Field evolution of low-energy excitations in the hyperhoneycomb magnet $\beta$-Li$_2$IrO$_3$

TL;DR

This study uses NMR and THz spectroscopy to investigate magnetic excitations in the hyperhoneycomb Kitaev magnet $eta$-Li$_2$IrO$_3$, revealing a transition from gapless to gapped excitations with increasing magnetic field.

Contribution

It provides the first detailed field-dependent analysis of low-energy magnetic excitations in $eta$-Li$_2$IrO$_3$, highlighting differences from honeycomb Kitaev magnets.

Findings

Gapless excitations at low fields below 2.8 T.

Presence of zone-center magnon peaks without continuum at high fields.

Field-induced opening of an excitation gap.

Abstract

$^7$Li nuclear magnetic resonance (NMR) and terahertz (THz) spectroscopies are used to probe magnetic excitations and their field dependence in the hyperhoneycomb Kitaev magnet $\beta$-Li$_2$IrO$_3$. Spin-lattice relaxation rate ($1/T_1$) measured down to 100\,mK indicates gapless nature of the excitations at low fields (below $H_c\simeq 2.8$\,T), in contrast to the gapped magnon excitations found in the honeycomb Kitaev magnet $\alpha$-RuCl$_3$ at zero applied magnetic field. At higher temperatures in $\beta$-Li$_2$IrO$_3$, $1/T_1$ passes through a broad maximum without any clear anomaly at the N\'eel temperature $T_N\simeq 38$\,K, suggesting the abundance of low-energy excitations that are indeed observed as two peaks in the THz spectra, both correspond to zone-center magnon excitations. At higher fields (above $H_c$), an excitation gap opens, and a re-distribution of the THz spectral weight is observed without any indication of an excitation continuum, in contrast to $\alpha$-RuCl$_3$ where an excitation continuum was reported.