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S Sorella

Publications and source records attributed to S Sorella.

At least 19 recordsLinked to original sources

Increasing d-wave superconductivity by on-site repulsion.

We study, through the variational Monte Carlo technique, an extended Hubbard model away from half filled band density which contains two competing nearest-neighbor interactions: a superexchange J favoring d-wave superconductivity and a repulsion V opposing it. We find that the on-site repulsion U effectively enhances the strength of J while suppressing that of V, thus favoring superconductivity. This result shows that attractions which do not involve charge fluctuations are very well equipped against strong electron-electron repulsion so much to get advantage from it.

Journal Article↗

Superconductivity in the two-dimensional t-J model.

Using computational techniques, it is shown that pairing is a robust property of hole-doped antiferromagnetic insulators. In one dimension and for two-leg ladder systems, a BCS-like variational wave function with long-bond spin singlets and a Jastrow factor provides an accurate representation of the ground state of the t-J model, even though strong quantum fluctuations destroy the off-diagonal superconducting long-range order in this case. However, in two dimensions it is argued-and numerically confirmed using several techniques, especially quantum Monte Carlo-that quantum fluctuations are not strong enough to suppress superconductivity.

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Stripes and spin incommensurabilities are favored by lattice anisotropies.

Structural distortions in cuprate materials give a natural origin for anisotropies in electron properties. We study a modified one-band t-J model in which we allow for different hoppings and antiferromagnetic couplings in the two spatial directions (t(x) not equal to t(y) and Jx not equal to J(y)). Incommensurate peaks in the spin structure factor show up only in the presence of a lattice anisotropy, whereas charge correlations, indicating enhanced fluctuations at incommensurate wave vectors, are present but almost unaffected with respect to the isotropic case.

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Resonating valence bond wave functions for strongly frustrated spin systems.

The resonating-valence-bond (RVB) theory for two-dimensional quantum antiferromagnets is shown to be the correct paradigm for large enough "quantum frustration." This scenario, proposed a long time ago but never confirmed by microscopic calculations, is strongly supported by a new type of variational wave function, which is extremely close to the exact ground state of the J(1)-J(2) Heisenberg model for 0.4 less than approximately J(2)/J(1) less than approximately 0.5. This wave function is proposed to represent the generic spin-half RVB ground state in spin liquids.

Journal Article↗

Nagaoka ferromagnetism in the two-dimensional infinite-U Hubbard model.

We present different numerical calculations based on variational quantum Monte Carlo simulations supporting a ferromagnetic ground state for finite and small hole densities in the two-dimensional infinite-U Hubbard model. Moreover, by studying the energies of different total spin sectors, these calculations strongly suggest that the paramagnetic phase is unstable against a phase with a partial polarization for large hole densities delta approximately 0.40 with evidence for a second-order transition to the paramagnetic large doping phase.

Journal Article↗

Spontaneous plaquette dimerization in the J1-J2 heisenberg model

We investigate the nonmagnetic phase of the spin-half frustrated Heisenberg antiferromagnet on the square lattice using exact diagonalization (up to 36 sites) and quantum Monte Carlo techniques (up to 144 sites). The spin gap and the susceptibilities for the most important crystal symmetry breaking operators are computed. A genuine and somehow unexpected "plaquette resonating valence bond," with spontaneously broken translation symmetry and no broken rotation symmetry, comes out from our numerical simulations as the most plausible ground state for J(2)/J(1) approximately 0.5.

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Etiology of acute otitis media in human immunodeficiency virus-infected children.

BACKGROUND: Acute otitis media (AOM) is one of the most common infections that are implicated as significant contributors to morbidity in HIV-infected children. To establish the optimal antibiotic therapy tympanocentesis is indicated as the first line diagnostic procedure, because unusual pathogens may play a role in advanced stages of deficient humoral or cellular immunity. METHODS: The microbiology of 60 episodes of AOM diagnosed in 21 symptomatic HIV-infected children (ages 9 months to 12 years) was compared with that of 121 episodes of AOM occurring in 113 immunocompetent HIV-negative children (ages 6 months to 12 years) in the last 5 years. RESULTS: The prevalence of the three most common pathogens (Streptococcus pneumoniae, Haemophilus influenzae and group A beta-hemolytic Streptococcus) was similar in HIV-infected and in normal children (56.5% vs. 54.9% of the ears). Staphylococcus aureus was significantly more frequent in AOM diagnosed in severely immunosuppressed stages. A significantly lower proportion of middle ear effusions obtained in HIV-infected children yielded no bacteria compared with normal children. Beta-lactamase production among isolates of H. influenzae was a rare phenomenon, both in HIV-infected and in normal children. No penicillin-resistant S. pneumoniae was found. CONCLUSIONS: In HIV-infected children with absent or moderate immunosuppression empiric antibiotic therapy should be based on the recommendations given for immunocompetent children of the same geographic area. In severe immunosuppressed stages, given the possible role of Staph. aureus, extended spectrum antibiotics should be considered.

Bacterial Infections↗

Comparative study of once-weekly azithromycin and once-daily amoxicillin treatments in prevention of recurrent acute otitis media in children.

Continuous chemoprophylaxis is effective in the prevention of new episodes of acute otitis media (AOM) in otitis-prone children, but compliance can be a problem and thus efficacy can be decreased. Intermittent chemoprophylaxis has so far shown conflicting results. Azithromycin, which has a peculiar pharmacokinetics, resulting, even after a single dose, in persistently elevated concentrations in respiratory tissues, could permit a periodic administration with higher compliance. We compared a 6-month course of once-weekly azithromycin (5 or 10 mg/kg of body weight) with that of once-daily amoxicillin (20 mg/kg) in a single-blind, randomized study of prophylaxis for recurrent AOM in 159 children aged 6 months to 5 years with at least three episodes of AOM in the preceding 6 months. In the amoxicillin group, 23 (31.1%) of 74 children developed 29 episodes of AOM, while in the 10-mg/kg azithromycin group, 11 (14.9%) of 74 children experienced 15 episodes. The 5-mg/kg/week azithromycin trial was prematurely interrupted after nine cases, due to the high occurrence rate of AOM (55.5%). During the 6-month prophylaxis period, the proportion of children with middle ear effusion declined similarly in both groups. No substantial modification of the nasopharyngeal flora was noted at the end of prophylaxis in both antimicrobial groups. In the 6-month-postprophylaxis follow-up period, about 40% of children in both groups again developed AOM. Azithromycin at 10 mg/kg once weekly can be regarded as a valid alternative to once-daily low-dose amoxicillin for the prophylaxis of AOM. Although in the present study no microbiological drawback was noted, accurate selection of children eligible for prophylaxis is mandatory to avoid the risk of emergence of resistant strains.

Amoxicillin↗