Transport and thermodynamic studies of stripe and checkerboard ordering in layered nickel oxides R_2-xSr_xNiO₄ (R = La and Nd)

We have investigated the Sr-doped layered nickelates Nd_2-xSr_xNiO₄ and La_2-xSr_xNiO₄ to elucidate qualitative differences between the stripe-type and checkerboard (CB)-type charge/spin ordered phases. From electrical resistivity, specific heat, magnetic susceptibility and inelastic neutron-scattering measurements, we construct a revised phase diagram around the border of the stripe and CB phases in Nd_2-xSr_xNiO₄, and show the marked difference in the holeconcentration dependence of physical properties between two phases. These results strongly indicate that the CB order is not a simple structural limit of the stripe order.