733 research outputs found
The Egypt labor market panel survey: introducing the 2012 round
This paper introduces the 2012 round of the Egypt Labor Market Panel Survey (ELMPS), a publicly-available nationally representative longitudinal household survey. We describe the key characteristics of the ELMPS, including the samples and questionnaires for each round. Additionally, we examine the attrition processes observed in the panel and discuss the creation of weights to correct for attrition. We compare our data to other statistical sources for Egypt to evaluate the sample's representativeness. To demonstrate how the ELMPS allows for an improved understanding of the labor market, we compare unemployment trends using the ELMPS and other data on unemployment in Egypt
Comment on "Quantum Monte Carlo Evidence for Superconductivity in the Three-Band Hubbard Model in Two Dimensions"
In a recent Letter, Kuroki and Aoki [Phys. Rev. Lett. 76, 440 (1996)]
presented quantum Monte-Carlo (QMC) results for pairing correlations in the
three-band Hubbard model, which describes the Cu-d_{x^2-y^2} and O-p_{x,y}
orbitals present in the CuO_2 planes of high-T_c materials. In this comment we
argue that (i) the used parameter set is not appropriate for the description of
high-T_c materials since it does not satisfy the minimal requirement of a
charge-transfer gap at half-filling, and (ii) the observed increase in the
d_{x^2-y^2} channel is dominantly produced by the pair-field correlations
without the vertex part. Hence, the claim of evidence of ODLRO is not
justified.Comment: 1 page latex and 2 eps-figures, uses epsfig, submitted to PR
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
Charge and Spin Structures of a Superconductor in the Proximity of an Antiferromagnetic Mott Insulator
To the Hubbard model on a square lattice we add an interaction, , which
depends upon the square of a near-neighbor hopping. We use zero temperature
quantum Monte Carlo simulations on lattice sizes up to , to show
that at half-filling and constant value of the Hubbard repulsion, the
interaction triggers a quantum transition between an antiferromagnetic Mott
insulator and a superconductor. With a combination of finite
temperature quantum Monte Carlo simulations and the Maximum Entropy method, we
study spin and charge degrees of freedom in the superconducting state. We give
numerical evidence for the occurrence of a finite temperature
Kosterlitz-Thouless transition to the superconducting state.
Above and below the Kosterlitz-Thouless transition temperature, , we
compute the one-electron density of states, , the spin relaxation
rate , as well as the imaginary and real part of the spin susceptibility
. The spin dynamics are characterized by the vanishing of
and divergence of in the low
temperature limit. As is approached develops a pseudo-gap
feature and below shows a peak
at finite frequency.Comment: 46 pages (latex) including 14 figures in encapsulated postscript
format. Submitted for publication in Phys. Rev.
Dynamic response of trapped ultracold bosons on optical lattices
We study the dynamic response of ultracold bosons trapped in one-dimensional
optical lattices using Quantum Monte Carlo simulations of the boson Hubbard
model with a confining potential. The dynamic structure factor reveals the
inhomogeneous nature of the low temperature state, which contains coexisting
Mott insulator and superfluid regions. We present new evidence for local
quantum criticality and shed new light on the experimental excitation spectrum
of 87Rb atoms confined in one dimension.Comment: 4 pages, 5 figure
Introducing the Egypt Labor Market Panel Survey 2018
This paper introduces the 2018 wave of the Egypt Labor Market Panel Survey (ELMPS), previously fielded in 1998, 2006, and 2012. The ELMPS has already become the primary source of data for a large number of scholarly and policy studies on the labor market and human development issues in Egypt, and this new wave will further enhance its value as a critical data public good. This longitudinal survey is nationally representative, tracking both households and individuals over two decades. In this paper, we describe the key characteristics of the 2018 wave, including sampling, fielding, and questionnaire design. Changes in the collection of retrospective data starting in 2018 are discussed, and we demonstrate that they improved the data quality. We examine the patterns of attrition and present the construction of weights designed to correct for attrition, as well as to ensure that the sample remains nationally representative. We compare the ELMPS data with other Egyptian data sources, namely, the 2017 Census and various rounds of the Labor Force Survey (LFS). The data provide important new insights into Egypt's labor market, economy, and society
Doping induced metal-insulator transition in two-dimensional Hubbard, , and extended Hubbard, , models
We show numerically that the nature of the doping induced metal-insulator
transition in the two-dimensional Hubbard model is radically altered by the
inclusion of a term, , which depends upon a square of a single-particle
nearest-neighbor hopping. This result is reached by computing the localization
length, , in the insulating state. At finite values of we find
results consistent with where is
the critical chemical potential. In contrast, for the Hubbard model. At finite values of , the presented
numerical results imply that doping the antiferromagnetic Mott insulator leads
to a superconductor.Comment: 19 pages (latex) including 7 figures in encapsulated postscript
format. Submitted for publication in Phys. Rev.
Spin and charge dynamics of the ferromagnetic and antiferromagnetic two-dimensional half-filled Kondo lattice model
We present a detailed numerical study of spin and charge dynamics of the
two-dimensional Kondo lattice model with hopping t and exchange J. At T=0 and J
> 0, the competition between the RKKY interaction and Kondo effect triggers a
quantum phase transition between magnetically ordered and disordered
insulators: J_c/t = 1.45(5). The quasiparticle gap scales as |J|. S(q,\omega),
evolves smoothly from its strong coupling form with spin gap at q = (\pi,\pi)
to a spin wave form. At J>0, A(\vec{k},\omega) shows a dispersion relation
following that of hybridized bands. For J < J_c this feature is supplemented by
shadows thus pointing to a coexistence of Kondo screening and magnetism. For J
< 0 A(\vec{k},\omega) is similar to that of non-interacting electrons in a
staggered magnetic field. Spin, T_S, and charge, T_C, scales are defined. For
weak to intermediate couplings, T_S marks the onset of antiferromagnetic
fluctuations and follows a J^2 law. At strong couplings T_S scales as J. T_C
scales as J both at weak and strong couplings. At and slightly below T_C we
observe i) a rise in the resistivity as a function of decreasing temperature,
ii) a dip in the integrated density of states at the Fermi energy and iii) the
occurrence of hybridized bands in A(k,\omega). It is shown that in the weak
coupling limit, the charge gap of order J is of magnetic origin. The specific
heat shows a two peak structure, the low temperature peak being of magnetic
origin. Our results are compared to various mean-field theories.Comment: 30 pages, 24 figure
Dynamic charge correlations near the Peierls transition
The quantum phase transition between a repulsive Luttinger liquid and an
insulating Peierls state is studied in the framework of the one-dimensional
spinless Holstein model. We focus on the adiabatic regime but include the full
quantum dynamics of the phonons. Using continuous-time quantum Monte Carlo
simulations, we track in particular the dynamic charge structure factor and the
single-particle spectrum across the transition. With increasing electron-phonon
coupling, the dynamic charge structure factor reveals the emergence of a charge
gap, and a clear signature of phonon softening at the zone boundary. The
single-particle spectral function evolves continuously across the transition.
Hybridization of the charge and phonon modes of the Luttinger liquid
description leads to two modes, one of which corresponds to the coherent
polaron band. This band acquires a gap upon entering the Peierls phase, whereas
the other mode constitutes the incoherent, high-energy spectrum with backfolded
shadow bands. Coherent polaronic motion is a direct consequence of quantum
lattice fluctuations. In the strong-coupling regime, the spectrum is described
by the static, mean-field limit. Importantly, whereas finite electron density
in general leads to screening of polaron effects, the latter reappear at half
filling due to charge ordering and lattice dimerization.Comment: 8 pages, 7 figures, final versio
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