Competing states in the two-dimensional frustrated Kondo-necklace model

The interplay between Kondo screening, indirect magnetic interaction, and geometrical frustration is studied in the two-dimensional Kondo-necklace model on the triangular lattice. Using infinite projected entangled pair states (iPEPS), we compute the ground state as a function of the antiferromagnetic local Kondo interaction J_K and the Ising-type direct spin-spin interaction I_z. As opposed to previous studies, we do not find partial Kondo screening (PKS) in the isotropic limit I_z =0 but the same behavior as in the unfrustrated case, i.e., a direct phase transition between the paradigmatic phases of the Doniach competition: (i) a disordered phase consisting of local spin singlets at strong J_K and (ii) a magnetically ordered phase at weak J_K. For I_z > 0, we find a PKS ground state but again oppositely to previous studies, we find that the PKS ground state is in strong competition with a second ground state candidate not found before. This state is characterized by a strongly polarized central spin in each hexagon and its antiparallel, weakly polarized (i.e., partially screened) neighbors. We name it the central spin phase.