Search results for "Hub"
showing 10 items of 207 documents
Observational constraints on the LLTB model
2010
We directly compare the concordance LCDM model to the inhomogeneous matter-only alternative represented by LTB void models. To achieve a "democratic" confrontation we explore LLTB models with non-vanishing cosmological constant and perform a global likelihood analysis in the parameter space of cosmological constant and void radius. In our analysis we carefully consider SNe, Hubble constant, CMB and BAO measurements, marginalizing over spectral index, age of the universe and background curvature. We find that the LCDM model is not the only possibility compatible with the observations, and that a matter-only void model is a viable alternative to the concordance model only if the BAO constrain…
Constraints on cosmological models from strong gravitational lensing systems
2012
Strong lensing has developed into an important astrophysical tool for probing both cosmology and galaxies (their structure, formation, and evolution). Using the gravitational lensing theory and cluster mass distribution model, we try to collect a relatively complete observational data concerning the Hubble constant independent ratio between two angular diameter distances $D_{ds}/D_s$ from various large systematic gravitational lens surveys and lensing by galaxy clusters combined with X-ray observations, and check the possibility to use it in the future as complementary to other cosmological probes. On one hand, strongly gravitationally lensed quasar-galaxy systems create such a new opportun…
Ferromagnetism of the Hubbard Model at Strong Coupling in the Hartree-Fock Approximation
2005
As a contribution to the study of Hartree-Fock theory we prove rigorously that the Hartree-Fock approximation to the ground state of the d-dimensional Hubbard model leads to saturated ferromagnetism when the particle density (more precisely, the chemical potential mu) is small and the coupling constant U is large, but finite. This ferromagnetism contradicts the known fact that there is no magnetization at low density, for any U, and thus shows that HF theory is wrong in this case. As in the usual Hartree-Fock theory we restrict attention to Slater determinants that are eigenvectors of the z-component of the total spin, {S}_z = sum_x n_{x,\uparrow} - n_{x,\downarrow}, and we find that the ch…
Dynamical mean-field theory calculation with the dynamical density-matrix renormalization group
2006
Abstract We study the Hubbard model at half band-filling on a Bethe lattice with infinite coordination number at zero temperature. We use the dynamical mean-field theory (DMFT) mapping to a single-impurity Anderson model with a bath whose properties have to be determined self-consistently. For a controlled and systematic implementation of the self-consistency scheme we use the fixed-energy approach to the DMFT. Using the dynamical density–matrix renormalization group method (DDMRG) we calculate the density of states (DOS) with a resolution ranging from 3% of the bare bandwidth W = 4 t at high energies to 0.01% for the quasi-particle peak. The DDMRG resolution and accuracy for the DOS is sup…
Hole-doped Hubbard ladders
2005
The formation of stripes in six-leg Hubbard ladders with cylindrical boundary conditions is investigated for two different hole dopings, where the amplitude of the hole density modulation is determined in the limits of vanishing DMRG truncation errors and infinitely long ladders. The results give strong evidence that stripes exist in the ground state of these systems for strong but not for weak Hubbard couplings. The doping dependence of these findings is analysed.
Spectral Function of the One-Dimensional Hubbard Model away from Half Filling
2004
We calculate the photoemission spectral function of the one-dimensional Hubbard model away from half filling using the dynamical density matrix renormalization group method. An approach for calculating momentum-dependent quantities in finite open chains is presented. Comparison with exact Bethe Ansatz results demonstrates the unprecedented accuracy of our method. Our results show that the photoemission spectrum of the quasi-one-dimensional conductor TTF-TCNQ provides evidence for spin-charge separation on the scale of the conduction band width.
Landau Fermi Liquid Theory and Beyond
2014
In this chapter we consider the Landau theory of the Fermi liquid that has a long history and remarkable results in describing a numerous properties of the electron liquid in ordinary metals and Fermi liquids of the \(^3\)He type. The theory is based on the assumption that elementary excitations determine the physics at low temperatures, resembling that of weakly interacting Fermi gas. These excitations behave as quasiparticles with a certain effective mass. The effective mass \(M^*\) exhibits a simple universal behavior, for it is independent of the temperature, pressure, and magnetic field strength and is a parameter of the theory. Microscopically deriving the equation determining the eff…
Orbital-selective Mott transitions in a doped two-band Hubbard model with crystal field splitting
2013
We investigate the effects of crystal field splitting in a doped two-band Hubbard model with different bandwidths within dynamical mean-field theory (DMFT), using a quantum Monte Carlo impurity solver. In addition to an orbital-selective Mott phase (OSMP) of the narrow band, which is adiabatically connected with the well-studied OSMP in the half-filled case without crystal field splitting, we find, for sufficiently strong interaction and a suitable crystal field, also an OSMP of the wide band. We establish the phase diagram (in the absence of magnetic or orbital order) at moderate doping as a function of interaction strength and crystal field splitting and show that also the wide-band OSMP …
Spin and charge orderings in the atomic limit of the U-V-J model
2011
In this paper we study a generalization of the 1D Hubbard model by considering density-density and Ising-type spin-spin nearest neighbor (NN) interactions, parameterized by $V$ and $J$, respectively. We present the T=0 phase diagram for both ferro ($J>0$) and anti-ferro ($J<0$) coupling obtained in the narrow-band limit by means of an extension to zero-temperature of the transfer-matrix method. Based on the values of the Hamiltonian parameters, we identify a number of phases that involve orderings of the double occupancy, NN density and spin correlations, being these latter very fragile.
Deciding the fate of the false Mott transition in two dimensions by exact quantum Monte Carlo methods
2015
We present an algorithm for the computation of unbiased Green functions and self-energies for quantum lattice models, free from systematic errors and valid in the thermodynamic limit. The method combines direct lattice simulations using the Blankenbecler Scalapino-Sugar quantum Monte Carlo (BSS-QMC) approach with controlled multigrid extrapolation techniques. We show that the half-filled Hubbard model is insulating at low temperatures even in the weak-coupling regime; the previously claimed Mott transition at intermediate coupling does not exist.