Thermal expansion studies of superconducting U1-xThxBe13 (0≤x≤0.052):: Implications for the interpretation of the T-x phase diagram

We report on high-resolution measurements of the coefficient of thermal expansion a of the heavy-fermion superconductor U1-xThxBe13 for temperatures 0.05 K less than or equal toT less than or equal to6 K and magnetic fields B less than or equal to8 T. Particular attention is paid to the properties of the low-temperature normal state and their evolution as a function of thorium concentration. By exploring a wide concentration range, 0 less than or equal tox less than or equal to0.052, that encompasses the region x(c1) =0.019<x<x(c2)=0.045 where temperature-dependent specific-heat measurements reveal two subsequent phase transitions at T-c1 > T-c2, our study discloses features in the T-x plane that have been overseen by all other techniques applied to this system so far: ii) The substitution of uranium by thorium in UBe13 induces an anomaly that manifests itself in a negative at T) contribution to the low-temperature normal-state expansivity. Its distinct field dependence signals a magnetic origin. Analyzing the relative lengths changes associated with this anomaly and that of the phase transition at T-c2 suggests a common (presumably magnetic) nature of both features, iii) The linear concentration dependence of the second low-energy scale T-c2, which gives rise to a pronounced maximum in alpha (T) of UBe13 at 2 K (at B = 0) could be followed up-by applying a magnetic field-to concentrations x>0.03. Most remarkably, T-max(x) vanishes at x approximate to0.043, i.e., almost exactly at x(c2). (iii) Upon increasing x to above 0.03 the normal- to superconducting-state transition at T-c2 progressively loses its signatures in alpha Our measurements, together with recent specific-heat results by Schreiner et al. [Schreiner ct al., Europhys. Lett. 48, 568 (1999)] indicate that superconductivity becomes gapless for x-->x(c2). Hence, the phase transition seen in specific heat as well as thermal-expansion measurements for samples with x>x(c2) has to be attributed to the T-c2 transition. Concomitant investigations of the ac susceptibility indicate that the normal- to superconducting-state transition for x>x(c2) now coincides with T-c2. As for the implications of our observations for the interpretation of the various low-temperature anomalies, we discuss two possible scenarios both of which imply an intimate interrelation of superconductivity with the symmetry broken state that forms below T-c2.