Kitaev’s discovery of an exactly solvable quantum spin liquid model on the honeycomb lattice [1] and a subsequent proposal for how his anisotropic bond-dependent model could arise in realistic materials [2] have led to a revolution in the field of frustrated quantum magnetism and a surge of research into effective spin-1/2 magnets with strong spin-orbit coupling. Furthermore, it has been realized that the Kitaev interaction could also give rise to quantum spin liquids in 3-dimensional analogs of the honeycomb lattice, including the hyper-honeycomb lattice which is realized in the material β-Li2IrO3. While this system orders at ambient pressure, adopting a complicated incommensurate magnetic structure with counter-rotating moments [3], there are indications that the magnetic order is suppressed under pressure [4]. We present high-pressure (2 GPa) 7Li nuclear magnetic resonance (NMR) measurements on single crystals of this material. The spectra show evidence for a structural phase transition around 200 K and a coexistence of phases, consistent with the results of other measurement techniques. The spectra and shift measurements demonstrate a strong suppression of the local magnetic susceptibility at high pressure. However, the spin-lattice relaxation (1/T1) shows a clear power-law temperature dependence. This is inconsistent with a gapped singlet ground state of dimers and trimers, as was previously proposed, and is instead evocative of a more exotic quantum spin liquid-like ground state.
I will compare the power-law relaxation rate observed in β-Li2IrO3 with power laws obtained in a number of gapless quantum spin liquid candidates studied by my research group and others. Importantly, I will show that these power laws are far from universal, and may even vary between two inequivalent crystallographic sites within the same material.
[1] A. Kitaev, Annals of Physics 321, 2 (2006).
[2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009).
[3] A. Biffin et al., Phys. Rev. B 90, 205116 (2014).
[4] M. Majumder et al., Phys. Rev. Lett. 120, 237202 (2018).