Space Charge Effect and Mirror Charge Effect in Photoemission Spectroscopy

We report the observation and systematic investigation of the space charge effect and mirror charge effect in photoemission spectroscopy. When pulsed light is incident on a sample, the photoemitted electrons experience energy redistribution after escaping from the surface because of the Coulomb interaction between them (space charge effect) and between photoemitted electrons and the distribution of mirror charges in the sample (mirror charge effect). These combined Coulomb interaction effects give rise to an energy shift and a broadening which can be on the order of 10 meV for a typical third-generation synchrotron light source. This value is comparable to many fundamental physical parameters actively studied by photoemission spectroscopy and should be taken seriously in interpreting photoemission data and in designing next generation experiments.Comment: Journal of Electron Spectroscopy and Related Phenomena 142(2004)27-3

Strong energy enhancement in a laser-driven plasma-based accelerator through stochastic friction

Conventionally, friction is understood as an efficient dissipation mechanism depleting a physical system of energy as an unavoidable feature of any realistic device involving moving parts, e.g., in mechanical brakes. In this work, we demonstrate that this intuitive picture loses validity in nonlinear quantum electrodynamics, exemplified in a scenario where spatially random friction counter-intuitively results in a highly directional energy flow. This peculiar behavior is caused by radiation friction, i.e., the energy loss of an accelerated charge due to the emission of radiation. We demonstrate analytically and numerically how radiation friction can enhance the performance of a specific class of laser-driven particle accelerators. We find the unexpected directional energy boost to be due to the particles' energy being reduced through friction whence the driving laser can accelerate them more efficiently. In a quantitative case we find the energy of the laser-accelerated particles to be enhanced by orders of magnitude.Comment: 14 pages, 3 figure

An extended Kalman filtering approach to modeling nonlinear dynamic gene regulatory networks via short gene expression time series

Copyright [2009] IEEE. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Brunel University's products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to [email protected]. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.In this paper, the extended Kalman filter (EKF) algorithm is applied to model the gene regulatory network from gene time series data. The gene regulatory network is considered as a nonlinear dynamic stochastic model that consists of the gene measurement equation and the gene regulation equation. After specifying the model structure, we apply the EKF algorithm for identifying both the model parameters and the actual value of gene expression levels. It is shown that the EKF algorithm is an online estimation algorithm that can identify a large number of parameters (including parameters of nonlinear functions) through iterative procedure by using a small number of observations. Four real-world gene expression data sets are employed to demonstrate the effectiveness of the EKF algorithm, and the obtained models are evaluated from the viewpoint of bioinformatics

Evidence of Electron Fractionalization from Photoemission Spectra in the High Temperature Superconductors

In the normal state of the high temperature superconductors Bi_2Sr_2CaCu_2O_{8+delta} and La_{2-x}Sr_{x}CuO_4, and in the related ``stripe ordered'' material La_1.25Nd_0.6Sr_0.15CuO_4, there is sharp structure in the measured single hole spectral function A(k,w) considered as a function of k at fixed small binding energy w. At the same time, as a function of w at fixed k on much of the putative Fermi surface, any structure in A(k,w), other than the Fermi cutoff, is very broad. This is characteristic of the situation in which there are no stable excitations with the quantum numbers of the electron, as is the case in the one dimensional electron gas.Comment: Published versio

The Dichotomy between Nodal and Antinodal Quasiparticles in Underdoped (La2−x_{2-x}Srx_x)CuO4_4 Superconductors

High resolution angle-resolved photoemission measurements on underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ system show that, at energies below 70 meV, the quasiparticle peak is well defined around the ($\pi$/2,$\pi$/2) nodal region and disappears rather abruptly when the momentum is changed from the nodal point to the ($\pi$,0) antinodal point along the underlying ``Fermi surface''. It indicates that there is an extra low energy scattering mechanism acting upon the antinodal quasiparticles. We propose that this mechanism is the scattering of quasiparticles across the nearly parallel segments of the Fermi surface near the antinodes.Comment: to appear in Phys. Rev. Let

A refined invariant subspace method and applications to evolution equations

The invariant subspace method is refined to present more unity and more diversity of exact solutions to evolution equations. The key idea is to take subspaces of solutions to linear ordinary differential equations as invariant subspaces that evolution equations admit. A two-component nonlinear system of dissipative equations was analyzed to shed light on the resulting theory, and two concrete examples are given to find invariant subspaces associated with 2nd-order and 3rd-order linear ordinary differential equations and their corresponding exact solutions with generalized separated variables.Comment: 16 page