Evidence for persistent spin fluctuations in uranium sesquicarbide

Abstract

The low-temperature magnetic susceptibility, heat capacity, and electrical resistivity of uranium sesquicarbide have been determined down to 2 K by combining measurements carried out on samples of U 2 C 3 , containing UC as minority phase, and on pure UC specimens. The presence of spin fluctuations with characteristic temperature T SF = 7 K is revealed by a nonanalytic contribution to the specific heat behaving (well below T SF ) as T 3 ln(T /T SF ), and confirmed by a T 2 increase of the electrical resistivity and a 1 − κT 2 decrease of the low-temperature magnetic susceptibility. The analysis of the specific-heat data above T SF provides a Debye temperature D ≈ 256 K and a moderate high value of the Sommerfeld coefficient, γ ≈ 42 mJ mol −1 U K −2 . The zero-temperature many-body enhancement of the electron mass is found to be m * /m ≈ 2.7. Our data rule out the occurrence of magnetic ordering in U 2 C 3 . First-principles electronic structure calculations support the absence of a magnetically ordered ground state but suggest that dynamical spin fluctuations are responsible for the electron mass enhancement.

Structure of UC2 and U2C3:XRD, 13C NMR and EXAFS study

“…The U 2 C 3 phase has the body centred cubic Pu 2 C 3 type structure (I 43d, SG n°220) [13,14] and cannot be obtained directly by arc melting [3]. Many controversies have been reported in the literature on the synthesis of U 2 C 3 [16] and its magnetic properties [17]. Now it is well known that U 2 C 3 can be generated from a start ing material UC x (x = 1.5 2) by grinding, pressing, and heating un der high vacuum.…”