Electronic orders and phase transitions in a honeycomb Kondo lattice system

Abstract

We study the electronic orders in a honeycomb-Kondo lattice. For the ground state, we use variational quantum Monte Carlo to find the transition from antiferromagnetic insulator to Kondo insulator is continuous, in contrast to the discontinuous transition in mean-field theory. Moreover, the hybridization parameter between the conduction electron and the Kondo spin is nonzero even within the antiferromagnetic phase. At finite temperatures, we resort to dynamical mean-field theory, which not only captures local quantum fluctuations but also accesses the thermodynamic limit directly. There are three phases, namely, antiferromagnetic insulator, Kondo insulator, and paramagnetic phase. The transition from antiferromagnetic phase to paramagnetic phase is likely discontinuous, while that from antiferromagnetic phase to Kondo insulator phase remains to be continuous at finite temperatures. There is a crossover from the paramagnetic phase, where spin excitations are gapless, to the Kondo insulator phase, where spin excitations are gapped. Our results indicate a significant effect of fluctuations beyond mean-field theory in the honeycomb-Kondo lattice. Since the transition from the antiferromagnetic phase to the Kondo insulating phase occurs at a sizable Kondo coupling, where the Kondo lattice model is inequivalent to the Anderson lattice model, our results are complementary to that for a honeycomb-Anderson lattice.