81 research outputs found
Employee Decisions with Respect to 401(k) Plans: Evidence From Individual-Level Data
401(k) plans have been the most rapidly growing type of employer- provided pension plan during the last decade. This paper utilizes employee-level data from the 401(k) plan at a medium-sized U.S. manufacturing firm to analyze the participation and contribution decisions of workers eligible for this plan. Our analysis reveals two important features of 401(k) participant behavior. First, contribution decisions of eligible employees are relatively insensitive to the rate of employer matching on worker contributions. Most employees maintain the same participation status and contribution rate year after year, despite substantial changes in the employer's match rate at the firm we study. This suggests that employer matching may not be a critical factor in explaining the growth of 401(k) plans. Second, we find that institutional constraints on contributions, imposed either by the employer or by the IRS, are an extremely important influence on contributor behavior. About three quarters of eligible employees contributed at rates that place them at one of the 'corners' or 'kinks' in the 401(k) opportunity set. This finding must be recognized in any analysis of how changes in 401(k) plan provisions are likely to affect contribution levels.
Influence of spin structures and nesting on Fermi surface and a pseudogap anisotropy in t-t'-U Hubbard model
Influence of two type of spin structures on the form of the Fermi surface
(FS) and a photoemission intensity map is studied for t-t'-U Hubbard model.
Mean field calculations are done for the stripe phase and for the spiral spin
structure. It is shown, that unlike a case of electron doping, the hole-doped
models are unstable with respect to formation of such structures. The pseudogap
anisotropies are different for h- and e- doping. In accordance with ARPES data
for La2SrxCuO4 the stripe phase is characterized by quasi-one-dimensional
segments of FS at k=(\pi,0) and by suppression of spectral weight in diagonal
direction. It is shown that spiral structures display the polarisation
anisotropy: different segments of FS correspond to electros with different spin
polarisations.Comment: 12 pages, 4 figure
From nodal liquid to nodal Mottness in a frustrated Hubbard model
We investigate the physics of frustrated 3-leg Hubbard ladders in the band
limit, when hopping across the ladder's rungs (t) is of the same
order as hopping along them (t) much greater than the onsite Coulomb repulsion
(U). We show that this model exhibits a striking electron-hole asymmetry close
to half-filling: the hole-doped system at low temperatures develops a
Resonating Valence Bond (RVB)-like d-wave gap (pseudogap close to (,0))
coinciding with gapless nodal excitations (nodal liquid); in contrast, the
electron-doped system is seen to develop a Mott gap at the nodes, whilst
retaining a metallic character of its majority Fermi surface. At lower
temperatures in the electron-doped case, d-wave superconducting correlations --
here, coexisting with gapped nodal excitations -- are already seen to arise.
Upon further doping the hole-doped case, the RVB-like state yields to d-wave
superconductivity. Such physics is reminiscent of that exhibited by the high
temperature cuprate superconductors--notably electron-hole asymmetry as noted
by Angle Resolved PhotoEmission Spectroscopy (ARPES) and the resistivity
exponents observed. This toy model also reinforces the importance of a more
thorough experimental investigation of the known 3-leg ladder cuprate systems,
and may have some bearing on low dimensional organic superconductors.Comment: 26 pages, 16 figure
Phase Separation Models for Cuprate Stripe Arrays
An electronic phase separation model provides a natural explanation for a
large variety of experimental results in the cuprates, including evidence for
both stripes and larger domains, and a termination of the phase separation in
the slightly overdoped regime, when the average hole density equals that on the
charged stripes. Several models are presented for charged stripes, showing how
density waves, superconductivity, and strong correlations compete with quantum
size effects (QSEs) in narrow stripes. The energy bands associated with the
charged stripes develop in the middle of the Mott gap, and the splitting of
these bands can be understood by considering the QSE on a single ladder.Comment: significant revisions: includes island phase, 16 eps figures, revte
Flux Phase as a Dynamic Jahn-Teller Phase: Berryonic Matter in the Cuprates?
There is considerable evidence for some form of charge ordering on the
hole-doped stripes in the cuprates, mainly associated with the low-temperature
tetragonal phase, but with some evidence for either charge density waves or a
flux phase, which is a form of dynamic charge-density wave. These three states
form a pseudospin triplet, demonstrating a close connection with the E X e
dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of
Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller
effect as a form of flux phase. A simple model of the Cu-O bond stretching
phonons allows an estimate of electron-phonon coupling for these modes,
explaining why the half breathing mode softens so much more than the full
oxygen breathing mode. The anomalous properties of provide a coupling
(correlated hopping) which acts to stabilize density wave phases.Comment: Major Revisions: includes comparisons with specific cuprate phonon
modes, 16 eps figures, revte
Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping
from a pseudogap state in the underdoped cuprates to a superconducting state at
optimal and overdoping, has been interpreted as evidence that the pseudogap
must be due to precursor pairing. We suggest an alternative explanation, that
the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the
Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the
pseudogap could actually be due to any of a number of nesting instabilities,
including charge or spin density waves or more exotic phases. We present a
detailed analysis of this competition for one particular model: the pinned
Balseiro-Falicov model of competing charge density wave and (s-wave)
superconductivity. We show that most of the anomalous features of both
tunneling and photoemission follow naturally from the model, including the
smooth crossover, the general shape of the pseudogap phase diagram, the
shrinking Fermi surface of the pseudogap phase, and the asymmetry of the
tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1
and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be
described in detail by this model, but we suggest a simple generalization to
account for inhomogeneity, which does provide an adequate description. We show
that it should be possible, with a combination of photoemission and tunneling,
to demonstrate the extent of pinning of the Fermi level to the Van Hove
singularity. A preliminary analysis of the data suggests pinning in the
underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure
Angle-resolved photoemission spectroscopy of the cuprate superconductors
This paper reviews the most recent ARPES results on the cuprate
superconductors and their insulating parent and sister compounds, with the
purpose of providing an updated summary of the extensive literature in this
field. The low energy excitations are discussed with emphasis on some of the
most relevant issues, such as the Fermi surface and remnant Fermi surface, the
superconducting gap, the pseudogap and d-wave-like dispersion, evidence of
electronic inhomogeneity and nano-scale phase separation, the emergence of
coherent quasiparticles through the superconducting transition, and many-body
effects in the one-particle spectral function due to the interaction of the
charge with magnetic and/or lattice degrees of freedom. The first part of the
paper introduces photoemission spectroscopy in the context of strongly
interacting systems, along with an update on the state-of-the-art
instrumentation. The second part provides a brief overview of the scientific
issues relevant to the investigation of the low energy electronic structure by
ARPES. The rest of the paper is devoted to the review of experimental results
from the cuprates and the discussion is organized along conceptual lines:
normal-state electronic structure, interlayer interaction, superconducting gap,
coherent superconducting peak, pseudogap, electron self energy and collective
modes. Within each topic, ARPES data from the various copper oxides are
presented.Comment: Reviews of Modern Physics, in press. A HIGH-QUALITY pdf file is
available at http://www.physics.ubc.ca/~damascel/RMP_ARPES.pd
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