La 2− xSr xCoO 4 compounds with x ≳0.25 exhibit diagonally modulated incommensurate AFM order with a modulation period that varies linearly with doping from x ≃0.25 up to at least x=0.5 (refs 2, 7). In the compositions of interest here the Co 2+ ions have spin S=3/2 and the Co 3+ ions carry no spin ( S=0 refs 1, 6). Substitution of Sr for La adds positive charge (holes) on the CoO 2 layers, converting Co 2+ into Co 3+ and suppressing the AFM order. The parent compound La 2CoO 4 is an insulator with Néel-type antiferromagnetic (AFM) order below T N=275 K (ref. These layers are isostructural with the CuO 2 layers in the copper oxide high-temperature superconductors. The crystal structure of La 2− xSr xCoO 4 contains well-separated CoO 2 layers with Co arranged on a square lattice. The presence or absence of charge stripes, therefore, is of crucial importance for understanding the hourglass spectrum in La 2− xSr xCoO 4. to propose a stripe-free model based on a nanoscopically phase-separated ground state comprising coexisting regions with local composition x=0 and x=0.5 (ref. However, the subsequent failure to observe charge stripe order in diffraction experiments led Drees et al. Initially 1, it was explained by a model of short-range spin and charge stripe correlations similar to those found in certain hole-doped copper-oxide superconductors near one-eighth doping 4. The hourglass spectrum has been observed in measurements on La 2− xSr xCoO 4 with x=0.25–0.4 (refs 1, 2, 3) however, its origin has become a matter of contention.
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