnetic systems, R, = R,, + R,4q * where x * = x/(1 + 4nx). If skew scattering dominates both p and R,. then R, is proportional to p. For side jump scatter- This single impurity approach does not apply to the coherent regime.
It has been suggested [2,4,5] that a two band model with light and heavy bands could explain the large Hall effect in CeAI, and UBe,,.
Sharp structure in the density of states, suggested to explain the strongly temperature dependent specific heat and thermopower in Kondo lattices [19] could also cause R,, to change rapidly with temperature.
Hall and resistivity measurements were made on two samples of each system, with essentially the same results. The preparation and properties of the UBe,, single crystals are described in ref. [7]. The CeCu, was cooled slowly from the melt in a Ta crucible. Our sample, a large grain polycrystalline disk, showed some anisotropy in the susceptibility. The x data in fig. 1 is for the field in the plane of the disk, the direction with the largest x. The Hall measurements were made with the field perpendicular to the disk. In conclusion.
the first Hall measurements on a heavy fermion system, CeCu,, showing the transition to the coherently scattering regime, have been made. On cooling from the incoherently scattering high resistivity region to the low resistivity region, the Hall constant changes from strongly positive to strongly negative. In both systems studied. the behavior of p and R, are correlated with the transition into the heavy fermion state, as determined by C/T.