5,556 research outputs found
Comment on ``Evidence for Narrow Baryon Resonances in Inelastic pp Scattering''
Compton scattering data are sensitive to the existence of low-mass resonances
reported by Tatischeff et al. We show that such states, with their reported
properties, are excluded by previous Compton scattering experiments.Comment: One page, submitted to PR
Compton Scattering from the Deuteron and Extracted Neutron Polarizabilities
Differential cross sections for Compton scattering from the deuteron were
measured at MAX-lab for incident photon energies of 55 MeV and 66 MeV at
nominal laboratory angles of , , and . Tagged
photons were scattered from liquid deuterium and detected in three NaI
spectrometers. By comparing the data with theoretical calculations in the
framework of a one-boson-exchange potential model, the sum and difference of
the isospin-averaged nucleon polarizabilities, and (in units of fm),
have been determined. By combining the latter with the global-averaged value
for and using the predictions of the Baldin sum rule for
the sum of the nucleon polarizabilities, we have obtained values for the
neutron electric and magnetic polarizabilities of (total) (model) and (total) (model), respectively.Comment: 4 pages, 2 figures, revtex. The text is substantially revised. The
cross sections are slightly different due to improvements in the analysi
Quasi-free Compton Scattering from the Deuteron and Nucleon Polarizabilities
Cross sections for quasi-free Compton scattering from the deuteron were
measured for incident energies of 236--260 MeV at the laboratory angle -135
degrees. The recoil nucleons were detected in a liquid-scintillator array
situated at 20 degrees. The measured differential cross sections were used,
with the calculations of Levchuk et al., to determine the polarizabilities of
the bound nucleons. For the bound proton, the extracted values were consistent
with the accepted value for the free proton. Combining our results for the
bound neutron with those from Rose et al., we obtain one-sigma constraints of
alpha_n = 7.6-14.0 and beta_n = 1.2-7.6.Comment: 4 pages, 3 figures, accepted in PR
АНАЛИЗ ЭЛЕКТРОННОЙ СТРУКТУРЫ ЭЛЕКТРИЧЕСКИ ИНДУЦИРОВАННОЙ КВАНТОВОЙ ТОЧКИ ВО ВНЕШНЕМ МАГНИТНОМ ПОЛЕ
Numerical modeling of the electronic structure of a quantum dot, induced by an electric field of a nanosized disc-shaped gate, is carried out in the presence of external magnetic field. The dependences of an electronic energy spectrum on electric and magnetic fields are calculated using the finite element method. It has been found that a series of anti-crossing points for electronic levels takes place at relatively small magnetic fields. The existence of groups of close-energy levels (electronic shells) has been found. It has been shown that despite the essential distinction of the gate potential from the parabolic one, a model of a near-surface anisotropic harmonic oscillator can be effectively used for a qualitative description of the electronic structure of the electrically induced quantum dot. With the use of this model, the evolution of energy spectrum and wave function structure with magnetic and electric fields is described. In particular, the anisotropic oscillator model allows to predict anti-crossing points of electronic levels in external fields, as well as quasi-degeneracy of states having different values of the angular momentum projection. Проведено численное моделирование электронной структуры квантовой точки, индуцированной электрическим полем наноразмерного дискообразного затвора и находящейся во внешнем магнитном поле. С помощью метода конечных элементов рассчитаны зависимости энергетического спектра электрона от величины магнитного поля и потенциала на затворе. Обнаружено наличие последовательности точек квазипересечения электронных уровней при относительно слабых магнитных полях, а также существование групп близких уровней энергии (электронных оболочек). Показано, что, несмотря на существенное отличие потенциала затвора от параболического потенциала, для качественного описания электронной структуры электрически индуцированной квантовой точки возможно использование модели приповерхностного анизотропного гармонического осциллятора. На основании этой модели описаны закономерности эволюции структуры волновых функций при изменении потенциала затвора и магнитного поля. В частности, модель анизотропного осциллятора позволяет предсказать появление точек квазипересечения электронных уровней при изменении внешних полей, а также квазивырождение состояний с различными значениями проекции орбитального момента импульса.
Low-Energy Compton Scattering of Polarized Photons on Polarized Nucleons
The general structure of the cross section of scattering with
polarized photon and/or nucleon in initial and/or final state is systematically
described and exposed through invariant amplitudes. A low-energy expansion of
the cross section up to and including terms of order is given which
involves ten structure parameters of the nucleon (dipole, quadrupole,
dispersion, and spin polarizabilities). Their physical meaning is discussed in
detail. Using fixed-t dispersion relations, predictions for these parameters
are obtained and compared with results of chiral perturbation theory. It is
emphasized that Compton scattering experiments at large angles can fix the most
uncertain of these structure parameters. Predictions for the cross section and
double-polarization asymmetries are given and the convergence of the expansion
is investigated. The feasibility of the experimental determination of some of
the struture parameters is discussed.Comment: 41 pages of text, 9 figures; minor revisions prior to publication in
Phys. Rev.
ВЛИЯНИЕ МАГНИТНОГО ПОЛЯ НА ЛОКАЛИЗАЦИЮ ВОЛНОВОЙ ФУНКЦИИ ЭЛЕКТРОНА В СИСТЕМЕ НАНОЗАТВОР–ДОНОР
Numerical modeling of magnetic field effect on the states of shallow donor, affected by external electric field of a disc-shaped gate, is carried out. Bound electron energies and wave functions are calculated using the finite element method. Characteristics of relocation of the ground state wave function from the donor to the gate are obtained as a function of magnetic field. Проведено численное моделирование влияния магнитного поля на состояния мелкого донора, находящегося во внешнем электрическом поле дискообразного затвора. Для расчета энергий и волновых функций связанного электрона использовался метод конечных элементов. Получены характеристики, определяющие изменение локализации волновой функции основного состояния электрона в рассматриваемой системе, в зависимости от величины магнитного поля.
P.505 The potential role of BDNF and prolactin genotypes on antidepressant response in depressive patients
Dirac-Foldy term and the electromagnetic polarizability of the neutron
We reconsider the Dirac-Foldy contribution to the neutron electric
polarizability. Using a Dirac equation approach to neutron-nucleus scattering,
we review the definitions of Compton continuum (), classical
static (), and Schr\"{o}dinger () polarizabilities
and discuss in some detail their relationship. The latter is the
value of the neutron electric polarizability as obtained from an analysis using
the Schr\"{o}dinger equation. We find in particular , where is the magnitude of the magnetic moment
of a neutron of mass . However, we argue that the static polarizability
is correctly defined in the rest frame of the particle, leading to
the conclusion that twice the Dirac-Foldy contribution should be added to
to obtain the static polarizability .Comment: 11 pages, RevTeX, to appear in Physical Review
ИСПОЛЬЗОВАНИЕ МЕТОДА КОНЕЧНЫХ РАЗНОСТЕЙ ДЛЯ РАСЧЕТА ЭЛЕКТРОННЫХ СОСТОЯНИЙ В МДП-СТРУКТУРЕ С ОДИНОЧНЫМ ДОНОРОМ
Numerical modeling of electronic state evolution due to non-uniform external electric field in the structure metal-insulator-semiconductor with solitary donor center is carried out. Considering a nanometer disc-shaped gate as a source of the electric field, the problem for the Laplace equation in multilayered medium is solved numerically to determine the distribution of the gate potential. The energy spectrum of a bound electron is calculated from the problem for the stationary Schrödinger equation. Finite difference schemes are constructed to solve both the problems. Difference scheme for the Schrödinger equation takes into account cusp condition for the wave function at the donor location. To solve the problem for the Laplace equation, asymptotic boundary conditions for approximating the external field potential at large distances from the gate in different layers are suggested. These conditions allow to reduce the calculation domain for the electrostatic problem essentially. The effect of the boundary conditions on the accuracy of calculating the potential and energies is investigated. Using the developed difference schemes, the dependences of the energy spectrum of the bound electron on the gate potential are calculated, and the values of critical potential at which the wave function of the electron is relocated are determined. It has been found on the basis of calculation results, that governing parameter for the description of electronic behavior is the potential difference between the donor and semiconductor surface. It has been shown that critical potential difference does not depend on dielectric thickness and permittivity.Проводится численное моделирование эволюции электронных состояний под действием неоднородного внешнего электрического поля в структуре металл-диэлектрик-полупроводник (МДП-структуре) при наличии одиночного донорного центра. В качестве источника электрического поля рассматривается нанометровый дискообразный затвор, для расчета потенциала которого решается задача для уравнения Лапласа в многослойной среде. Энергетический спектр связанного электрона находится из задачи для стационарного уравнения Шредингера. Для решения обеих задач строятся разностные схемы. Разностная схема для уравнения Шредингера учитывает неаналитичность волновой функции в точке расположения донора. При решении задачи для уравнения Лапласа предлагаются асимптотические граничные условия, аппроксимирующие потенциал внешнего поля на больших расстояниях от затвора в различных слоях, что позволяет существенно уменьшить расчетную область. Исследуется влияние этих граничных условий на точность вычисления потенциала и энергий. С помощью построенных разностных схем рассчитываются зависимости энергетического спектра связанного электрона от потенциала на затворе, определяются значения критического потенциала, при котором происходит передислокация волновой функции электрона. На основании результатов вычислений устанавливается, что управляющим параметром для описания поведения электрона является разность потенциалов между донором и поверхностью полупроводника. Показывается, что критическая разность потенциалов практически не зависит от толщины и проницаемости диэлектрического слоя.
Nucleon Polarizabilities from Deuteron Compton Scattering within a Green's-Function Hybrid Approach
We examine elastic Compton scattering from the deuteron for photon energies
ranging from zero to 100 MeV, using state-of-the-art deuteron wave functions
and NN-potentials. Nucleon-nucleon rescattering between emission and absorption
of the two photons is treated by Green's functions in order to ensure gauge
invariance and the correct Thomson limit. With this Green's-function hybrid
approach, we fulfill the low-energy theorem of deuteron Compton scattering and
there is no significant dependence on the deuteron wave function used.
Concerning the nucleon structure, we use Chiral Effective Field Theory with
explicit \Delta(1232) degrees of freedom within the Small Scale Expansion up to
leading-one-loop order. Agreement with available data is good at all energies.
Our 2-parameter fit to all elastic data leads to values for the
static isoscalar dipole polarizabilities which are in excellent agreement with
the isoscalar Baldin sum rule. Taking this value as additional input, we find
\alpha_E^s= (11.3+-0.7(stat)+-0.6(Baldin)) x 10^{-4} fm^3 and \beta_M^s =
(3.2-+0.7(stat)+-0.6(Baldin)) x 10^{-4} fm^3 and conclude by comparison to the
proton numbers that neutron and proton polarizabilities are essentially the
same.Comment: 47 pages LaTeX2e with 20 figures in 59 .eps files, using graphicx.
Minor modifications; extended discussion of theoretical uncertainties of
polarisabilities extraction. Version accepted for publication in EPJ
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