Magnetic order in the Shastry-Sutherland model

The ground state properties of the Shastry-Sutherland model in the presence of an external field are investigated by means of variational states built up from unpaired spins (monomers) and singlet pairs of spins (dimers). The minimum of the energy is characterized by specific monomer-dimer configurations, which visualize the magnetic order in the sectors with fixed magnetization M=S/N. A change in the magnetic order is observed if the frustrating coupling alpha exceeds a critical value alpha_c(M), which depends on M. Special attention is paid to the ground state configurations at M=1/4, 1/6, 1/8.Comment: 9 pages, 9 figures, RevTe

The magnetization process in the 2-dimensional J_1-J_2 model

We study the alpha = J_2/J_1-dependence of the magnetization process in the J_1-J_2 model on a square lattice with frustrating couplings J_2 along the diagonals. Perturbation expansions around alpha=J_2/J_1=0 and 1/alpha=0$ yield an adequate description of the magnetization curve in the antiferromagnetic and collinear antiferromagnetic phase, respectively. The transition from one phase to the other (0.5 < alpha < 0.7) leaves pronounced structures in the longitudinal and transverse structure factors at p=(pi,pi) and p=(0,pi).Comment: 10 pages, 10 figures, RevTe

Charge density plateaux and insulating phases in the t−Jt-J model with ladder geometry

We discuss the occurrence and the stability of charge density plateaux in ladder-like $t-J$ systems (at zero magnetization M=0) for the cases of 2- and 3-leg ladders. Starting from isolated rungs at zero leg coupling, we study the behaviour of plateaux-related phase transitions by means of first order perturbation theory and compare our results with Lanczos diagonalizations for $t-J$ ladders ($N=2\times 8$) with increasing leg couplings. Furthermore we discuss the regimes of rung and leg couplings that should be favoured for the appearance of the charge density plateaux.Comment: 10 pages, 7 figures, RevTex

Formation of clusters in the two dimensional t-J model: The mechanism for phase separation

The emergence of phase separation is investigated in the framework of a 2D t-J model by means of a variational product ansatz, which covers the infinite lattice by two types of L x L clusters. Clusters of the first type are completely occupied with electrons, i.e. they carry maximal charge Q_e=L^2 and total spin 0, and thereby form the antiferromagnetic background. Holes occur in the second type of clusters -- called ``hole clusters''. They carry a charge Q_h<L^2. The charge Q_h and the number N(Q_h) of hole clusters is fixed by minimizing the total energy at given hole density and spin exchange coupling \alpha=J/t. For \alpha not too small (\alpha>0.5) it turns out that hole clusters are occupied with an even number Q_h<L^2 of electrons and carry a total spin 0. For increasing \alpha the charge Q_h(\alpha) of the hole clusters decreases.Some points on the boundary curve can be extracted from Q_h(\alpha).Comment: 11 pages and 3 figure

A numerical study of the formation of magnetisation plateaus in quasi one-dimensional spin-1/2 Heisenberg models

We study the magnetisation process of the one dimensional spin-1/2 antiferromagnetic Heisenberg model with modulated couplings over j=1,2,3 sites. It turns out that the evolution of magnetisation plateaus depends on j and on the wave number q of the modulation according to the rule of Oshikawa, et al. A mapping of two- and three-leg zig-zag ladders on one dimensional systems with modulated couplings yields predictions for the occurence of magnetization plateaus. The latter are tested by numerical computations with the DMRG algorithm.Comment: 7 pages, 10 figures, accepted for publication in Euro. Phys. J.

Hypertensive disorders of pregnancy and risk of diabetes in Indian women:a cross-sectional study

BACKGROUND: Epidemiological data from high-income countries suggest that women with hypertensive disorders of pregnancy (HDP) are more likely to develop diabetes later in life. OBJECTIVE: We investigated the association between pre-eclampsia and eclampsia (PE&E) during pregnancy and the risk of diabetes in Indian women. DESIGN: Cross-sectional study. SETTING: India. METHODS: Data from India's third National Family Health Survey (NFHS-3, 2005-2006), a cross-sectional survey of women aged 15-49 years, are used. Self-reported symptoms suggestive of PE&E were obtained from 39 657 women who had a live birth in the 5 years preceding the survey. The association between PE&E and self-reported diabetes status was assessed using multivariable logistic regression models adjusting for dietary intake, body mass index (BMI), tobacco smoking, alcohol drinking, frequency of TV watching, sociodemographic characteristics and geographic region. RESULTS: The prevalence of symptoms suggestive of PE&E in women with diabetes was 1.8% (n=207; 95% CI 1.5 to 2.0; p<0.0001) and 2.1% (n=85; 95% CI 1.8 to 2.3; p<0.0001), respectively, compared with 1.1% (n=304; 95% CI 1.0 to 1.4) and 1.2% (n=426; 95% CI 1.1 to 1.5) in women who did not report any PE&E symptoms. In the multivariable analysis, PE&E was associated with 1.6 times (OR=1.59; 95% CI 1.31 to 1.94; p<0.0001) and 1.4 times (OR=1.36; 95% CI 1.05 to 1.77; p=0.001) higher risk for self-reported diabetes even after controlling for dietary intake, BMI and sociodemographic characteristics. CONCLUSIONS: HDP is strongly associated with the risk of diabetes in a large nationally representative sample of Indian women. These findings are important for a country which is already tackling the burden of young onset of diabetes in the population. However, longitudinal medical histories and a clinical measurement of diabetes are needed in this low-resource setting