We have measured the specific heat of pure and thoriated (3.3% thorium) ${\mathrm{UBe}}_{13}$ in the temperature interval 0.3–1.8 K in applied magnetic fields up to 20 T, and find striking qualitative differences between the two systems. For pure ${\mathrm{UBe}}_{13}$ the electronic coefficient of specific heat ${\gamma }_{\mathrm{el}(=C_{\mathrm{el}\left(\mathrm{T}\right)/\mathrm{T})}}$ is noticeably suppressed by a field of 20 T at 1.8 K; below 1 K, however, this reduction from the low-field, normal-state value tends to zero. For the 3.3% thoriated sample, the high-temperature value of ${\gamma }_{\mathrm{el}}$ is relatively insensitive to strong magnetic fields, but at lower temperatures the suppression of ${\gamma }_{\mathrm{el}}$ becomes much stronger, reaching nearly 40% at 0.35 K in a field of 20 T. This behavior is similar to that observed in most cerium-based heavy-fermion systems.

• Received 14 August 1989

DOI:https://doi.org/10.1103/PhysRevB.40.9358