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 λSOC in Kondo insulators remain elusive. The main reason is that variations of (chemical) pressure or doping strongly influence the Kondo coupling JK and the chemical potential μ---both essential parameters determining the ground state of the material---and thus possible λSOC tuning effects have remained unnoticed. Here, we present the successful growth of the substitution series Ce₃Bi₄(Pt_1-xPdₓ)₃ (0≤x≤1) of the archetypal (noncentrosymmetric) Kondo insulator Ce₃Bi₄Pt₃. The Pt-Pd substitution is isostructural, isoelectronic, and isosize, and it therefore is likely to leave JK and μ essentially unchanged. By contrast, the large mass difference between the $5d$ element Pt and the $4d$ element Pd leads to a large difference in λSOC, which thus is the dominating tuning parameter in the series. Surprisingly, with increasing x (decreasing λSOC), we observe a Kondo insulator to semimetal transition, demonstrating an unprecedented drastic influence of the SOC. The fully substituted end compound Ce₃Bi₄Pd₃ shows thermodynamic signatures of a recently predicted Weyl-Kondo semimetal.