Abstract
Low-temperature specific-heat measurements on U1 − xThxBe13 reveal a linear contribution Clin = γrest(x)·T at lowest temperatures which varies systematically with thorium concentration. Size and concentration dependence of this contribution can be explained by resonant defect scattering of the Th impurities in an anisotropic superconductor with gap nodes. The most meaningful observation, however, is that γrest(x) shows relatively sharp minima at both critical concentrations (xC1 ≈ 1.8 at %, xC2 ≈ 4.5 at%) in the phase diagram of U1 − xThxBe13; in between these concentrations a double-maximum structure in the temperature-dependent specific heat is observed. These sharp minima are strong indications of a sudden change in the superconducting ground state at the critical concentrations. This direct information about the gap nodes is important for theory to explain the complex phase diagram of U1 − xThxBe13.