Kondo Insulator to Semimetal Transformation Tuned by Spin-Orbit Coupling

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 \(\lambda_{\rm{SOC}}\) 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 \(\mu\) -- both essential parameters determining the ground state of the material -- and thus possible \(\lambda_{\rm{SOC}}\) 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 \(\mu\) essentially unchanged. By contrast, the large mass difference between the \(5d\) element Pt and the \(4d\) element Pd leads to a large difference in \(\lambda_{\rm{SOC}}\), which thus is the dominating tuning parameter in the series. Surprisingly, with increasing \(x\) (decreasing \(\lambda_{\rm{SOC}}\)), 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.