arXiv subjects

Elbio Dagotto

Publications and source records attributed to Elbio Dagotto.

At least 235 records · Page 13Linked to original sources

arXiv1997

Hole Doping Evolution of the Quasiparticle Band in Models of Strongly Correlated Electrons for the High-T_c Cuprates

Quantum Monte Carlo (QMC) and Maximum Entropy (ME) techniques are used to study the spectral function $A({\bf p},ω)$ of the one band Hubbard model in strong coupling including a next-nearest-neighbor electronic hopping with amplitude $t'/t= -0.35$. These values of parameters are chosen to improve the comparison of the Hubbard model with angle-resolved photoemission (ARPES) data for $Sr_2 Cu O_2 Cl_2$. A narrow quasiparticle (q.p.) band is observed in the QMC analysis at the temperature of the simulation $T=t/3$, both at and away from half-filling. Such a narrow band produces a large accumulation of weight in the density of states at the top of the valence band. As the electronic density $< n >$ decreases further away from half-filling, the chemical potential travels through this energy window with a large number of states, and by $< n > \sim 0.70$ it has crossed it entirely. The region near momentum $(0,π)$ and $(π,0)$ in the spectral function is more sensitive to doping than momenta along the diagonal from $(0,0)$ to $(π,π)$. The evolution with hole density of the quasiparticle dispersion contains some of the features observed in recent ARPES data in the underdoped regime. For sufficiently large hole densities the ``flat'' bands at $(π,0)$ cross the Fermi energy, a prediction that could be tested with ARPES techniques applied to overdoped cuprates. The population of the q.p. band introduces a {\it hidden} density in the system which produces interesting consequences when the quasiparticles are assumed to interact through antiferromagnetic fluctuations and studied with the BCS gap equation formalism. In particular, a region of extended s-wave is found to compete with d-wave in the overdoped regime, i.e. when the chemical potential has almost entirely crossed the q.p.

cond-mat.str-el↗

arXiv1996

Spectral Functions of One-dimensional Models of Correlated Electrons

Using the Ogata-Shiba wave function, the spectral functions of the one-dimensional infinite U Hubbard model are calculated for various concentrations. It is shown that the ``shadow band'' feature due to 2k_F fluctuations becomes more intense close to half-filling. Comparing these results with exact diagonalization data obtained on finite clusters for the finite U Hubbard model and for the t-J model, it is also shown that this feature remains well-defined for physically reasonable values of the parameters (U/t\simeq 10, J/t\simeq 0.4). The ``shadow'' structure in the spectral functions should thus be observable in angle-resolved photoemission experiments for a variety of quasi-one dimensional compounds.

cond-mat.str-el↗

arXiv1996

Ferromagnetism in Electronic Models for Manganites

Ground state properties of the Kondo model for manganese oxides in one dimension are studied using numerical techniques. The large Hund coupling ($J_{H}$) limit is specially analyzed. A robust region of fully saturated ferromagnetism (FM) is identified at all densities. For open boundary conditions it is shown exactly that the ground state is FM at $J_{H} = \infty$. Hole-spin phase separation competing with FM was also observed when a large exchange $J$ between the $Mn^{3+}$ ions is used. As the spin of the transition metal ion grows, the hole mobility decreases providing a tentative explanation for the differences between Cu-oxides and Mn-oxides.

cond-mat↗

arXiv1996

Rapid Suppression of the Spin Gap in Zn-doped CuGeO_3 and SrCu_2O_3

The influence of non-magnetic impurities on the spectrum and dynamical spin structure factor of a model for CuGeO$_3$ is studied. A simple extension to Zn-doped ${\rm Sr Cu_2 O_3}$ is also discussed. Using Exact Diagonalization techniques and intuitive arguments we show that Zn-doping introduces states in the Spin-Peierls gap of CuGeO$_3$. This effect can beunderstood easily in the large dimerization limit where doping by Zn creates ``loose'' S=1/2 spins, which interact with each other through very weak effective antiferromagnetic couplings. When the dimerization is small, a similar effect is observed but now with the free S=1/2 spins being the resulting S=1/2 ground state of severed chains with an odd number of sites. Experimental consequences of these results are discussed. It is interesting to observe that the spin correlations along the chains are enhanced by Zn-doping according to the numerical data presented here. As recent numerical calculations have shown, similar arguments apply to ladders with non-magnetic impurities simply replacing the tendency to dimerization in CuGeO$_3$ by the tendency to form spin-singlets along the rungs in SrCu$_2$O$_3$.

cond-mat↗

arXiv1996

Photoemission Spectra in t-J Ladders with Two Legs

Photoemission spectra for the isotropic two-leg t-J ladder are calculated at various hole-doping levels using exact diagonalization techniques. Low-energy sharp features caused by short-range antiferromagnetic correlations are observed at finite doping levels close to half-filling, above the naive Fermi momentum. These features should be observable in angle-resolved photoemission experiments. In addition, the formation of a d-wave pairing condensate as the ratio J/t is increased leads to dynamically generated spectral weight for momenta close to $k_F$ where the $d_{x^2-y^2 }$-order parameter is large.

cond-mat↗

arXiv1996

Hole Dispersion and Symmetry of the Superconducting Order Parameter for Underdoped CuO$_2$ Bilayers and 3D Antiferromagnets

We calculate the dispersion of a hole in ${\rm CuO_2}$ bilayers and 3D antiferromagnets, using the self-consistent Born approximation (SCBA) to the $t-J$ model. Superconductivity and the symmetry of its order parameter are studied introducing a nearest-neighbor density-density attraction induced by short range antiferromagnetic (AF) fluctuations, as described in recent studies for the single layer cuprates. The well-known pairing in the ${ d_{x^2-y^2}}$ channel observed for one plane remains robust when three dimensional interactions are turned on. In bilayers, as the exchange along the direction perpendicular to the planes grows, eventually a transition to a ``s-wave'' state is observed with an order parameter mixture of ${ d_{3z^2-r^2}}$ and ${ s_{x^2+y^2 + z^2}}$. For an isotropic 3D antiferromagnet the $d_{x^2 -y^2}$ and $d_{3z^2 - r^2}$ channels are degenerate.

cond-mat↗

arXiv1996

Electronic Hamiltonian for Transition Metal Oxide Compounds

An effective electronic Hamiltonian for transition metal oxide compounds is presented. For Mn-oxides, the Hamiltonian contains spin-2 ``spins'' and spin-3/2 ``holes'' as degrees of freedom. The model is constructed from the Kondo-lattice Hamiltonian for mobile $e_g$ electrons and localized $t_{2g}$ spins, in the limit of a large Hund's coupling. The effective electron bond hopping amplitude fluctuates in sign as the total spin of the bond changes. In the large spin limit, the hopping amplitude for electrons aligned with the core ions is complex and a Berry phase is accumulated when these electrons move in loops. The new model is compared with the standard double exchange Hamiltonian. Both have ferromagnetic ground states at finite hole density and low temperatures, but their critical temperatures could be substantially different due to the frustration effects induced by the Berry phase.

cond-mat↗

arXiv1996

Influence of Hole Doping on Antiferromagnetic Real-Space Approaches for the High-Tc Cuprates

Recently proposed scenarios for the cuprates make extensive use of a ``flat'' quasiparticle (q.p.) dispersion and short-range hole-hole interactions in real-space, both caused by antiferromagnetic (AF) correlations. The density of states (DOS) at half-filling has a robust peak which boosts the superconducting critical temperature $T_c$ to large values as holes are introduced into the (rigid) q.p. band. Here, the stability of such scenarios is studied after a $finite$ but small hole density is introduced. The overall conclusion is that the main features of real-space AF-based approaches remain qualitatively similar, namely a large $T_c$ is found and superconductivity (SC) appears in the ${\rm d_{x^2 - y^2}}$ channel. As the hole density grows the chemical potential $μ$ crosses a broad peak in the DOS. We also observe that extended s-wave SC competes with d-wave in the overdoped regime.

cond-mat↗

arXiv1995

A Possible Phononic Mechanism for $d_{x^2 - y^2}$-Superconductivity in the Presence of Short-Range AF Correlations

We discuss the high temperature superconductors in a regime where the antiferromagnetic (AF) correlation length is only a couple of lattice spacings. In the model proposed here, these short-range AF fluctuations play an essential role in the dressing of the carriers, but the attraction needed for superconductivity (SC) arises from a transverse phonon oxygen mode with a finite buckling angle as it appears in ${\rm YBa_2Cu_3O_{7-δ}}$. A simple fermion-phonon model analog to the Holstein model is introduced to account for this effect. We argue that the model has a $d_{x^2 - y^2}$-wave superconducting groundstate. The critical temperature ($T_c$) and the O-isotope effect coefficient ($α_O$) vs hole density ($x$) are in qualitative agreement with experiments for the cuprates. The minimum (maximum) of $α_O$ ($T_c$) at optimal doping is caused by a large peak in the density of states of holes dressed by AF fluctuations, as discussed in previous van Hove scenarios.

cond-mat↗

arXiv1995

A Model of Correlated Fermions with $d_{x^2-y^2}$ Superconductivity

Motivated by the phenomenology of the high-Tc cuprates, a two dimensional fermionic model with attractive interactions is here discussed. The exact solution to the two particle problem leads to a bound state in the $d_{x^2 - y^2}$ subspace. Numerical techniques suggest that the model has $d_{x^2 - y^2}$ superconductivity (SC) in the ground state at low fermionic density. Within a self-consistent RPA diagrammatic study, the density dependence of the critical temperature is calculated. We argue that in the context of d-wave SC this model fulfills the role that the attractive on-site Hubbard model has played for s-wave SC. We also show that another candidate, the attractive ``t-U-V'' model, which has d-wave SC at the mean-field level is actually not useful as a realization of this family of condensates for a variety of reasons.

cond-mat↗

arXiv1995

Predictions for Neutron Scattering and Photoemission Experiments on CuGeO_3

Applying numerical techniques to a model recently proposed for the one dimensional spin-Peierls compound CuGeO_3, we calculate dynamical properties that can be directly compared with inelastic neutron scattering data and angle-resolved photoemission experiments. The momentum and energy dependence of the dynamical structure factor S(q,omega) are discussed, as well as the static structure factor S(q). The spectral function A(q,omega) calculated from the one particle Green's function at half-filling is shown at several values of the hole hopping amplitude t. The results have some unique features characteristic of one dimensional systems including small weight near the Fermi energy. The presence of ``shadow bands'' induced by short distance antiferromagnetic correlations is predicted to appear in ARPES experiments for CuGeO_3 and also for Sr_2CuO_3.

cond-mat↗

arXiv1995

Magnetic Raman Scattering in Two-Dimensional Spin-1/2 Heisenberg Antiferromagnets: Spectral Shape Anomaly and Magnetostrictive Effects

We calculate the Raman spectrum of the two-dimensional (2D) spin-1/2 Heisenberg antiferromagnet by exact diagonalization and quantum Monte Carlo techniques on clusters of up to 144 sites and, on a 16-site cluster, by considering the phonon-magnon interaction which leads to random fluctuations of the exchange integral. Results are in good agreement with experiments on various high-T_c precursors, such as La_2CuO_4 and YBa_2Cu_3O_{6.2}. In particular, our calculations reproduce the broad lineshape of the two-magnon peak, the asymmetry about its maximum, the existence of spectral weight at high energies, and the observation of nominally forbidden A_{1g} scattering.

cond-mat.supr-con↗

arXiv1994

Shadow Bands in the Cuprates

A consequence of strong antiferromagnetic correlations in models of high-Tc cuprates is the appearance in photoemission (PES) calculations of considerable more weight above the Fermi momentum ${\rm {\bf p}_F }$ than expected for non-interacting electrons. This effect, first discussed by Kampf and Schrieffer (Phys. Rev. {\bf B 41}, 6399 (1990)) under the name of ``shadow bands'', is here analyzed in the two dimensional Hubbard and t-J models using Monte Carlo and exact diagonalization techniques. It is concluded that for underdoped compounds weight above ${\rm {\bf p}_F }$ could be observable in experimental PES data, while in the overdoped regime it will be likely hidden in the experimental background. In the intermediate region the signal is weak. Our results are thus compatible with recent experimental data by Aebi et al. (Phys. Rev. Lett. {\bf 72}, 2757 (1994)) for Bi2212 at optimal doping. However, to definitely prove the existence of this effect in the cuprates, experiments in the underdoped regime are necessary.

cond-mat↗

arXiv1994

On the Liaison Between Superconductivity and Phase Separation

Models of strongly correlated electrons that tend to phase separate are studied including a long-range 1/r repulsive interaction. It is observed that charge-density-wave states become stable as the strength of the 1/r term, ${\rm V_{coul}}$, is increased. Due to this effect, the domain of stability of the superconducting phases that appear near phase separation at ${\rm V_{coul} = 0}$ is not enlarged by a 1/r interaction as naively expected. Nevertheless, superconductivity exists in a wide region of parameter space, even if phase separation is suppressed. Our results have implications for some theories of the cuprates.

cond-mat↗

arXiv1994

Antiferromagnetic and van Hove Scenarios for the Cuprates: Taking the Best of Both Worlds

A theory for the high temperature superconductors is proposed. Holes are spin-1/2, charge e, quasiparticles strongly dressed by spin fluctuations. Based on their dispersion, it is claimed that the experimentally observed van Hove singularities of the cuprates are likely originated by antiferromagnetic (AF) correlations. From the two carriers problem in the 2D t-J model, an effective Hamiltonian for holes is defined with %no free parameters. This effective model has superconductivity in the ${\rm d_{x^2-y^2}}$ channel, a critical temperature ${\rm T_c \sim 100K}$ at the optimal hole density, ${\rm x=0.15}$, and a quasiparticle lifetime linearly dependent with energy. Other experimental results are also $quantitatively$ reproduced by the theory.

cond-mat↗

arXiv1994

Flat quasiparticle dispersion and a hidden small energy scale in the 2D t-J model

A model of weakly interacting hole quasiparticles is proposed to describe the normal state of the high temperature superconductors. The effect of strong correlations is contained in the dispersion relation of the holes, which is obtained accurately using a numerical technique and the t-J model on $12\times 12$ clusters. Saddle-points generated by many-body effects induce quasiparticle $flat$ bands similar to those observed in recent angle-resolved photoemission experiments. The near degeneracy between momentum states along the ${\rm X-Y}$ direction is a hidden small energy scale in the problem, which produces a strongly temperature dependent Hall coefficient, as well as a large Fermi surface similar to those observed in the cuprates.

cond-mat↗

arXiv1994

Optical conductivity of the Hubbard model at finite temperature

The optical conductivity, $σ(ω)$, of the two dimensional one-band Hubbard model is calculated at finite temperature using exact diagonalization techniques on finite clusters. The in-plane d.c. resistivity, $ρ_{ab}$, is also evaluated. We find that at large U/t and temperature T, $ρ_{ab}$ is approximately linear with temperature, in reasonable agreement with experiments on high-T$_c$ superconductors. Moreover, we note that $σ(ω)$ displays charge excitations, a mid-infrared (MIR) band and a Drude peak, also as observed experimentally. The combination of the Drude peak and the MIR oscillator strengths leads to a conductivity that decays slower than $1/ω^2$ at energies smaller than the insulator gap near half-filling.

cond-mat↗

arXiv1993

Correlated Electrons in High Temperature Superconductors

Theoretical ideas and experimental results concerning high temperature superconductors are reviewed. Special emphasis is given to calculations carried out with the help of computers applied to models of strongly correlated electrons proposed to describe the two dimensional ${\rm Cu O_2}$ planes. The review also includes results using several analytical techniques. The one and three band Hubbard models, and the ${\rm t-J}$ model are discussed, and their behavior compared against experiments when available. Among the conclusions of the review, we found that some experimentally observed unusual properties of the cuprates have a natural explanation through Hubbard-like models. In particular abnormal features like the mid-infrared band of the optical conductivity $σ(ω)$, the new states observed in the gap in photoemission experiments, the behavior of the spin correlations with doping, and the presence of phase separation in the copper oxide superconductors may be explained, at least in part, by these models. Finally, the existence of superconductivity in Hubbard-like models is analyzed. Some aspects of the recently proposed ideas to describe the cuprates as having a $\dx2y2$ superconducting condensate at low temperatures are discussed. Numerical results favor this scenario over others....(continues).

cond-mat↗