Specific heat and thermal conductivity of UCu_4+x Al_8−x compounds

Abstract

We report on thermal conductivity and specific heat measurements for eight UCu_4+x Al_8−x compounds (0 ≤ x ≤ 2.0) as a function of temperature and magnetic field. For this series of compounds, previous magnetic and transport studies indicated a transition from magnetic to a non-magnetic heavy fermion state near x _cr ≈ 1.15. This paper presents supplementary specific heat and thermal conductivity studies. The ratio of the specific heat over temperature C/T data on the non magnetic compound with x _cr ≈ 1.15 show logarithmic dependence with T, a hallmark of non-Fermi liquid (NFL) behavior due to the proximity of a quantum critical point. Compounds with higher Cu content (x > x _cr ) exhibit unusual temperature scaling in the specific heat possibly due to an increase in disorder between Cu and Al. Thermal conductivity data show stark contrast in the behaviors between the magnetic (x = 0.5) and non-magnetic compound (x = 1.75). Our results confirm that a simple free-electron picture is inadequate for the description of the low-temperature thermal conductivity properties in non-magnetic UCu_4+x Al_8−x compounds.