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

DIFFICULTIES IN WRITING AN ESSAY OF ENGLISH-MAJORED SOPHOMORES AT TAY DO UNIVERSITY, IN VIETNAM

Writing is an important skill in English that helps people express thoughts, emotion and viewpoint to readers. However, students face some difficulties when writing. Hence, the survey research presents the process of the study about “Difficulties in writing an essay of English-majored sophomores at Tay Do University, in Viet Nam”. It was conducted to find out some difficulties in learning writing (from 200 to 250 - word essay) of 102 the sophomores from Bachelor of English 10 at Tay Do University. Questionnaire, paper interview and essay samples were the instruments of the study. The results showed that sophomores had many problems in writing such as vocabulary, grammar structures, ideas arrangement, background knowledge, and others. Basing on the results, some solutions would be suggested to help students to get a good writing skill. Viết là một kỹ năng quan trọng trong tiếng Anh giúp người viết thể hiện suy nghĩ, cảm xúc và quan điểm với người đọc. Tuy nhiên, sinh viên thường gặp một số khó khăn khi viết. Do đó, nghiên cứu “Khó khăn khi viết bài luận của sinh viên năm thứ hai chuyên ngành Ngôn Ngữ Anh tại Trường Đại học Tây Đô, Việt Nam” được thực hiện nhằm tìm ra một số khó khăn khi học môn viết (bài luận từ 200 đến 250 từ) của 102 sinh viên Cử nhân Tiếng Anh năm thứ hai, khóa 10 của Trường Đại học Tây Đô. Công cụ nghiên cứu gồm bảng câu hỏi, phỏng vấn trên giấy và phân tích bài luận. Kết quả cho thấy sinh viên năm thứ hai gặp nhiều vấn đề về viết như từ vựng, cấu trúc ngữ pháp, sắp xếp ý tưởng, kiến thức nền tảng và những vấn đề khác. Dựa trên kết quả đạt được, một số giải pháp sẽ được đề xuất để giúp sinh viên có được kỹ năng viết tốt hơn. Article visualizations

In-band pumped conical refraction Nd:KGW laser

We have demonstrated an in-band pumped conical refraction (CR) Nd: KGW laser. The CR laser was diode-pumped at 910 nm and produced an output power of 1.15 W at 1069 nm