Gapless neutral fermions in insulating YbIr₃Si₇revealed by specific heat and thermal conductivity

Kondo insulators have been intensively re-investigated recently due to non-trivial topology [1], unconventional quantum oscillations (QOs) [2], and evidence for existence of itinerant charge-neutral fermions (CNFs) [3]. Although it has been proposed the CNFs may produce the QO signals, the origin of the CNFs is highly elusive and under debate.
YbIr₃Si₇ is a newly discovered Kondo lattice compound, where c-f interaction leads to insulating ground state with antiferromagnetic (AF) ordering [4]. The AF ordering can be easily tuned by an external magnetic field. Here we present specific heat C and thermal conductivity κ measurements of YbIr₃Si₇. Between 2 and 3 T, where the AF order is suppressed, C is strongly enhanced and a finite residual linear term in the thermal conductivity κ/T (T → 0) is observed. A likely possible explanation for the present results is that CNFs are coupled directly to the spin degrees of freedom of the underlying Kondo lattice. YbIr₃Si₇ thus provides an intriguing platform to study the CNFs and their relationship with magnetism in strongly correlated insulators.

[1] M. Dzero et al., Annu. Rev. Con. Mat. Phys. 7, 249 (2016).
[2] Z. Xiang et al., Science 362, 65 (2018).
[3] Y. Sato et al., Nature Physics 15, 954 (2019).
[4] M. Stavinoha et al., arXiv:1908.11336v2.