Quantum spin liquid in frustrated one dimensional LiCuSbO$_4$*

A quantum magnet, LiCuSbO$_4$, with chains of edge-sharing $S\!=\!1/2$ CuO$_6$ octahedra is reported. Short-range ordering is observed while no phase transition or spin freezing occurs down to 100 mK in zero magnetic field. Specific heat indicates a distinct low-temperature high-field phase near the 12 T saturation field. Neutron scattering shows incommensurate spin correlations with $q\!=\!0.47\pm0.01~\pi/a$ and places an upper limit of 70~$\mu$eV on a potential spin gap. Exact diagonalization of easy plane $S\!=\!1/2$ chains with competing nearest neighbor ferro- and next-nearest neighbor antiferromagnetic interactions (J$_1\!=\!-75$~K, J$_2\!=\!34$~K) accounts for the $T\!>\!2$~K bulk and neutron data. Close to a quantum critical point, free from long-range order and with an achievable saturation field, LiCuSbO$_4$ is a promising candidate material to test long-standing predictions for chiral and nematic states in quantum spin chains