BORON ISOTOPE EFFECT IN NI AND PD BASED BOROCARBIDE SUPERCONDUCTORS

The boron partial isotope effect (B-10-B-11) was investigated in YNi2B2C and in YPd5B3C0.3. Samples with pure B-10 and B-11, as well as natural boron, and a 1:1 mixture of B-10 and B-11 were investigated. We find a shift of the magnetic transition temperature of Delta T-c = T-c(B-10) - T-c(B-11) = 690 mK for the Pd based samples, and of 380 mK for the Ni based samples. This corresponds to partial isotope exponents alpha(B) = 0.25 +/- 0.04 (Ni based samples) and alpha(B) = 0.32 +/- 0.04 (Pd based samples). This supports the classification of these materials as electron-phonon mediated superconductors. Preliminary investigations of the carbon isotope effect (C-12-C-13) did not show a change in T-c.