Angle-resolved photoemission study of MX-chain compound [Ni(chxn)2Br]Br2: absence of spin-charge separation signals

We report on the results of angle-resolved photoemission experiments on a quasi-one-dimensional MX-chain compound [Ni(chxn)2Br]Br2 (chxn = 1R,2R-cyclohexanediamine), a one-dimensional Heisenberg system with S = 1/2 and J ∼ 3600 K, which shows a gigantic non-linear optical effect. A ”band” having about 500 meV energy dispersion is found in the first half of the Brillouin zone (0 ≤ kb/π < 1/2), but disappears at kb/π ∼ 1/2. Two dispersive features, expected from the spin-charge separation, as have been observed in other quasi-one-dimensional systems like Sr2CuO3, are not detected. These characteristic features are well reproduced by the d-p chain model calculations with a small charge-transfer energy ∆ compared with that of one-dimensional Cu-O based compounds. We propose that this smaller ∆ is the origin of the absence of clear spinand charge-separation in the photoemission spectra and strong non-linear optical effect in [Ni(chxn)2Br]Br2.