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 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(Pt1−xPdx)3 (0 ≤ x ≤ 1) of the archetypal (noncentrosymmetric) Kondo insulator Ce3Bi4Pt3. The Pt-Pd substitution is isostructural, isoelectronic, and isosize, and therefore 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.

Evidence for Weyl fermions in a canonical heavy-fermion semimetal YbPtBi

“…Meanwhile, evidence for Weyl fermions has also been found in some magnetic d -electron systems such as Mn 3 Sn 39 and YbMnBi 2 40 , 41 , where in the case of Mn 3 Sn a significant topological Hall effect is also observed 10 , 11 . On the other hand, it is of great interest to look for the kind of dichotomy observed here for YbPtBi in other potential Weyl heavy-fermion semimetals, such as Ce 3 Bi 4 Pd 3 where a similarly small v * was inferred from the specific heat 18 , yet evidence for the chiral anomaly at elevated temperatures has not yet been reported. Furthermore, the strength of the Kondo interaction in heavy fermion systems can be readily tuned by non-thermal control parameters, such as pressure and magnetic field, and in particular, a quantum critical point can be reached in YbPtBi at a critical field of 0.4 T 35 .…”

Weyl–Kondo semimetal in heavy-fermion systems

“…The resulting state could be a heavy-fermion metal or a Kondo insulator, depending on whether the chemical potential lies within or falls between the hybridized bands ( 3 – 5 ). Electronically intermediate between the two cases are heavy-fermion semimetals ( 6 – 13 ). Several of these have a broken inversion symmetry, including CeRu 4 Sn 6 ( 6 , 7 ) and Ce 3 Bi 4 Pd 3 ( 9 ).…”

Large Fermi surface expansion through anisotropic mixing of conduction and f electrons in the semimetallic Kondo lattice CeBi

“…The low carrier density significantly reduces the Kondo screening, while the small FS allows for easy detection of any change in the Fermi wavevector kF due to valence fluctuations. Low carrier density Kondo systems have attracted considerable interest recently, in view of novel phenomena such as unconventional quantum criticality and correlated topological states [10,11,12,13,14].…”