Topological spin liquid phase in a low-dimensional organic molecular compound

We report the discovery of a correlated insulator with a bulk gap at two-thirds filling in a geometrically frustrated Hubbard model that describes the low-energy physics of Mo${}_3$S${}_7$(dmit)${}_3$. This is very different from the Mott insulator expected at half-filling. We show that the insulating phase, which persists even for very weak electron-electron interactions ($U$), is adiabatically connected to the Haldane phase and is consistent with experiments on Mo${}_3$S${}_7$(dmit)${}_3$