Does the Heisenberg model describe the multimagnon spin dynamics in antiferromagnetic CuO layers ?

We compute the absorption spectrum for multimagnon excitations assisted by phonons in insulating layered cuprates using exact diagonalization in clusters of up to 32 sites. The resulting line shape is very sensitive to the underlying magnetic Hamiltonian describing the spin dynamics. For the usual Heisenberg description of undoped Cu-O planes we find, in accordance with experiment, a two-magnon peak followed by high energy side bands. However the relative weight of the side bands is too small to reproduce the experiment. An extended Heisenberg model including a sizable four-site cyclic exchange term is shown to be consistent with the experimental data.Comment: To appear in Physical Review Letter

Inhomogeneity-Induced Superconductivity?

A t-J-like model for inhomogeneous superconductivity of cuprate oxides is presented, in which local anisotropic magnetic terms are essential. We show that this model predicts pairing, consistent with experiments, and argue how the macroscopic phase-coherent state gradually grows upon lowering of the temperature. We show that appropriate inhomogeneities are essential in order to have significant pair binding in the thermodynamic limit. Particularly, {\it local} breaking of SU(2) spin symmetry is an efficient mechanism for inducing pairing of two holes, as well as explaining the magnetic scattering properties. We also discuss the connection of the resulting inhomogeneity-induced superconductivity to recent experimental evidence for a linear relation between magnetic incommensurability and the superconducting transition temperature, as a function of doping.Comment: 4 pages, REVTEX, 4 jpeg figures. To appear in Europhys. Let

Photoemission Spectroscopy from Inhomogeneous Models of Cuprates

We investigate the electronic dynamics in the underdoped cuprates focusing on the effects of one-dimensional charge stripes. We address recent experimental Angular-Resolved Photoemission Spectra results on (La$_{1.28}$Nd$_{0.6}$Sr$_{0.12}$)CuO$_4$. We find that various inhomogeneous models can account for the distribution of quasiparticle weights close to momentum ${\bf k}=(\pi,0)$ and symmetry related points. The observed flat dispersion region around the same ${\bf k}$ point can only be addressed by certain classes of those inhomogeneous models which locally break spin symmetry. Homogeneous models including hopping elements up to second neighbors cannot reproduce the experimental quasiparticle weight, since most of it is centered around ${\bf k}=(\frac {\pi}{2},\frac {\pi} {2})$.Comment: 5 pages, color figure

Striped superconductors in the extended Hubbard model

We present a minimal model of a doped Mott insulator that simultaneously supports antiferromagnetic stripes and d-wave superconductivity. We explore the implications for the global phase diagram of the superconducting cuprates. At the unrestricted mean-field level, the various phases of the cuprates, including weak and strong pseudogap phases, and two different types of superconductivity in the underdoped and the overdoped regimes, find a natural interpretation. We argue that on the underdoped side, the superconductor is intrinsically inhomogeneous -- striped coexistence of of superconductivity and magnetism -- and global phase coherence is achieved through Josephson-like coupling of the superconducting stripes. On the overdoped side, the state is overall homogeneous and the superconductivity is of the classical BCS type.Comment: 5 pages, 3 eps figures. Effect of t' on stripe filling + new references are adde

Spin dynamics of hole doped Y2BaNiO5

Starting from a multiband Hamiltonian containing the relevant Ni and O orbitals, we derive an effective Hamiltonian $H_{eff}$ for the low energy physics of doped Y$_{2}$BaNiO$_{5}.$ For hole doping, $H_{eff}$ describes O fermions interacting with S=1 Ni spins in a chain, and cannot be further reduced to a simple one-band model. Using numerical techniques, we obtain a dynamical spin structure factor with weight inside the Haldane gap. The nature of these low-energy excitations is identified and the emerging physical picture is consistent with most of the experimental information in Y% $_{2-x}$Ca$_{x}$BaNiO$_{5}$Comment: 4 pages, 2 figure

Pairing in Inhomogeneous Superconductors

Starting from a t-J model, we introduce inhomogeneous terms to mimic stripes. We find that if the inhomogeneous terms break the SU(2) spin symmetry the binding between holes is tremendously enhanced in the thermodynamic limit. In any other model (including homogeneous models) the binding in the thermodynamic limit is small or neglible. By including these inhomogeneous terms we can reproduce experimental neutron scattering data. We also discuss the connection of the resulting inhomogeneity-induced superconductivity to recent experimental evidence for a linear relation between magnetic incommensurability and the superconducting transition temperature, as a function of doping.Comment: 4 pages, 2 figure

Microscopic Scenario for Striped Superconductors

We argue that the superconducting state found in high-$T_c$ cuprates is inhomogeneous with a corresponding inhomogeneous superfluid density. We introduce two classes of microscopic models which capture the magnetic and superconducting properties of these strongly correlated materials. We start from a generalized t-J model, in which appropriate inhomogeneous terms mimic stripes. We find that inhomogeneous interactions that break magnetic symmetries are essential to induce substantial pair binding of holes in the thermodynamic limit. We argue that this type of model reproduces the ARPES and neutron scattering data seen experimentally.Comment: 4 pages, 2 psfigures. To appear in Physica

Quantum Phase Diagram of the t-Jz Chain Model

We present the quantum phase diagram of the one-dimensional $t$-$J_z$ model for arbitrary spin (integer or half-integer) and sign of the spin-spin interaction $J_z$, using an {\it exact} mapping to a spinless fermion model that can be solved {\it exactly} using the Bethe ansatz. We discuss its superconducting phase as a function of hole doping $\nu$. Motivated by the new paradigm of high temperature superconductivity, the stripe phase, we also consider the effect the antiferromagnetic background has on the $t$-$J_z$ chain intended to mimic the stripe segments.Comment: 4 pages, 2 figure

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.Comment: 4 pages, 6 embedded figure