Relaxation and recombination in ultrasmall InAs quantum dots

We report a photoluminescence study of self-organized nanometer-size InAs quantum dots grown by molecular beam epitaxy on a GaAs substrate. High optical excitation has been used in order to observe emission from higher states of the quantum dots. The energy difference between adjacent states turns out to be of the order of 40–50 meV for dot diameters around 20 nm. The photoluminescence decay time at the fundamental transition is found to be of the order of 700 ps, decreasing down to 100 ps for the highest confined states. Finally, a cascade-like mechanism for the carrier relaxation in these structures is strongly suggested by the time resolved data.     

An interval logic for real-time system specification

Formal techniques for the specification of real time systems must be capable of describing system behavior as a set of relationships expressing the temporal constraints among events and actions, including properties of invariance, precedence, periodicity, liveness, and safety conditions. The paper describes a Temporal-Interval Logic with Compositional Operators (TILCO) designed expressly for the specification of real time systems. TILCO is a generalization of classical temporal logics based on the operators, eventually and henceforth; it allows both qualitative and quantitative specification of time relationships. TILCO is based on time intervals and can concisely express temporal constraints with time bounds, such as those needed to specify real time systems. This approach can be used to verify the completeness and consistency of specifications, as well as to validate system behavior against its requirements and general properties. TILCO has been formalized by using the theorem prover Isabelle/HOL. TILCO specifications satisfying certain properties are executable by using a modified version of the Tableaux algorithm. The paper defines TILCO and its axiomatization, highlights the tools available for proving properties of specifications and for their execution, and provides an example of system specification and validation     

Ricerca del minimo vincolato di una funzione dotata di derivate prima e seconda

The paper deals with a method for the constrained minimization of an-variable non-linear function. The method is quite fast to find the minimum also in its neighborhood and therefore it may be used for the solution of non-linear systems. The way to deal with the linear or non-linear constraints makes calculation time reasonable also in difficult problems. The method is essentially based on the local second-order approximation of the objective function (and so it is exact for quadratic functions) and on diagonalization of the associated quadratic form.