Nematic heavy fermions and coexisting magnetic order in CeSiI

Motivated by the recent discovery of magnetism and heavy quasiparticles in the van der Waals material CeSiI, we develop an effective model that incorporates the conduction electrons residing at the silicene layer interacting with the local moments of the Ce ions. Ce sites are arranged on two layers of triangular lattices, above and below the silicene layer, and they are located at the center of the honeycomb lattice. This arrangement results in an effective extended Kondo interaction along with a predominant ferromagnetic Ruderman-Kittel-Kasuya-Yosida interaction. Via the mean-field theory of Abrikosov fermions, our analysis indicates that the ground state of the monolayer can exhibit a nonmagnetic nematic heavy fermion phase that breaks C6 rotational symmetry for a small Heisenberg exchange and a magnetically ordered phase for a large Heisenberg exchange. For intermediate values, a coexistence of magnetic order and a uniform heavy Fermi liquid is stabilized where they reside on separate Ce layers. We show that this phase can further be enhanced by an external electric field. Our results provide a natural mechanism for the coexistence of magnetic order and heavy fermions in CeSiI and highlight the possibility of unconventional nonmagnetic heavy fermions with broken rotational symmetry.