2,112 research outputs found

### Insulator to Metal Transition Induced by Disorder in a Model for Manganites

The physics of manganites appears to be dominated by phase competition among
ferromagnetic metallic and charge-ordered antiferromagnetic insulating states.
Previous investigations (Burgy {\it et al.}, Phys. Rev. Lett. {\bf 87}, 277202
(2001)) have shown that quenched disorder is important to smear the first-order
transition between those competing states, and induce nanoscale inhomogeneities
that produce the colossal magnetoresistance effect. Recent studies (Motome {\it
et al.} Phys. Rev. Lett. {\bf 91}, 167204 (2003)) have provided further
evidence that disorder is important in the manganite context, unveiling an
unexpected insulator-to-metal transition triggered by disorder in a one-orbital
model with cooperative phonons. In this paper, a qualitative explanation for
this effect is presented. It is argued that the transition occurs for disorder
in the form of local random energies. Acting over an insulating states made out
of a checkerboard arrangement of charge, with ``effective'' site energies
positive and negative, this form of disorder can produce lattice sites with an
effective energy near zero, favorable for the transport of charge. This
explanation is based on Monte Carlo simulations and the study of simplified toy
models, measuring the density-of-states, cluster conductances using the
Landauer formalism, and other observables. The applicability of these ideas to
real manganites is discussed.Comment: 14 pages, 23 figures, submitted to Physical Review

### Dynamical Mean-Field Study of the Ferromagnetic Transition Temperature of a Two-Band Model for Colossal Magnetoresistance Materials

The ferromagnetic (FM) transition temperature (Tc) of a two-band
Double-Exchange (DE) model for colossal magnetoresistance (CMR) materials is
studied using dynamical mean-field theory (DMFT), in wide ranges of coupling
constants, hopping parameters, and carrier densities. The results are shown to
be in excellent agreement with Monte Carlo simulations. When the bands overlap,
the value of Tc is found to be much larger than in the one-band case, for all
values of the chemical potential within the energy overlap interval. A nonzero
interband hopping produces an additional substantial increase of Tc, showing
the importance of these nondiagonal terms, and the concomitant use of multiband
models, to boost up the critical temperatures in DE-based theories.Comment: 4 pages, 4 eps figure

### Surprises on the Way from 1D to 2D Quantum Magnets: the Novel Ladder Materials

One way of making the transition between the quasi-long range order in a
chain of S=1/2 spins coupled antiferromagnetically and the true long range
order that occurs in a plane, is by assembling chains to make ladders of
increasing width. Surprisingly this crossover between one and two dimensions is
not at all smooth. Ladders with an even number of legs have purely short range
magnetic order and a finite energy gap to all magnetic excitations. Predictions
of this novel groundstate have now been verified experimentally. Holes doped
into these ladders are predicted to pair, and possibly superconduct.Comment: Review Article, Science, TeX file, 18 pages, 6 figures available upon
reques

### Fragility of the A-type AF and CE Phases of Manganites: An Exotic Insulator-to-Metal Transition Induced by Quenched Disorder

Using Monte Carlo simulations and the two eg-orbital model for manganites,
the stability of the CE and A-type antiferromagnetic insulating states is
analyzed when quenched disorder in the superexchange JAF between the t2g
localized spins and in the on-site energies is introduced. At vanishing or
small values of the electron-(Jahn-Teller)phonon coupling, the previously
hinted "fragility" of these insulating states is studied in detail, focusing on
their charge transport properties. This fragility is here found to induce a
rapid transition from the insulator to a (poor) metallic state upon the
introduction of disorder. A possible qualitative explanation is presented based
on the close proximity in energy of ferromagnetic metallic phases, and also on
percolative ideas valid at large disorder strength. The scenario is compared
with previously discussed insulator-to-metal transitions in other contexts. It
is argued that the effect unveiled here has unique properties that may define a
new class of giant effects in complex oxides. This particularly severe effect
of disorder must be present in other materials as well, in cases involving
phases that arise as a compromise between very different tendencies, as it
occurs with striped states in the cuprates.Comment: 13 pages, 17 figures, RevTex 4, submitted for publicatio

### Novel Phase Between Band and Mott Insulators in Two Dimensions

We investigate the ground state phase diagram of the half-filled repulsive
Hubbard model in two dimensions in the presence of a staggered potential
$\Delta$, the so-called ionic Hubbard model, using cluster dynamical mean field
theory. We find that for large Coulomb repulsion, $U\gg \Delta$, the system is
a Mott insulator (MI). For weak to intermediate values of $\Delta$, on
decreasing $U$, the Mott gap closes at a critical value $U_{c1}(\Delta)$ beyond
which a correlated insulating phase with possible bond order (BO) is found.
Further, this phase undergoes a first-order transition to a band insulator (BI)
at $U_{c2}(\Delta)$ with a finite charge gap at the transition. For large
$\Delta$, there is a direct first-order transition from a MI to a BI with a
single metallic point at the phase boundary

### Phase Diagram of the Two-Leg Kondo Ladder

The phase diagram of the two-leg Kondo ladder is investigated using
computational techniques. Ferromagnetism is present, but only at small
conduction electron densities and robust Kondo coupling $J$. For densities
$n\gtrsim0.4$ and any Kondo coupling, a paramagnetic phase is found. We also
observed spin dimerization at densities $n$=1/4 and $n$=1/2. The spin structure
factor at small $J$ peaks at $\vec{q}$=$(2n,0)\pi$ for $n\lesssim0.5$, and at
$\vec{q}$=$(n,1)\pi$ for $n\gtrsim0.5$. The charge structure factor suggests
that electrons behave as free particles with spin-1/2 (spin-0) for small
(large) $J$.Comment: 5 pages, 4 fig

### Origin of the multiferroic spiral spin-order in the RMnO3 perovskites

The origin of the spiral spin-order in perovskite multiferroic manganites
$R$MnO$_{3}$ ($RE=$ Tb or Dy) is here investigated using a two $e_{\rm
g}$-orbitals double-exchange model. Our main result is that the experimentally
observed spiral phase can be stabilized by introducing a relatively weak
next-nearest-neighbor superexchange coupling ($\sim10%$ of the nearest-neighbor
superexchange). Moreover, the Jahn-Teller lattice distortion is also shown to
be essential to obtain a realistic spiral period. Supporting our conclusions,
the generic phase diagram of undoped perovskite manganites is obtained using
Monte Carlo simulations, showing phase transitions from the A-type
antiferromagnet, to the spiral phase, and finally to the E-type
antiferromagnet, with decreasing size of the $R$ ions. These results are
qualitatively explained by the enhanced relative intensity of the
superexchanges.Comment: 6 pages, 4 figure

### Effect of Jahn-Teller coupling on Curie temperature in the Double Exchange Model

We consider the two-band double exchange model for manganites with
Jahn-Teller (JT) coupling and explore the suppression of the ferromagnetism
because of the JT distortion. The localized spins of the \emph{t}_{2g}
electrons are represented in terms of the Schwinger bosons, and two
spin-singlet Fermion operators are introduced instead of the $e_{g}$ electrons'
operators. In terms of the new Fermi fields the on-site Hund's interaction is
in a diagonal form and one accounts for it exactly. Integrating out the
spin-singlet fermions, we derive an effective Heisenberg model for a vector
which describes the local orientations of the total magnetization. The exchange
constants are different for different space directions and depend on the
density $n$ of \emph{e}_{g} electrons and JT energy. At zero temperature,
with increasing the density of the \emph{e}_{g} electrons the system
undergoes phase transition from ferromagnetic phase $(0<n<n_c)$ to A-type
antiferromagnetic phase $(n_c<n)$. The critical value $n_c$ decreases as JT
energy is increased. At finite temperature we calculate the Curie temperature
as a function of electron density for different JT energy. The results show
that JT coupling strongly suppresses the spin fluctuations and decreases the
Curie temperature.Comment: 4 pages, 3 figure

### Study of ARPES data and d-wave superconductivity using electronic models in two dimensions

We review the results of an extensive investigation of photoemission spectral
weight using electronic models for the high-Tc superconductors. Here we show
that some recently reported unusual features of the cuprates namely the
presence of (i) flat bands, (ii) small quasiparticle bandwidths, and (iii)
antiferromagnetically induced weight, have all a natural explanation within the
context of holes moving in the presence of robust antiferromagnetic
correlations. Introducing interactions among the hole carriers, a model is
constructed which has ${\rm d_{x^2 - y^2}}$ superconductivity, an optimal
doping of $\sim 15\%$ (caused by the presence of a large density of states at
the top of the valence band), and a critical temperature $\sim 100K$.Comment: 11 pages Z-compressed postscript, to appear in the Proceedings to the
Stanford Conference on Spectroscopies in Novel superconductor

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