Kondo-lattice-mixed-valence resistance scaling in heavy-fermion CeCu6 under pressure.

PHYSICAL REVIE% B Kondo-lattice VOLUME 31, NUMBER — mixed-valence resistance scaling in heavy-fermion JANUARY 1985 CeCu6 under pressure J. D. Thompson and Z. Fisk Los Alamos Rational Laboratory, Los Alamos, Xew Mexico 87545 (Received 14 August 1984) From measurements of the electrical resistance of heavy-fermion CeCu6 subjected to hydrostatic pressures, we infer a continuous pressure-induced transition from Kondo-lattice to more strongly mixed-valence behavior. The resistance is found to scale over an appreciable temperature interval with a pressure-dependent characteristic temperature that reflects this transition. Similarities in the resistive behaviors of CeCu6 and CeA13 are discussed. There now appears to be a distinct class of materials characterized by huge electronic specific heats (y's) and correspondingly large effective electronic masses. ' Each of the known representatives of this class of heavy- fermion materials contain either 4f or 5f elements. Whether the large-y enhancements are a result of the in- trinsic electronic band structure or many-body effects remains an open question. What is clear, however, from thermal variations in properties such as specific heat, magnetic susceptibility, and resistivity is that electronic excitations from the heavy-mass ground state develop on a characteristic temperature scale that is small, typically much lower than phonon energies. Such observations sug- gest that sufficient energy could be supplied by routine high-pressure techniques to perturb noticeably the elec- tronic spectrum of these materials. Recent1y, CeCu6 was discovered to have an electronic specific heat y(T =0) of about 1. 6 J/mole K, making it one of the largest-y materials known. Many of the physi- cal properties of CeCu6 are strikingly similar to those ' of CeA13, which for some time has been considered a proto- typical Kondo-lattice system. Percheron et al. have shown that Kondo-like thermal variations in the electrical resistivity of CeA13 are strongly pressure dependent. Al- though a complete analysis of their data was hampered by irreversibilities in the resistivity large pressure-induced value of CeA13, they were able to conclude that the cerium 4f level moved closer to the Fermi level with applied pres- sure. seen by Percheron The troublesome irreversibility et al. was attributed to pressure-induced formation of an allotropic form of CeA13, with unknown properties. Be- cause of the possibility that CeCu6 might not suffer this complication and consequently represent a cleaner system to study, we have measured the effect of pressure on the electrical resistance of CeCu6. As we will show, in many ways the analogy between CeA13 and CeCu6 persists at high pressures. However, because the resistance of CeCu6 is reversible with pressure, a somewhat different and more straightforward interpretation of the high-pressure prop- erties of CeCu6 is possible. Four-probe ac resistance measurements were made on a single crystal of CeCu6 grown by slow cooling from a melt. The direction of current flow with respect to crys- X-ray analysis tallographic axes was not determined. showed only lines characteristic of the CeCu6 orthorhom- bic structure. The resistance was determined over the 300 K and at hydrostatic pres- temperature interval 1 — sures up to 18 kbar, generated by a self-clamping pressure cell whose operation has been described in detail else- where. resistance We show in Fig. 1 the temperature-dependent of CeCu6 at five fixed clamp pressures. Numbers beside each curve correspond to the order in which pressure was applied. Curve 5 fits smoothly between curves 2 and 3, indicating the absence of pressure hysteresis seen in CeA13. There are several additional qualitative features of these data that should be noted. At P =0, the resistance decreases with decreasing temperature until it reaches a shallow minimum centered around 195 K. Near 15 K, a well-defined resistive peak develops below which the resis- tance falls rapidly. This behavior is similar to that of CeA13 except the resistance of CeA13 shows no minimum below room temperature and the peak for P =0 occurs at 35 K. With increasing pressure, the peak resistance of CeCu6 becomes less prominent and is shifted to higher 0 kbar (1) kbar (2) II r CeCu II ~ ~ 0 g 8. 5 kbar (5) 1 1. 9 kbar (3) I SI ~]:~ I7. 4 'fe m m 0 kbar r kbar (4) 0. 8 -I I I I kbar kbar 0. 4 &. I I / S. S kbar I: gI I I I I I T(K) I S FIG. 1. Resistance clamp logical shows Curve vs temperature for CeCu6 at five different pressures. Numbers in parentheses indicate the chrono- order in which the curves were acquired. The inset resistance. an expanded view of the low-temperature 5 is not shown for clarity only. The American Physical Society