Origin of electron spin-orbit anisotropy in pyramidal InAs quantum dots

We investigate the electron spin–orbit interaction anisotropy of pyramidal InAs quantum dots using a fully three-dimensional Hamiltonian. The dependence of the spin–orbit interaction strength on the orientation of externally applied in-plane magnetic fields is consistent with recent experiments, and it can be explained from the interplay between Rashba and Dresselhaus spin–orbit terms in dots with asymmetric confinement. Based on this, we propose manipulating the dot composition and height as efficient means for controlling the spin–orbit anisotropy.This work was supported by UJI-Bancaixa Project no. P1-1B2011-01, MINECO Project no. CTQ2011-27324, and FPU Grant (C. Segarra)

Valence band mixing of cubic GaN/AlN quantum dots

We study the spin purity of the hole ground state in nearly axially symmetric GaN/AlN quantum dots (QDs). To this end, we develop a six-band Burt-Foreman Hamiltonian describing the valence band structure of zinc-blende nanostructures with cylindrical symmetry, and calculate the effects of eccentricity variationally. We show that that the aspect ratio is a key factor for spin purity. In typical QDs with small aspect ratio the ground state is essentially a heavy hole (HH) whose spin purity is even higher than that of InGaAs QDs of similar size. When the aspect ratio increases, mixing with light-hole (LH) and split-off (SO) subbands becomes important and, additionally, the ground state becomes sensitive to QD anisotropy, which further enhances the mixing. We finally show that despite the large GaN hole effective mass, an efficient magnetic modulation is feasible in QDs with aspect ratio ∼ 1, which can be used to modify the ground state symmetry and hence the optical spectrum properties

Theory of electrons, holes and excitons in GaAs polytype quantum dots

Single and multi-band (Burt-Foreman) k.p Hamiltonians for GaAs crystal phase quantum dots are developed and used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization, valence band mixing and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorious Nano Lett. 15, 2652 (2015)] it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al. Adv. Mater. 27, 2195 (2015)], including non-heavy hole ground state, strong hole spin admixture and a type-II to type-I exciton transition, which can be used to improve the absorption strength and reduce the radiative lifetime of GaAs polytypes

UDWDM-PON using low-cost coherent transceivers with limited tunability and heuristic DWA

A new Passive Optical Network (PON) for access, making use of Ultra Dense Wavelength Division Multiplexing (UDWDM) by densely spacing channels at few GHz, and introducing the “wavelength-to-the-user” concept, is proposed. The key challenge will be developing low-cost coherent transceivers, providing an excellent selectivity while avoiding filters, and furnishing high sensitivity, which will allow high splitting ratios, large number of users and long distance reach. The Optical Distribution Network (ODN) at the outside plant is based on splitters and kept compatible with legacy systems. Optical Network Unit (ONU) designs realized with coherent transceivers using one or two lasers are presented and the corresponding Optical Line Terminal (OLT) architectures are introduced. The ONUs at customer premises own lasers with limited thermal tunability and their wavelengths are randomly distributed in a band. By using heuristic Dynamic Wavelength Assignment (DWA) schemes and extending the original working band, the required optical band is obtained and optimized. In activation processes, ONU acceptances up to 99.9% are achieved. Furthermore, in operation scenario under indoors and also under outdoors environmental conditions, ONU blocking probabilities below 0.1% and ONU availability ratios (OARs) up to 99.9% are demonstrated. The PON is dimensioned according to the number of deployed users and system reach; moreover, power safety and also fiber nonlinearities constraints are evaluated, illustrating the characteristics of the projected network. Finally, the coexistence with legacy networks is discussed.Peer ReviewedPostprint (author's final draft

Bagagem desfeita: a experiência da imigração por artistas congoleses

Two Congolese artists in São Paulo, Yannick Delass e Shambuyi Wetu encounter each other in performances that we record and recreate. Here, music and art are moments of empowerment, subtle shifts in the visible and auditory spaces of immigration policies.Dois artistas congoleses em São Paulo, Yannick Delass e Shambuyi Wetu, encontram-se em performances que registramos e recriamos. Aqui, música e arte são momentos de empoderamento, mudanças sutis na visibilidade e no espaço auditivo das políticas da imigração

Anisotropy of spin–orbit induced electron spin relaxation in [001] and [111] grown GaAs quantum dots

We report a systematic study of the spin relaxation anisotropy between single electron Zeeman sublevels in three-dimensional cuboidal GaAs quantum dots (QDs). The QDs are subject to an in-plane magnetic field. As the field orientation varies, the relaxation rate oscillates periodically, showing 'magic' angles where the relaxation rate is suppressed by several orders of magnitude. This behavior is found in QDs with different shapes, heights, crystallographic orientations and external fields. The origin of these angles can be traced back to the symmetries of the spin admixing terms of the Hamiltonian. Our results evidence that cubic Dresselhaus terms play an important role in determining the spin relaxation anisotropy, which can induce deviations of the 'magic' angles from the crystallographic directions reported in recent experiments (P Scarlino et al 2014 Phys. Rev. Lett. 113 256802).This work was supported by UJI-Bancaixa Project No. P1-1B2011-01, MINECO Project No. CTQ2011-27324, and FPU Grant (C S)

Hole spin relaxation in InAS/GaAs quantum dot molecules

We calculate the spin–orbit induced hole spin relaxation between Zeeman sublevels of vertically stacked InAs quantum dots. The widely used Luttinger–Kohn Hamiltonian, which considers coupling of heavy- and light-holes, reveals that hole spin lifetimes (T 1) of molecular states significantly exceed those of single quantum dot states. However, this effect can be overcome when cubic Dresselhaus spin–orbit interaction is strong. Misalignment of the dots along the stacking direction is also found to be an important source of spin relaxation

Influence of Polytypism on the Electronic Structure of CdSe/CdS and CdSe/CdSe Core/Shell Nanocrystals

We address theoretically differences and similarities on the electronic structure of CdSe/CdS dot-in-dot nanocrystals (NCs) with wurtzite/wurtzite (WZ/WZ), zinc-blende/zinc-blende (ZB/ZB) and polytype ZB/WZ crystalline phases, as they are currently being synthesized and used in optoelectronic devices. We show that the electronic structure of polytypic CdSe/CdS NCs closely resembles that of WZ or ZB NCs with regard to quantum confinement and strain, resulting in similar single-exciton wave functions. The main differences arise in the nature and magnitude of built-in electric fields. We predict that these fields are stronger in polytypes than in pure WZ or ZB NCs due to the sharp spontaneous polarization mismatch between the cubic core and the hexagonal shell lattices. Polarization in NCs is currently believed to be screened by several surface effects. In polytypical structures, however, the polarization mismatch at the interface may create effective charges that are sufficiently far from the outer surface to be quenched. To make a definitive assessment on this controversial issue, we propose experiments in polytypic ZB/WZ NCs where both core and shell are made of CdSe. In such a case, band offsets are small, strain is absent, and our calculations predict pyroelectricity should become the driving force, inducing transitions from type-I to type-II excitons with increasing core or shell size.We thank I. Moreels and P. Guyot-Sionnest for useful discussions. Support from MINECO project CTQ2014-60178-P, UJI project P1-1B2014-24 is acknowledged

STUDENTS’ PERCEPTION ON PLAGIARISM IN ACADEMIC WRITING AT ENGLISH LEARNING EDUCATION DEPARTMENT (ELED) UNIVERSITY OF MUHAMMADIYAH MALANG

Plagiarism is a bad action that can bring a big impact for the students’ education and habits. However, we cannot say that all of the students have the same perception of plagiarism. Plagiarism appeared with some factors whether they are intentional or unintentional. Therefore, in order to determine what those factors are, we need to do research about it. In this research, the researcher attempted a). to find the students’ factors of plagiarism Academic Writing and b). to find out the students’ perception of plagiarism when they are dealing with plagiarism. The researcher applied the mixed method approach for this research, and questionnaire as the instrument. The sample of this research was the fifth-semester students who are taking the Intermediate Writing course. The result reveals that a). there are seven factors of plagiarism; Internet and technology, regulation, academic skill, teaching factors, pride, pressure, and other reasons Moreover, b). The students’ perception of plagiarism shows that the students do not agree with the act of plagiarism and the students are against plagiarism

Piezoelectric Control of the Exciton Wave Function in Colloidal CdSe/CdS Nanocrystals

Using multiband k·p calculations, we show that strain-engineered piezoelectricity is a powerful tool to modulate the electron−hole spatial separation in a wide class of wurtzite CdSe/CdS nanocrystals. The inherent anisotropy of the hexagonal crystal structure leads to anisotropic strain and, consequently, to a pronounced piezoelectric field along the c axis, which can be amplified or quenched through a proper design of the core−shell structure. The use of large cores and thick shells promotes a gradual departure from quantum confined nanocrystals to a regime dominated by piezoelectric confinement. This allows excitons to evolve from the usual type-I and quasi-type-II behavior to a type-II behavior in dot-in-dots, dot-in-rods, rod-in-rods, and dot-in-plates. Piezoelectric fields explain experimental observations for giant-shell nanocrystals, whose time-resolved photoluminescence reveals long exciton lifetimes for large cores, contrary to the expectations of standard quantum confinement models. They also explain the large differences in exciton lifetimes reported for different classes of CdSe/CdS nanocrystals.Support from MINECO project CTQ2014-60178-P, UJI project P1-1B2014-24 and a FPU grant (C.S.) is acknowledged. The present publication is further realized with the support of the Ministero degli Affari Esteri e della Cooperazione Internazionale (IONX-NC4SOL, I.M.)