43 research outputs found
Quadratic operators used in deducing exact ground states for correlated systems: ferromagnetism at half filling provided by a dispersive band
Quadratic operators are used in transforming the model Hamiltonian (H) of one
correlated and dispersive band in an unique positive semidefinite form coopting
both the kinetic and interacting part of H. The expression is used in deducing
exact ground states which are minimum energy eigenstates only of the full
Hamiltonian. It is shown in this frame that at half filling, also dispersive
bands can provide ferromagnetism in exact terms by correlation effects .Comment: 7 page
Exact ground states for the four-electron problem in a Hubbard ladder
The exact ground state of four electrons in an arbitrary large two leg
Hubbard ladder is deduced from nine analytic and explicit linear equations. The
used procedure is described, and the properties of the ground state are
analyzed. The method is based on the construction in r-space of the different
type of orthogonal basis wave vectors which span the subspace of the Hilbert
space containing the ground state. In order to do this, we start from the
possible microconfigurations of the four particles within the system. These
microconfigurations are then rotated, translated and spin-reversed in order to
build up the basis vectors of the problem. A closed system of nine analytic
linear equations is obtained whose secular equation, by its minimum energy
solution, provides the ground state energy and the ground state wave function
of the model.Comment: 10 pages, 7 figure
Localized spin ordering in Kondo lattice models
Using a non-Abelian density matrix renormalization group method we determine
the phase diagram of the Kondo lattice model in one dimension, by directly
measuring the magnetization of the ground-state. This allowed us to discover a
second ferromagnetic phase missed in previous approaches. The phase transitions
are found to be continuous. The spin-spin correlation function is studied in
detail, and we determine in which regions the large and small Fermi surfaces
dominate. The importance of double-exchange ordering and its competition with
Kondo singlet formation is emphasized in understanding the complexity of the
model.Comment: Revtex, 4 pages, 4 eps figures embedde
Correlations for the orthogonal-unitary and symplectic-unitary transitions at the hard and soft edges
For the orthogonal-unitary and symplectic-unitary transitions in random
matrix theory, the general parameter dependent distribution between two sets of
eigenvalues with two different parameter values can be expressed as a
quaternion determinant. For the parameter dependent Gaussian and Laguerre
ensembles the matrix elements of the determinant are expressed in terms of
corresponding skew-orthogonal polynomials, and their limiting value for
infinite matrix dimension are computed in the vicinity of the soft and hard
edges respectively. A connection formula relating the distributions at the hard
and soft edge is obtained, and a universal asymptotic behaviour of the two
point correlation is identified.Comment: 37 pgs., 1fi
Quantum Ising model in a transverse random field: A density-matrix renormalization group analysis
The spin-1/2 quantum Ising chain in a transverse random magnetic field is
studied by means of the density-matrix renormalization group. The system
evolves from an ordered to a paramagnetic state as the amplitude of the random
field is increased. The dependence of the magnetization on a uniform magnetic
field in the z direction and the spontaneous magnetization as a function of the
amplitude of the transverse random magnetic field are determined. The behavior
of the spin-spin correlation function both above and at criticality is studied.
The scaling laws for magnetization and correlation functions are tested against
previous numerical and renormalization-group results.Comment: 5 pages with 7 figures inside them, proper format of authors' names
use
Ferromagnetism in the Two-Dimensional Periodic Anderson Model
Using the constrained-path Monte Carlo method, we studied the magnetic
properties of the two-dimensional periodic Anderson model for electron fillings
between 1/4 and 1/2. We also derived two effective low energy theories to
assist in interpreting the numerical results. For 1/4 filling we found that the
system can be a Mott or a charge transfer insulator, depending on the relative
values of the Coulomb interaction and the charge transfer gap between the two
non-interacting bands. The insulator may be a paramagnet or antiferromagnet. We
concentrated on the effect of electron doping on these insulating phases. Upon
doping we obtained a partially saturated ferromagnetic phase for low
concentrations of conduction electrons. If the system were a charge transfer
insulator, we would find that the ferromagnetism is induced by the well-known
RKKY interaction. However, we found a novel correlated hopping mechanism
inducing the ferromagnetism in the region where the non-doped system is a Mott
insulator. Our regions of ferromagnetism spanned a much smaller doping range
than suggested by recent slave boson and dynamical mean field theory
calculations, but they were consistent with that obtained by density matrix
renormalization group calculations of the one-dimensional periodic Anderson
model
Magnetism in the dilute Kondo lattice model
The one dimensional dilute Kondo lattice model is investigated by means of
bosonization for different dilution patterns of the array of impurity spins.
The physical picture is very different if a commensurate or incommensurate
doping of the impurity spins is considered. For the commensurate case, the
obtained phase diagram is verified using a non-Abelian density-matrix
renormalization-group algorithm. The paramagnetic phase widens at the expense
of the ferromagnetic phase as the -spins are diluted. For the incommensurate
case, antiferromagnetism is found at low doping, which distinguishes the dilute
Kondo lattice model from the standard Kondo lattice model.Comment: 11 pages, 2 figure
Phase diagram of the anisotropic Kondo chain
We establish the phase diagram of the one-dimensional anisotropic Kondo
lattice model at T=0 using a generalized two-dimensional classical Coulomb gas
description. We analyze the problem by means of a renormalization group (RG)
treatment. We find that the phase diagram contains regions of paramagnetism,
partial and full ferromagnetic order.Comment: Final version to appear in Physical Review Letter
Spin Reduction Transition in Spin-3/2 Random Heisenberg Chains
Random spin-3/2 antiferromagnetic Heisenberg chains are investigated using an
asymptotically exact renormalization group. Randomness is found to induce a
quantum phase transition between two random-singlet phases. In the strong
randomness phase the effective spins at low energies are S_eff=3/2, while in
the weak randomness phase the effective spins are S_eff=1/2. Separating them is
a quantum critical point near which there is a non-trivial mixture of S=1/2,
S=1, and S=3/2 effective spins at low temperatures.Comment: 4 pages, 3 figures. Typos correcte