Field-induced spin continuum in twin-free Na$_3$Co$_2$SbO$_6$ revealed by magneto-THz spectroscopy

TL;DR

This study uses magneto-THz spectroscopy to explore spin excitations in twin-free Na$_3$Co$_2$SbO$_6$, revealing a field-induced transition from spin waves to a continuum and highlighting strong magnetic anisotropy relevant for quantum spin liquid research.

Contribution

It provides the first detailed magneto-THz spectroscopic analysis of Na$_3$Co$_2$SbO$_6$, demonstrating field-tunable spin excitations and anisotropic magnetic behavior in a candidate Kitaev quantum spin liquid.

Findings

Observation of a low-energy continuum excitation above $T_N$

Transformation of spin waves into a magnetic continuum under in-plane magnetic field

Detection of strong magnetic anisotropy between crystallographic axes

Abstract

The honeycomb magnet Na$_3$Co$_2$SbO$_6$ recently emerged as a promising candidate for realizing Kitaev quantum spin liquid with relatively low levels of structural disorder. While the precise spin Hamiltonian remains controversial, the potential existence of a quantum spin liquid or other novel quantum magnetic phases continues to stimulate investigation. Here, we study the temperature and magnetic field-dependent spin excitations of Na$_3$Co$_2$SbO$_6$ on a twin-free single crystal using magneto-terahertz (THz) spectroscopy, focusing on magnetic anisotropy and field-induced unusual phases. We observe a low-energy continuum excitation above $T_N$ and a 0.5 THz (2 meV) spin wave excitation in magnetic order under zero field. Upon applying an in-plane magnetic field, the spin waves transform to a magnetic continuum over an intermediate field range, above which the system enters a spin-polarized state. Crucially, the spin excitation spectra reveal striking anisotropy between the $\textbf{a}$ and $\textbf{b}$ crystallographic axes, demanding description by a strongly anisotropic spin model. These findings establish Na$_3$Co$_2$SbO$_6$ as a model system for investigating field-tunable quantum magnetism and potential spin liquid behavior in highly anisotropic systems.