Continuous transition between antiferromagnetic insulator and paramagnetic metal in the pyrochlore iridate Eu${}_2$Ir${}_2$O${}_7$

Our single crystal study of the magnetothermal and transport properties of the pyrochlore iridate Eu${}_2$Ir${}_2$O${}_7$ reveals a continuous phase transition from a paramagnetic metal to an antiferromagnetic insulator for a sample with stoichiometry within $\sim$1$\%$ resolution. The insulating phase has strong proximity to an antiferromagnetic semimetal, which is stabilized by several $\%$ level of the off-stoichiometry. Our observations suggest that in addition to electronic correlation and spin-orbit coupling the magnetic order is essential for opening the charge gap.

Figure 1

Temperature dependence of the electrical resistivity of single crystalline Eu${}_2$Ir${}_2$O${}_7$ for three samples with different off-stoichiometry $x$, normalized by the value at 300 K. Inset: Temperature dependence of the temperature derivative of the resistivity $d\rho /dT$ normalized by the resistivity at 300 K.

Figure 2

Inverse resistivity ratio $1/\mathrm{RRR}=\rho \left(2\mathrm{K}\right)/\rho \left(300\mathrm{K}\right)$ vs the off-stoichiometry $\left|x\right|$ for various single crystals of Eu${}_{2(1-x)}$Ir${}_{2(1+x)}$O${}_{7+\delta }$. Blue and red symbols correspond to Ir rich ($x>0$) and Eu rich ($x<0$) samples, respectively. The blue shaded belt is a guide to the eye.

Figure 3

$\ln \rho \left(T\right)/\rho \left(300\mathrm{K}\right)$ vs $1/T^{1/4}$ of single crystalline Eu${}_2$Ir${}_2$O${}_7$ for three samples with different off-stoichiometry $x$. The solid lines are guides to the eye. Inset: Temperature dependence of the activation energy $E_{\mathrm{a}}$. For definition, see text.

Figure 4

Field dependence of the Hall resistivity ${\rho }_{xy}$ for single crystalline Eu${}_2$Ir${}_2$O${}_7$ with $1/\mathrm{RRR}=200$ obtained under external field along [111] at various temperatures between 90 and 140 K across the transition temperature of $T_{\mathrm{MN}}=120$ K. Inset: Temperature dependence of the Hall coefficient $R_{\mathrm{H}}={\rho }_{xy}/B$ obtained under a field of 9 T applied along [111].

Figure 5

Temperature dependence of the magnetic susceptibility $\chi =M/H$ of single crystalline Eu${}_2$Ir${}_2$O${}_7$ with $1/\mathrm{RRR}>100$, measured using ZFC and FC sequences under a field of 0.1 T aligned along [100], [111], and [110]. The broken lines indicate the Van-Vleck contribution ${\chi }_{\mathrm{VV}}$ estimated using the excited magnetic multiplet levels for a nonmagnetic $J=0$ ground state of Eu${}^{3+}$ ion and the component obtained after subtracting the Van-Vleck contribution from the observed $\chi \left(T\right)$, labeled as ${\chi }_{\mathrm{obs}}-{\chi }_{\mathrm{VV}}$. Inset: $\chi \left(T\right)$ for three different qualities of crystals under a field of 0.1 T along [110]. Both FC and ZFC results are shown.

Figure 6

Temperature dependence of the specific heat divided by temperature, $C_P/T$, of the single crystalline Eu${}_2$Ir${}_2$O${}_7$ with $1/\mathrm{RRR}>500$. Inset: $C_P/T$ vs $T^2$ for crystals with $1/\mathrm{RRR}>500$ and with $1/\mathrm{RRR}<33$. The solid lines are guides to the eye.