arXiv subjects

C. D. Batista

Publications and source records attributed to C. D. Batista.

133 records · Page 8Linked to original sources

arXiv1998

Electron-Doped Manganese Perovskites: The Polaronic State

Using the Lanczos method in linear chains we study the ground state of the double exchange model including an antiferromagnetic super-exchange in the low concentration limit. We find that this ground state is always inhomogeneous, containig ferromagnetic polarons. The extention of the polaron spin distortion, the dispersion relation and their trapping by impurities, are studied for diferent values of the super exchange interaction and magnetic field. We also find repulsive polaron polaron interaction.

cond-mat↗

arXiv1997

Specific Heat of defects in the Haldane System Y_2BaNiO_5

We calculate the specific heat of the antiferromagnetic spin-1 chain compound Y_2BaNi_{1-x}Zn_xO_5 in the presence of a magnetic field. The low-energy spectrum of a Heisenberg Hamiltonian, which includes realistic anisotropies, has been solved using the density matrix renormalization group. The observed Schottky anomaly is very well described by this theory. For large chains ($N>50$), contrary to previous interpretations of the specific heat data, we find S=1/2 states. These results are thus also consistent with electron- spin-resonance data for NENP.

cond-mat↗

arXiv1997

Charge and spin excitations of insulating lamellar copper oxides

A consistent description of low-energy charge and spin responses of the insulating Sr_2CuO_2Cl_2 lamellar system is found in the framework of a one-band Hubbard model which besides $U$ includes hoppings up to 3^{rd} nearest-neighbors. By combining mean-field calculations, exact diagonalization (ED) results, and Quantum Monte Carlo simulations (QMC), we analyze both charge and spin degrees of freedom responses as observed by optical conductivity, ARPES, Raman and inelastic neutron scattering experiments. Within this effective model, long-range hopping processes flatten the quasiparticle band around $(0,π)$. We calculate also the non-resonant A_{1g} and B_{1g} Raman profiles and show that the latter is composed by two main features, which are attributed to 2- and 4-magnon scattering.

cond-mat.str-el↗

arXiv1997

Superconductivity and incommensurate spin fluctuations in a generalized t-J model for the cuprates

We consider the slave-fermion Schwinger-boson decomposition of an effective model obtained through a systematic low-energy reduction of the three-band Hubbard Hamiltonian. The model includes a three-site term t'' similar to that obtained in the large-U limit of the Hubbard model but of opposite sign for realistic or large O-O hopping. For parameters close to the most realistic ones for the cuprates, the mean-field solution exhibits d+s superconductivity (predominantly d_{x^2-y^2}) with a dependence on doping x very similar to the experimentally observed. We also obtained incommensurate peaks at wave vectors near $π(1,1 +(-) 2x)$ in the spin structure factor, which also agree with experiment.

cond-mat.str-el↗

arXiv1996

Excitons in insulating cuprates

We study the electronic excitations near the charge-transfer gap in insulating CuO$_2$ planes, starting from a six-band model which includes $% p_π$ and $d_{xy}$ orbitals and Cu-O nearest-neighbor repulsion $U_{pd}$. While the low lying electronic excitations in the doped system are well described by a modified $t-J$ model, the excitonic states of the insulator include hybrid $d_{xy}-$ $p_π$ states of $A_{2g}$ symmetry. We also obtain excitons of symmetries $B_{1g}$ and $E_u$, and eventually $A_{1g}$, which can be explained within a one-band model. The results agree with observed optical absorption and Raman excitations.

cond-mat↗

arXiv1995

Superconductivity from correlated hopping

We consider a chain described by a next-nearest-neighbor hopping combined with a nearest-neighbor spin flip. In two dimensions this three-body term arises from a mapping of the three-band Hubbard model for CuO$_2$ planes to a generalized $t-J$ model and for large O-O hopping favors resonance-valence-bond superconductivity of predominantly $d$-wave symmetry. Solving the ground state and low-energy excitations by analytical and numerical methods we find that the chain is a Luther-Emery liquid with correlation exponent $K_ρ = (2-n)^2/2$, where $n$ is the particle density.

supr-con↗

arXiv1994

Superconductivity in the cuprates

We evaluate numerically several superconducting correlation functions in a generalized $t-J$ model derived for hole-doped CuO$_2$ planes. The model includes a three-site term $t''$ similar to that obtained in the large $U$ limit of the Hubbard model but of opposite sign for realistic O-O hopping. For realistic parameters we obtain strong evidence of superconductivity of predominantly $d_{x^2-y^2}$ character. The ground state has a large overlap with a very simple resonating-valence-bond wave function with off-diagonal long-range order. This function reproduces the main features of the magnetic and superconducting correlation functions.

cond-mat↗