Experimental observation on Jeff=1/2 state in CuAl2O4*

The spin-orbit entangled pseudo-spin state, J_eff=1/2 has driven intriguing phenomena such as high-Tc superconductivity, quantum spin liquid, and topological semimetal. This studies have been conducted on 4d or 5d compounds but 3d systems haven't been focused on. The J_eff=1/2 states in 3d systems including cuprates haven't been considered to emerge in condensed matter physics. One reason is the spin-orbit coupling constant of 3d is smaller than that of Ru (4d) or Ir (5d) ions and the other is that many 3d systems have distorted-crystal structures which lead quenched orbital angular momentum. However, we suggest CuAl2O4 is a candidate material to induce Jeff=1/2 states. Cubic symmetry of the crystal structure verified by single crystal XRD supports Jeff=1/2 nature. Moreover, X-ray absorption spectroscopy also shows this nature following the electric dipole selection rule.