Frustrated magnetism and resonating valence bond physics in two-dimensional kagome-like magnets

We explore the phase diagram and the low-energy physics of three Heisenberg antiferromagnets which, like the kagome lattice, are networks of corner-sharing triangles but contain two sets of inequivalent short-distance resonance loops. We use a combination of exact diagonalization, analytical strong-coupling theories, and resonating valence bond approaches, and scan through the ratio of the two inequivalent exchange couplings. In one limit, the lattices effectively become bipartite, while at the opposite limit heavily frustrated nets emerge. In between, competing tunneling processes result in short-ranged spin correlations, a manifold of low-lying singlets (which can be understood as localized bound states of magnetic excitations), and the stabilization of valence bond crystals with resonating building blocks.Comment: Published versio

KCrF_3: Electronic Structure, Magnetic and Orbital Ordering from First Principles

The electronic, magnetic and orbital structures of KCrF_3 are determined in all its recently identified crystallographic phases (cubic, tetragonal, and monoclinic) with a set of {\it ab initio} LSDA and LSDA+U calculations. The high-temperature undistorted cubic phase is metallic within the LSDA, but at the LSDA+U level it is a Mott insulator with a gap of 1.72 eV. The tetragonal and monoclinic phases of KCrF_3 exhibit cooperative Jahn-Teller distortions concomitant with staggered 3x^2-r^2/3y^2-r^2 orbital order. We find that the energy gain due to the Jahn-Teller distortion is 82/104 meV per chromium ion in the tetragonal/monoclinic phase, respectively. These phases show A-type magnetic ordering and have a bandgap of 2.48 eV. In this Mott insulating state KCrF_3 has a substantial conduction bandwidth of 2.1 eV, leading to the possibility for the kinetic energy of charge carriers in electron- or hole-doped derivatives of KCrF_3 to overcome the polaron localization at low temperatures, in analogy with the situation encountered in the colossal magnetoresistive manganites.Comment: 7 pages, 11 figure

Non conventional screening of the Coulomb interaction in low dimensional and finite size system

We study the screening of the Coulomb interaction in non polar systems by polarizable atoms. We show that in low dimensions and small finite size systems this screening deviates strongly from that conventionally assumed. In fact in one dimension the short range interaction is strongly screened and the long range interaction is anti-screened thereby strongly reducing the gradient of the Coulomb interaction and therefore the correlation effects. We argue that this effect explains the success of mean field single particle theories for large molecules.Comment: 4 pages, 5 figure

Values and Governance Systems

Cooperative behavior as well as other cultural rules and values are made explicit in organizational units that procure and provide a common service. The common service is procured by a club, which consists of a user-value function, a representation governance, and a budget-allocation function. The common service is provided by a firm, which consists of a value-production function, an implementation governance, and a remuneration function. They extend the concepts of the consumer and the producer in neoclassical theory. The functions mentioned are determined by cooperative games. A governance is represented by a structure of principal-agent relations. The representation governance is a bottom-up hierarchy, empowered by the members of the club. The implementation governance is a top-down hierarchy, empowered by the governor of the firm. The optimal size of the club organization and the firm organization in a competitive environment is determined under certain conditions. This description of a club and a firm allows characterization of some cultural dimensions in the society. For one such cultural dimension, viz. the degree of collectivism in society, the impact on the optimal organization is shown.Governance;cooperation;hierarchy;club;firm;institution;service economy;values;culture;organization

The effects of food habits and socioeconomic status on overweight. Differences between the native Dutch and immigrants in the Netherlands

Overweight is a worldwide growing epidemic. The Netherlands is among the countries with the highest prevalence for overweight, together with the USA, UK, and Germany. This paper investigates differences in overweight between native Dutch and three immigrant groups in the Netherlands, and the effects of food habits and socioeconomic status on overweight. The results show that all immigrant groups have a higher prevalence for overweight than the Dutch, apart from Moroccans. Males are overweight more frequently than females. Takeaway food, eating out, and fresh vegetables decrease BMI, while convenience food, ready-to-eat meals, and delivery food (in some cases) increase BMI. In all groups, BMI increases with age. For Surinamese/Antilleans and Turks BMI increases with children living at home, whereas for native Dutch BMI decreases with children living at home. The national health expenditures due to overweight is 200 million to 4 billion Euro per year, which is 1 to 5 percent of the national health expenditures. The government and health insurance companies should try to prevent overweight and encourage healthy behavior.overweight, ethnicity, food habits, Food Consumption/Nutrition/Food Safety, C20, D12, I12,

The demand for health: differences between the native Dutch and immigrants in the Netherlands

This paper estimates the demand for health by using a health capital model for different population groups (native Dutch, Surinamese/Antillean, Moroccan, and Turkish) in the Netherlands. Also the effect of overweight on health utility is investigated. We found a decrease in the demand for health for age, overweight, and smoking, we found an increase in the demand for health for level of education and marital status. The analyses show a strong effect of gender. Being female in all groups is negatively related to health utility. Turkish and Moroccan ethnicity is negatively related to health status.demand for health, health production, ethnicity, overweight, food habits, Health Economics and Policy, C24, C25, I10, I12,

Electronic Correlations in Oligo-acene and -thiophene Organic Molecular Crystals

From first principles calculations we determine the Coulomb interaction between two holes on oligo-acene and -thiophene molecules in a crystal, as a function of the oligomer length. The relaxation of the molecular geometry in the presence of holes is found to be small. In contrast, the electronic polarization of the molecules that surround the charged oligomer, reduces the bare Coulomb repulsion between the holes by approximately a factor of two. In all cases the effective hole-hole repulsion is much larger than the calculated valence bandwidth, which implies that at high doping levels the properties of these organic semiconductors are determined by electron-electron correlations.Comment: 5 pages, 3 figure