Low-temperature thermal expansion of the topological material candidates -PtBi2 and -PdBi2

Abstract

We report on the low-temperature (T< 120 K) thermal expansion of the bismuth-based topological semimetal β-PtBi2 and topological superconductor β-PdBi2 candidates. The linear thermal-expansion coefficient of tetragonal β-PdBi2 shows a pronounced anisotropy between the a- and c-axis while the volume thermal-expansion coefficient αV (β-PdBi2) is considerable larger than αV (β-PtBi2). The coefficient αV (β-PtBi2) nearly matches the experimental specific heat, from which a Debye temperature θD=199 K is obtained. On the other hand, αV (β-PdBi2) reasonably fits the Debye model with θD=138 K, extracted from the low-temperature specific heat. An almost constant Grüneisen parameter Γ≈ 2 is obtained for both compounds. No magnetostriction is observed in any of both compounds up to μ0H=16 T. We compare our results with other Bi-based topological materials.