Theoretical evidence of spin-orbital-entangled J(eff)=1/2 state in the 3d transition metal oxide CuAl2O4

Abstract

The spin-orbital-entangled Kramers doublet, known as the J(eff) = 1/2 pseudospin driven by large spin-orbit coupling (SOC), appears in layered iridates and alpha-RuCl3, manifesting a relativistic Mott insulating phase. Such entanglement, however, seems barely attainable in 3d transition metal oxides, where the SOC is small and the orbital angular momentum is easily quenched. Based on the density-functional-theory calculations, we report the CuAl2O4 spinel as the possible example of a J(eff) = 1/2 Mott insulator in 3d transition metal compounds. With the help of strong electron correlations, the J(eff) = 1/2 state can survive the competition with an orbital-momentum-quenched S = 1/2 state in the d(9) configuration of CuO4 tetrahedron. From the dynamical mean-field theory calculations, the electron-addition spectra probing unoccupied states are well described by the j(eff) = 1/2 hole state, whereas electron-removal spectra have a rich multiplet structure. The fully relativistic entity found in CuAl2O4 provides insight into the untapped regime where the spin-orbital-entangled Kramers pair coexists with strong electron correlation. ©2019 American Physical Society