Heavy-Fermion Behavior in a Honeycomb Kondo Lattice CePt6Al3

We have synthesized single crystals of CePt6Al3 in which Ce atoms form a honeycomb lattice in a trigonal basal plane. The measurements of magnetic susceptibility chi, electrical resistivity rho, thermopower S, and specific heat C are reported. For both B parallel to c and B perpendicular to c, chi(T) data at temperatures above 100K follow the Curie-Weiss law with effective moments close to that of a trivalent Ce ion. The magnetic part of rho(T) and S(T) show a maximum at around 100 K, which is attributed to the Kondo effect in the excited crystal-field levels. The energy of the total crystal-field splitting is estimated to be approximately 100K from the magnetic entropy. At low temperatures, rho(T) decreases in proportion to T-2 without showing any anomaly down to 0.05 K. C/T exhibits a broad maximum of 0.5 J/K-2 mol at around 0.55 K. The ground state appears to be a heavy-fermion state where the Kondo effect and frustration effect cooperatively inhibit the Ce magnetic moments in the honeycomb lattice from long-range ordering.