Precursor phenomena to pressure- induced superconductivity in the thermal expansion of UGe2

Abstract

Thermal expansions of single crystalline UGe2 have been measured at high pressure up to 2 GPa. It is found at ambient pressure that there is a large anisotropy in the α(T) curve depending on the ciystal axis and discontinuous change corresponding to ferromagnetic ordering near 50 K(= TC). Furthermore a small change in α (T) is observed near 25 K (= T*) below TC, which may be related with a phase transition. We studied the pressure dependence of TC and T*, which are strongly dependent on pressure and diappear near 1.8 GPa and 1.2 GPa, respectively. Anomalous pressure dependence is also found in the α (T) and ρ(T) still above TC. The origin of these anomalies will be discussed briefly in connection with the ferromagnetism and pressure-induced superconductivity.