566 research outputs found
Understanding panel conditioning: an examination of social desirability bias in self-reported height and weight in panel surveys using experimental data
Typically reliant on self-reports from panel data, a growing body of literature suggests that relative body weight can have negative effects on labour market outcomes. Given the interest in the effects of relative weight in the social sciences, this paper addresses the question of whether repeated interviewing affects the quality of these data. A theory that focuses on the sensitivity of the questions rather than the survey context is proposed. Examining experimental panel data from Understanding Society using quantile-regression, the findings for women are consistent with the argument that conditioning reduces social desirability effects. The ameliorative effects of panel conditioning on social desirability bias in self-reported height and bodyweight appear to strengthen the association between relative weight and employment for men, but not women, however
Magnetic Properties of (VO)_2P_2O_7 from Frustrated Interchain Coupling
Neutron-scattering experiments on (VO)_2P_2O_7 reveal both a gapped magnon
dispersion and an unexpected, low-lying second mode. The proximity and
intensity of these modes suggest a frustrated coupling between the alternating
spin chains. We deduce the minimal model containing such a frustration, and
show that it gives an excellent account of the magnon dispersion, static
susceptibility and electron spin resonance absorption. We consider two-magnon
states which bind due to frustration, and demonstrate that these may provide a
consistent explanation for the second mode.Comment: RevTeX, 5 pages, 6 figures, compressed from first versio
Excitations in one-dimensional S=1/2 quantum antiferromagnets
The transition from dimerized to uniform phases is studied in terms of
spectral weights for spin chains using continuous unitary transformations
(CUTs). The spectral weights in the S=1 channel are computed perturbatively
around the limit of strong dimerization. We find that the spectral weight is
concentrated mainly in the subspaces with a small number of elementary triplets
(triplons), even for vanishing dimerization. So, besides spinons, triplons may
be used as elementary excitations in spin chains. We conclude that there is no
necessity to use fractional excitations in low-dimensional, undoped or doped
quantum antiferromagnets.Comment: 4 pages, 1 figure include
Excitation Spectra of Structurally Dimerized and Spin-Peierls Chains in a Magnetic Field
The dynamical spin structure factor and the Raman response are calculated for
structurally dimerized and spin-Peierls chains in a magnetic field, using exact
diagonalization techniques. In both cases there is a spin liquid phase composed
of interacting singlet dimers at small fields h < h_c1, an incommensurate
regime (h_c1 < h < h_c2) in which the modulation of the triplet excitation
spectra adapts to the applied field, and a fully spin polarized phase above an
upper critical field h_c2. For structurally dimerized chains, the spin gap
closes in the incommensurate phase, whereas spin-Peierls chains remain gapped.
In the spin liquid regimes, the dominant feature of the triplet spectra is a
one-magnon bound state, separated from a continuum of states at higher
energies. There are also indications of a singlet bound state above the
one-magnon triplet.Comment: RevTex, 10 pages with 8 eps figure
Fractional and Integer Excitations in Quantum Antiferromagnetic Spin 1/2 Ladders
Spectral densities are computed in unprecedented detail for quantum
antiferromagnetic spin 1/2 two-leg ladders. These results were obtained due to
a major methodical advance achieved by optimally chosen unitary
transformations. The approach is based on dressed integer excitations.
Considerable weight is found at high energies in the two-particle sector.
Precursors of fractional spinon physics occur implying that there is no
necessity to resort to fractional excitations in order to describe features at
higher energies.Comment: 6 pages, 4 figures included, minor text changes, improved figure
Microscopic model for Bose-Einstein condensation and quasiparticle decay
Sufficiently dimerized quantum antiferromagnets display elementary S=1
excitations, triplon quasiparticles, protected by a gap at low energies. At
higher energies, the triplons may decay into two or more triplons. A strong
enough magnetic field induces Bose-Einstein condensation of triplons. For both
phenomena the compound IPA-CuCl3 is an excellent model system. Nevertheless no
quantitative model was determined so far despite numerous studies. Recent
theoretical progress allows us to analyse data of inelastic neutron scattering
(INS) and of magnetic susceptibility to determine the four magnetic couplings
J1=-2.3meV, J2=1.2meV, J3=2.9meV and J4=-0.3meV. These couplings determine
IPA-CuCl3 as system of coupled asymmetric S=1/2 Heisenberg ladders
quantitatively. The magnetic field dependence of the lowest modes in the
condensed phase as well as the temperature dependence of the gap without
magnetic field corroborate this microscopic model.Comment: 6 pages, 5 figure
Magnetic properties of (VO)_2P_2O_7: two-plane structure and spin-phonon interactions
Detailed experiments on single-crystal (VO)_2P_2O_7 continue to reveal new
and unexpected features. We show that a model composed of two, independent
planes of spin chains with frustrated magnetic coupling is consistent with
nuclear magnetic resonance and inelastic neutron scattering measurements. The
pivotal role of PO_4 groups in mediating intrachain exchange interactions
explains both the presence of two chain types and their extreme sensitivity to
certain lattice vibrations, which results in the strong magnetoelastic coupling
observed by light scattering. We compute the respective modifications of the
spin and phonon dynamics due to this coupling, and illustrate their observable
consequences on the phonon frequencies, magnon dispersions, static
susceptibility and specific heat.Comment: 10 pages, 9 figure
Generic susceptibilities of the half-filled Hubbard model in infinite dimensions
Around a metal-to-insulator transition driven by repulsive interaction (Mott
transition) the single particle excitations and the collective excitations are
equally important. Here we present results for the generic susceptibilities at
zero temperature in the half-filled Hubbard model in infinite dimensions.
Profiting from the high resolution of dynamic density-matrix renormalization at
all energies, results for the charge, spin and Cooper-pair susceptibilities in
the metallic and the insulating phase are computed. In the insulating phase, an
almost saturated local magnetic moment appears. In the metallic phase a
pronounced low-energy peak is found in the spin response.Comment: 12 pages, 12 figures; slight changes and one additional figure due to
referees' suggestion
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