Direct experimental observation of the molecular J(eff) = 3/2 ground state in the lacunar spinel GaTa(4)Se(8)

Strong spin–orbit coupling lifts the degeneracy of t (2g) orbitals in 5d transition-metal systems, leaving a Kramers doublet and quartet with effective angular momentum of J (eff) = 1/2 and 3/2, respectively. These spin–orbit entangled states can host exotic quantum phases such as topological Mott state, unconventional superconductivity, and quantum spin liquid. The lacunar spinel GaTa(4)Se(8) was theoretically predicted to form the molecular J (eff) = 3/2 ground state. Experimental verification of its existence is an important first step to exploring the consequences of the J (eff) = 3/2 state. Here, we report direct experimental evidence of the J (eff) = 3/2 state in GaTa(4)Se(8) by means of excitation spectra of resonant inelastic X-ray scattering at the Ta L(3) and L(2) edges. We find that the excitations involving the J (eff) = 1/2 molecular orbital are absent only at the Ta L(2) edge, manifesting the realization of the molecular J (eff) = 3/2 ground state in GaTa(4)Se(8).