Field Evolution Of Low-Energy Excitations In The Hyperhoneycomb Magnet Beta-Li2iro3

Li-7 nuclear magnetic resonance and terahertz (THz) spectroscopies are used to probe magnetic excitations and their field dependence in the hyperhoneycomb Kitaev magnet beta-Li2IrO3. Spin-lattice relaxation rate (1/T-1) measured down to 100 mK indicates the gapless nature of the excitations at low fields (below H-c similar or equal to 2.8 T), in contrast to the gapped magnon excitations found in the honeycomb Kitaev magnet alpha-RuCl3 at zero applied magnetic field. At higher temperatures in beta-Li2IrO3, 1/T-1 passes through a broad maximum without any clear anomaly at the Neel temperature T-N similar or equal to 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 redistribution of the THz spectral weight is observed without any indication of an excitation continuum, in contrast to alpha-RuCl3 where an excitation continuum was reported.