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arXiv:cond-mat/0111320 · Physical review. B, Condensed matter

Friedel oscillations and charge density waves in chains and ladders

Steven R. White, Ian K. Affleck, Douglas J. Scalapino

Published Apr 16, 2002• 174 citations
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Abstract

The density matrix renormalization (DMRG) group method for ladders works much more efficiently with open boundary conditions. One consequence of these boundary conditions is ground-state charge density oscillations that often appear to be nearly constant in magnitude or to decay only slightly away from the boundaries. We analyze these using bosonization techniques, relating their detailed form to the correlation exponent and distinguishing boundary induced generalized Friedel oscillations from true charge density waves. We also discuss a different approach to extracting the correlation exponent from the finite size spectrum which uses exclusively open boundary conditions and can therefore take advantage of data for much larger system sizes. A general discussion of the Friedel oscillation wave vectors is given, and a convenient Fourier transform technique is used to determine it. DMRG results are analyzed on Hubbard and $t\ensuremath{-}J$ chains and 2 leg $t\ensuremath{-}J$ ladders. We present evidence for the existence of a long-ranged charge density wave state in the $t\ensuremath{-}J$ ladder at a filling of $n=0.75$ and near $J/t\ensuremath{\approx}0.25.$

DOI
10.1103/physrevb.65.165122

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The density matrix renormalization (DMRG) group method for ladders works much more efficiently with open boundary conditions. One consequence of these boundary conditions is ground-state charge density oscillations that often appear to be nearly constant in magnitude or to decay only slightly away from the boundaries. We analyze these using bosonization techniques, relating their detailed form to the correlation exponent and distinguishing boundary induced generalized Friedel oscillations from true charge density waves. We also discuss a different approach to extracting the correlation exponent from the finite size spectrum which uses exclusively open boundary conditions and can therefore take advantage of data for much larger system sizes. A general discussion of the Friedel oscillation wave vectors is given, and a convenient Fourier transform technique is used to determine it. DMRG results are analyzed on Hubbard and t\ensuremath−Jt\ensuremath{-}Jt\ensuremath−J chains and 2 leg t\ensuremath−Jt\ensuremath{-}Jt\ensuremath−J ladders. We present evidence for the existence of a long-ranged charge density wave state in the t\ensuremath−Jt\ensuremath{-}Jt\ensuremath−J ladder at a filling of n=0.75n=0.75n=0.75 and near J/t\ensuremath≈0.25.J/t\ensuremath{\approx}0.25.J/t\ensuremath≈0.25.
Using the density-matrix renormalization-group (DMRG) technique, we calculate critical exponents for the one-dimensional Hubbard model with open boundary conditions with and without additional boundary potentials at both ends. A direct comparison with open boundary condition Bethe ansatz calculations provides a good check for the DMRG calculations on large system sizes. On the other hand, the DMRG calculations provide an independent check of the predictions of conformal field theory, which are needed to obtain the critical exponents from the Bethe ansatz. From the Bethe ansatz we predict the behavior of the 1/L1/L1/L-corrected mean value of the Friedel oscillations (for the density and the magnetization) and the characteristic wave vectors, and show numerically that these conjectures are fulfilled with and without boundary potentials. The quality of the numerical results allows us to determine the behavior of the coefficients of the Friedel oscillations as a function of the Hubbard interaction.
Density matrix renormalization group (DMRG) calculations on four-leg $t\text{\ensuremath{-}}J$ and Hubbard ladders have found a phase exhibiting ``stripes'' at intermediate doping. Such behavior can be viewed as generalized Friedel oscillations, with wavelength equal to the inverse hole density, induced by the open boundary conditions. So far, this phase has not been understood using the conventional weak-coupling bosonization approach. Based on studies of a general bosonization proof, finite size spectrum, an improved analysis of weak-coupling renormalization group equations, and the decoupled two-leg ladder limit, we here find new types of phases of four-leg ladders, which exhibit stripes. They also inevitably exhibit ``bipairing,'' meaning that there is a gap to add one or two electrons (but not four) and that both single electron and electron pair correlation functions decay exponentially, while correlation functions of charge-4 operators exhibit a power-law decay. Whether or not bipairing occurs in the stripe phase found in DMRG is an important open question.
We revisit the ground state of the Hubbard model on two-legged ladders in this paper. We perform density matrix renormalization group (DMRG) calculations on large system sizes with large numbers of kept states and perform extrapolation of DMRG results with truncation errors in the converged region. We find that the superconducting correlation exponent Ksc{K}_{sc}Ksc​ extracted from the pair-pair correlation is very sensitive to the position of the reference bond, reflecting a huge boundary effect on it. By systematically removing the effects from boundary conditions, finite sizes, and truncation errors in DMRG, we obtain more accurate values of Ksc{K}_{sc}Ksc​ and ${K}_{\ensuremath{\rho}}$ with DMRG. With these exponents, we confirm that the two-legged Hubbard model is in the Luther-Emery liquid phase with ${K}_{sc}\ifmmode\cdot\else\textperiodcentered\fi{}{K}_{\ensuremath{\rho}}=1$ from tiny doping near half filling to 1/81/81/8 hole doping. When the doping is increased to $\ensuremath{\delta}⪆1/6$, the behaviors of charge, pairing, and spin correlations do not change qualitatively, but the relationship ${K}_{sc}\ifmmode\cdot\else\textperiodcentered\fi{}{K}_{\ensuremath{\rho}}=1$ is likely to be violated. With the further increase of the doping to $\ensuremath{\delta}=1/3$, the quasi-long-ranged charge correlation turns into a true long-ranged charge order, and the spin gap is closed, while the pair-pair correlation still decays algebraically. Our work provides a standard way to analyze the correlation functions when studying systems with open boundary conditions.
We report a density-matrix renormalization-group (DMRG) study of S=1/2 antiferromagnetic Heisenberg spin chains. We show the Friedel oscillations in open spin chains within bosonization theory. The bosonization predictions are in excellent agreement with the DMRG calculation. The chain length dependence of the ground state and several low-energy excited state energies are compared with conformal field theory results. Based upon the DMRG calculations for various quantities, the interplay between the system size and the number of optimal states kept in the DMRG is carefully discussed. \textcopyright{} 1996 The American Physical Society.
Using both the density-matrix renormalization group method and the constrained-path quantum Monte Carlo method, we studied the ground-state energies and the spin and hole densities of a $12\ifmmode\times\else\texttimes\fi{}3$ Hubbard model with open boundary conditions and six holes doped away from half-filling. Results obtained with these two methods agree well in the small and intermediate U regimes. For U/t> 6U/t>~6U/t> 6 we find a ground-state with charge inhomogeneities consistent with stripes.
The ground-state phase diagram of a two-leg fermionic dipolar ladder with inter-site interactions is studied using density matrix renormalization group (DMRG) techniques. We use a state-of-the-art implementation of the DMRG algorithm and finite size scaling to simulate large system sizes with high accuracy. We also consider two different model systems and explore stable phases in half and quarter filling factors. We find that in the half filling, the charge and spin gaps emerge in a finite value of the dipole-dipole and on-site interactions. In the quarter filling case, s-wave superconducting state, charge density wave, homogenous insulating and phase separation phases occur depend on the interaction values. Moreover, in the dipole-dipole interaction, the D-Mott phase emerges when the hopping terms along the chain and rung are the same, whereas, this phase has been only proposed for the anisotropic Hubbard model. In the half filling case, on the other hand, there is either charge-density wave or charged Mott order phase depends on the orientation of the dipole moments of the particles with respect to the ladder geometry.