Robust D-wave Pairing Correlations in a Hole-Doped Spin-Fermion Model for Cuprates

Pairing Correlations are studied numerically in the hole-doped spin-fermion model for cuprates. Simulations performed on up to 12x12 clusters provide robust indications of D-wave superconductivity away from half-filling. The pairing correlations are the strongest in the direction perpendicular to the dynamic stripe-like inhomogeneities that appear in the ground state at some densities. An optimal doping, where the correlations reach a maximum value, was observed at about 25% doping, in qualitative agreement with hight T_{c} cuprates' experiments. On the other hand, pairing correlations are suppressed by static stripe inhomogeneities.Comment: 4 pages, 4 figure

Superconducting Puddles and "Colossal'' Effects in Underdoped Cuprates

Phenomenological models for the antiferromagnetic (AF) vs. d-wave superconductivity competition in cuprates are studied using conventional Monte Carlo techniques. The analysis suggests that cuprates may show a variety of different behaviors in the very underdoped regime: local coexistence or first-order transitions among the competing orders, stripes, or glassy states with nanoscale superconducting (SC) puddles. The transition from AF to SC does not seem universal. In particular, the glassy state leads to the possibility of "colossal'' effects in some cuprates, analog of those in manganites. Under suitable conditions, non-superconducting Cu-oxides could rapidly become superconducting by the influence of weak perturbations that align the randomly oriented phases of the SC puddles in the mixed state. Consequences of these ideas for thin-film and photoemission experiments are discussed.Comment: RevTeX 4, revised expanded version, 8 pages, 8 figure

Colossal Effects in Transition Metal Oxides Caused by Intrinsic Inhomogeneities

The influence of quenched disorder on the competition between ordered states separated by a first-order transition is investigated. A phase diagram with features resembling quantum-critical behavior is observed, even using classical models. The low-temperature paramagnetic regime consists of coexisting ordered clusters, with randomly oriented order parameters. Extended to manganites, this state is argued to have a colossal magnetoresistance effect. A scale T* for cluster formation is discussed. This is the analog of the Griffiths temperature, but for the case of two competing orders, producing a strong susceptibility to external fields. Cuprates may have similar features, compatible with the large proximity effect of the very underdoped regime.Comment: 4 pages, 4 figure

Resistivity of Mixed-Phase Manganites

The resistivity $\rho_{dc}$ of manganites is studied using a random-resistor-network, based on phase-separation between metallic and insulating domains. When percolation occurs, both as chemical composition and temperature vary, results in good agreement with experiments are obtained. Similar conclusions are reached using quantum calculations and microscopic considerations. Above the Curie temperature, it is argued that ferromagnetic clusters should exist in Mn-oxides. Small magnetic fields induce large $\rho_{dc}$ changes and a bad-metal state with (disconnected) insulating domains.Comment: 4 pages, 4 eps figure

Magnetic Domains and Stripes in the Spin-Fermion Model for Cuprates

Monte Carlo simulations applied to the Spin-Fermion model for cuprates show the existence of antiferromagnetic spin domains and charge stripes upon doping. The stripes are partially filled, with a filling of approximately 1/2 hole per site, and they separate spin domains with a $\pi$ phase shift among them. The stripes observed run either along the x or y axes and they are separated by a large energy barrier. No special boundary conditions or external fields are needed to stabilize these structures at low temperatures. When magnetic incommensurate peaks are observed at momentum $\pi(1,1-\delta)$ and symmetrical points, charge incommensurate peaks appear at $(0,2 \delta)$ and symmetrical points, as experimentally observed. The strong charge fluctuations responsible for the formation of the stripes also induce a pseudogap in the density of states.Comment: Four pages with four figures embedded in tex

Properties of a two orbital model for oxypnictide superconductors: Magnetic order, B_2g spin-singlet pairing channel, and its nodal structure

A two orbital model for the new Fe-based superconductors is studied using the Lanczos method as well as pairing mean-field approximations. Our main goals are (i) to provide a comprehensive analysis of this model using numerical techniques with focus on half-filling and on the state with two more electrons than half-filling and (ii) to investigate the nodal structure of the mean-field superconducting state and compare the results with angle-resolved photoemission data. In particular, we provide evidence that at half-filling spin 'stripes', as observed experimentally, dominate over competing states. Depending on parameters, the state with two more electrons added to half filling is either triplet or singlet. Since experiments suggest spin singlet pairs, our focus is on this state. Under rotation, it transforms as the B_2g representation of the D_4h group. We also show that the s+/- pairing operator transforms as A_1g and becomes dominant only in an unphysical regime of the model where the undoped state is an insulator. For robust values of the effective electronic attraction producing the Cooper pairs, assumption compatible with recent angle-resolved photoemission (ARPES) results that suggesting small Cooper-pair size, the nodes of the two-orbital model are found to be located only at the electron pockets. Since recent ARPES efforts have searched for nodes at the hole pockets or only in a few directions at the electron pockets, our results for the nodal distribution may help to guide future experiments. More in general, the investigations reported here aim to establish several of the properties of the two orbital model. Only a detailed comparison with experiments will clarify how far this simple model present a valid description of the Fe pnictides

Pseudogap Formation in Models for Manganites

The density-of-states (DOS) and one-particle spectral function $\rm A({\bf k}, \omega)$ of the one- and two-orbital models for manganites, the latter with Jahn-Teller phonons, are evaluated using Monte Carlo techniques. Unexpectedly robust pseudogap (PG) features were found at low- and intermediate-temperatures, particularly at or near regimes where phase-separation occurs as $\rm T$$\to$0. The PG follows the chemical potential and it is caused by the formation of ferromagnetic metallic clusters in an insulating background. It is argued that PG formation should be generic of mixed-phase regimes. The results are in good agreement with recent photoemission experiments for $\rm La_{1.2} Sr_{1.8} Mn_2 O_7$.Comment: Accepted for publication in Phys. Rev. Lett., 4 pages, Revtex, with 4 figures embedde

Deviations from Fermi-liquid behavior above TcT_c in 2D short coherence length superconductors

We show that there are qualitative differences between the temperature dependence of the spin and charge correlations in the normal state of the 2D attractive Hubbard model using quantum Monte Carlo simulations. The one-particle density of states shows a pseudogap above \tc with a depleted $N(0)$ with decreasing $T$. The susceptibility \cs and the low frequency spin spectral weight track $N(0)$, which explains the spin-gap scaling: 1/T_1T \sim \cs(T). However the charge channel is dominated by collective behavior and the compressibility $dn/d\mu$ is $T$-independent. This anomalous ``spin-charge separation'' is shown to exist even at intermediate $|U|$ where the momentum distribution n(\bk) gives evidence for degenerate Fermi system.Comment: 4 pages (twocolumn format), 5 Postscript figure