Anomalous field dependence of specific heat of CeNiSn below 1 K

Anomalous field dependence of specific heat of CeNiSn below 1 K

Journal of Magnetism and Magnetic Materials 177-181 (1998) 395 396 ELSEVIER ~ Journalof magnetism and magnetic ~l~ materials Anomalous field depen...

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Journal of Magnetism and Magnetic Materials 177-181 (1998) 395 396

ELSEVIER

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Journalof magnetism and magnetic ~l~ materials

Anomalous field dependence of specific heat of CeNiSn below 1 K Koichi Izawa a'*, Takashi Suzuki a, Toshizo Fujita a, Toshiro Takabatake b, Go Nakamoto c, H i r o n o b u Fujii c, K u n i h i k o Maezawa a aDepartment of Physics, Faculty of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739, Japan bDepartment of Materials Science, Hiroshima University, Higashi-Hiroshima 739, Japan cFaculty of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima 739, Japan dFaculty of Engineering, Toyama Prefectural University, Toyama 939-03, Japan

Abstract Specific heat measurements have been performed below 2 K in magnetic field up to 5 T using high-quality singlecrystalline CeNiSn which shows metallic conduction at low temperatures. Remarkable field dependence has been observed in the specific heat, suggesting the possibilit3~ that a new electronic ground state due to a many-body effect is realized in CeNiSn below 0.5 K. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Rare earth

ternary compounds; Ground state

A variety of experiments [ 1] suggest that a pseudo-gap opens in the Kondo resonance state of CeNiSn at low temperatures. The origin of the gap formation and its electronic details remain to be clarified. One of the experimental difficulties we have encountered so far is that the electronic properties are very sensitive to the purity of samples [2]. The best sample obtained recently shows metallic conduction at tow temperatures in contrast with semiconductor-like conduction of dirty samples used in earlier works. The large impurity effect is argued theoretically by Ikeda and Miyake [3]. In the study of the intrinsic features of this material, the sample quality is of essential importance. In our previous papers [4, 5], we reported the specific heat for a high-quality single crystal of CeNiSn, which was measured from 2 to 20 K in magnetic field/~oH up to 14 T. The field dependence obtained is well reproduced simply by taking account of the Zeeman splitting of the density of states (DOS) with the pseudo-gap in the Kondo resonance band. This appears to indicate the validity of the rigid-band model for the field up to t4 T in

*Corresponding author. Fax: + 81 824 24 0716; e-mail: [email protected].

properties; Density of states; Specific heat

low temperature

this temperature region above 2 K. In the present work, we measured the specific heat at lower temperatures in magnetic field to study the electronic state inside the gap. The single crystal of CeNiSn was grown by a Czochralski technique using a radio-frequency furnace. The crystal was purified by solid-state electrotransport. Details of sample preparation were reported elsewhere [2]. The specific heat measurements were carried out below 2 K in magnetic field up to 5 T using an adiabatic calorimeter mounted in a 3He-4He dilution refrigerator. Fig. 1 shows the ratio of specific heat to temperature C/T obtained at various magnetic fields of#oH = 0, 1, 2, 3, 4 and 5 T. In zero field, the slope of C/T changes the sign around 0.5 K and C/T slightly increases with increasing T, suggesting that there exists some additional structure inside the pseudo-gap of the DOS around EF. With increasing field along the a-axis which is the magnetically easy axis, the value of C/T decreases rapidly below 0.5 K and shows a minimum around 2 T (see the inset in Fig. t). The amount of decrease is more than 30%. After marking the minimum, C/T increases again with increasing field. Above 1 K, C/T monotonically increases with the field. Temperature dependence of

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K. lzawa et al. /Journal of Magnetism and Magnetic Materials 177-181 (1998) 395-396

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Fig. 1. Specific heat of CeNiSn divided by temperature C/T in various magnetic fields up to 5 T along the a-axis. The solid curve is C/T calculated by using our DOS model shown in Fig. 2. The inset shows C/T at 0.2 K plotted against #oH. The dashed line is a guide for the eye.

C also remarkably changes from T- to T2-depenedence below ~ 0.6 K above 1 T. Let us discuss the T and H dependence of C/T. We tried to fit the experimental data by introducing a peak structure at EF into the previous D O S model [4, 5] as illustrated in Fig. 2. The calculated value C/T for #oH = 0 is shown by a solid curve in Fig. 1. The lattice and nuclear spin contributions to the specific heat are neglected in the calculation. Temperature dependence of C/T in zero field is well reproduced with the parameters D = 58 K, A = 23 K, W = 2.0 K, I~o = (A/Tt)No = 7.1 × 1 0 - 4 K - l a n d d b ~ / b T o = 0 . 1 9 , where A is a normalization factor to satisfy the condition ~N(E)dE = 1 and the parameter No is equal to 3.2 x 10 -3 K - 1 . However, the decrease (30%) in C/T at 2 T cannot be explained with dN/b~o = 0.19, as far as we assume only the Zeeman splitting of rigid partial bands as illustrated in Fig. 2. A possible explanation is that the newly introduced peak at EF in the D O S model is dynamically reduced by applying the magnetic field. Possibly, a new metallic state grows below ~ 0.5 K due to some many-body effect with characteristic temperature T * < 1 K. The increase in C/T above 2 T is reasonably explained by overlapping both the edges of the pseudo-gap at E ~< Eg~ and E ~> Ugh due to the Zeeman effect. In summary, unusual H and T dependence of C/T was found for high-quality single-crystalline CeNiSn below

Fig. 2. Schematic DOS with a residual DOS inside the pseudo gap. Solid and dashed curves indicate the peak structure and the fiat residual DOS, respectively. The parameters D, A and W are the half-width of the Lorentzian, the V-shaped gap and the bottom of the gap. The fiat residual DOS and the height of the peak structure are expressed as bTo and d]V, respectively.

2 K in magnetic field up to 5 T. The results suggest the growth of a new many-body structure inside the pseudogap. We would like to thank Professor K. Miyake for valuable discussions. This work is supported by a Grantin-Aid for Scientific Research from Ministry of Education, Science, Sports and Culture of Japan.

References

rl] T. Takabatake, G. Nakamoto, H. Tanaka, Y. Bando, H. Fujii, S. Nishigori, H. Goshima, T. Suzuki, T. Fujita, I. Oguro, T. Hiraoka, S.K. Malik, Physica B 199&200 (1994) 457. I-2] G. Nakamoto, T. Takabatake, H. Fujii, A. Minami, K. Maezawa, I. Oguro, A.A. Menovsky, J. Phys. Soc. Japan 64 (1995) 4834. I-3] H. Ikeda, K. Miyake, J. Phys. Soc. Japan 65 (1996) 1769. [4] K. Izawa, T. Suzuki, M. Kitamura, T. Fujita, T. Takabatake, G. Nakamoto, H. Fujii, K. Maezawa, J. Phys. Soc. Japan 65 (1996) 3119. 1-5] K. Izawa, T. Suzuki, M. Kitamura, T. Fujita, T, Takabatake, G. Nakamoto, H. Fujii, Physica B 230-232 (1997) 670.