Dilute stuffing in the pyrochlore iridate $E{u}_{2}I{r}_2{O}_7$

Abstract

The pyrochlore Eu${}_2$Ir${}_2$O${}_7$ has recently attracted significant attention as a candidate Weyl semimetal. The previous reports on this compound unanimously show a thermally induced metal to insulator (MI) transition, concomitant with antiferromagnetic (AFM) long-range ordering of the Ir-moments below T${}_{\text{N}}\sim$120 K. However, there are contradictory reports concerning the slope d$\rho /$dT of the resistivity plots ($\rho$) in the "metallic" state above the metal-insulator (MI) transition, and the value of $\rho$ in the insulating state, both of which show significant sample dependence. Here, we explore this issue by investigating six different Eu${}_2$Ir${}_2$O${}_7$ samples with slightly varying Eu:Ir ratio. High-resolution synchrotron powder diffraction are done to probe minor variations in the cell parameters of the various Eu${}_2$Ir${}_2$O${}_7$ samples investigated here. Specific heat (C${}_p$) and magnetic susceptibility of all the samples showed long-range antiferromagnetic ordering upon cooling below T${}_{\text{N}}\sim$120 K. The transitions are, however, found to be smeared out for the off-stoichiometric samples. We show that the sign of d$\rho /$dT above the metal-insulator (MI) transition is highly sensitive to the unit cell length, which, in turn, depends on the level of Eu-stuffing at the Ir-site. Samples with composition close to the ideal stoichiometry (Eu : Ir $=$ 1) showed a change of sign of d$\rho /$dT from negative to positive upon cooling below a certain temperature T ${}^{\ast }$ $>$ T${}_{\text{MI}}$. With increasing Eu-stuffing T${}^{\ast }$ decreased until a negative d$\rho /$dT persisted without any sign change down to T${}_{\text{MI}}$.

12 pages, 7 figures