Gated combo nanodevice for sequential operations on single electron spin

An idea for a nanodevice in which an arbitrary sequence of three basic quantum single qubit gates - negation, Hadamard and phase shift - can be performed on a single electron spin. The spin state is manipulated using the spin-orbit coupling and the electron trajectory is controlled by the electron wave function self-focusing mechanism due to the electron interaction with the charge induced on metal gates. We present results of simulations based on iterative solution of the time dependent Schr\"odinger equation in which the subsequent operations on the electron spin can be followed and controlled. Description of the moving electron wave packet requires evaluation of the electric field within the entire nanodevice in each time step

Stark effect on the exciton spectra of vertically coupled quantum dots: horizontal field orientation and non-aligned dots

We study the effect of an electric-field on an electron-hole pair in an asymmetric system of vertically coupled self-assembled quantum dots taking into account their non-perfect alignment. We show that the non-perfect alignment does not qualitatively influence the exciton Stark effect for the electric field applied in the growth direction, but can be detected by application of a perpendicular electric field. We demonstrate that the direction of the shift between the axes of non-aligned dots can be detected by rotation of a weak electric field within the plane of confinement. Already for a nearly perfect alignment the two-lowest energy bright exciton states possess antilocked extrema as function of the orientation angle of the horizontal field which appear when the field is parallel to the direction of the shift between the dot centers

Accuracy of the Hartree-Fock method for Wigner molecules at high magnetic fields

Few-electron systems confined in two-dimensional parabolic quantum dots at high magnetic fields are studied by the Hartree-Fock (HF) and exact diagonalization methods. A generalized multicenter Gaussian basis is proposed in the HF method. A comparison of the HF and exact results allows us to discuss the relevance of the symmetry of the charge density distribution for the accuracy of the HF method. It is shown that the energy estimates obtained with the broken-symmetry HF wave functions become exact in the infinite magnetic-field limit. In this limit the charge density of the broken-symmetry solution can be identified with the classical charge distribution.Comment: to appear in EPJ

Very high energy gamma-ray follow-up observations of novae and dwarf novae with the MAGIC telescopes

In the last few years the Fermi-LAT instrument has detected GeV gamma-ray emission from several novae. Such GeV emission can be interpreted in terms of inverse Compton emission from electrons accelerated in the shock or in terms of emission from hadrons accelerated in the same conditions. The latter might reach much higher energies and could produce a second component in the gamma-ray spectrum at TeV energies. We perform follow-up observations of selected novae and dwarf novae in search of the second component in TeV energy gamma rays. This can shed light on the acceleration process of leptons and hadrons in nova explosions. We have performed observations with the MAGIC telescopes of 3 sources, a symbiotic nova YY Her, a dwarf nova ASASSN-13ax and a classical nova V339 Del, shortly after their outbursts. We did not detect TeV gamma-ray emission from any of the objects observed. The TeV upper limits from MAGIC observations and the GeV detection by Fermi constrain the acceleration parameters for electrons and hadrons.Comment: Proceedings of the 34th International Cosmic Ray Conference, 30 July- 6 August, 2015, The Hague, The Netherlands. The content of this submission is similar to our paper in the Fermi Symposium of novae observations with MAGIC, which appeared as arXiv:1502.05853. arXiv admin note: substantial text overlap with arXiv:1502.0585

The paradoxes of management

Funding text: this work was funded by national funds through FCT-Fundação para a Ciência e Tecnologia under the project ref. UID/ECO/00124/2013 and by POR Lisboa ubder the project LISBOA-01-0145-FEDER-007722.This paper explores paradox, a way of seeing organizations that emphasizes interdependence, opposition and persistence, as both a source of novelty and absurdity. Paradoxes are processes with a dual potential for organizational harm or good, depending on the way they are framed and tackled. We explore this dual side of paradox and discuss some of its implications for the management of organizations.publishersversionpublishe

Very high energy gamma-ray emission from X-ray transients during major outbursts

Context: Some high mass X-ray binaries (HMXB) have been recently confirmed as gamma-ray sources by ground based Cherenkov telescopes. In this work, we discuss the gamma-ray emission from X-ray transient sources formed by a Be star and a highly magnetized neutron star. This kind of systems can produce variable hadronic gamma-ray emission through the mechanism proposed by Cheng and Ruderman, where a proton beam accelerated in the pulsar magnetosphere impacts the transient accretion disk. We choose as case of study the best known system of this class: A0535+26. Aims: We aim at making quantitative predictions about the very high-energy radiation generated in Be-X ray binary systems with strongly magnetized neutron stars. Methods: We study the gamma-ray emission generated during a major X-ray outburst of a HMXB adopting for the model the parameters of A0535+26. The emerging photon signal from the disk is determined by the grammage of the disk that modulates the optical depth. The electromagnetic cascades initiated by photons absorbed in the disk are explored, making use of the so-called "Approximation A" to solve the cascade equations. Very high energy photons induce Inverse Compton cascades in the photon field of the massive star. We implemented Monte Carlo simulations of these cascades, in order to estimate the characteristics of the resulting spectrum. Results: TeV emission should be detectable by Cherenkov telescopes during a major X-ray outburst of a binary formed by a Be star and a highly magnetized neutron star. The gamma-ray light curve is found to evolve in anti-correlation with the X-ray signal.Comment: 8 pages, 7 figures, accepted for publication in Astronomy and Astrophysical journa