5,403 research outputs found
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
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
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
Watermelon stomach seen by wireless‐capsule endoscopy
Endoscopy. 2003 Jan;35(1):100.
Watermelon stomach seen by wireless-capsule endoscopy.
Mascarenhas-Saraiva M, Lopes L, Mascarenhas-Saraiva A.
SourceDigestive Endoscopy and Motility Unit, Trindade Hospital, Rua Trinidade 115, 4000-541 Porto, Portugal. [email protected]
PMID:12510242[PubMed - indexed for MEDLINE
Quasi-one-dimensional system as a high-temperature superconductor
It is well-known that quasi-one-dimensional superconductors suffer from the
pairing fluctuations that significantly reduce the superconducting temperature
or even completely suppress any coherent behavior. Here we demonstrate that a
coupling to a robust pair condensate changes the situation dramatically. In
this case the quasi-one-dimensional system can be a high temperature
superconductor governed by the proximity to the Lifshitz transition at which
the Fermi level approaches the lower edge of the single-particle spectrum.Comment: 5 pages, 1 figur
PUDENDAL NERVE BLOCK AND OBSTETRIC SIMULATION
A primeira descrição do bloqueio do nervo pudendo foi em 1908. Esta é uma técnica indicada para analgesia nas fases mais adiantadas do trabalho de parto. O uso de técnicas analgésicas eficazes desde os estadios mais precoces do trabalho de parto, como as técnicas do neuro-eixo, deixaram o bloqueio do
pudendo para segundo plano. No entanto, é de fácil execução, habitualmente pelo obstetra, e com baixo risco de hemorragia ou infeção.
O bloqueio do pudendo é um procedimento com poucas oportunidades de treino na prática clínica, pelo que o retomar da prática na sua execução através do uso de simuladores obstétricos torna-se pertinente. Ainda, a sua utilização em cenários simulados permite a familiarização de equipas multidisciplinares
na sua aplicação em diversos contextos, emergentes ou não emergentes.
Os principais objetivos deste trabalho são (1) efetuar uma revisão sobre o bloqueio do nervo pudendo na analgesia de parto, focando as principais vantagens e limitações, (2) repensar a sua utilização na ausência ou contraindicação de outras técnicas analgésicas, (3) reavivar a técnica de execução através da simulação aplicada à Obstetrícia
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