Thermal and magnetic properties of CeNiSn have been measured on a highquality single crystal in magnetic fields in order to reveal the intrinsic ground state of this compound. The specific heat for the purified crystal has been found to remarkably depend on the field along the a axis in contrast with less field dependence for dirty samples. The electronic specific heat divided by temperature Cel/T increases around 2 K with increasing field along the a axis, indicating that the field enhances the density of states (DOS) around Fermi level EF in this temperature range. On the other hand, Cel/T around 7 K decreases with increasing field. To understand the origin of temperature and field dependence of Cel/T, we introduced a DOS model which consists of a V-shaped gap with a residual DOS in the Lorentzian peak around EF. The experimental result is well reproduced by assuming the Zeeman splitting of the DOS without changing the shape of the gap by magnetic field. This result indicates that the gap above 2 K is hardly affected by applying field at least up to 14 T.

The Cel/T below 2 K stays nearly constant in zero field. With further decreasing temperature, Cel/T slightly increases below 0.5 K, which suggests that, there is a peak structure at the bottom of the pseudo-gap around EF. However, the large reduction of Cel/T in the field has been found, suggesting that a rigid band picture is no more realistic below 1 K. The field dependence indicates that, the peak inside the pseudo-gap is collapsed by external field. In this study, we concluded that CeNiSn is classified as a new-type Kondo metal instead of a Kondo insulator.

Abstract
Contents / p1
1 Introduction / p5
　1.1 Ce-based compounds / p5
　1.2 CeNiSn / p6
2 Experimental / p10
　2.1 Sample preparation / p10
　2.2 Measurements / p10
3 Results / p14
　3.1 Specific heat / p14
　3.2 Magnetocaloric effect / p27
　3.3 Magnetic susceptibility / p29
4 Discussion / p31
　4.1 Magnetic field effect on the pseudo-gap / p31
　4.2 Peak in magnetic susceptibility / p36
　4.3 The electronic state inside the pseudo-gap / p38
5 Conclusion / p42