Kondo Insulator to Semimetal Transformation Tuned by Spin-Orbit Coupling

Abstract

Recent theoretical studies of topologically nontrivial electronic states in Kondo insulators have pointed to the importance of spin-orbit coupling (SOC) for stabilizing these states. However, systematic experimental studies that tune the SOC parameter $\hat{I}{»}_{\mathrm{SOC}}$ in Kondo insulators remain elusive. The main reason is that variations of (chemical) pressure or doping strongly influence the Kondo coupling $J_{\text{K}}$ and the chemical potential $\hat{I}¼$ -- both essential parameters determining the ground state of the material -- and thus possible $\hat{I}{»}_{\mathrm{SOC}}$ tuning effects have remained unnoticed. Here we present the successful growth of the substitution series Ce${}_3$Bi${}_4$(Pt${}_{1-x}$Pd${}_x$)${}_3$ ($0\le x\le 1$) of the archetypal (noncentrosymmetric) Kondo insulator Ce${}_3$Bi${}_4$Pt${}_3$. The Pt-Pd substitution is isostructural, isoelectronic, and isosize, and therefore likely to leave $J_{\text{K}}$ and $\hat{I}¼$ essentially unchanged. By contrast, the large mass difference between the $5d$ element Pt and the $4d$ element Pd leads to a large difference in $\hat{I}{»}_{\mathrm{SOC}}$, which thus is the dominating tuning parameter in the series. Surprisingly, with increasing $x$ (decreasing $\hat{I}{»}_{\mathrm{SOC}}$), we observe a Kondo insulator to semimetal transition, demonstrating an unprecedented drastic influence of the SOC. The fully substituted end compound Ce${}_3$Bi${}_4$Pd${}_3$ shows thermodynamic signatures of a recently predicted Weyl-Kondo semimetal.

6 pages, 5 figures plus Supplemental Material