Transport and Thermodynamic Studies of Stripe and Checkerboard Ordering in Layered Nickel Oxides R2-xSrxNiO4 (R = La and Nd)

We have investigated the Sr-doped layered nickelates Nd2-xSrxNiO4 and La2-xSrxNiO4 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 Nd2-xSrxNiO4, and show the marked difference in the hole-concentration 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.