Low-temperature thermal expansion of the topological material candidates $\beta$-PtBi$_2$ and $\beta$-Bi$_2$Pd

TL;DR

This study investigates the low-temperature thermal expansion properties of topological semimetal $eta$-PtBi$_2$ and superconductor $eta$-Bi$_2$Pd, revealing anisotropic expansion, Debye temperatures, and stable magnetostriction behavior.

Contribution

It provides detailed measurements of thermal expansion and related thermodynamic parameters for $eta$-PtBi$_2$ and $eta$-Bi$_2$Pd, enhancing understanding of their lattice dynamics and topological properties.

Findings

$eta$-Bi$_2$Pd shows anisotropic thermal expansion between axes.

Debye temperatures are 199 K for $eta$-PtBi$_2$ and 138 K for $eta$-Bi$_2$Pd.

No magnetostriction observed up to 16 T.

Abstract

We report on the low-temperature ($T <$ 120 K) thermal expansion of the bismuth-based topological semimetal $\beta$-PtBi$_2$ and topological superconductor $\beta$-Bi$_2$Pd candidates. The linear thermal-expansion coefficient of tetragonal $\beta$-Bi$_2$Pd shows a pronounced anisotropy between the $a$- and $c$-axis while the volume thermal-expansion coefficient $\alpha_V$($\beta$-Bi$_2$Pd) is considerable larger than $\alpha_V$($\beta$-PtBi$_2$). The coefficient $\alpha_V$($\beta$-PtBi$_2$) nearly matches the experimental specific heat, from which a Debye temperature $\theta_D =$ 199 K is obtained. On the other hand, $\alpha_V$($\beta$-Bi$_2$Pd) reasonably fits the Debye model with $\theta_D =$ 138 K, extracted from the low-temperature specific heat. An almost constant Gr\"uneisen parameter $\Gamma \approx$ 2 is obtained for both compounds. No magnetostriction is observed in any of both compounds up to $\mu_0 H =$ 16 T. We compare our results with other Bi-based topological materials.