125 research outputs found
SO(6)-Generalized Pseudogap Model of 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 reflects an underlying SO(6) instability structure of
the (pi,0) saddle points. The pseudogap is probably not associated with
superconductivity, but is related to competing nesting instabilities, which are
responsible for the stripe phases. We earlier introduced a simple Ansatz of
this competition in terms of a pinned Balseiro-Falicov (pBF) model of competing
charge density wave and (s-wave) superconductivity. This model gives a good
description of the phase diagram and the tunneling and photoemission spectra.
Here, we briefly review these results, and discuss some recent developments:
experimental evidence for a non-superconducting component to the pseudogap; and
SO(6) generalizations of the pBF model, including flux phase and d-wave
superconductivity.Comment: 6 pages LaTex, 4 ps figures (U. of Miami Conference HTS99
Allocating Opportunity: The Role and Impact of School Counselors in Promoting Access to AP Coursework
In the K–12 education setting, professional school counselors are uniquely positioned to support high quality educational opportunities for all students. At the secondary level, student participation in Advanced Placement (AP) programming can be viewed as one such example of opportunity. School counselors serve as student advocates by channeling information and creating access to educational opportunity like AP. This important work takes place in the context of a bureaucratic policy environment that necessarily shapes the way AP opportunity is allocated in the local context. Charged with promoting equity and access to educational opportunity for all students, school counselors operate in a space of tension, and even conflict, when district policy, school site policy, and organizational norms related to AP participation signal less-than-open access. In this environment, school counselor advocacy and leadership become increasingly important determinants of opportunity and academic outcomes, particularly for students in the margins. The aim of this qualitative study was to examine the extent to which school counselors (a) are enabled and/or constrained in their ability to create student access to AP coursework, (b) use discretionary decision making as they navigate local AP course-taking policy, (c) consider efficiency and equity as values associated with policy and counseling practice, and (d) identify and perceive justice in their local context of professional work
Matrix Element and Strong Electron Correlation Effects in ARPES from Cuprates
We discuss selected results from our recent work concerning the ARPES
(angle-resolved photoemission) spectra from the cuprates. Our focus is on
developing an understanding of the effects of the ARPES matrix element and
those of strong electron correlations in analyzing photointensities. With
simulations on BiSrCaCuO (Bi2212), we show that the
ARPES matrix element possesses remarkable selectivity properties, such that by
tuning the photon energy and polarization, emission from the bonding or the
antibonding states can be enhanced. Moreover, at low photon energies (below 25
eV), the Fermi surface (FS) emission is dominated by transitions from just the
O-atoms in the CuO planes. In connection with strong correlation effects,
we consider the evolution with doping of the FS of
NdCeCuO (NCCO) in terms of the -- Hubbard
model Hamiltonian. We thus delineate how the FS evolves on electron doping from
the insulating state in NCCO. The Mott pseudogap is found to collapse around
optimal doping suggesting the existence of an associated quantum critical
point.Comment: 5 pages, 4 figures, accepted to be published in Journal of Physics
and Chemistry of Solid
Compact system design based on digital in-line holographic microscopy configuration
We present our study regarding a compact system design for cell counting and simultaneous 3D imaging, based on digital in-line holographic microscopy configuration. The system is built around the known experimental configuration which includes a pinhole but we also investigate the configuration with a monomode fiber as a light source. Considered samples consist of a very low concentration of cells in flow in a microchannel. The main challenge in our design is to obtain the digital hologram of one cell on a regular video camera sensor in proper resolution conditions, as opposed to the usual configurations where the aim is to visualize a large area. This fact is possible with shorter distances between pinhole and sample and with pinholes with diameters slightly larger than 1micron. These can now be realized by considering the microtechnological processes for microchannel and pinhole fabrication on the same substrate with high refractive index - to increase the numerical aperture of the system The geometrical parameters are established after the numerical analysis of the diffracted field from a single cell and of the entire system numerical aperture values
Fate of Quasiparticle at Mott Transition and Interplay with Lifshitz Transition Studied by Correlator Projection Method
Filling-control metal-insulator transition on the two-dimensional Hubbard
model is investigated by using the correlator projection method, which takes
into account momentum dependence of the free energy beyond the dynamical
mean-field theory. The phase diagram of metals and Mott insulators is analyzed.
Lifshitz transitions occur simultaneously with metal-insulator transitions at
large Coulomb repulsion. On the other hand, they are separated each other for
lower Coulomb repulsion, where the phase sandwiched by the Lifshitz and
metal-insulator transitions appears to show violation of the Luttinger sum
rule. Through the metal-insulator transition, quasiparticles retain nonzero
renormalization factor and finite quasi-particle weight in the both sides of
the transition. This supports that the metal-insulator transition is caused not
by the vanishing renormalization factor but by the relative shift of the Fermi
level into the Mott gap away from the quasiparticle band, in sharp contrast
with the original dynamical mean-field theory. Charge compressibility diverges
at the critical end point of the first-order Lifshitz transition at finite
temperatures. The origin of the divergence is ascribed to singular momentum
dependence of the quasiparticle dispersion.Comment: 24 pages including 10 figure
Quantum communication networks with optical vortices
Quantum communications bring a paradigm change in internet security by using
quantum resources to establish secure keys between parties. Present-day quantum
communications networks are mainly point-to-point and use trusted nodes and key
management systems to relay the keys. Future quantum networks, including the
quantum internet, will have complex topologies in which groups of users are
connected and communicate with each-other. Here we investigate several
architectures for quantum communication networks. We show that photonic orbital
angular momentum (OAM) can be used to route quantum information between
different nodes. Starting from a simple, point-to-point network, we will
gradually develop more complex architectures: point-to-multipoint,
fully-connected and entanglement-distribution networks. As a particularly
important result, we show that an -node, fully-connected network can be
constructed with a single OAM sorter and OAM values. Our results pave the
way to construct complex quantum communication networks with minimal resources.Comment: 10 pages, 9 figure
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.
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