Two Electrons in a Quantum Dot: A Unified Approach

Low-lying energy levels of two interacting electrons confined in a two-dimensional parabolic quantum dot in the presence of an external magnetic field have been revised within the frame of a novel model. The present formalism, which gives closed algebraic solutions for the specific values of magnetic field and spatial confinement length, enables us to see explicitly individual effects of the electron correlation.     

Kronecker representation and decompositional analysis of closed queueing networks with phase-type service distributions and arbitrary buffer sizes

Two approximative fixed-point iterative methods based on decomposition for closed queueing networks with Coxian service distributions and arbitrary buffer sizes are extended to include phase-type service distributions. The irreducible Markov chain associated with each subnetwork in the respective decompositions is represented hierarchically using Kronecker products. The two methods are implemented in a software tool capable of computing the steady-state probability vector of each subnetwork by a multilevel method at each fixed-point iteration and are compared with other methods for accuracy and efficiency. Numerical results indicate that there is a niche filled by the two approximative methods. The authors thank Jean-Michel Fourneau for pointing out Marie’s method and Brouwer’s fixed-point theorem. The first author gratefully acknowledges grant TüBA-GEBİP from the Turkish Academy of Sciences.     

Analysis of meeting protocols by formalisation, simulation, and verification

Organizations depend on regular meetings to carry out their everyday tasks. When carried out successfully, meetings offer a common medium for participants to exchange ideas and make decisions. However, many meetings suffer from unfocused discussions or irrelevant dialogues. To study meetings in detail, we first formalize general properties of meetings and a generic meeting protocol to specify how roles in a meeting should interact to realize these properties. This generic protocol is used as a starting point to study real-life meetings. Next, an example meeting is simulated using the generic meeting protocol. The general properties are formally verified in the simulation trace. Next, these properties are also verified formally against empirical data of a real meeting in the same context. A comparison of the two traces reveals that a real meeting is more robust since when exceptions happen and the rules of the protocol are violated, these exceptions are handled effectively. Given this observation, a more refined protocol is specified that includes exception-handling strategies. Based on this refined protocol a meeting is simulated that closely resembles the real meeting. This protocol is then validated against another set of data from another real meeting. By iteratively adding exception handling rules, the protocol is enhanced to handle a variety of situations successfully.     

On system reliability in stress–strength setup

In this paper we study stress–strength reliability for a general coherent system. The exact expression as well as bounds and approximations for system reliability are presented. We also illustrate the estimation procedure for exponential stress–strength distributions.     

Worst-case robust decisions for multi-period mean–variance portfolio optimization

In this paper, we extend the multi-period mean–variance optimization framework to worst-case design with multiple rival return and risk scenarios. Our approach involves a min–max algorithm and a multi-period mean–variance optimization framework for the stochastic aspects of the scenario tree. Multi-period portfolio optimization entails the construction of a scenario tree representing a discretised estimate of uncertainties and associated probabilities in future stages. The expected value of the portfolio return is maximized simultaneously with the minimization of its variance. There are two sources of further uncertainty that might require a strengthening of the robustness of the decision. The first is that some rival uncertainty scenarios may be too critical to consider in terms of probabilities. The second is that the return variance estimate is usually inaccurate and there are different rival estimates, or scenarios. In either case, the best decision has the additional property that, in terms of risk and return, performance is guaranteed in view of all the rival scenarios. The ex-ante performance of min–max models is tested using historical data and backtesting results are presented.     

Analysis of the quadrupole deformation of <Emphasis Type="Italic">Δ</Emphasis>(1232) within an effective Lagrangian model for pion photoproduction from the nucleon

We present an extraction of the E2/M1 ratio of the Δ(1232) from experimental data applying an effective Lagrangian model. We compare the result obtained with different nucleonic models and we reconcile the experimental results with the lattice QCD calculations.     

Anodic stripping voltammetric determination of total lead, copper and selenium in whole blood and blood serum

A two-step procedure including appropriate wet-digestions, separation of selenium from interfering ions such as heavy metal ions with pentyl alcohol and anodic stripping voltammetric (ASV) determination of Pb²⁺, Cu²⁺ and SeO₃ ^2– is developed. The elements in digested whole blood and serum sample solutions were determined by using a standard addition method. 1 × 10^–9 mol/L SeO^2– ₃, Cu²⁺ and Pb²⁺ were successfully determined with relative standard deviations of approximately 1–2% (n = 6–8). Received: 19 August 1996 / Revised: 24 February 1997 / Accepted: 28 February 1997     

The theoretical calculation of transition probabilities for some excited p-d transitions in atomic nitrogen

The atomic transition probabilities are calculated for individual lines between some quartet terms of 3p↦4d and 3p↦5d transition arrays using weakest bound electron potential model theory (WBEPMT). In the determination of relevant parameters which are needed for calculation of transition probabilities, we employed numerical non-relativistic Hartree-Fock wave functions for expectation values of radius in both ground and excited states unlike to NCA method used on traditional WBEPMT procedure. We have obtained very good agreement between our results and the accepted values taken from NIST.     

Synthesis and size differentiation of Ge nanocrystals in amorphous SiO2

Germanosilicate layers were grown on Si substrates by plasma enhanced chemical vapor deposition (PECVD) and annealed at different temperatures ranging from 700–1010 °C for durations of 5 to 60 min. Transmission electron microscopy (TEM) was used to investigate Ge nanocrystal formation in SiO₂:Ge films. High-resolution cross section TEM images, electron energy-loss spectroscopy and energy dispersive X-ray analysis (EDX) data indicate that Ge nanocrystals are present in the amorphous silicon dioxide films. These nanocrystals are formed in two spatially separated layers with average sizes of 15 and 50 nm, respectively. EDX analysis indicates that Ge also diffuses into the Si substrate. PACS 68.73.Lp; 61.46.Hk; 61.46.-w; 68.65.Hb; 61.82.Rx     

Efficiency improvement of quantum well solar cell with the AuGeNi metallization and Si3N4 ARC design

This study demonstrates the power conversion efficiency enhancement on In_0.19Ga_0.81As/GaAs quantum well solar cells (QWSC). The solar cell structure was grown on n-type (100)-oriented GaAs substrate by using solid-source molecular beam epitaxy technique and divided into square pieces. In order to understand whether the eight quantum wells were grown or not, scanning electron microscopy (SEM), and secondary ion mass spectrometry characterizations were done at room temperature. After that, the Si₃N₄ antireflection layers were coated onto both two square pieces of In_0.19Ga_0.81As/GaAs QWSC structure and p-GaAs substrate at different temperatures by the radio frequency magnetron sputtering system. The optical properties of the Si₃N₄ coated and uncoated p-GaAs samples have been evaluated by means of ultraviolet-visible spectrometry measurements at room temperature. According to ultraviolet-visible spectrometry results, the best Si₃N₄ antireflection coated one was obtained at 100 °C substrate temperature. Thus, the In_0.19Ga_0.81As/GaAs QWSC structure with and without Si₃N₄ layer, which was coated at 100 °C substrate temperature, was selected for other measurements and processes. Moreover, the In_0.19Ga_0.81As/GaAs QWSC samples with and without Si₃N₄ antireflection coating were separately fabricated with different metallization materials for obtaining the solar cell electrical output parameters. AuGe and AuGeNi metallization materials were used for the fabrication processes. After fabrication, the electrical output parameters were extracted from the current-voltage measurements at room temperature both in dark and under AM1.5 – 1 Sun illumination. The proposed design which includes the AuGeNi metallization and Si₃N₄ antireflection layer enhanced the power conversion efficiency by 44.40%.