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

Abstract

We report on high-resolution measurements of the coefficient of thermal expansion α of the heavy-fermion superconductor U1−xThxBe13 for temperatures 0.05K<~T<~6K and magnetic fields B<~8T. 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<~x<~0.052, that encompasses the region xc1=0.019<x<xc2=0.045 where temperature-dependent specific-heat measurements reveal two subsequent phase transitions at Tc1>Tc2, our study discloses features in the T-x plane that have been overseen by all other techniques applied to this system so far: (i) The substitution of uranium by thorium in UBe13 induces an anomaly that manifests itself in a negative α(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 Tc2 suggests a common (presumably magnetic) nature of both features. (ii) The linear concentration dependence of the second low-energy scale Tmax, which gives rise to a pronounced maximum in α(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, Tmax(x) vanishes at x≈0.043, i.e., almost exactly at xc2. (iii) Upon increasing x to above 0.03 the normal- to superconducting-state transition at Tc1 progressively loses its signatures in α. Our measurements, together with recent specific-heat results by Schreiner et al. [Schreiner et al., Europhys. Lett. 48, 568 (1999)] indicate that superconductivity becomes gapless for x→xc2. Hence, the phase transition seen in specific heat as well as thermal-expansion measurements for samples with x>xc2 has to be attributed to the Tc2 transition. Concomitant investigations of the ac susceptibility indicate that the normal- to superconducting-state transition for x>xc2 now coincides with Tc2. 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 Tc2.

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