Hot Spots and Pseudogaps for Hole- and Electron-Doped High-Temperature Superconductors

Using cluster perturbation theory, it is shown that the spectral weight and pseudogap observed at the Fermi energy in recent Angle Resolved Photoemission Spectroscopy (ARPES) of both electron and hole-doped high-temperature superconductors find their natural explanation within the t-t'-t''-U Hubbard model in two dimensions. The value of the interaction U needed to explain the experiments for electron-doped systems at optimal doping is in the weak to intermediate coupling regime where the t-J model is inappropriate. At strong coupling, short-range correlations suffice to create a pseudogap but at weak coupling long correlation lengths associated with the antiferromagnetic wave vector are necessary.Comment: RevTeX 4, 4 pages, 5 figures (2 in color

Virginia College Access Resource Study: Region One Report

This research brief shares the results of a MERC study commissioned by the State Council of Higher Education for Virginia (SCHEV) and supported by Virginia 529 about college access in central Virginia. The purpose of college access providers is discussed as well the current areas of need. A list of access providers in Virginia are listed at the end of the brief

Field Identification in Non-Unitary Diagonal Cosets

We study the nonunitary diagonal cosets constructed from admissible representations of Kac-Moody algebras at fractional level, with an emphasis on the question of field identification. Generic classes of field identifications are obtained from the analysis of the modular S matrix. These include the usual class related to outer automorphisms, as well as some intrinsically nonunitary field identifications. They allow for a simple choice of coset field representatives where all field components of the coset are associated with integrable finite weights.Comment: 34 page

Integrability of the quantum KdV equation at c = -2

We present a simple a direct proof of the complete integrability of the quantum KdV equation at $c=-2$, with an explicit description of all the conservation laws.Comment: 9 page

Strong-Coupling Perturbation Theory of the Hubbard Model

The strong-coupling perturbation theory of the Hubbard model is presented and carried out to order (t/U)^5 for the one-particle Green function in arbitrary dimension. The spectral weight A(k,omega) is expressed as a Jacobi continued fraction and compared with new Monte-Carlo data of the one-dimensional, half-filled Hubbard model. Different regimes (insulator, conductor and short-range antiferromagnet) are identified in the temperature--hopping integral (T,t) plane. This work completes a first paper on the subject (Phys. Rev. Lett. 80, 5389 (1998)) by providing details on diagrammatic rules and higher-order results. In addition, the non half-filled case, infinite resummations of diagrams and the double occupancy are discussed. Various tests of the method are also presented.Comment: 28 pages, 19 figure

Les pavages, les quasi-cristaux et le 18th problème de Hilbert

Le 18e problème de Hilbert est constitué de trois questions vaguement liées : Le nombre de groupes a région fondamentale (bornée) dans E mest-il fini ? Existet-il un pavage sur les paves duquel aucun groupe n’agisse de façon transitive ? Quels sont les juxtapositions les plus denses de corps congruents dans E3 ? Ces questions ont orienté la cristallographie mathématique vers de nouvell es directions et ont été excessivement efficaces: de nos jours, les quasicristaux posent des problèmes mathématiques qui se situent précisément dans les champs indiqués par Hilbert. En effet, plusieurs des nouveaux problèmes sont des reformulations de ceux de Hilbert. On a fait de considérables progrès dans les demières années, mais une question clé - comment les parties du problème sont liées entre elles - n’e st pas encore complètement comprise.Hilbert’s 18th problem consisted of three loosely related questions: Is the number of groups in En with (bounded) fundamental region finite? Does there exist a tiling on whose tiles no group acts transitively? What are the densest packings of congruent bodies in E3? These questions pointed mathematical crystallography in new directions and have been unreasonably effective: in our time quasicrystals pose mathematical problems in precisely the areas indicated by Hilbert. Indeed, many of the new problems are reformulations of Hilbert’s. Considerable progress has been made in the last few years, but a key issue-how the parts of the problem are related to one another-is still not completely understood.Peer Reviewe

Rethinking School Improvement: The Case for Networked Improvement Communities

The question of why school improvement efforts have not been as effective as we would hope is a complicated one that could be addressed from a number of perspectives. The purpose of this paper is to explore some of the underlying problems that prevent current school reform effort from achieving sustained impact, and to describe a promising model of school improvement, called the Networked Improvement Community (NIC). The NIC model – which has just started gaining traction in the world of K-12 school reform – establishes small inquiry groups within organizations to engage in cycles of improvement that involve implementing strategies designed to improve outcomes, collecting data on the effects of the strategies, reflecting on the data, and then planning next actions (Bryk, Gomez & Gunrow, 2010). In addition, the NIC model Rethinking School Improvement 4 supports the sharing of information about the effects of initiatives across networks (e.g. schools, school districts), and uses this shared learning to thoughtfully scale up successful practices. While the NIC model shares some principles and strategies with other popular school reform efforts, in its fully-realized form it represents a significant departure from the way that school improvement efforts have traditionally been approached. To some extent it is model that seems to have developed out of an understanding of both the strengths and the weaknesses of prior organizational improvement efforts. This paper will be guided by three questions: (1) What are the qualities of current school improvement efforts that have prevented them from achieving broad and sustained success across the system? (2) What is the Networked Improvement Community model and how does it respond to the weaknesses of prior reform efforts? (3) What are the key recommendations for integrating the Networked Improvement Community model into K-12 school improvement efforts