Direct evidence of a near-ideal J eff = 1 / 2 ground state in triangular-lattice Na 2 BaCo ( PO 4 ) 2

We investigated the local Co 3 d electronic structure of Na 2 BaCo ( PO 4 ) 2 using polarization-dependent x-ray absorption spectroscopy (XAS) in combination with full multiplet cluster calculations. We employed the line-fitting inverse partial fluorescence yield (IPFY) technique to obtain accurate XAS spectra from strong insulating materials. Our combined experimental and theoretical analysis reveals a very small effective trigonal distortion of only 11 meV in the CoO 6 octahedra, indicating a close to ideal condition to render a ground state with the J eff = 1 / 2 character. With our cluster model we were also able to simulate magnetic susceptibility measurements along different directions in the crystal. These findings highlight Na 2 BaCo ( PO 4 ) 2 as a promising platform for exploring exotic magnetic phenomena associated with J eff = 1 / 2 ground states on triangular lattices.