Excitonic Effects in Quantum Wires

We review the effects of Coulomb correlation on the linear and non-linear optical properties of semiconductor quantum wires, with emphasis on recent results for the bound excitonic states. Our theoretical approach is based on generalized semiconductor Bloch equations, and allows full three-dimensional multisubband description of electron-hole correlation for arbitrary confinement profiles. In particular, we consider V- and T-shaped structures for which significant experimental advances were obtained recently. Above band gap, a very general result obtained by this approach is that electron-hole Coulomb correlation removes the inverse-square-root single-particle singularity in the optical spectra at band edge, in agreement with previous reports from purely one-dimensional models. Strong correlation effects on transitions in the continuum are found to persist also at high densities of photoexcited carriers. Below bandgap, we find that the same potential- (Coulomb) to kinetic-energy ratio holds for quite different wire cross sections and compositions. As a consequence, we identify a shape- and barrier-independent parameter that governs a universal scaling law for exciton binding energy with size. Previous indications that the shape of the wire cross-section may have important effects on exciton binding are discussed in the light of the present results.Comment: Proc. OECS-5 Conference, G\"ottingen, 1997 (To appear in Phys. Stat. Sol. (b)

Robust reconstruction of the discrete state for a class of nonlinear uncertain switched systems

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A Prognostic Model for Estimating the Time to Virologic Failure in HIV-1 Infected Patients Undergoing a New Combination Antiretroviral Therapy Regimen

<p>Abstract</p> <p>Background</p> <p>HIV-1 genotypic susceptibility scores (GSSs) were proven to be significant prognostic factors of fixed time-point virologic outcomes after combination antiretroviral therapy (cART) switch/initiation. However, their relative-hazard for the time to virologic failure has not been thoroughly investigated, and an expert system that is able to predict how long a new cART regimen will remain effective has never been designed.</p> <p>Methods</p> <p>We analyzed patients of the Italian ARCA cohort starting a new cART from 1999 onwards either after virologic failure or as treatment-naïve. The time to virologic failure was the endpoint, from the 90^thday after treatment start, defined as the first HIV-1 RNA > 400 copies/ml, censoring at last available HIV-1 RNA before treatment discontinuation. We assessed the relative hazard/importance of GSSs according to distinct interpretation systems (Rega, ANRS and HIVdb) and other covariates by means of Cox regression and random survival forests (RSF). Prediction models were validated via the bootstrap and c-index measure.</p> <p>Results</p> <p>The dataset included 2337 regimens from 2182 patients, of which 733 were previously treatment-naïve. We observed 1067 virologic failures over 2820 persons-years. Multivariable analysis revealed that low GSSs of cART were independently associated with the hazard of a virologic failure, along with several other covariates. Evaluation of predictive performance yielded a modest ability of the Cox regression to predict the virologic endpoint (c-index≈0.70), while RSF showed a better performance (c-index≈0.73, p < 0.0001 vs. Cox regression). Variable importance according to RSF was concordant with the Cox hazards.</p> <p>Conclusions</p> <p>GSSs of cART and several other covariates were investigated using linear and non-linear survival analysis. RSF models are a promising approach for the development of a reliable system that predicts time to virologic failure better than Cox regression. Such models might represent a significant improvement over the current methods for monitoring and optimization of cART.</p

Fault detection and reconstruction for a three-tank system via high-order sliding-mode observer

This paper presents an approach to the simultaneous faults detection, isolation and reconstruction in the hydraulic vertical three-tank system. An observer is designed which contains a corrective term based on a second-order sliding mode control algorithm featuring global convergence properties. Thanks to the proposed global observer, the reconstruction of certain faults and/or disturbances can be performed without requiring any filtration. The effectiveness of the proposed method is shown by means of a simulation example and then validated by performing real experiments

Fault diagnosis for the vertical three-tank system via high-order sliding-mode observation

This paper presents an approach to the fault diagnosis and disturbance observation for the hydraulic vertical three-tank system. An observer is designed which contains a corrective term based on a second-order sliding mode control algorithm featuring global convergence properties. Thanks to the proposed global observer, the reconstruction of certain faults and/or disturbances can be performed directly from the continuous observer output injection signal, without requiring any filtration. The effectiveness of the proposed method is shown by means of a simulation example and is then validated by performing real experiments

Exact reconstruction of actuator faults by reduced-order sliding mode observer

This paper presents an approach to the finite time detection and isolation of actuator faults in a class of nonlinear uncertain systems. After a suitable preliminary state transformation, devoted to put the system dynamics in an appropriate normal form, a reduced-order observer is designed which has an output injection term based on a second-order sliding mode control algorithm featuring global convergence properties. Thanks to the proposed observer, the reconstruction of the actuator faults can be performed exactly and in finite time directly from the observer output injection signal, i.e. without requiring any filtration. The effectiveness of the proposed method is shown by means of a simulation example

On a new sliding-mode differentiation scheme

We propose a new real-time differentiator, called the discontinuous high-gain observer (DHGO), which is derived from the classical HGO structure by introducing a discontinuous error term. Non-smooth Lyapunov analysis shows that the DHGO is a &quot;theoretically-exact&quot; differentiator in the absence of noise, meaning that the origin of the error state-space is a globally stable equilibrium point. A simulative comparison with the classical HGO and the 'Super-Twisting&quot; second-order sliding mode differentiator has been carried out. Simulations results point out that the new proposed differentiator features a particularly effective trade-off between accuracy and noise immunity

Position/attitude control of a jet-propelled surface-vessel prototype via the simplex-vector sliding-mode approach

This note concerns the design and implementation of a position/attitude sliding-mode controller for a surface vessel prototype. The prototype is equipped with a special, recently patented (V. Arrichiello, et al., 2005), propulsion system based on hydrojets with variable output section. The sliding mode control design is based on the vector simplex method (G, Bartolini, et al., 1997). First we describe the structure and the working principle of the prototype. Then, we present an approximate dynamic model and describe the detailed derivation of the motion controller. Finally, the major implementation issues are discussed and some experimental results are shown

Load swing damping in overhead cranes by sliding mode technique

Moving a suspended load is not an easy task when strict specifications on the swing angle and on the transfer time need to be satisfied. Nevertheless, these type of requirements are always present in industry because they are related to operation safety and cost. Intuitively, minimizing the cycle time and the load swing are conflicting requirements, and their satisfaction requires proper control actions, especially if some uncertainties in the system dynamics are present. In this paper we propose a simple control scheme based on second order sliding modes which is proved to be effective also in the case of poor knowledge of the system dynamics and/or parameters. Such controller has been tested on a laboratory-size model of an overhead crane by means of commercial devices, and some experimental results are reported within the paper