| We present a novel mechanism of resistivity minimum for conduction electrons coupled with spin-ice-type localized Ising moments. In several metallic Ir pyrochlore oxides, such as Ln2Ir2O7 (Ln=Pr, Nd), electrical resistivity shows a minimum around Tmin = 40K. These compounds are well described by a Kondo lattice model where Ir 5d conduction electrons interact with Ln 4f localized moments. Nevertheless, it is difficult to ascribe the resistivity minimum to canonical scenario of Kondo effect in a straightforward way. Firstly, strong Ising anisotropy of localized moments should reduce Kondo temperature considerably. Secondly, magnetic susceptibility shows diverging behavior below Tmin, in contrast to saturation in typical Kondo impurity systems. Thirdly, the peak of specific heat is located at 2K, much below Tmin, implying the absence of entropy release accompanied with Kondo singlet formation. To resolve these difficulties, we propose a new scenario, focusing on the role of spin ice correlation in this system. By applying the cluster dynamical mean-field theory to Ising Kondo lattice model on a pyrochlore lattice, we found that the resistivity shows a minimum at a characteristic temperature below which spin ice correlation sets in. We also successfully reproduced the diverging magnetic susceptibility and the separation of energy scales between resistivity and specific heat. We discuss our non-Kondo mechanism in comparison with the experimental data of Pr2Ir2O7 and Nd2Ir2O7. |
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