1,333 research outputs found
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
Relevance of Cooperative Lattice Effects and Correlated Disorder in Phase-Separation Theories for CMR Manganites
Previous theoretical investigations of colossal magnetoresistance (CMR)
materials explain this effect using a ``clustered'' state with preformed
ferromagnetic islands that rapidly align their moments with increasing external
magnetic fields. While qualitatively successful, explicit calculations indicate
drastically different typical resistivity values in two- and three-dimensional
lattices, contrary to experimental observations. This conceptual bottleneck in
the phase-separated CMR scenario is resolved here considering the cooperative
nature of the Mn-oxide lattice distortions. This induces power-law correlations
in the quenched random fields used in toy models with phase competition. When
these effects are incorporated, resistor-network calculations reveal very
similar results in two and three dimensions, solving the puzzle.Comment: RevTeX 4, 4 figure
Bogoliubov Quasiparticle Excitations in the Two-Dimensional t-J Model
Using a proposed numerical technique for calculating anomalous Green's
functions characteristic of superconductivity, we show that the low-lying
excitations in a wide parameter and doping region of the two-dimensional
model are well described by the picture of dressed Bogoliubov
quasiparticles in the BCS pairing theory. The pairing occurs predominantly in
-wave channel and the energy gap has a size
between quarter and half fillings. Opening of
the superconducting gap in the photoemission and inverse-photoemission spectrum
is demonstrated.Comment: 6 pages, RevTe
Self-energy and Fermi surface of the 2-dimensional Hubbard model
We present an exact diagonalization study of the self-energy of the
two-dimensional Hubbard model. To increase the range of available cluster sizes
we use a corrected t-J model to compute approximate Greens functions for the
Hubbard model. This allows to obtain spectra for clusters with 18 and 20 sites.
The self-energy has several `bands' of poles with strong dispersion and
extended incoherent continua with k-dependent intensity. We fit the self-energy
by a minimal model and use this to extrapolate the cluster results to the
infinite lattice. The resulting Fermi surface shows a transition from hole
pockets in the underdoped regime to a large Fermi surface in the overdoped
regime. We demonstrate that hole pockets can be completely consistent with the
Luttinger theorem. Introduction of next-nearest neighbor hopping changes the
self-energy stronlgy and the spectral function with nonvanishing
next-nearest-neighbor hopping in the underdoped region is in good agreement
with angle resolved photoelectron spectroscopy.Comment: 17 pages, 18 figure
Dynamic Spin Response for Heisenberg Ladders
We employ the recently proposed plaquette basis to investigate static and
dynamic properties of isotropic 2-leg Heisenberg spin ladders. Simple
non-interacting multi-plaquette states provide a remarkably accurate picture of
the energy/site and dynamic spin response of these systems. Insights afforded
by this simple picture suggest a very efficient truncation scheme for more
precise calculations. When the small truncation errors are accounted for using
recently developed Contractor Renormalization techniques, very accurate results
requiring a small fraction of the computational effort of exact calculations
are obtained. These methods allow us to determine the energy/site, gap, and
spin response of 2x16 ladders. The former two values are in good agreement with
density matrix renormalization group results. The spin response calculations
show that nearly all the strength is concentrated in the lowest triplet level
and that coherent many-body effects enhance the response/site by nearly a
factor of 1.6 over that found for 2x2 systems.Comment: 9 pages with two enclosed postscript figure
A Plaquette Basis for the Study of Heisenberg Ladders
We employ a plaquette basis-generated by coupling the four spins in a
lattice to a well-defined total angular momentum-for the study of
Heisenberg ladders with antiferromagnetic coupling. Matrix elements of the
Hamiltonian in this basis are evaluated using standard techniques in
angular-momentum (Racah) algebra. We show by exact diagonalization of small
( and ) systems that in excess of 90% of the ground-state
probability is contained in a very small number of basis states. These few
basis states can be used to define a severely truncated basis which we use to
approximate low-lying exact eigenstates. We show how, in this low-energy basis,
the isotropic spin-1/2 Heisenberg ladder can be mapped onto an anisotropic
spin-1 ladder for which the coupling along the rungs is much stronger than the
coupling between the rungs. The mapping thereby generates two distinct energy
scales which greatly facilitates understanding the dynamics of the original
spin-1/2 ladder. Moreover, we use these insights to define an effective
low-energy Hamiltonian in accordance to the newly developed COntractor
REnormalization group (CORE) method. We show how a simple range-2 CORE
approximation to the effective Hamiltonian to be used with our truncated basis
reproduces the low-energy spectrum of the exact theory at the \alt
1% level.Comment: 12 pages with two postscript figure
Spin Dynamics of Double-Exchange Manganites with Magnetic Frustration
This work examines the effects of magnetic frustration due to competing
ferromagnetic and antiferromagnetic Heisenberg interactions on the spin
dynamics of the double-exchange model. When the local moments are non-colinear,
a charge-density wave forms because the electrons prefer to sit on lines of
sites that are coupled ferromagnetically. With increasing hopping energy, the
local spins become aligned and the average spin-wave stiffness increases. Phase
separation is found only within a narrow range of hopping energies. Results of
this work are applied to the field-induced jump in the spin-wave stiffness
observed in the manganite PrCaMnO with .Comment: 10 pages, 3 figure
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 where the -order parameter is large.Comment: 9 pages, RevTex, to be published in Phys. Rev. B (RC
Evolution of a metastable phase with a magnetic phase coexistence phenomenon and its unusual sensitivity to magnetic field cycling in the alloys Tb5-xLuxSi3 (x <= 0.7)
Recently, we reported an anomalous enhancement of the positive
magnetoresistance beyond a critical magnetic field in Tb5Si3 in the
magnetically ordered state, attributable to 'inverse metamagnetism'. This
results in unusual magnetic hysteresis loops for the pressurized specimens,
which are relevant to the topic of 'electronic phase separation'. In this
paper, we report the influence of small substitutions of Lu for Tb, to show the
evolution of these magnetic anomalies. We find that, at low temperatures, the
high-field high-resistivity phase could be partially stabilized on returning
the magnetic field to zero in many of these Lu substituted alloys, as measured
through the electrical resistivity ({\rho}). Also, the relative fractions of
this phase and the virgin phase appear to be controlled by a small tuning of
the composition and temperature. Interestingly, at 1.8 K a sudden 'switch-over'
of the value of {\rho} for this mixed phase to that for the virgin phase for
some compositions is observed at low fields after a few field cycles,
indicating metastability of this mixed phase
- …