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Search Results: 1 - 10 of 208469 matches for " L. Belkhir "
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Quantum Hall effect in single wide quantum wells
M. Abolfath,L. Belkhir,N. Nafari
Physics , 1996, DOI: 10.1103/PhysRevB.55.10643
Abstract: We study the quantum Hall states in the lowest Landau level for a single wide quantum well. Due to a separation of charges to opposite sides of the well, a single wide well can be modelled as an effective two level system. We provide numerical evidence of the existence of a phase transition from an incompressible to a compressible state as the electron density is increased for specific well width. Our numerical results show a critical electron density which depends on well width, beyond which a transition incompressible double layer quantum Hall state to a mono-layer compressible state occurs. We also calculate the related phase boundary corresponding to destruction of the collective mode energy gap. We show that the effective tunneling term and the interlayer separation are both renormalised by the strong magnetic field. We also exploite the local density functional techniques in the presence of strong magnetic field at $\nu=1$ to calculate renormalized $\Delta_{SAS}$. The numerical results shows good agreement between many-body calculations and local density functional techniques in the presence of a strong magnetic field at $\nu=1$. we also discuss implications of this work on the $\nu=1/2$ incompressible state observed in SWQW.
Avascular necrosis of femoral head in patients with human immunodeficiency virus type 1 (HIV-1) infection: a single-centre experience
D Wilmes,P Docquier,L Belkhir,S Jonckheere
Journal of the International AIDS Society , 2012, DOI: 10.7448/ias.15.6.18325
Abstract: Purpose of the study: Introduction of highly active antiretroviral therapy (HAART) has led to an improvement of life expectancy and quality of life in patients living with HIV. Concomitantly, concerns are arising about long-term side effects of chronic use of antiretroviral therapy. Avascular necrosis of the femoral head (ANFH) and other epiphyses is increasingly reported as one of these debilitating complications. The objective of this study is to analyse clinical characteristics and outcome of patients with avascular necrosis (AN) followed in our centre. Patients and methods: We analysed retrospectively the charts of 1020 HIV-1 infected patients followed in our centre and focused on symptomatic patients with radiologicaly proven AN. We analysed risk factors, demographic and clinical characteristics, treatment and outcome in these patients. Summary of results: Ten patients with AN were identified (prevalence of 0.98%). The average interval between HIV diagnosis and diagnosis of AN was 89.1 months (1–254). Four patients had no evidence of risk factors (40%) whereas 6 (60%) had at least one risk factor. One patient had three cumulated risk factors and for him the onset time for AN was shorter (36 months). All patients had been treated by antiretroviral therapy when AN was diagnosed, but one of the patients developed symptoms prior to start of antiretroviral treatment. All classes of antiretroviral drugs have been used: protease inhibitors (mean use duration of 34.7 months before the ONFH onset), non-nucleoside reverse transcriptase inhibitors (12 months), and nucleoside reverse transcriptase inhibitors (40.5 months). ANFH was unilateral in 4 patients and bilateral in 6 patients. In one of these 6 cases, multiple AN locations were present (table). In eight patients, total hip arthroplasty (THA) (88.8%) was the definitive treatment. Diagnosis of ANFH in the two patients who were not operated is very recent (5 and 13 months) but surgery is already indicated. The average interval between ANFH diagnosis and the first THA was 10.5 months. Conclusion: AN, and particularly ANFH, is a rare but debilitating complication in HIV-1 infected patients. The role of ARV in the pathogenesis of AN remains unclear. However, classical risk factors play an essential role and accumulation of several risk factors could shorten the time before onset of AN. In the presence of advanced disease (stage III–IV) the final treatment remains arthroplasty.
Late presentation for HIV diagnosis: a single-centre experience
A Vincent,S Jonckheere,D Wilmes,L Belkhir
Journal of the International AIDS Society , 2012, DOI: 10.7448/ias.15.6.18368
Abstract: Purpose of the study: Antiretroviral therapy reduces mortality and morbidity in HIV-infected individuals, most markedly when initiated early, before advanced immunodeficiency has developed. All international guidelines (IAS 2010, EACS 2011, DHHS 2012) tend to recommend starting ART in patients with high CD4 cell count. Some challenges need to be met before reaching this goal, particularly HIV-infected patients with late presentation-diagnosis. The objective of this study was to determine the frequency of and demographic features associated with delayed presentation to care in our centre. Methods: All patients, newly diagnosed with HIV between January 2007 and December 2011 and on follow-up in our AIDS Reference Centre, were included. ‘Late presenter patient’ was defined as patient with CD4 count<350/mm3 at the time of diagnosis. Demographic (age, sex, ethnicity, migration) and clinical characteristics (transmission, CD4 cell count, viral load, CDC stage) were collected. We then compared these features with those identified in our centre between 1997-2006. Summary of results: Of the 601 patients diagnosed between 1997–2006, 57.1% were late presenters for HIV diagnosis. Among the 359 patients included between 2007–2011, 42.9% patients were late presenters. Demographic characteristic are summarized in Table 1. In the univariate analysis, patient age >50 years, female sex, immigrant status and heterosexual contact were associated with late presentation for HIV diagnosis. In the multivariate analysis, patients aged >50 and migrant women were the only independent risk factors for late presentation. Except gender, other risk factors remain identical to those that were identified in our centre between 1997–2006 [1]. Conclusion: A considerable proportion of patients continue to be diagnosed with advanced HIV disease, despite the fact that risk factors for late presentation have been identified clearly. In order to be able to treat all patient at high CD4 cell counts as recommended in all guidelines, we need to develop policies focused directly to categories of people at high risk of late presentation. Conflict of interest: None. All co-authors have participated in, and agree with the content and conclusions. This work is original and does not infringe any copyright. Acknowledgement: A. Sasse, Institut de Santé Publique, Belgium
Adherence with screening guidelines for hepatitis C testing among HIV-infected patients
S Jonckheere,A Vincent,D Wilmes,L Belkhir
Journal of the International AIDS Society , 2012, DOI: 10.7448/ias.15.6.18423
Abstract: Purpose of the study: Co-infection with HIV / hepatitis C virus (HCV) occurs commonly due to similar routes of transmission, mainly in MSM and IVDU patients. In 2009, EACS guidelines introduced the notion of systematic annual HCV screening among HIV-infected patients. This study evaluated staff knowledge, adherence to HCV screening recommendations and seroconversion rates for HCV in our HIV Reference Centre. Methods: Eight physicians (HIV specialists) were interviewed on recommendations and perceived adherence to EACS clinical guidelines on HCV screening [1]. We then reviewed medical records of our cohort of HIV-infected patients on regular follow-up in our centre each year, from 2008 to 2011. We considered a patient to be on regular follow-up when records showed at least two clinical reviews and one HIV viral load testing during the year. Demographic features and HCV serology tests were collected from the operating software of our institution (Medical Explorer v3r9, 2008). Diagnosis of HCV was retained when serology became positive and HCV RNA was detected. Summary of results: Though knowledge of current guidelines was excellent (100%), staff claimed a 87.5% adherence rate to these recommendations. Rate of screening rose gradually between 2008 and 2011, especially after introduction of EACS guidelines in 2009 (Table 1 and Fig. 1). The maximal screening rate was in 2011, with 44% of patients tested among the general HIV population and 57% among MSM bisexual patients. This trend was statistically significant in both populations (p<0.01). The year 2011 displayed a marked increase in diagnosis of HCV infection, with 8 new patients diagnosed in a 963-patient-large cohort (all were MSM). Conclusion: In our centre, knowledge of EACS guidelines on screening for HCV was good but adherence to these recommendations is poor, though it improves over time. It is consistent with published rates of compliance to clinical guidelines on screening policies for HCV among STD/HIV specialists (47–54%) [2]. However, it remains low compared to expected rates of 70–100%. Education of clinicians is warranted to increase awareness and further improve adherence to guidelines. Peer review and computer-based algorithms / reminders could be used in order to increase systematic screening.
Vitamin D deficiency in a cohort of HIV-infected patients: clinical analysis
D Wilmes,P Forget,A Vincent,L Belkhir
Journal of the International AIDS Society , 2012, DOI: 10.7448/ias.15.6.18205
Abstract: Purpose of the study: Observational studies have noted very high rates of low serum 25-hydroxyvitamin D [25(OH)D3] levels in both general and HIV-infected populations. In HIV-infected patients, low 25(OH)D3 levels are secondary to a combination of usual risk factors and HIV-specific risk factors, like antiretroviral therapy [1]. The objective of our study is to analyse the magnitude of vitamin D deficiency or insufficiency and the role of various factors such as age, sex, ethnicity, season, and antiretroviral medications in our cohort of HIV-infected patients. Methods: We prospectively collected data on 25-hydroxyvitamin D levels sampled between January 2009 and June 2011 from our cohort of 930 HIV-infected patients. Vitamin D dosage was performed using immunoassay (‘Diasorin’ - Saluggia, Italy). We divided vitamin D levels into 3 categories: 25-hydroxyvitamin D levels <20 mg/nl were considered deficient, insufficient between 20 and 29 ng/ml. Levels ≥30 ng/ml were defined as normal [2]. Data on demographic features (age, ethnicity, season, heterosexuality vs homosexuality), clinical features and laboratory findings (CD4 cell count, viral load, HAART, BMI) were collected from patients’ medical records using our institutional database ‘Medical explorer v3r9, 2009’. Summary of results: Overall, 848 patients were included in our study (Table 1). Low levels of serum 25(OH)D3 were seen in 89.3% of the study population, from which 69.5% were deficient and 19.8% were insufficient. On univariate analysis, female sex, high BMI, black African, heterosexuality, undetectable viral load and antiretroviral treatment were all predictors of vitamin D deficiency and insufficiency. Treatment with efavirenz and tenofovir were the most associated with low vitamin D levels. On multivariate analysis (multiple linear regression model) only female sex (OR=1.14; 95% CI 0.84–0.96; p<0.001), dosage during winter months (OR=1.14; 95% CI 1–1.15; p<0.05) and HAART (OR=1.12; 95% CI 1.04–1.19; p=0.002) were identified as independent risk factors of low 25(OH)D3 levels. Conclusion: Vitamin D deficiency is frequent in HIV-infected populations (69.5%). Patients on antiretroviral therapy are at higher risk of vitamin D deficiency. In our cohort, black women and dosage during winter were also independent risk factors for low vitamin D levels.
Half-Integral Spin-Singlet Quantum Hall Effect
L. Belkhir,X. G. Wu,J. K. Jain
Physics , 1993, DOI: 10.1103/PhysRevB.48.15245
Abstract: We provide numerical evidence that the ground state of a short range interaction model at $\nu=1/2$ is incompressible and spin-singlet for a wide range of repulsive interactions. Furthermore it is accurately described by a trial wave function studied earlier. For the Coulomb interaction we find that this wave function provides a good description of the lowest lying spin-singlet state, and propose that fractional quantum Hall effect would occur at $\nu=1/2$ if this state became the global ground state.
Police in the Islamic Maghreb
Libdry Belkhir
Historical Kan Periodical , 2010,
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Quantum and Many-Body Effects on the Capacitance of a Quantum Dot
Lotfi Belkhir
Physics , 1994, DOI: 10.1103/PhysRevB.50.8885
Abstract: We calculate exactly, using finite size techniques, the quantum mechanical and many-body effects to the self-capacitance of a spherical quantum dot in the regime of extreme confinement, where the radius of the sphere is much smaller than the effective Bohr radius. We find that the self-capacitance oscillates as a function of the number of electrons close to its classical value. We also find that the electrostatic energy extrapolates to zero when $N=1$, suggesting that the energy scales like $e^{2}N(N-1)$. This establishes, at least for this configuration, that the semiclassical description of Coulomb charging effects in terms of capacitances holds to a good approximation even at very small scales.
The Star Height Hierarchy Vs. The Variable Hierarchy
Walid Belkhir
Computer Science , 2009,
Abstract: The star height hierarchy (resp. the variable hierarchy) results in classifying $\mu$-terms into classes according to the nested depth of fixed point operators (resp. to the number of bound variables). We prove, under some assumptions, that the variable hierarchy is a proper refinement of the star height hierarchy. We mean that the non collapse of the variable hierarchy implies the non collapse of the star height hierarchy. The proof relies on the combinatorial characterization of the two hierarchies.
Closure Under Minors of Undirected Entanglement
Walid Belkhir
Computer Science , 2009,
Abstract: Entanglement is a digraph complexity measure that origins in fixed-point theory. Its purpose is to count the nested depth of cycles in digraphs. In this paper we prove that the class of undirected graphs of entanglement at most $k$, for arbitrary fixed $k \in \mathbb{N}$, is closed under taking minors. Our proof relies on the game theoretic characterization of entanglement in terms of Robber and Cops games.
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