arXiv subjects

Elbio Dagotto

Publications and source records attributed to Elbio Dagotto.

At least 217 records · Page 12Linked to original sources

arXiv1999

Competition between ferromagnetic and charge-orbital ordered phases in Pr$_{1-x}$Ca$_{x}$MnO$_3$ for $x$=1/4, 3/8, and 1/2

Spin, charge, and orbital structures in models for doped manganites are studied by a combination of analytic mean-field and numerical relaxation techniques. At realistic values for the electron-phonon and antiferromagnetic $t_{2g}$ spin couplings, a competition between a ferromagnetic (FM) phase and a charge-orbital ordered (COO) insulating state is found for $x$=1/4, 3/8, and 1/2, as experimentally observed in Pr$_{1-x}$Ca$_{x}$MnO$_3$ for $x$=0.3$\sim$0.5. The theoretical predictions for the spin-charge-orbital ordering pattern are compared with experiments. The FM-COO energy difference is surprisingly small for the densities studied, result compatible with the presence of a robust colossal-magnetoresistive effect in Pr$_{1-x}$Ca$_{x}$MnO$_3$ in a large density interval.

cond-mat.str-el↗

arXiv1999

Explanation of Giant Cluster Coexistence in Doped Manganites and Other Compounds

Computational studies show the generation of large coexisting metallic and insulating clusters with equal electronic density in models for manganites. The clusters are induced by disorder on exchange and hopping amplitudes near first-order transitions of the non-disordered strongly coupled system. The random-field Ising model is used to explain the qualitative aspects of our results. Percolative characteristics are natural in this context. Our results explain the recently experimentally discovered micrometer size inhomogeneities in manganites. The conclusions are general and apply to a variety of compounds.

cond-mat.str-el↗

arXiv1999

Topological Scenario for Stripe Formation in Manganese Oxides

The spin-charge-orbital complex structures of manganites are studied using topological concepts. The key quantity is the "winding number" w associated with the Berry-phase connection of an e_g electron parallel-transported through Jahn-Teller centers, along zigzag one-dimensional paths in an antiferromagnetic environment of t_{2g} spins. From these concepts, it is shown that the "bi-stripe" and "Wigner-crystal" states observed experimentally have different w's. Predictions for the spin structure of the charge-ordered states for heavily doped manganites are discussed.

cond-mat.str-el↗

arXiv1999

Ferromagnetic, A-type, and Charge-Ordered CE-type States in Doped Manganites using Jahn-Teller Phonons

The two-orbital model for manganites with both non-cooperative and cooperative Jahn-Teller phonons is studied at hole density x=0.5 using Monte Carlo techniques. The phase diagram is obtained varying the electron-phonon coupling and the $\rm t_{2g}$-spins exchange. The insulating CE-type charge- and orbital-ordered state with the $z$-axis charge $stacking$ observed in narrow-bandwidth manganites is stabilized in the simulations. Its charge gap $Δ_{\rm CO}$ is much larger than the critical temperature $\rm k_B T_{CO}$. Metallic-like A-type and ferromagnetic states are also obtained in the same framework, and the phase boundaries among them have first-order characteristics.

cond-mat.str-el↗

arXiv1999

Pseudogap Formation in Models for Manganites

The density-of-states (DOS) and one-particle spectral function $\rm A({\bf k}, ω)$ 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 $\rm La_{1.2} Sr_{1.8} Mn_2 O_7$.

cond-mat.str-el↗

arXiv1999

Experiments on Ladders Reveal a Complex Interplay between a Spin-Gapped Normal State and Superconductivity

In recent years, the study of ladder materials has developed into a well-established area of research within the general context of Strongly Correlated Electrons. This effort has been triggered by an unusual cross-fertilization between theory and experiments. In this paper, the main experimental results obtained in the context of ladders are reviewed from the perspective of a theorist. Emphasis is given to the many similarities between the two-dimensional high-$\rm T_c$ cuprates and the two-leg ladder compounds, including Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ (14-24-41) which has a superconducting phase at high pressure and a small hole density. Examples of these similarities include regimes of linear resistivity vs temperature in metallic ladders and a normal state with spin-gap or pseudogap characteristics. Some controversial results in this context are also discussed. It is remarked that the ladder 14-24-41 is the first superconducting copper-oxide material with a non-square-lattice layered arrangement, and certainly much can be learned from a careful analysis of this compound. A short summary of the main theoretical developments in this field is also included, as well as a brief description of the properties of non-copper-oxide ladders. Suggestions by the author on possible experiments are described in the text. Overall, it is concluded that the enormous experimental effort carried out on ladders has already unveiled quite challenging and interesting physics that adds to the rich behavior of electrons in transition-metal-oxides, and in addition contributes to the understanding of the two-dimensional cuprates. However, still considerable work needs to be carried out to fully understand the interplay between charge and spin degrees of freedom in these materials.

cond-mat.str-el↗

arXiv1999

Cooperative Jahn-Teller Effect on the Magnetic Structure of Manganese Oxides

The magnetic structure of LaMnO$_3$ is investigated on three-dimensional clusters of MnO$_6$ octahedra by using a combination of relaxation and Monte Carlo techniques. It is found that the cooperative Jahn-Teller phonons lead to the stabilization of A-type antiferromagnetic and C-type orbital structures in the physically relevant region of parameter space for LaMnO$_3$ with small corrections due to tilting effects. The results suggest that strong Coulomb interactions are not necessary for a qualitative description of undoped manganites. In fact, it is shown that the present result is not essentially changed even if the Coulomb interaction is explicitly included.

cond-mat.str-el↗

arXiv1999

A-type Antiferromagnetic and C-type Orbital-Ordered State in LaMnO$_3$ Using Cooperative Jahn-Teller Phonons

The effect of Jahn-Teller phonons on the magnetic and orbital structure of LaMnO$_3$ is investigated using a combination of relaxation and Monte Carlo techniques on three-dimensional clusters of MnO$_6$ octahedra. In the physically relevant region of parameter space for LaMnO$_3$, and after including small corrections due to tilting effects, the A-type antiferromagnetic and C-type orbital structures were stabilized, in agreement with experiments.

cond-mat.str-el↗

arXiv1999

Indications of Spin-Charge Separation in the 2D Extended t-J Model

The 2D extended $\rm t-J$ model is studied computationally in a broad region of parameter space, motivated by recent photoemission experiments for the undoped cuprate $\rm Ca_2 Cu O_2 Cl_2$ (F. Ronning et al., Science {\bf 282}, 2067 (1998)). The one-hole ground state is shown to develop robust antiferromagnetic (AF) correlations between spins separated by the mobile hole (i.e. across the hole). This effect tends to decouple charge from spin, and the quasiparticle weight becomes negligible, particularly at momenta $(0,π)-(π,0)$. Studies with more holes show precursors of metallic stripe formation, with holes sharing their individual spin arrangements, and AF-correlations generated across the stripe.

cond-mat.str-el↗

arXiv1999

Pseudogap Formation in an Electronic System with d-wave Attraction at Low-density

On the basis of an electronic model with separable attractive interaction, the precursors at high temperature and strong coupling of the d-wave superconducting state are investigated in the one-particle spectral function $A({\bf k},ω)$ and the total density of states $ρ(ω)$, with the use of the self-consistent $t$-matrix approximation. In the low-density region, it is found that a gap-like structure at the Fermi level appears in $A({\bf k},ω)$ and $ρ(ω)$ above the superconducting transition temperature. It is shown that the pseudogap energy scale is determined by the binding energy of the Cooper-pair.

cond-mat↗

arXiv1999

The Phase Separation Scenario for Manganese Oxides

Recent computational studies of models for manganese oxides have revealed a rich phase diagram, not anticipated in early calculations in this context performed in the 1950's and 60's. In particular, the transition between the antiferromagnetic insulator state of the hole-undoped limit and the ferromagnetic metal at finite hole-density was found to occur through a mixed-phase process. When extended Coulomb interactions are included, a microscopically charge inhomogeneous state should be stabilized. These phase separation tendencies, also present at low electronic densities, influence the properties of the ferromagnetic region by increasing charge fluctuations. Experimental data reviewed here using several techniques for manganites and other materials are consistent with this scenario. Similarities with results previously discussed in the context of cuprates are clear from this analysis, although the phase segregation tendencies in manganites seem stronger.

cond-mat.str-el↗

arXiv1999

Hole-Density Evolution of the One-Particle Spectral Function in Doped Ladders

The spectral function $A({\bf q}, ω)$ of doped $t-J$ ladders is presented on clusters with up to $2 \times 20$ sites at zero temperature applying a recently developed technique that uses up to $\sim 6 \times 10^6$ rung-basis states. Similarities with photoemission results for the 2D cuprates are observed, such as the existence of a gap at $(π,0)$ near half-filling (caused by hole pair formation) and flat bands in its vicinity. These features should be observable in ARPES experiments on ladders. The main result of the paper is the nontrivial evolution of the spectral function from a narrow band at $x=0$, to a quasi-noninteracting band at $x \geq 0.5$. It was also observed that the low-energy peaks of a cluster spectra acquire finite line-widths as their energies move away from the chemical potential.

cond-mat.str-el↗

arXiv1998

Indications of Unconventional Superconductivity in Doped and Undoped Triangular Antiferromagnets

The possibility of superconductivity in doped and undoped triangular antiferromagnets is discussed. Using the Bethe-Salpeter (B-S) equation, it is shown that the exchange of RPA paramagnons on a triangular lattice Hubbard model leads to strong pairing correlations at and near half-filling. The dominant states for this system correspond to d-wave singlet (even-frequency) and s-wave triplet (odd-frequency) pairing. Analytical techniques applied to the hole-doped t-J model yield similar results. A t_1-t_2 Hubbard model interpolating between square (t_1=0) and triangular (t_1=t_2) lattices has a tendency to only d-wave singlet pairing for t_1/t_2 <= 0.8. Experimental consequences for organic compounds $κ$-(BEDT-TTF)$_2$X are discussed.

cond-mat.supr-con↗

arXiv1998

Unified Quantum Mechanical Picture for Confined Spinons in Dimerized and Frustrated Spin S=1/2 Chains

A quantum mechanical picture is presented to describe the behavior of confined spinons in a variety of S=1/2 chains. The confinement is due to dimerization and frustration and it manifests itselfas a nonlinear potential V(x)~ |x|^b, centered at chain ends (b <= 1) or produced by modulation kinks (b > 1). The calculation extends to weak or zero frustration some previous ideas valid for spinons in strongly frustrated spin chains. The local magnetization patterns of the confined spinons are calculated. A (minimum) enhancement of the local moments of about 11/3 over a single S=1/2 is found. Estimates for excitation energies and binding lengths are obtained.

cond-mat.str-el↗

arXiv1997

Enhancement of Antiferromagnetic Correlations Induced by Nonmagnetic Impurities: Origin and Predictions for NMR Experiments

Spin models that have been proposed to describe dimerized chains, ladders, two dimensional antiferromagnets, and other compounds are here studied when some spins are replaced by spinless vacancies, such as it occurs by $Zn$ doping. A small percentage of vacancies rapidly destroys the spin gap, and their presence induces enhanced antiferromagnetic correlations near those vacancies. The study is performed with computational techniques which includes Lanczos, world-line Monte Carlo, and the Density Matrix Renormalization Group methods. Since the phenomenon of enhanced antiferromagnetism is found to occur in several models and cluster geometries, a common simple explanation for its presence may exist. It is argued that the resonating-valence-bond character of the spin correlations at short distances of a large variety of models is responsible for the presence of robust staggered spin correlations near vacancies and lattice edges. The phenomenon takes place regardless of the long distance properties of the ground state, and it is caused by a ``pruning'' of the available spin singlets in the vicinity of the vacancies. The effect produces a broadening of the low temperature NMR signal for the compounds analyzed here. This broadening should be experimentally observable in the structurally dimerized chain systems $Cu(NO_3)_2\cdot2.5H_2O$, $CuWO_4$, $(VO)_2P_2O_7$, and $Sr_{14}Cu_{24}O_{41}$, in ladder materials such as $Sr Cu_2 O_3$, in the spin-Peierls systems $CuGeO_3$ and $NaV_2 O_5$, and in several others since it is a universal effect common to a wide variety of models and compounds.

cond-mat.str-el↗

arXiv1997

Revisiting two holes in a locally antiferromagnetic background: the role of retardation and Coulomb repulsion effects

The problem of two holes in the presence of strong antiferromagnetic fluctuations is revisited using computational techniques. Two-dimensional clusters and 2-leg ladders with up to 32 sites are studied with the Lanczos and Truncated Lanczos algorithms. The main motivation is the recently discussed spatial distribution of holes in ladders where the maximum probability for the hole-hole distance is obtained at d=sqrt(2) in units of the lattice spacing, a counter-intuitive result considering that the overall symmetry of the two-hole bound state is d_{x^2 - y^2}. The existence of holes in the same sublattice is argued to be a consequence of non-negligible retardation effects in the t-J model. The existence of strong spin singlets in the region where the two hole bound state is located is here confirmed, and a simple explanation for its origin in the case of planes is proposed. It is predicted that they should appear regardless of the long distance properties of the spin system under consideration, as long as the bound state is d_{x^2 - y^2}. In particular, it is shown that they are present in an Ising spin background. The time retardation in the family of t-J models leads naturally to low-energy hole states with nonzero momentum and spin one, providing a possible explanation for apparent SO(5)-symmetric features observed recently in this context. Finally, the influence of a short-range Coulombic repulsion is analyzed. The hole distribution in the d_{x^2 - y^2} bound state is reanalyzed in the presence of such repulsion. Very short hole-hole distances lose their relevance in the presence of a realistic hole-hole interaction.

cond-mat.str-el↗

arXiv1997

Pseudogap above Tc in a model with d_{x^2-y^2} pairing

We study the anomalous normal state properties of a simple two-dimensional model whose ground state is a d-wave superconductor. Using a self-consistent, conserving formulation, we show that pairing correlations above T_c lead to the appearance of a highly anisotropic pseudogap in the electronic spectral function and the destruction of the Fermi surface. We discuss the similarities and differences between our results and ARPES experiments on underdoped cuprates.

cond-mat.supr-con↗

arXiv1997

Local Enhancement of Antiferromagnetic Correlations by Nonmagnetic Impurities

The local enhancement of antiferromagnetic correlations near vacancies observed in a variety of spin systems is analyzed in a single framework. Variational calculations suggest that the resonating-valence-bond character of the spin correlations at short distances is responsible for the enhancement. Numerical results for uniform spin chains, with and without frustration, dimerized chains, ladders, and two dimensional clusters are in agreement with our conjecture. This short distance phenomenon occurs independently of the long distance behavior of the spin correlations in the undoped system. Experimental predictions for a variety of compounds are briefly discussed.

cond-mat.str-el↗