1,495 research outputs found
Coexistence of distinct charge fluctuations in -(BEDT-TTF)X
Using the Lanczos exact-diagonalization and density-matrix renormalization
group methods, we study the extended Hubbard model at quarter filling defined
on the anisotropic triangular lattice. We focus on charge ordering (CO)
phenomena induced by onsite and intersite Coulomb interactions. We determine
the ground-state phase diagram including three CO phases, i.e., diagonal,
vertical, and three-fold CO phases, based on the calculated results of the hole
density and double occupancy. We also calculate the dynamical density-density
correlation functions and find possible coexistence of the diagonal and
three-fold charge fluctuations in a certain parameter region where the onsite
and intersite interactions compete. Furthermore, the characteristic features of
the optical conductivity for each CO phase are discussed.Comment: 9 pages, 7 figure
Tomonaga-Luttinger parameters for doped Mott insulators
The Tomonaga--Luttinger parameter determines the critical behavior
in quasi one-dimensional correlated electron systems, e.g., the exponent
for the density of states near the Fermi energy. We use the numerical
density-matrix renormalization group method to calculate from the
slope of the density-density correlation function in momentum space at zero
wave vector. We check the accuracy of our new approach against exact results
for the Hubbard and XXZ Heisenberg models. We determine in the phase
diagram of the extended Hubbard model at quarter filling, , and
confirm the bosonization results on the critical
line and at infinitesimal doping of the
charge-density-wave (CDW) insulator for all interaction strengths. The doped
CDW insulator exhibits exponents only for small doping and strong
correlations.Comment: 7 pages, 4 figure
Metal-insulator transition in the Edwards model
To understand how charge transport is affected by a background medium and
vice versa we study a two-channel transport model which captures this interplay
via a novel, effective fermion-boson coupling. By means of (dynamical) DMRG we
prove that this model exhibits a metal-insulator transition at half-filling,
where the metal typifies a repulsive Luttinger liquid and the insulator
constitutes a charge density wave. The quantum phase transition point is
determined consistently from the calculated photoemission spectra, the scaling
of the Luttinger liquid exponent, the charge excitation gap, and the
entanglement entropy.Comment: 4 pages, 3 figures, contributions to SCES 201
Kitaev interactions between j=1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations
NaIrO, a honeycomb 5 oxide, has been recently identified as a
potential realization of the Kitaev spin lattice. The basic feature of this
spin model is that for each of the three metal-metal links emerging out of a
metal site, the Kitaev interaction connects only spin components perpendicular
to the plaquette defined by the magnetic ions and two bridging ligands. The
fact that reciprocally orthogonal spin components are coupled along the three
different links leads to strong frustration effects and nontrivial physics.
While the experiments indicate zigzag antiferromagnetic order in NaIrO,
the signs and relative strengths of the Kitaev and Heisenberg interactions are
still under debate. Herein we report results of ab initio many-body electronic
structure calculations and establish that the nearest-neighbor exchange is
strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the
Heisenberg contribution is significantly weaker and antiferromagnetic. The
calculations further reveal a strong sensitivity to tiny structural details
such as the bond angles. In addition to the large spin-orbit interactions, this
strong dependence on distortions of the IrO plaquettes singles out the
honeycomb 5 oxides as a new playground for the realization of
unconventional magnetic ground states and excitations in extended systems.Comment: 13 pages, 2 tables, 3 figures, accepted in NJ
MMIC low-noise amplifiers and applications above 100 GHz
In this paper we will present recent work on low noise amplifiers developed for very high frequencies above 100 GHz. These amplifiers were developed with a unique InP-based HEMT MMIC process. The amplifiers have been developed for both cryogenic and room temperature amplifier applications with state-of-art performance demonstrated from 100 GHz to 215 GHz
Anomalous behaviors of the charge and spin degrees of freedom in the CuO double chains of PrBaCuO
The density-matrix renormalization-group method is used to study the
electronic states of a two-chain Hubbard model for CuO double chains of
PrBaCuO. We show that the model at quarter filling has the charge
ordered phases with stripe-type and in-line--type patterns in the parameter
space, and in-between, there appears a wide region of vanishing charge gap; the
latter phase is characteristic of either Tomonaga-Luttinger liquid or a
metallic state with a spin gap. We argue that the low-energy electronic state
of the CuO double chains of PrBaCuO should be in the metallic state
with a possibly small spin gap.Comment: REVTEX 4, 10 pages, 9 figures; submitted to PR
Charge Ordering in the One-Dimensional Extended Hubbard Model: Implication to the TMTTF Family of Organic Conductors
We study the charge ordering (CO) in the one-dimensional (1D) extended
Hubbard model at quarter filling where the nearest-neighbor Coulomb repulsion
and dimerization in the hopping parameters are included. Using the cluster
mean-field approximation to take into account the effect of quantum
fluctuations, we determine the CO phase boundary of the model in the parameter
space at T=0 K. We thus find that the dimerization suppresses the stability of
the CO phase strongly, and in consequence, the realistic parameter values for
quasi-1D organic materials such as (TMTTF)PF are outside the region of
CO. We suggest that the long-range Coulomb interaction between the chains
should persist to stabilize the CO phase.Comment: 5 pages, 4 eps figures, to appear in 15 Nov. 2001 issue of PR
Density-matrix renormalisation group approach to quantum impurity problems
A dynamic density-matrix renormalisation group approach to the spectral
properties of quantum impurity problems is presented. The method is
demonstrated on the spectral density of the flat-band symmetric single-impurity
Anderson model. We show that this approach provides the impurity spectral
density for all frequencies and coupling strengths. In particular, Hubbard
satellites at high energy can be obtained with a good resolution. The main
difficulties are the necessary discretisation of the host band hybridised with
the impurity and the resolution of sharp spectral features such as the
Abrikosov-Suhl resonance.Comment: 16 pages, 6 figures, submitted to Journal of Physics: Condensed
Matte
Landau mapping and Fermi liquid parameters of the 2D t-J model
We study the momentum distribution function n(k) in the 2D t-J model on small
clusters by exact diagonalization. We show that n(k) can be decomposed
systematically into two components with Bosonic and Fermionic doping
dependence. The Bosonic component originates from the incoherent motion of
holes and has no significance for the low energy physics. For the Fermionic
component we exlicitely perform the one-to-one Landau mapping between the low
lying eigenstates of the t-J model clusters and those of an equivalent system
of spin-1/2 quasiparticles. This mapping allows to extract the quasiparticle
dispersion, statistics, and Landau parameters. The results show conclusively
that the 2D t-J model for small doping is a Fermi liquid with a `small' Fermi
surface and a moderately strong attractive interaction between the
quasiparticles.Comment: Revtex file, 5 pages with 5 embedded eps-files, hardcopies of figures
(or the entire manuscript) can be obtained by e-mail request to:
[email protected]
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