PubMed Health⌕ Search

Biomedical subjects

B Barbara

Publications and source records attributed to B Barbara.

At least 19 recordsLinked to original sources
PubMed2003

Quantum dynamics of atomic magnets: cotunneling and dipolar-biased tunneling.

Multispin tunneling cross relaxations in an ensemble of weakly coupled Ho3+ ions, mediated by weak anisotropic dipolar interactions, can be evidenced by ac-susceptibility measurements in a high temperature regime. Based on a four-body representation, including the rare-earth nuclear spin, two-ion tunneling mechanisms can be attributed to both dipolar-biased tunneling and cotunneling processes. The coreversal involving entangled pairs of magnetic moments is discussed with a particular emphasis, giving new evidence to elucidate the many-body quantum dynamics in dipolar spin glasses.

Journal Article↗
PubMed2003

Superconductivity in doped sp3 semiconductors: the case of the clathrates.

We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba(8)@Si-46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant lambda calculated from perturbative density functional theory. Further, the study of I(8)@Si-46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp(3) silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C60.

Journal Article↗
PubMed2001

Nuclear spin driven quantum relaxation in LiY0.998Ho0.002F4.

Staircaselike hysteresis loops of the magnetization of a LiY0.998Ho0.002F4 single crystal are observed at subkelvin temperatures and low field sweep rates. This behavior results from quantum dynamics at avoided level crossings of the energy spectrum of single Ho3+ ions in the presence of hyperfine interactions. Enhanced quantum relaxation in constant transverse fields allows the study of the relative magnitude of tunnel splittings. At faster sweep rates, nonequilibrated spin-phonon and spin-spin transitions, mediated by weak dipolar interactions, lead to magnetization oscillations and additional steps.

Journal Article↗
PubMed2000

Phonon-assisted tunneling in the quantum regime of Mn12 acetate

Longitudinal or transverse magnetic fields applied on a crystal of Mn12 acetate allows one to observe independent tunnel transitions between m = -S+p and m = S-n-p ( n = 6-10, p = 0-2 in longitudinal field and n = p = 0 in transverse field). We observe a smooth transition (in longitudinal) from coherent ground-state to thermally activated tunneling. Furthermore, two ground-state relaxation regimes show a crossover between quantum spin relaxation far from equilibrium and near equilibrium, when the environment destroys multimolecule correlations. Finally, we stress that the complete Hamiltonian of Mn12 should contain odd spin operators of low order.

Journal Article↗
PubMed2000

Butterfly hysteresis loop and dissipative spin reversal in the S = 1/2, V15 molecular complex

Time resolved magnetization measurements have been performed on a spin 1/2 molecular complex, so-called V15. Despite the absence of a barrier, magnetic hysteresis is observed over a time scale of several seconds. A detailed analysis in terms of a dissipative two-level model is given, in which fluctuations and splittings are of the same energy. Spin-phonon coupling leads to long relaxation times and to a particular "butterfly" hysteresis loop.

Journal Article↗
PubMed1997

Inactivation of alpha1-antiproteinase (alpha1-AT) and changes in antioxidants' plasma levels in subarachnoid hemorrhage.

Recent studies have suggested that a quantitative or a qualitative imbalance between the activity of proteases and its inhibitors hypothetically might be involved in intracranial aneurysm rupture. In the present study we test the hypothesis that the systemic reduction of alpha1-antitrypsin activity might be related to the elevated oxidative potential exerted by cigarette smoking and/or to a systemic low antioxidant capacity. We studied, in a series of 57 patients bearing intracranial aneurysms, the relationship between alpha1-antitrypsin activity, cigarette smoking and the following variables measured in plasma: vitamin A, vitamin E, thiol groups, urate and lipid peroxide levels. Serum levels of alpha1-antitrypsin are higher in patients with subarachnoid hemorrhage than in cases of unruptured aneurysms, while the levels of vitamin A and vitamin E are significantly lower in patients that suffered subarachnoid hemorrhage than in controls. Both vitamin A and E levels are related to the occurrence of rupture of the aneurysm, as elicited by logistic regression analysis (P=0.017 and P=0.014, respectively), with a protective effect of higher levels of the variables, as shown by their odds ratio (0.028 and 0.84, respectively). No significant changes in the strength of the association could be appreciated when controlling for smoking habit. None of the other tested variables could be related to the occurrence of the aneurysm rupture. Both alpha1-antitrypsin serum level and the level of vitamin A appeared to be independently related to alpha1-antitrypsin collagenase inhibitory capacity percentage (P=0.03 and P=0.025), with no independent influence of the type of aneurysm and the smoking habit. The results of the present study show that the qualitative pattern of alpha1-antitrypsin is significantly related to the serum level of liposoluble vitamin A, while the type of aneurysm and the smoking habit have no independent influence. This suggests that in a situation in which systemic levels of vitamin A are reduced, the risk of a reduced activity of alpha1-antitrypsin as controller of proteases is elevated, with the consequent increased risk of aneurysm bleeding.

Analysis of Variance↗
PubMed1995

Epidemiology of gallstone disease in Italy: prevalence data of the Multicenter Italian Study on Cholelithiasis (M.I.COL.)

This study was designed to obtain a general overview of gallstone disease in Italy. A total of 18 cohorts in 10 Italian regions were enrolled in this survey. Four cohorts were excluded from analysis because of a participation rate of less than 50 percent. Field activities started in December 1984 and terminated in April 1987. A precoded questionnaire was administered to each subject by a trained member of the medical staff. Participants underwent an ultrasonographic examination of the upper abdomen and blood sampling. Each research group was provided with an identical ultrasonographic machine. Standardized criteria were established for both the examination and diagnosis. The inter- and intraobserver variation in the ultrasonographic procedure was assessed using a reliability study. The overall kappa score for interobserver agreement was good (0.649), while intraobserver agreement was good or excellent (kappa scores > 0.60) in 75 percent of the observers. A total of 29,739 of the 46,139 (64.4 percent) enrolled subjects were examined. Among those with a clear-cut definition of the gallbladder status, 6.5 percent of the males and 10.5 percent of the females had gallstones. Additional 3.0 and 8.4 percents, respectively, had already undergone cholecystectomy. Prevalence of gallstone disease (gallstones plus cholecystectomy) increased linearly with age in both sexes. Among subjects with gallstones, 76.7 percent of men and 72 percent of women were not aware of having gallstones; 87.0 percent of men and 84.9 percent of women were asymptomatic. This study confirms that gallstone disease is a highly prevalent condition. Gallstones rarely cause symptoms, and subjects are mostly unaware of their presence.

Adult↗