  We report our theoretical results on the emergence of a partially-disordered
state at zero temperature and its detailed nature in the periodic Anderson
model on a triangular lattice at half filling. The partially-disordered state
is characterized by coexistence of a collinear antiferromagnetic order on an
unfrustrated honeycomb subnetwork and nonmagnetic state at the remaining sites.
This state appears with opening a charge gap between a noncollinear
antiferromagnetic metal and Kondo insulator while changing the hybridization
and Coulomb repulsion. We also find a characteristic crossover in the
low-energy excitation spectrum as a result of coexistence of magnetic order and
nonmagnetic sites. The result demonstrates that the partially-disordered state
is observed distinctly even in the absence of spin anisotropy, in marked
contrast to the partial Kondo screening state found in the previous study for
the Kondo lattice model.
