Manipulation of J(eff) =3/2 states by tuning the tetragonal distortion

The spin-orbit entangled quantum states in 4d/5d compounds, e.g., the J(eff) = 1/2 and J(eff) = 3/2 states, have attracted great interests for their unique physical roles in unconventional superconductivity and topological states. Here, the key role of tetragonal distortion is clarified, which determines the ground states of 4d(1)/5d(1) systems to be the J(eff) = aone (e.g., K2NbCl6) or the S = 1/2 one (e.g., Rb2NbCl6). By tuning the tetragonal distortion via epitaxial strain, the occupation weights of d(xy)/d(yz)/d(xz) orbitals can be subtly modulated, competing with the spin-orbit coupling. Consequently, quantum phase transitions between the S = 1/2 state and the J(eff) = 3/2 state, as well as between different J(eff) = astates, can be achieved, resulting in significant changes of local magnetic moments. Our prediction points out a reliable rout e to engineer new functionality of J(eff) states in these quantum materials.