Variational ground states of the two-dimensional Hubbard model

Recent refinements of analytical and numerical methods have improved our understanding of the ground-state phase diagram of the two-dimensional (2D) Hubbard model. Here we focus on variational approaches, but comparisons with both Quantum Cluster and Gaussian Monte Carlo methods are also made. Our own ansatz leads to an antiferromagnetic ground state at half filling with a slightly reduced staggered order parameter (as compared to simple mean-field theory). Away from half filling, we find d-wave superconductivity, but confined to densities where the Fermi surface passes through the antiferromagnetic zone boundary (if hopping between both nearest-neighbour and next-nearest-neighbour sites is considered). Our results agree surprisingly well with recent numerical studies using the Quantum Cluster method. An interesting trend is found by comparing gap parameters (antiferromagnetic or superconducting) obtained with different variational wave functions. They vary by an order of magnitude and thus cannot be taken as a characteristic energy scale. In contrast, the order parameter is much less sensitive to the degree of sophistication of the variational schemes, at least at and near half filling.Comment: 18 pages, 4 figures, to be published in New J. Phy

Pulmonary tuberculosis in prisons of the ex-USSR state Georgia: results of a nation-wide prevalence survey among sentenced inmates

SETTING: The penitentiary system of the ex-USSR state of Georgia. OBJECTIVES: To measure the prevalence of active pulmonary tuberculosis and drug-resistant disease among prisoners. To identify factors associated with active tuberculosis and multidrug resistance (MDR). DESIGN: A comprehensive multiphasic screening survey for tuberculosis. RESULTS: The prevalence of smear- or culture-positive tuberculosis was 5995 per 100,000 prisoners (n = 448 cases among 7473 inmates). Of all the strains, 215 (77.9%) were resistant to at least one drug and 37 (13.0%) were MDR. Independent risk markers associated smear- or culture-positive tuberculosis with included prison stay of 2 years or more, body mass index <20 kg/m2, accommodation in a large size prison, previous anti-tuberculosis treatment, cough of 2 weeks or more and loss of appetite. Risk markers associated with MDR were a prison stay of less than 2 years and being over 25 years of age. CONCLUSIONS: In Georgia, the excess risk for tuberculosis among prisoners is unprecedented, suggesting that prisons represent a significant reservoir of tuberculosis. Only a comprehensive strategy for tuberculosis control in prisons, including prison reform, control of anti-tuberculosis drugs, and prompt and efficient diagnosis and treatment of patients can have an impact on the tuberculosis burden in the prison system