Mixed-valent metallic pyrochlore iridate: A possible route to non-Fermi liquids

Abstract

Non-Fermi liquid behavior in some fermionic systems has attracted significant interest in last few decades. Certain pyrochlore iridates with stronger spin-orbit coupling strength have recently been added to the list. Here, we provide evidence of such a non-Fermi liquid ground state in another mixed-valent metallic pyrochlore iridate ${\mathrm{Pb}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$, through the combined investigation of electronic, magnetic, and thermodynamic properties as a function of temperature ($T$) and applied magnetic field ($H$). Resistivity measurement showed a linear temperature dependence down to 15 K below which it shows $\ensuremath{\rho}\ensuremath{\sim}{T}^{3/2}$ dependence while magnetic susceptibility diverges as $\ensuremath{\chi}(T$) $\ensuremath{\sim}{T}^{\ensuremath{-}\ensuremath{\alpha}}$ ($\ensuremath{\alpha}<1$) below 10 K. While a strong negative ${\mathrm{\ensuremath{\Theta}}}_{\text{CW}}$ has been observed from Curie-Weiss fitting, the absence of any long-range order down to 80 mK only indicates the presence of strong inherent geometric frustration in the system. Heat capacity data showed ${C}_{p}\ensuremath{\sim}Tln({T}_{0}/T)+\ensuremath{\beta}{T}^{3}$ dependence below 15 K down to 1.8 K. More importantly spin-orbit coupling strength by x-ray absorption spectroscopy was found to be weaker in ${\mathrm{Pb}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ compared to other pyrochlore iridates. In the absence of any large moment rare earth magnetic ion, ${\mathrm{Pb}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ presents a rare example of an iridate system showing non-Fermi liquid behavior due to disordered distribution of ${\mathrm{Ir}}^{4+}$ and ${\mathrm{Ir}}^{5+}$ having markedly different strengths of spin-orbit coupling which might offer a prescription for achieving new non-Fermi liquid systems.