High field magnetotransport and specific heat in YbAgCu4

The electrical resistivity (ρ{variant}) and magnetoresistance of polycrystalline YbAgCu4 have been measured at temperatures between 25 mK and 300 K, and at magnetic fields (B) up to 18 T. The magnetoresistance (ρ{variant}(B) - ρ{variant}(0))/ρ{variant}(0)) is positive at all temperatures below 200 K and reaches its maximum of 60% at 18 T and 25 mK. The field- and temperature-dependent resistivity does not scale in a simple way. The opposite sign of the magnetoresistance at ambient and high pressure can be explained qualitatively by crystal-field effects lifting the degeneracy of the J = 7 2 groundstate. The linear coefficient of the specific heat (γ) measured at fields up to 10 T shows a quadratic field dependence. We did not find a linear relation between γ2 and A, the T2-coefficient of the temperature-dependent resistivity, with the applied magnetic field as the implicit parameter. © 1995.

YbAgCu 4 is one of the few Yb-based intermetallic compounds with a large linear coefficient of the specific heat 3'--245 mJ/mol K z [1]. Its temperaturedependent magnetic susceptibility and specific heat are described well by the Coqblin-Schrieffer model with J = 7/2 and a characteristic energy scale T O ~ 160K [1,2]. Inelastic neutron scattering [3] finds no evidence for well-defined crystal-field excitations consistent with the susceptibility results. Application of pressure causes a rapid decrease in T .... the temperature at which the resistivity is maximal, and an increase in the T2-coefficient of the resistivity (A) [4,5], suggesting that OTo/aP<O. At sufficiently high pressures, it is distinctly possible that T 0 becomes much smaller than the crystal-field splitting of the J-multiplet, the ground state degeneracy is at least partially lifted and spin fluctuations increasingly dominate electrical transport * Corresponding author. at low temperatures. This possibility could provide a partial explanation for the significantly different magnetoresistive behavior of YbAgCu 4 at low and high pressures. At ambient pressure the magnetoresistance is positive for T<20K and fields <10T [4] but for pressures >70 kbar, the magnetoresistance is strongly negative [5]. To explore in more detail the origin of these opposite behaviors at low and high pressure (at large and small To, respectively) the specific heat (C), of YbAgCu 4 was measured in fields to 10T for temperatures 4 K ~< T ~< 10 K and the electrical resistivity at fields up to 18T and temperatures between 25 mK and 300 K.
The preparation of polycrystalline samples has been described previously [5]. The electrical resistivity was measured using a four lead AC resistance bridge (LR-400) operating at 17Hz. The magnetic field was applied perpendicularly to the current (transverse geometry) and was generated by a 20 T superconducting magnet at the National High Magnetic was measured in a small mass calorimeter utilizing a relaxation method. Fig. l(a) shows the temperature-dependent resistivity p of YbAgCu 4 in magnetic fields from 0 to 18 T. The specific heat divided by temperature is plotted in Fig. 2 as a function of T 2 for various applied fields. Solid lines are least square fits to the data and yield the linear coefficients y, which are shown in Fig. 3 to increase linearly with B 2. With the usual assumption that y ~ 1 / T o, this implies that T O is inversely~proportional to B 2. From the linear relation ),~VA found [7] for several heavy fermion compounds at zero field, A would, be expected to increase as B 4. Fig. 3 shows the measured change in A as a function of B 2.
Although A(B) increases superlinearly in B 2 for B <~ 12T, at higher fields A varies approximately as B 2. The inset of Fig. 3 clearly demonstrates the absence of a linear correlation between y and X/A for B ~< 10 T. This is contrary to what was found [8] when pressure was the implicit variable.
Qualitatively the different field responses of YbAgCu 4 at zero and high pressures can be understood as follows. Okiji and Kawakami [9] have shown for the J= 5/2 Coqblin-Schrieffer model that y increases approximately quadratically with field for B < 0.4 T o (B < 95 T for T o = 160 K). A similar situation is expected to hold for J = 7/2, i.e. YbAgCu 4 at ambient pressure. From the assumed relationship between 3' and A, therefore it would be expected that A increases with B, as found at ambient pressure. On the other hand, for J = 1/2, y decreases strongly with field

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. 13 • v" 12~-260 [9,10] and A should also be found decreasing with the field, as observed at high pressures [5]. Although, a change in ground state degeneracy appears to account qualitatively for observations at ambient and high pressure, there remain quantitative questions to be addressed. The 10% increase in y at 10T is larger than predicted, at least for J = 5/2. The large change in Oo in the applied field, for either ambient or high pressures, lacks a simple explanation, as does the field dependence of A and, more generally, of p(T). Additional high field measurements on heavy fermion systems are now in progress to identify to what extent these features are general [11].