OBSERVATION OF LOW-LYING LEVELS IN UBE13

Doping UBe13 with Th, La, Pr, or Ce destroys the as-yet not understood anomaly in the specific heat at 2 K. Using U0.97Th0.03Be13 as a background, we note that the anomaly in UBe13 fits very well with a Schottky temperature dependence, with fit parameters centered around a level splitting of 7 K but with only ∼5% of the amplitude expected if all of the U ions participated. The decrease observed in the resistivity in UBe13 below 2 K is consistent with the freezing out of some type of degrees of freedom, which could also include magnetic scattering. Based on the known lack of magnetic-field dependence of the magnetization and the specific heat in the UBe13, however, these low-lying f states cannot simply be understood as noncompensated magnetic moments as observed in, e.g., CeBe13. We infer that this anomaly is intrinsic based on its behavior with annealing and improved purity, and on the fact that the entropy under this Schottky anomaly is shifted upon doping into more than a factor of 2 increase in the low-temperature m*(γ) in U0.97Th0.03Be13. The occurrence of a similar anomaly in another heavy-fermion superconductor, CeCu2Si2, makes the anomaly of some interest. © 1995 The American Physical Society.