Study of the triangular lattice tV model near x=1/3

We study extended Hubbard model on a triangular lattice near doping $x=1/3$, which may be relevant for the recently discovered superconductor Na$_x$CoO$_2 \cdot y$H$_2$O. By generalizing this model to $N$ fermionic species, we formulate a meanfield description in the limit of large $N$. In meanfield, we find two possible phases: a renormalized Fermi liquid and a \rt3rt3 charge density wave state. The transition between the two phases is driven by increasing the nearest neighbor repulsion and is found to be first order for doping $x=1/3$, but occurs close to the point of the local instability of the uniform liquid. We also study fluctuations about the uniform meanfield state in a systematic 1/N expansion, focusing on the residual interaction of quasiparticles and possible superconducting instabilities due to this interaction. Upon moving towards the CDW instability, the increasing charge fluctuations favor a particular $f$-wave triplet state. (This state was recently discussed by Tanakaet al, cond-mat/0311266). We also report a direct Gutzwiller wavefunction study of the spin-1/2 model.Comment: 9 pages, 5 figure

Cuprates as doped U(1) spin liquids

We explore theoretically the notion that the underdoped cuprates may be viewed as doped U(1) spin liquid Mott insulators. We pursue a conceptually clear version of this idea that naturally incorporates several aspects of the phenomenology of the cuprates. We argue that the low doping region may be fruitfully discussed in terms of the universal physics associated with a chemical potential tuned Mott transition between a U(1) spin liquid insulator and a d-wave superconductor. A precise characterization of the deconfinement in the U(1) spin liquid is provided by the emergence of a conserved gauge flux. This extra conservation law should hold at least approximately in the underdoped materials. Experiments that could possibly detect this conserved gauge flux are proposed.Comment: 11 pages, 7 figure

Monte Carlo Estimation of the Density of the Sum of Dependent Random Variables

We study an unbiased estimator for the density of a sum of random variables that are simulated from a computer model. A numerical study on examples with copula dependence is conducted where the proposed estimator performs favourably in terms of variance compared to other unbiased estimators. We provide applications and extensions to the estimation of marginal densities in Bayesian statistics and to the estimation of the density of sums of random variables under Gaussian copula dependence

Report on the Workshop on Refugee and Asylum Policy in Practice in Europe and North America

Western nations have struggled to accomplish the dual goals of refugee and asylum policies: (1) identifying and protecting Convention refugees as well as those fleeing civil conflict; and (2) controlling for abuse. The Workshop on Refugee and Asylum Policy in Practice in Europe and North America was organized to facilitate a transatlantic dialogue to explore just how well these asylum systems are balancing the dual goals. The workshop exa!llined key elements of the U.S. and European asylum systems: decision making on claims, deterrence of abuse, independent review, return of rejected asylum seekers, scope of the refugee concept, social rights and employment, international cooperation, and data and evaluation. The Workshop was convened by the Institute for the Study of International Migration (ISIM) of Georgetown University and the Center for the Study of Immigration, Integration and Citizenship Policies (CEPIC) of the Centre Nationale de Recherche Scientifique, with the support of the German Marshall Fund of the United States. It was held on July 1-3, 1999, at Oxford University. Workshop participants included government officials, scholars, and representatives from non-governmental organizations (NGOs) actively involved in analyzing and implementing refugee and asylum policies. This report outlines the major points of discussion and the areas of consensus at the Workshop, and emphasizes the issues in need of further analysis and agreement. Through this report, the Workshop seeks to encourage further discussion on refugee and asylum policies in practice in order to clarify, develop, and improve the existing mechanisms for protection