Metal-insulator transition in pyrochlore iridates Ln(2)Ir(2)O(7) (Ln = Nd, Sm, and Eu)

We report that Ln(2)Ir(2)O(7) for Ln = Nd, Sm, and Eu have metal-insulator transitions (MITs) at 36, 117, and 120K, respectively. Their electrical resistivities and thermoelectric powers are not discontinuous and exhibit no thermal hysteresis at their transition temperatures T-MI, indicating second-order phase transitions. In this letter, we focus on the MIT Of Sm2Ir2O7. The specific heat and the magnetic susceptibility show a clear anomaly at TMI. These results indicate that this MIT involves a magnetic ordering produced by 5d electrons in Ir.