Periodic Electrostatic Potential Research Articles

Anomalous Ion Diffusion and Radial-Electric-Field Generation in a Turbulent Edge Plasma Potential Weakly Correlated in Time and Space

Using a simple model of tokamak-edge-plasma turbulence in the form of a spatially periodic and time-independent electrostatic potential, we have found recently a new type of anomalous impurity diffusion in this regime. In the present contribution, we estimate this diffusion in the test-particle approach for a real turbulent potential obtained in an experiment. A significant difference in the impurity dynamics between the Hamiltonian and drift approaches is observed. As an interesting consequence of these dynamics, the possibility of radial-electric-field generation in the turbulence regimes appears. This effect can be of importance in scenarios related to transport barriers.

A fast multipole method combined with a reaction field for long-range electrostatics in molecular dynamics simulations: The effects of truncation on the properties of water

We present a combination of the structure adapted multipole method with a reaction field (RF) correction for the efficient evaluation of electrostatic interactions in molecular dynamics simulations under periodic boundary conditions. The algorithm switches from an explicit electrostatics evaluation to a continuum description at the maximal distance that is consistent with the minimum image convention, and, thus, avoids the use of a periodic electrostatic potential. A physically motivated switching function enables charge clusters interacting with a given charge to smoothly move into the solvent continuum by passing through the spherical dielectric boundary surrounding this charge. This transition is complete as soon as the cluster has reached the so-called truncation radius Rc. The algorithm is used to examine the dependence of thermodynamic properties and correlation functions on Rc in the three point transferable intermolecular potential water model. Our test simulations on pure liquid water used either the RF correction or a straight cutoff and values of Rc ranging from 14 Å to 40 Å. In the RF setting, the thermodynamic properties and the correlation functions show convergence for Rc increasing towards 40 Å. In the straight cutoff case no such convergence is found. Here, in particular, the dipole–dipole correlation functions become completely artificial. The RF description of the long-range electrostatics is verified by comparison with the results of a particle-mesh Ewald simulation at identical conditions.

Density modulation and electrostatic self-consistency in a two-dimensional electron gas subject to a periodic quantizing magnetic field

We calculate the single-particle states of a two-dimensional electron gas (2DEG) in a perpendicular quantizing magnetic field, which is periodic in one direction of the electron layer. We discuss the modulation of the electron density in this system and compare it with that of a 2DEG in a periodic electrostatic potential. We take account of the induced potential within the Hartree approximation, and calculate self-consistently the density fluctuations and effective energy bands. The electrostatic effects on the spectrum depend strongly on the temperature and on the ratio between the cyclotron radius ${R}_{c}$ and the length scale ${a}_{\ensuremath{\delta}\ensuremath{\rho}}$ of the density variations. We find that ${a}_{\ensuremath{\delta}\ensuremath{\rho}}$ can be equal to the modulation period $a$, but also much smaller. For ${R}_{c}\ensuremath{\sim}{a}_{\ensuremath{\delta}\ensuremath{\rho}}$ the spectrum in the vicinity of the chemical potential remains essentially the same as in the noninteracting system, while for ${R}_{c}\ensuremath{\ll}{a}_{\ensuremath{\delta}\ensuremath{\rho}}$ it may be drastically changed by the Hartree potential: For noninteger filling factors the energy dispersion is reduced, like in the case of an electrostatic modulation, whereas for even-integer filling factors, on the contrary, the dispersion may be amplified.

Electrons in mesoscopically inhomogeneous magnetic fields

We investigated the low-field magnetoresistivity of a two-dimensional electron gas (2DEG) underneath microstructured ferromagnetic gratings. The strength and the direction of an externally applied magnetic field determines the strength and the shape of the micromagnet's stray field. Commensurability effects due to the imposed periodic magnetic field are in competition with similar effects resulting from a strain-induced periodic electrostatic potential, associated with the patterned ferromagnets on top of the heterojunction. The presence of both a periodic magnetic field and a periodic electrostatic potential gives rise to the interesting interference phenomena presented here.

Microwave Effects on Spinodal Decomposition

AbstractMicrostructural differences between microwave and conventionally annealed solid solution TiO2-SnO2 and TiO2-SnO2-(Al2O3)0,02-samples demixed by spinodal decomposition are reported. The decomposition is retarded upon microwave annealing as compared to conventional heat treatment. This result is in accordance with theoretical predictions, if a superposition of the microwave field with an internal periodic electrostatic potential resulting from kinetic effects is assumed.

Adaptive Riemannian metric for all-electron calculations.

We present two techniques that make feasible the application of the adaptive Riemannian metric technique to all-electron local-density-functional calculations. The first overcomes both the real- (r=0) and Fourier- (G=0) space divergences of the nuclear Coulomb potential by computing the electron-ion energy as the smooth periodic electrostatic potential due to the electrons measured at the positions of the ions. The second overcomes the problem of slow convergence of the extreme metrics which the r=0 Coulomb divergence necessitates by giving an explicit prescription for a suitable metric for arbitrary ionic configurations. All-electron-diamond calculations then serve as a proving ground for these ideas and demonstrate the viability of adaptive Riemannian methods for bypassing the pseudopotential approximation in solid-state calculations.

Photoconductive response of PbTe doping superlattices

Doping superlattices made of PbTe exhibit excellent photoconductive response with a peak detectivity D*≥8×1010 cm Hz1/2 W−1, which is close to the theoretical limit at 6 μm wavelength. Lifetime and far-infrared magneto-optical investigations show that photogenerated electron-hole pairs are separated in the periodic electrostatic potential, leading to a lifetime enhancement by nearly two orders of magnitude as compared to homogeneous bulk crystals.

Inhomogeneous Two-Stream Instability

It is shown that the well-known two-stream instability can under certain conditions be stabilized by the introduction of a periodic stationary electrostatic potential which is produced by the particle beams themselves.