Hyperhoneycomb iridate beta-Li2IrO3 as a platform for Kitaev spin liquid

Realization of quantum spin liquid has been a long-sought dream in condensed matter physics, where exotic excitations and unconventional superconductivity upon doping are expected. Honeycomb iridates recently emerged as a possible materialization of Kitaev spin liquid with frustrated ``$bond-dependent\; ferromagnetic\; interaction$''. However, the real materials, $\alpha$-Na$_2$IrO$_3$ and $\alpha$-Li$_2$IrO$_3$, undergo antiferromagnetic ordering likely due to the presence of other dominant magnetic interactions and lattice distortion. We discovered a new form of Li$_2$IrO$_3$, $\beta$-Li$_2$IrO$_3$, which comprises a three-dimensional analogue of honeycomb lattice dubbed as "hyperhoneycomb". Each Ir$^{4+}$ ion of the hyperhoneycomb lattice has three neighboring like ions rotated by 120$^{\circ}$ and thus the local structure is identical with 2D honeycomb, indicating that the hyperhoneycomb lattice is a new platform for Kitaev physics. $\beta$-Li$_2$IrO$_3$ diplays a spiral magnetic order below 38 K, which likely originates from dominance of ferromagnetic Kitaev interaction. We argure that $\beta$-Li$_2$IrO$_3$ locates in a close proximity to Kitaev spin liquid. We also discuss the spin liquid behavior observed in a new honeycomb iridate obtained by chemical modulation.