Magnetic field dependence of the many-electron states in a magnetic quantum dot: The ferromagnetic-antiferromagnetic transition

The electron-electron correlations in a many-electron (Ne = 1, 2,..., 5) quantum dot confined by a parabolic potential is investigated in the presence of a single magnetic ion and a perpendicular magnetic field. We obtained the energy spectrum and calculated the addition energy which exhibits cusps as function of the magnetic field. The vortex properties of the many-particle wave function of the ground state are studied and for large magnetic fields are related to composite fermions. The position of the impurity influences strongly the spin pair correlation function when the external field is large. In small applied magnetic field, the spin exchange energy together with the Zeeman terms leads to a ferromagnetic-antiferromagnetic(FM-AFM) transition. When the magnetic ion is shifted away from the center of the quantum dot a remarkable re-entrant AFM-FM-AFM transition is found as function of the strength of the Coulomb interaction. Thermodynamic quantities as the heat capacity, the magnetization, and the susceptibility are also studied. Cusps in the energy levels show up as peaks in the heat capacity and the susceptibility.Comment: 16 pages, 24 figure

Cyclotron resonance of a magnetic quantum dot

The energy spectrum of a one-electron quantum dot doped with a single magnetic ion is studied in the presence of an external magnetic field. The allowed cyclotron resonance (CR) transitions are obtained together with their oscillator strength (OS) as function of the magnetic field, the position of the magnetic ion, and the quantum dot confinement strength. With increasing magnetic field a ferromagnetic - antiferromagnetic transition is found that results in clear signatures in the CR absorption. It leads to discontinuities in the transition energies and the oscillator strengths and an increase of the number of allowed transitions.Comment: 11 pages, 14 figure

Many-body effects in the cyclotron resonance of a magnetic dot

Intraband cyclotron resonance (CR) transitions of a two-electron quantum dot containing a single magnetic ion is investigated for different Coulomb interaction strengths and different positions of the magnetic ion. In contrast to the usual parabolic quantum dots where CR is independent of the number of electrons, we found here that due to the presence of the magnetic ion Kohn's theorem no longer holds and CR is different for systems with different number of electrons and different effective electron-electron Coulomb interaction strength. Many-body effects result in \emph{shifts in the transition energies} and \emph{change the number of CR lines}. The position of the magnetic ion inside the quantum dot affects the structure of the CR spectrum by changing the position and the number of crossings and anti-crossings in the transition energies and oscillator strengths.Comment: 10 pages, 10 figures. Phys. Rev. B (in press

Impurity effects on semiconductor quantum bits in coupled quantum dots

We theoretically consider the effects of having unintentional charged impurities in laterally coupled two-dimensional double (GaAs) quantum dot systems, where each dot contains one or two electrons and a single charged impurity in the presence of an external magnetic field. Using molecular orbital and configuration interaction method, we calculate the effect of the impurity on the 2-electron energy spectrum of each individual dot as well as on the spectrum of the coupled-double-dot 2-electron system. We find that the singlet-triplet exchange splitting between the two lowest energy states, both for the individual dots and the coupled dot system, depends sensitively on the location of the impurity and its coupling strength (i.e. the effective charge). A strong electron-impurity coupling breaks down equality of the two doubly-occupied singlets in the left and the right dot leading to a mixing between different spin singlets. As a result, the maximally entangled qubit states are no longer fully obtained in zero magnetic field case. Moreover, a repulsive impurity results in a triplet-singlet transition as the impurity effective charge increases or/and the impurity position changes. We comment on the impurity effect in spin qubit operations in the double dot system based on our numerical results.Comment: published version on Physical Review B journal, 25 pages, 26 figure

Multiplexed, affordable, and portable platform for real time quantification of counterfeit and substandard medicines

The World Health Organization estimates that about 10-30% of pharmaceuticals in the world are either substandard or counterfeit. The number is even higher in the developing countries. From a public health perspective, a key contributor to the development and proliferation drug resistant strains of infections, including tuberculosis (TB), malaria and other infections that are leading killers in resource limited settings is poor quality medicines. Most of the main causes are profit driven corruption in many pharmaceutical companies, the poor manufacture and quality control, and/or the inappropriate storage conditions. Poor quality drugs lead to loss of life, create morbidity, strain the financial structure of the health system and lead to long-term drug resistance that affects us all. The current technology for screening poor quality drugs can be divided into 2 categories: the high end, precise and high cost technologies (such as High Performance Liquid Chromatography) and lower cost and qualitative technologies (such as Thin-Layered Chromatography). The high-end methods can give a precise measurement of active pharmaceutical ingredient (API) concentration and the presence of impurities in the tablets, but require trained personnel, advanced machine and lab set up, not suitable for field testing where most of poor quality pharmaceuticals have been found. The lower cost techniques require little training and simple equipment to operate at a relatively inexpensive price, but only gives qualitative results. In addition, most of current methods do not look at the dissolution profile of the tablets simultaneously with the concentration of API. Therefore, we propose to develop an assay that can quantify the concentrations of multiple APIs simultaneously and measure dissolution rates. In order to address current gaps in knowledge, my research proposal has three main parts in the assay development: 1) Development of an fluorescent/luminescent assay for detection of counterfeit/substandard antimalarial using small-molecules-based methods and field testing in Ghana; 2) Development of a fluorescent assay for detection of water-soluble pharmaceuticals using SELEX; and 3) Design a detection platform using microfluidic chips for real time quantification of multiple active pharmaceutical ingredients. For proof-of-concept, an antimalarial drug (artesunate and amodiaquine) and antibacterial antibiotics (sulfamethoxazole and trimethoprim) are selected to demonstrate the probe development and test the chip performance. Overall, the assay will be rapid, robust, portable, inexpensive, multiplexed, quantitative, specific, and sensitive. At a big picture level, emphasizing drug quality and creating robust mechanisms of drug testing will improve health outcomes and enhance treatment efficacy in resource limited settings

Community-Based Tourism and Destination Attractiveness: From Theory to Practice

Community-based tourism (CBT) is an approach for tourism development that highlights the active participation of the community as a producer, owner, decision-maker in the tourism development of a destination. It has been considered as a sustainable tourism development orientation because it creates employment and increases income for local communities, making them more aware of the responsibility to preserve and enrich their local tourism resources. However, one of the crucial indicators of the sustainability of a CBT venture is commercial viability which is mostly influenced by destination attractiveness to the tourists. Firstly, this paper aims at identifying core attractive attributes from which CBT is frequently developed. A case-based study is carried out on thirty documented successful CBT practices in many parts of the world. From that, this paper points out five types of core attractive attribute of the destination on which CBT can be developed. Then, this paper will discuss how CBT can make a destination more attractive. Finally, the paper will suggest some actions for tourism policymakers of a CBT destination. Keywords: community-based tourism, destination attractiveness, sustainable tourism, core attractive attributes, community participation DOI: 10.7176/JTHS/49-04 Publication date:May 31st 202

Lifting the Curse of the SOX Through Employee Assessments of the Internal Control Environment

This is the published version