Symmetry consideration and eg bands in NdNiO3 and YNiO3

Group theoretical analyses are applied to the magnetic and electronic structures of NdNiO3 and YNiO3, whose electronic structures have been studied very recently by the LSDA+U method. The Jahn-Teller distortion cannot satisfy the experimentally observed magnetic diffraction. The resultant ground state magnetic stuructures are monoclinic P_ba in NdNiO3 case and P_b2_1/a in YNiO3 case, respectively.Comment: 6 pages, 3 figures, Proc. Int. Symp. ISSP-Kashiwa 200

Charge and spin stripe in La2−x_{2-x}Srx_{x}NiO4_{4} (x=1/3,1/2)

Electronic structure of stripe ordered La$_{2-x}$Sr$_{x}$NiO$_{4}$ is investigated. The system with x=1/3 is insulator, in LSDA+U calculations, and shows charge and spin stripe, consistent with the experimental results. Highly correlated system of x=1/2 is studied by using exact diagonalization of multi-orbital many body Hamiltonian derived from LDA calculations and including on-site and inter-site Coulomb interactions. The fluctuation of the residual spin on Ni$^{3+}$ (hole) site couples with the charge fluctuation between Ni$^{3+}$ and Ni$^{2+}$ states and this correlation lowers the total energy. The resultant ground state is insulator with charge and spin stripe of the energy gap 0.9eV, consistent with observed one. The on-site Coulomb interaction stabilizes integral valency of each Ni ion (Ni$^{3+}$ and Ni$^{2+}$), but does not induce the charge order. Two quantities, inter-site Coulomb interaction and anisotropy of hopping integrals, play an important role to form the charge and spin stripe order in a system of x=1/2.Comment: 12 pages, 10 figures, fully revise

Antiferromagnetism in two-dimensional t-J model: pseudospin representation

We discuss a pseudospin representation of the two-dimensional t-J model. We introduce pseudospins associated with empty sites, deriving a new representation of the t-J model that consists of local spins and spinless fermions. We show, within a mean-field approximation, that our representation of t-J model corresponds to the {\it isotropic} antiferromagnetic Heisenberg model in an effective magnetic field. The strength and the direction of the effective field are determined by the hole doping ${\delta}$ and the orientation of pseudospins associated with empty sites, respectively. We find that the staggered magnetization in the standard representation corresponds to the component of magnetization perpendicular to the effective field in our pseudospin representation. Using a many-body Green's function method, we show that the staggered magnetization decreases with increasing hole doping ${\delta}$ and disappears at ${\delta \approx 0.06-0.15}$ for $t/J=2-5$. Our results are in good agreement with experiments and numerical calculations in contradistinction to usual mean-field methods.Comment: 6 pages, 3 figure