Temperature dependence of the lower critical field of UPt3 doped with boron

Doping of a small amount of boron in UPt3 produces samples possessing somewhat higher superconducting transition temperatures and lower residual resistivities (ρ0). We have performed dc-magnetization measurements and determined the lower critical field (Hc1) of a boron-doped, single-crystal of UPt3, along the b and c axes. We have observed a kink in the temperature dependence of Hc1 for both b and c directions. We also observe a double transition in ZFC dc-magnetization experiments for the external field parallel to the b axis. This step in the shielding curve coincides with the kink in Hc1 and the second peak in the specific heat measurements on the same sample. No such feature is observed with the field along the c axis. As an upward kink in Hc1 cannot be due to different metallurgical phases in the sample, it might be interpreted as a manifestation of the anisotropic superconductivity in this material. (orig.)