University of Tokyo
Recently, considerable attention has been focused on the heavy-fermion behavior in transition metal compounds, such as LiV2O4 and Y(Sc)Mn2. In the so-called heavy-fermion systems with rare-earth ions, it is well established that the origin of large quasiparticle mass is attributed to the entropy carried by the localized f-electron moments. In contrast, the mechanism of the heavy-fermion behavior is still unclear for the transition metal compounds. The main problem is that these systems have no obvious entropy reservoir, corresponding to the f-electron moments in the rare-earth materials. To shed light on this issue, we investigate the effect of a common feature in the heavy transition metal systems, that is, geometrical frustration in the lattice structure. We consider the Hubbard model on the kagome lattice, as a minimum model with geometrical frustration and electron correlation, and study the electronic and magnetic properties by means of the cluster dynamical mean-field theory with the continuous-time quantum Monte Carlo method as an impurity solver. From the careful analysis of the density matrix, we found that, in the correlated metallic region near the Mott transition, the spin-chirality degree of freedom becomes dominant as decreasing temperature and shows a crossover temperature, which is much lower than the characteristic temperature of spin degree of freedom left out by the suppression of charge fluctuations. An important observation is that the quasiparticle coherence peak grows rapidly below this crossover temperature, strongly suggesting a close relationship between the spin chirality and heavy-fermion behavior. From these results, we discuss a possibility of the formation of heavy-fermion state by an emergent object in geometrically-frustrated systems such as the spin chirality. We also discuss characteristic crossovers in magnetic fluctuations near the Mott transition.