Thermal conductivity across the metal-insulator transition in single crystalline hyperkagome Na$_{3+x}$Ir$_3$O$_8$

TL;DR

This study investigates how thermal conductivity varies across the metal-insulator transition in single crystals of Na$_{3+x}$Ir$_3$O$_8$, revealing the influence of magnetic excitations and disorder on phonon transport.

Contribution

It provides detailed measurements of thermal conductivity in both metallic and insulating states, highlighting the role of magnetic excitations and disorder in phonon scattering near the transition.

Findings

Reduced phonon thermal conductivity in insulating samples

Phonon scattering off magnetic excitations and disorder

Coupling between lattice and spin degrees of freedom

Abstract

The hyperkagome antiferromagnet Na$_{4}$Ir$_3$O$_8$ represents the first genuine candidate for the realisation of a three-dimensional quantum spin-liquid. It can also be doped towards a metallic state, thus offering a rare opportunity to explore the nature of the metal-insulator transition in correlated, frustrated magnets. Here we report thermodynamic and transport measurements in both metallic and weakly insulating single crystals down to 150 mK. While in the metallic sample the phonon thermal conductivity ($\kappa^{ph}$) is almost in the boundary scattering regime, in the insulating sample we find a large reduction $\kappa^{ph}$ over a very wide temperature range. This result can be ascribed to the scattering of phonons off nanoscale disorder or off the gapless magnetic excitations that are seen in the low-temperature specific heat. This works highlights the peculiarity of the metal-insulator transition in Na$_{3+x}$Ir$_3$O$_8$ and demonstrates the importance of the coupling between lattice and spin degrees of freedom in the presence of strong spin-orbit coupling.