Planar thermal Hall effect from phonons in a Kitaev candidate material.

Area of Science:

• Condensed Matter Physics
• Quantum Materials
• Spin Liquids

Background:

• The thermal Hall effect is a key probe for exotic excitations in quantum spin liquids.
• In Kitaev materials like $\alpha$-RuCl$_{3}$, the thermal Hall conductivity has been linked to Majorana fermions, chiral magnons, or phonons.
• A planar thermal Hall effect (magnetic field parallel to heat current) was theoretically expected for fermions and magnons, but not phonons.

Purpose of the Study:

• To investigate the origin of the planar thermal Hall effect in the Kitaev candidate material Na$_{2}$Co$_{2}$TeO$_{6}$.
• To determine if phonons can contribute to or cause the planar thermal Hall effect.
• To re-evaluate previous findings in $\alpha$-RuCl$_{3}$ based on new evidence.

Main Methods:

• Experimental measurement of thermal Hall conductivity in Na$_{2}$Co$_{2}$TeO$_{6}$ under varying magnetic field and heat current configurations.
• Comparison of the measured thermal Hall conductivity with the material's phonon-dominated thermal conductivity.
• Analysis of the directional dependence of the planar thermal Hall effect.

Main Results:

• A planar thermal Hall effect was observed in Na$_{2}$Co$_{2}$TeO$_{6}$.
• The observed effect closely matched the phonon-dominated thermal conductivity, strongly suggesting a phonon origin.
• A significant difference in the planar thermal Hall effect was found depending on the relative orientation of the heat current and magnetic field.

Conclusions:

• Phonons can generate a planar thermal Hall effect, contrary to previous theoretical assumptions.
• The findings necessitate a re-examination of the planar thermal Hall effect observed in $\alpha$-RuCl$_{3}$.
• The direction of heat current is a critical factor in determining the planar thermal Hall effect in these materials.