Magnetic excitations of the field-induced states in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>BaCo</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi>AsO</mml:mi><mml:mn>4</mml:mn></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> probed by time-domain terahertz spectroscopy

Abstract

Searching for Kitaev quantum spin liquids (QSLs) is a fascinating and challenging problem. Much effort has been devoted to honeycomb lattice candidates with strong spin-orbit coupling in $5d$-electron iridates and $4d$-electron ${\mathrm{RuCl}}_{3}$. Recently, theoretical studies suggested that $3{d}^{7}$ Co-based honeycomb materials with a high spin state $S=3/2$ and effective orbital angular momentum $L=1$ could also be promising candidates of a Kitaev QSL. One of the candidates, ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$, was revisited recently. The long-range magnetic order in ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$ can be suppressed by a very weak in-plane magnetic field, suggesting its proximity to a Kitaev QSL. Here, we perform time-domain terahertz spectroscopy measurements to study the magnetic excitations on ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$. We observe different magnon excitations upon increasing external magnetic field. In particular, the system is easily driven to a field-polarized paramagnetic phase, after the long-range magnetic order is suppressed by a weak field ${H}_{c2}$. The spectra beyond ${H}_{c2}$ are dominated by single-magnon and two-magnon excitations. Therefore, in the measured low-temperature region, the field-induced state is not a Kitaev QSL. We also compared the excitation spectra of ${\mathrm{BaCo}}_{2}{({\mathrm{AsO}}_{4})}_{2}$ with that of the widely studied $4d$-electron Kitaev candidate ${\mathrm{RuCl}}_{3}$ and addressed their similarities in magnetic excitations.

Topics & Concepts