Quest for the Origin of Heavy Fermion Behavior in d-Electron Systems

Spin fluctuation is presumed to be one of the key properties in understanding the microscopic origin of heavy-fermion-like behavior in the class of transition-metal compounds, including LiV₂O₄, Y(Sc)Mn₂, and YMn₂Zn₂₀. In this review, we demonstrate by our recent study of muon spin rotation/relaxation that the temperature (T) dependence of the longitudinal spin relaxation rate (λ ≡ 1/T₁) in these compounds exhibits a common trend of leveling off to a constant value (λ ∼ const.) below a characteristic temperature, T^*. This is in marked contrast to the behavior predicted for normal metals from the Korringa relation, λ ∝ T/ν, where the spin fluctuation rate (ν) in the Pauli paramagnetic state is given as a constant, ν ≃ 1/[hD(E_F)] [with D(E_F) being the density of states at the Fermi energy]. Thus, the observed behavior of λ implies that the spin fluctuation rate becomes linearly dependent on temperature, ν ∝ T, suggesting that heavy quasiparticles develop in a manner satisfying D(E_F) ∝ (m^*)^σ ∝ 1/T at lower temperatures (σ determined by the electronic dispersion). Considering that the theory of spin correlation for intersecting Hubbard chains as a model of pyrochlore lattice predicts ν ∝ T, our finding strongly indicates the crucial role of t_2g bands which preserve the one-dimensional character at low energies due to the geometrical frustration specific to the undistorted pyrochlore lattice.