237 research outputs found
First and second order ferromagnetic transition at T=0 in a 1D itinerant system
We consider a modified version of the one-dimensional Hubbard model, the
t1-t2 Hubbard chain, which includes an additional next-nearest-neighbor
hopping. It has been shown that at weak coupling this model has a Luttinger
liquid phase or a spin liquid phase depending upon the ratio of t2 to t1.
Additionally if the on-site interaction U is large enough, the ground state is
fully polarized. Using exact diagonalization and the density-matrix
renormalization group, we show that the transition to the ferromagnetic phase
is either of first or second order depending on whether the Luttinger liquid or
spin liquid is being destabilized. Since we work at T=0, the second order
transition is a quantum magnetic critical point
Low density ferromagnetism in the Hubbard model
A single-band Hubbard model with nearest and next-nearest neighbour hopping
is studied for , 2, 3, using both analytical and numerical techniques. In
one dimension, saturated ferromagnetism is found above a critical value of
for a band structure with two minima and for small and intermediate densities.
This is an extension of a scenario recently proposed by M\"uller--Hartmann. For
three dimensions and non-pathological band structures, it is proven that such a
scenario does not work.Comment: 4 pages, 3 postscript figure
DMRG study of ferromagnetism in a one-dimensional Hubbard model
The one dimensional Hubbard model with nearest and (negative) next-nearest
neighbour hopping has been studied with the density-matrix renormalization
group (DMRG) method. A large region of ferromagnetism has been found for finite
density and finite on-site interaction.Comment: 5 pages LateX, 3 postscript figure
Phase diagram of the half-filled Hubbard chain with next-nearest-neighbor hopping
We investigate the ground-state phase diagram of the half-filled
one-dimensional Hubbard model with next-nearest-neighbor hopping using the
Density-Matrix Renormalization Group technique as well as an unrestricted
Hartree-Fock approximation. We find commensurate and incommensurate disordered
magnetic insulating phases and a spin-gapped metallic phase in addition to the
one-dimensional Heisenberg phase. At large on-site Coulomb repulsion , we
make contact with the phase diagram of the frustrated Heisenberg chain, which
has spin-gapped phases for sufficiently large frustration. For weak ,
sufficiently large next-nearest-neighbor hopping leads to a band
structure with four Fermi points rather than two, producing a spin-gapped
metallic phase. As is increased in this regime, the system undergoes a
Mott-Hubbard transition to a frustrated antiferromagnetic insulator
Interaction induced collapse of a section of the Fermi sea in in the zig-zag Hubbard ladder
Using the next-nearest neighbor (zig-zag) Hubbard chain as an one
dimemensional model, we investigate the influence of interactions on the
position of the Fermi wavevectors with the density-matrix renormalization-group
technique (DMRG). For suitable choices of the hopping parameters we observe
that electron-electron correlations induce very different renormalizations for
the two different Fermi wavevectors, which ultimately lead to a complete
destruction of one section of the Fermi sea in a quantum critical point
Frustrated Hubbard ladders and superconductivity in -BEDT-TTF organic compounds
Half-filled two-leg Hubbard ladders have spin-gapped short-range
antiferromagnetic correlations while three-leg ladders have power law
antoferromagnetic correlations, and both systems have d_{x^2-y^2}-power law
pairing correlations when they are doped. Thus these ladders exhibit some of
the phenomenology seen in the layered cuprates. Here we report results for
half-filled frustrated Hubbard ladders, based upon ladder segments taken from a
tight-binding model of kappa-BEDT-TTF. Although these ladders are half-filled,
varying the degree of frustration can drive them across an insulator-metal
transition. We suggest that the spin, charge and pairing correlations of these
frustrated ladders near the insulator-metal transition provide support for the
notion that kappa-BEDT-TTF is a strongly correlated superconductor
Spectral function of the spiral spin state in the trestle and ladder Hubbard model
Eder and Ohta have found a violation of the Luttinger rule in the spectral
function for the t-t'-J model, which was interpreted as a possible breakdown of
the Tomonaga-Luttinger(TL) description in models where electrons can pass each
other. Here we have computed the spin correlation along with the spectral
function for the one-dimensional t-t' Hubbard model and two-leg Hubbard ladder.
By varying the Hubbard U we have identified that such a phenomenon is in fact a
spinless-fermion-like behavior of holes moving in a spiral spin configuration
that has a spin correlation length of the system size.Comment: 3 pages, RevTex, 8 figures in Postscript, to be published in Phys.
Rev. B (rapid communication
Phase diagram of the one-dimensional Hubbard model with next-nearest-neighbor hopping
We study the one-dimensional Hubbard model with nearest-neighbor and
next-nearest-neighbor hopping integrals by using the density-matrix
renormalization group (DMRG) method and Hartree-Fock approximation. Based on
the calculated results for the spin gap, total-spin quantum number, and
Tomonaga-Luttinger-liquid parameter, we determine the ground-state phase
diagram of the model in the entire filling and wide parameter region. We show
that, in contrast to the weak-coupling regime where a spin-gapped liquid phase
is predicted in the region with four Fermi points, the spin gap vanishes in a
substantial region in the strong-coupling regime. It is remarkable that a large
variety of phases, such as the paramagnetic metallic phase, spin-gapped liquid
phase, singlet and triplet superconducting phases, and fully polarized
ferromagnetic phase, appear in such a simple model in the strong-coupling
regime.Comment: 11 pages, 8 figure
Effect of the W-term for a t-U-W Hubbard ladder
Antiferromagnetic and d_{x2-y2}-pairing correlations appear delicately
balanced in the 2D Hubbard model. Whether doping can tip the balance to pairing
is unclear and models with additional interaction terms have been studied. In
one of these, the square of a local hopping kinetic energy H_W was found to
favor pairing. However, such a term can be separated into a number of simpler
processes and one would like to know which of these terms are responsible for
enhancing the pairing. Here we analyze these processes for a 2-leg Hubbard
ladder
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