Effect of disorder on the quantum spin liquid candidate Na${}_4$Ir${}_3$O${}_8$

Abstract

We report on the effects of introducing magnetic and non-magnetic disorder in the hyperkagome iridate quantum spin liquid (QSL) candidate Na${}_4$Ir${}_3$O${}_8$ by partially replacing Ir${}^{4+}$ ($S=1/2$) with Ru${}^{4+}$ ($S=1$) or Ti${}^{4+}$ ($S=0$). Specifically, we synthesized Na${}_4$(Ir${}_{1-x}$Ru${}_x$)${}_3$O${}_8(x=0.05,0.10,0.2,0.3)$ and Na${}_4$Ir${}_{2.7}$Ti${}_{0.3}$O${}_8$ samples and measured electrical transport, AC and DC magnetization, and heat capacity down to $T=1.8$ K. Na${}_4$Ir${}_3$O${}_8$ is associated with a large Weiss temperature $\theta =-650$ K, a broad anomaly in magnetic heat capacity C${}_{mag}$ at T $\approx 25$ K, low temperature power-law heat capacity, and spin glass freezing below $T_f\approx 6$ K. We track the change in these characteristic features as Ir is partially substituted by Ru or Ti. We find that for Ru substitution, $\theta$ increases and stays negative, the anomaly in C${}_{mag}$ is suppressed in magnitude and pushed to lower temperatures, low temperature $C\sim T^{\alpha }$ with $\alpha$ between $2$ and $3$ and decreasing towards $2$ with increasing $x$, and $T_f$ increases with increase in Ru concentration $x$. For Ti substitution we find that $\theta$ and T${}_f$ become smaller and the anomaly in $C_{mag}$ is completely suppressed. In addition, introducing non-magnetic Ti leads to the creation of orphan spins which show up in the low temperature magnetic susceptibility.

6 pages, 6 figures, 2 Tables