Non-Fermi-liquid behavior in CeNi2Ge2 single crystals

Journal of Magnetism and Magnetic Materials 177 181 (1998) 409-410

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Non-Fermi-liquid behavior in CeNi2Ge2 single crystals H. Sato a'*, Y. Aoki a, J. Urakawa a, T.D. Matsuda a, H. Sugawara a, T. F u k u h a r a b, K. Maezawa b aDepartment of Physics, Tokyo Metropolitan University, Minami-Ohsawa 1-1, Hachioji-shi, Tokyo 192-03, Japan bFaculty of Engineering, Toyama Prefectural University, Toyama 939, Japan

Abstract

The specific heat (C) and Hall coefficient (Rn) in CeNi2Ge2 single crystals have been investigated for the field (B) along the c- and a-axis. At B = 0, C/T varies approximately as T 1/2 down to 0.2 K, suggesting non-Fermi-liquid behavior. On application of B, we found a recovery of Fermi-liquid behavior in C, which is only weakly anisotropic. In contrast, the temperature dependence of RH is anisotropic; the sign is negative for Bllc and positive for B[la below 300 K. © 1998 Elsevier Science B.V. All rights reserved.

Keywords. Non-Fermi liquid, Specific heat - low temperature; Hall effect; Anisotropy - RE compounds

The heavy-fermion compounds CeNi2Gez and CeRuzSi2, having similar unit cell volumes ( ~ 170 ~a) of ThCrzSi2 structure, show close similarities; i.e. neither magnetic order nor superconductivity down to ~ 10 mK and almost the same specific-heat coefficient of 350mJ/KZmol. The metamagnetic-like transition near 77 kOe in CeRu2Si2 [1, 2] is one of the most interesting observation in heavy-Fermion systems, whose mechanism is still a matter of controversy. Recently, a similar metamagnetic-like transition has been found also in CeNizGe2 near 420 kOe [3]. On the other hand, the anisotropy in the magnetic susceptibility was reported to be highly different between the two compounds; i.e. Xc/Z, is ~ 1 for CeNi2Ge2 and is > 20 for CeRuzSi2. To compare further the anisotropy and the non-fermi liquid behavior, we have measured the anisotropy of the specific heat (C) and the transport properties in CeNizGe2 single crystals under the magnetic field (B) up to 80 kOe and the temperature (T) down to ~ 200 mK. Single crystals were grown by Czochralski pulling method using a tetra-arc furnace, and were purified by the solid-state electrotransport. The electrical resistivity and Hall effect have been measured by the ordinary DC

four-probe method. The magnetic measurements have been made with a quantum design SQUID magnetometer. Fig. 1 shows C/T versus ~ at several values of magnetic field for Brrc-axis. At B = 0, C/T continuously increases roughly as ~ with decreasing temperature showing no tendency to saturate down to the lowest temperature. Such a non-Fermi-liquid behavior was

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*Corresponding author. Tel.: +81 426 77 2507; fax: + 81 426 77 2483; e-mail: [email protected].

Fig. 1. C/T versus. ~ plot for BPIc.The inset shows TD versus B phase diagram.

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already reported by Steglich et al. on a polycrystalline sample [4]. Similar non-Fermi-liquid behavior in C / T has been found also in CeRu2Si2 just under the metamagnetic transition field of 77 kOe [2]. U p o n application of B, a Fermi-liquid behavior gradually restores at lower temperatures. Above 40 kOe, there appears a shallow maximum slightly below the temperature TD where C / T starts to deviate from the higher temperature x / T dependence. The specific-heat reported for a polycrystalline sample [4] is close to our data for Bllc, suggesting a small anisotropy. To further check the anisotropy, we have also measured C for Blla. We found the data for Bl]a (not shown) are almost the same as that for Bile, which is in striking contrast with the large anisotropy in CeRu2Si2. This might not be so strange, since the anisotropy in the magnetic susceptibility 00 is small compared to that in CeRu2Si2 as shown in Fig. 2. In fact, the crystal field ground state estimated from )~(T) for CeRu2Si2 is basically _+ 2s with a strong Ising character, while for CeNi2Ge2 up to 18% of _+ 23is mixed with _+ ~. The first excited state is estimated to be about 200 K above the ground state for both systems. Fig. 3 shows the temperature dependence of the Hall coefficient at l0 kOe for both B[la- and c-axis. There exist several points to be noticed compared to CeRuzSi2. The anisotropy of RH in CeNi2Ge2 is larger compared to CeRuzSi2, despite the smaller anisotropy in 7~. R~ is only weakly temperature dependent with positive sign, while R~ shows a large temperature dependence, suggesting the extraordinary Hall effect due to the incoherent Kondo scattering. However, it must be noted that R . for

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the Ce heavy-Fermion compounds usually increases to a 'positive value' with decreasing temperature and decreases at lower temperatures after a positive peak due to the coherent effect. At the moment, we have no definitive explanation why the extraordinary part of Ru in CeNi2Ge2 has different sign. For the coherent state in heavy fermion, Kontani et al. reported a scaling relation b e t w e e n R n and the resistivity p as RH ~ Ap 2 [5], which was reported to be fulfilled also for CeRu2Si2. For CeNi2Ge2, the relation is not fulfilled for Bllc above 0.5 K and found that the breakdown of the relation might be related with the non-Fermi-liquid effect; in fact, the resistivity shows non-Fermi-liquid behavior of p ~ T 15 at low fields [4, 6]. Another unexplainable feature of RH is the low-temperature peak for both directions; ~ 7 K for B}la and ~ 15 K for BIIc, which seem to be too low as coherent peaks. That might be related to the upturn found in the temperature dependence of Z below 10 K (see Fig. 2), however, the origin is not yet clear. More work is necessary to clarify these points. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.

References

[1] S. Kambe et al., J. Low Temp. Phys. 102 (1996) 477. I-2] Y. Aoki, et al., J. Magn. Magn. Mater. 177 181 (1998), these proceedings. I-3] T. Fukuhara et al., J. Phys. Soc. Japan 65 (1996) 1559. [-4] F. Steglich et al., J. Phys.: Condens. Matter 8 (1996) 9909. 1-5] H. Kontani et al., J. Phys. Soc. Japan 63 (1994) 2627. [6] S.R. Julian et al., J. Phys.: Condens. Matter 8 (1996) 9675.