•R. Lortz¹, G. Goll², K. Grube¹, C. Meingast¹, H. v. Löhneysen^1,2 und T. Takabatake^3
1Forschungszentrum Karlsruhe, Institut für Festkörperphysik, 76021 Karlsruhe
²Physikalisches Institut, Universität Karlsruhe, 76128 Karlsruhe
³Department of Quantum Matter, ADSM, Hiroshima University Higashi-Hiroshima 739-8526, Japan

CeBiPt is a low-carrier-density semimetal ( n = 7.7·10¹⁷ cm^-3). Shubnikov-de Haas (SdH) experiments [1] suggest an unusual temperature dependence of the Fermi surface cross section parallel to [100]. The 4f¹ electron configuration of the Ce³⁺ ion forms a ²F_5/2 groundstate with J = 5/2 according to Hund's rules. The sixfold level degeneracy is lifted by the cubic crystal symmetry into a doublet and a quadruplet state. We measured the thermal expansivity a down to 2 K in zero and applied magnetic field B. In B > 0 the fourfold degeneracy of the G₈ groundstate is lifted by the Zeeman splitting. The low-energy thermal exitations are seen as a minimum in the temperature dependence of the longitudinal thermal expansivity. The minimum in a(T) is shifted to higher T as B is increased. The magnetostriction, i. e. the relative length change DL(B)/L(0) as a function of B, was determined as well. Although a relative large length change was measured for both directions DL ||[100] and DL ||[110], no significant anisotropy was found which could account for the anisotropic behavior of the Fermi surface seen in the SdH experiments [1].
[1] G. Goll et al., Europhys. Lett. 57, 233 (2002).