Resonating-valence-bond states and ferromagnetic correlations in the doped triangular Mott insulator

The two-dimensional t-J model on a triangular lattice is studied using high-temperature expansions. By studying the entropy and spin susceptibility, we find that the sign of the hopping integral t is very crucial. In the case of t>0, the peak of the spin susceptibility moves to the high-temperature region with hole doping, which indicates the appearance of the resonating-valence-bond state. In contrast, for t<0, the peak of the spin susceptibility disappears with hole doping and the entropy at low temperatures behaves as S=gammaT with large coefficient gamma, representing a large effective mass. This behavior is understood from the competition between Nagaoka's ferromagnetism and singlet formation.