  Experimental and theoretical studies are performed on La3Ir3O11, an iridate
hosting a +4.33 fractional valence state for Ir ions and a three-dimensional
frustrated structure composed of edge-shared Ir2O10 dimers. These features are
expected to enhance inter-site hoppings and reduce magnetic moments of Ir ions.
However, a spin-orbit driven Mott insulating transport is observed, which is
supported by our first principles calculations. Most importantly, geometrical
frustration and competing interactions result in a non-Curie-Weiss paramagnetic
ground state, revealing no magnetic order down to 2 K. This unusual state is
further demonstrated by a theoretical modeling process, suggesting a possible
candidate for the spin liquid state.
