Extended interface states enhance valley splitting in Si/SiO2

Interface disorder and its effect on the valley degeneracy of the conduction band edge remains among the greatest theoretical challenges for understanding the operation of spin qubits in silicon. Here, we investigate a counterintuitive effect occurring at Si/SiO2 interfaces. By applying tight binding methods, we show that intrinsic interface states can hybridize with conventional valley states, leading to a large ground state energy gap. The effects of hybridization have not previously been explored in details for valley splitting. We find that valley splitting is enhanced in the presence of disordered chemical bonds, in agreement with recent experiments.Comment: 4 pages, 4 figure

Donors in Ge as Qubits: Establishing Physical Attributes

Quantum electronic devices at the single impurity level demand an understanding of the physical attributes of dopants at an unprecedented accuracy. Germanium-based technologies have been developed recently, creating a necessity to adapt the latest theoretical tools to the unique electronic structure of this material. We investigate basic properties of donors in Ge which are not known experimentally, but are indispensable for qubit implementations. Our approach provides a description of the wavefunction at multiscale, associating microscopic information from Density Functional Theory and envelope functions from state of the art multivalley effective mass calculations, including a central cell correction designed to reproduce the energetics of all group V donor species (P, As, Sb and Bi). With this formalism, we predict the binding energies of negatively ionized donors (D- state). Furthermore, we investigate the signatures of buried donors to be expected from Scanning Tunneling Microscopy (STM). The naive assumption that attributes of donor electrons in other semiconductors may be extrapolated to Ge is shown to fail, similar to earlier attempts to recreate in Si qubits designed for GaAs. Our results suggest that the mature techniques available for qubit realizations may be adapted to germanium to some extent, but the peculiarities of the Ge band structure will demand new ideas for fabrication and control

Theory of one and two donors in Silicon

We provide here a roadmap for modeling silicon nano-devices with one or two group V donors (D). We discuss systems containing one or two electrons, that is, D^0, D^-, D_2^+ and D_2^0 centers. The impact of different levels of approximation is discussed. The most accurate instances -- for which we provide quantitative results -- are within multivalley effective mass including the central cell correction and a configuration interaction account of the electron-electron correlations. We also derive insightful, yet less accurate, analytical approximations and discuss their validity and limitations -- in particular, for a donor pair, we discuss the single orbital LCAO method, the Huckel approximation and the Hubbard model. Finally we discuss the connection between these results and recent experiments on few dopant devices.Comment: 13 pages, 6 figure

Impact of the valley degree of freedom on the control of donor electrons near a Si/SiO_2 interface

We analyze the valley composition of one electron bound to a shallow donor close to a Si/barrier interface as a function of an applied electric field. A full six-valley effective mass model Hamiltonian is adopted. For low fields, the electron ground state is essentially confined at the donor. At high fields the ground state is such that the electron is drawn to the interface, leaving the donor practically ionized. Valley splitting at the interface occurs due to the valley-orbit coupling, V_vo^I = |V_vo^I| e^{i theta}. At intermediate electric fields, close to a characteristic shuttling field, the electron states may constitute hybridized states with valley compositions different from the donor and the interface ground states. The full spectrum of energy levels shows crossings and anti-crossings as the field varies. The degree of level repulsion, thus the width of the anti-crossing gap, depends on the relative valley compositions, which vary with |V_vo^I|, theta and the interface-donor distance. We focus on the valley configurations of the states involved in the donor-interface tunneling process, given by the anti-crossing of the three lowest eigenstates. A sequence of two anti-crossings takes place and the complex phase theta affects the symmetries of the eigenstates and level anti-crossing gaps. We discuss the implications of our results on the practical manipulation of donor electrons in Si nanostructures.Comment: 8 pages, including 5 figures. v2: Minor clarifying changes in the text and figures. Change of title. As published in PR

The UNOR 40 plan (1971-1972) by Hestnes Ferreira: As a more structured expansion proposal for a planning unit in Lisbon

The aim of this paper is to present the work of Hestnes Ferreira and his team, namely for the UNOR 40 planning unit in Lisbon, as a study case of an infrastructural enhancement in Mainland Portugal during the early 1970s. The UNOR design teams were recruited outside the municipal staff. For UNOR40 the team was coordinated by Raúl Hestnes Ferreira and included architects Rodrigo Rau and Vicente Bravo, landscape architect Gonçalo Ribeiro Teles, and urban geographer, Jorge Gaspar. These oversaw the planning of a large area between Campo Grande and Benfica, using a traffic study developed by French consultants. The main results of the UNOR 40 Plan were to redefine the layout of the North-South Hub, Combatentes and Lusíada Avenues, as a way of ordering the urban network of this sector, including the urban access to Telheiras. The plan also comprised the creation of an institutional square, based on a program that included museums, institutes, office buildings, and a church. However, the applicability of the UNOR 40 Plan was practically nil, with the exception of the layout of some road links.info:eu-repo/semantics/publishedVersio

Memory of space and place: tourism as factor of life or death of cities

The memory of space and place are reflected in our experience. Tourism is currently an uncontrollable mobility factor - whether human, identities, cultures, meanings, information, finance and objects - that cause changes and changes in space and place. Gaston Bachelard (1964), Edward Casey (2000) as philosophers and phenomenologist have also traced this powerful connection between memory and place. The work of Jane Jacobs (2001) is essential in this reading. Yi-Fu Tuan (2001) had a different approach and suggests that memorable architecture should strengthen our memories, enhance the self, and provide layers of meaning to a space. The purpose of this article is to understanding the spatial relationships and tensions between them, and how tourism can be a factor of life or death in the cities. How dynamics allows us to gain important insights into the processes that shape the spaces and places, without losing the original identity. This article will treat conceptual aspects that relate to the theme, bringing to this context the concepts of space and place.info:eu-repo/semantics/publishedVersio

Aggregated functional data model for Near-Infrared Spectroscopy calibration and prediction

Calibration and prediction for NIR spectroscopy data are performed based on a functional interpretation of the Beer-Lambert formula. Considering that, for each chemical sample, the resulting spectrum is a continuous curve obtained as the summation of overlapped absorption spectra from each analyte plus a Gaussian error, we assume that each individual spectrum can be expanded as a linear combination of B-splines basis. Calibration is then performed using two procedures for estimating the individual analytes curves: basis smoothing and smoothing splines. Prediction is done by minimizing the square error of prediction. To assess the variance of the predicted values, we use a leave-one-out jackknife technique. Departures from the standard error models are discussed through a simulation study, in particular, how correlated errors impact on the calibration step and consequently on the analytes' concentration prediction. Finally, the performance of our methodology is demonstrated through the analysis of two publicly available datasets.Comment: 27 pages, 7 figures, 7 table