Physica B 171 (1991) North-Holland
381-383
Thermal expansion and infrared optical properties heavy-fermion CeNiSn F.G. Aliev”, A.I. Belogorochov”, V.V. Moshchalkov”, M.A. Lopez de la Torreb, S. Vieirab and R. Villarb
of
R.V. Scolozdra”,
“Low Temperature Physics Laboratory, Department of Physics, Moscow State University, 117234 Moscow, USSR ‘Laboratorio de Bajas Temperaturas, Dpto de Fisica de la Materia Condensada, Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid, Spain
Thermal expansion (Y(0.5 < T < 50) K properties and infrared (IR) optical spectra (4.2 < T < 310) K of CeNiSn - a new heavy-fermion compound with a small gap at Fermi level - are presented. Our data strongly suggest the absence of a magnetic transition down to 2 K. At lower temperatures a peak near 1.3 K in the a/T temperature dependence and a change of sign of (Y below 1 K was observed. These features possibly indicate the appearance of magnetic correlations below 1 K.
ground state of CeNiSn through thermal expansion experiments and IR optical properties.
1. Introduction Most of the heavy-fermion (HF) or valencefluctuation (VF) compounds, demonstrating the appearance of a correlation gap at the Fermi level at low temperatures (e.g., SmS, SmB,, see ref. [l]), consist of elements with YbB,,, covalent properties. Recently, a new HF compound, intermetallic CeNiSn, was found to be characterized by a gap E, - 6-10 K at the Fermi level [2-91. The appearance of a gap was confirmed by measurements of the heat capacity [3,5,6], thermoelectric power [3] and Hall coefficient [6, lo]. Studying the properties of single crystalline CeNiSn, Takabatake et al. [9] have found that the gap is anisotropic. In that paper also the onset of antiferromagnetic correlations near 12K was suggested. On the other hand, NMR data [ll] on these crystals did not reveal any magnetic transition down to 0.5 K. It is obvious that CeNiSn is near a magnetic instability. In fact, a small substitution of Cu for Ni leads to an antiferromagnetic Kondo state [6,7], but a substitution of Ce by La leads to a nonmagnetic Kondo state [5]. In this paper we will try to determine the 0921-4526/91/$03.50
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2. Experiment CeNiSn was characterized as a valence-fluctuating compound with the &-TiNiSn structure by Scolozdra et al. [12]. This crystal structure can be presented as consisting of Ce layers in the b-c plane separated by two layers of Ni and Sn atoms. Details of the preparation of our polycrystalline CeNiSn samples have been published previously [3,4]. Thermal expansion measurements were made using the three terminal capacitance method, as described in ref. [13]. In our thermal expansion cell, the sample and a copper block containing two thermometers are held between a flat copper plate (high-potential electrode) and a copper base. Three silicon columns attached to this base hold the guard ring which surrounds the lowpotential electrode, both of which are located above the high-potential electrode and parallel to it. A thin Al,O, plate is inserted between sample and base for electrical insulation. The
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I Properties of heavy-fermion CeNiSn
thermal expansion experiments were performed in a pumped He’ cryostat provided with coaxial leads for the capacitance measurements extending down to the cold chamber. A GR 1621 bridge was used to detect changes in capacitance. Optical properties were studied on an infrared Fourier spectrometer ‘Brucker IFS-113V’. Figure 1 shows the temperature dependence of thermal expansion coefficient a(T) at temperatures between 0.5 and 15 K. At T = 5-10 K some deviation from linear temperature dependence is seen. More intriguing features were observed below 2 K: a maximum in a(T) (T,,, = 1.3 K) and a change of sign below 1 K. The amplitude of the negative a(T) anomaly of CeNiSn is unusually large, but down to 0.5 K no minimum was found. The main features of the thermal expansion are seen more clearly on the normalized LY / T( T) dependence (fig. 2). In these coordinates the thermal expansion coefficient seems to be linear in temperature for 15 < T < 40 K and demonstrates two maxima, at about 7 and 1.2 K (see fig. 2b). The energy dependence of the reflection R(w), studied at 150 < w < 500 cm-‘, revealed the presence of a strong phonon line at w = 343 cm-‘. Figure 3 shows of fragment of R(w) at four different temperatures 4.2 < T < 310 K. Decreasing of temperature makes the phonon line sharper, but below the onset temperature of 1 xl
1x10m6
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I-
-IxlO-@
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0.5
(b)1
1.0
1.5
2.0
2.5
T(K)
Fig. vs. the we
2. (a) Thermal expansion coefficient of CeNiSn as (Y/T T. Dashed line is intended as a guide to the eye showing linear dependence of a(T) between 15 and 40K. In (b) represent-a/T vs. T below 2.5 K.
o-5
1
A
7 Y
0
Y
,B
I
0
I
:
/
-1x1o-3
5
I
1
10
15
T(K)
Fig. 1. Thermal below 15 K.
expansion
coefficient
of CeNiSn
as a vs. T
Fig. 3. Energy dependences at various temperatures.
of reflectance
R(w) for CeNiSn
F. G. Aliev et al. I Properties of heavy-fermion
magnetic correlations (T = 12 K), proposed by Takabatake et al. [lo], no splitting or appearance of other phonon lines was observed.
3. Discussions In our opinion, our infrared optic data indicate the absence of a magnetic transition in the temperature interval where the gap in the energy spectrum of CeNiSn opens. In the case of URu,Si, a magnetic transition with a very small ordered moment of about 0.02~rJU atom induces a new line in the phonon spectra [14]. The absence of magnetic correlations around 10 K is also strongly suggested by thermal expansion data. The maxima in a(T) just below the Kondo temperature, observed in all classic HFC like CeAl,, CeCu,Si, (see ref. [15]), is clear indication for the appearance of an Abrikosov-Suhl resonance near the Fermi level and the formation of a ground state with a high effective mass. The anomaly in the thermal expansion around 1.3 K and the change of sign at T < 1 K probably indicates the onset of magnetic correlations. The Hall coefficient also displays a change of sign below 3 K [6, lo]. The presence of some magnetic correlations in the ground state, possibly, a common feature of ‘so-called’ nonmagnetic HFS, results in giant negative thermal expansion coefficients, like in CeAl, [16] or CeNiSn. Magnetic type quasiparticles may be a reason for a transformation of the density of states g(E) near the Fermi level, resulting in a linear energy dependence g(E) - E in the vicinity of E,. This type of g(E) dependence was recently observed by Kitaoka et al. in NMR experiments [ll]. A transformation of the energy spectrum from usual to a linear gap below 2 K is also possible and one of the main reasons for the resistivity saturation at T < 2 K. In conclusion, the results presented in our paper strongly suggest the possible development of some kind of antiferromagnetic type correlations in the ground state of the narrow gap heavy-fermion CeNiSn, which may change the density of states near the Fermi level inside the gap.
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Acknowledgements
The authors gratefully acknowledge helpful discussions with N.B. Brandt, D.I. Choumski, H. Fujii and T. Takabatake for sending reprints. One of the authors (F.G.A.) gratefully acknowledges a financial support from the Autonoma University of Madrid. The work at Madrid was supported by grant MAT-88.0717 from Plan National de Materiales.
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