The European Physical Journal D最新文献

A new method for determining the particle flux density in the beam of a neutral beam injector (NBI) for magnetically confined fusion plasmas is presented. The method uses a force probe to measure the momentum transfer from beam particles to a small target and has been applied at the BATMAN experimental facility at the MPI Garching. The experiments presented in this report are performed in the non-neutralized negative ion beam. It is found that the force measured at a distance of 1.4 m from the negative ion source correlates well with the momentum flux that is calculated from the ion energy and the averaged extracted ion current density.

Schematic of the force-probe diagnostic for momentum-flux measurements in the negative hydrogenion beam at the BATMAN facility. A water-cooled shield with a 5 mm aperture limits the beam spot on an L-shaped tungsten target mounted on an interferometric cantilever probe, enabling charge-independent local flux determination 1.4m downstream of the source

With the quantum state diffusion measurement theory (QSD), the measurement problem in liquid nuclear magnetic resonance (NMR) quantum computers was addressed and then it was shown that due to stochastical fluctuations, the measured magnetic moment value is comparable to noise for long times. Therefore, we suggested that the measurement time should be short to distinguish signal from noise.

QSD in NMR quantum computers.

Variational expressions for functionals yielding the second-order and fourth-order corrections to the nondegenerate eigenenergies of a Hamiltonian subject to a given perturbation have been derived. Discrete basis sets consisting of the suitably chosen eigenfunctions of the unperturbed Hamiltonian along with a nonlinear variational parameter can be used to evaluate the stationary functional quite accurately. The method has been applied conveniently to calculate the dipole hyperpolarizabilities of the hydrogenic atoms interacting with screened Coulomb potentials (SCP) quite accurately. Three SCPs are considered, namely the static screened Coulomb potential, the exponential cosine screened Coulomb potential and the generalized exponential cosine screened Coulomb potential. It is found that a basis set consisting of 20 eigenfunctions of the p-states of the hydrogenic atoms yields stationary functional corresponding to the second-order correction to the energy, whereas a basis set consisting of 20 d-state eigenfunctions and 19 s-state eigenfunctions of the hydrogenic atoms produces stationary functional corresponding to the fourth-order correction to the energy. Moreover, the stationary values of the functionals are highly accurate (up to 14th place of decimal) and converge rapidly with the increase in the number of terms in the basis sets.

When the space surrounding an atom or molecule is restricted, its electron density is modified compared to its unrestricted counterpart, which further alters its physical and chemical properties. To model the effect of space restriction, or confinement, on atoms and molecules, often the hard-boundary condition is employed to solve the corresponding Schrödinger equation. In the present work, we analyze the modifications in a chemical bond when the space surrounding the molecule is restricted such that some part of the space is forbidden to the electrons. To this end, we choose the simplest hydrogen molecular ion H(_{2}^{+}), in the presence of an external hard-sphere simulating the restriction of space available to the electrons due to Pauli exclusion by an atom close to the molecule. The hard-sphere is moved along the molecular axis and the axis transverse to it to study the changes in the ground-state energy, stretching frequency, kinetic, and potential energies of an H(_{2}^{+}) molecule. For this purpose, we employ a variational approach using a suitably constructed energy estimator that goes beyond the Born–Oppenheimer approximation. We find that as the sphere moves closer to the molecule along the molecular axis, the ground-state energy and the vibrational frequencies get enhanced. On the other hand, for the same case, the potential energy decreases, and the kinetic energy increases. When the sphere moves along the axis transverse to the molecular axis, the changes in the energy and vibrational frequencies show similar trends; however, the magnitude of the changes is significantly smaller than in the corresponding molecular axis case. For the transverse case, both potential and kinetic energies get enhanced marginally. These results demonstrate that the axial approach of the hard surface is more efficient in inducing a bond breaking of the H(_{2}^{+}) molecular ion. The shift of the electron bond by the external sphere explains these results.

In this work, we consider simple systems that are governed by Hamiltonians with time periodicity. Our analysis is mainly focused on the density matrix approach and aims to solve the von Neumann equation of motion from which one can extract the state of the system when the system is in a pure state. We start our analysis with the standard Rabi-oscillation problem. We consider a density matrix corresponding to the entire model system and solve the von Neumann equation of motion. We have then made use of the Lewis-Reisenfeld invariant approach and arrived at the exact same result, which implies that the density matrix of the system can indeed be identified with the Lewis invariant. Then, we consider a two-level system with a constant magnetic field in the z-direction and a time-dependent magnetic field in the x-direction. We solve the von Neumann equation of motion for this system and calculate the various coherence measures, and plot them to investigate the time dependence and reliability of different coherence measures.

Correspondence between the density operator and the Lewis invariant operator of the system

The elastic scattering and resonant charge transfer differential and integral cross sections in (textrm{H}(1s) + mathrm{H^+}) collisions are computed for the center-of-mass energy range of (10^{-10}-10) eV. Fully quantal and semiclassical approaches are utilized in these calculations. The reliability of the semiclassical approximation for very low collision energies is discussed. The results are compared with available data from the literature.