Excitations in time-dependent density-functional theory

An approximate solution to the time-dependent density functional theory (TDDFT) response equations for finite systems is developed, yielding corrections to the single-pole approximation. These explain why allowed Kohn-Sham transition frequencies and oscillator strengths are usually good approximations to the true values, and why sometimes they are not. The approximation yields simple expressions for G\"orling-Levy perturbation theory results, and a method for estimating expectation values of the unknown exchange-correlation kernel.Comment: 4 pages, 1 tabl

R-matrix Floquet theory for laser-assisted electron-atom scattering

A new version of the R-matrix Floquet theory for laser-assisted electron-atom scattering is presented. The theory is non-perturbative and applicable to a non-relativistic many-electron atom or ion in a homogeneous linearly polarized field. It is based on the use of channel functions built from field-dressed target states, which greatly simplifies the general formalism.Comment: 18 pages, LaTeX2e, submitted to J.Phys.

Collective Excitations of Strongly Interacting Fermi Gases of Atoms in a Harmonic Trap

The zero-temperature properties of a dilute two-component Fermi gas in the BCS-BEC crossover are investigated. On the basis of a generalization of the Hylleraas-Undheim method, we construct rigorous upper bounds to the collective frequencies for the radial and the axial breathing mode of the Fermi gas under harmonic confinement in the framework of the hydrodynamic theory. The bounds are compared to experimental data for trapped vapors of Li6 atoms.Comment: 11 pages, 2 figure

Scanning tunneling spectroscopy of superconducting LiFeAs single crystals: Evidence for two nodeless energy gaps and coupling to a bosonic mode

The superconducting compound, LiFeAs, is studied by scanning tunneling microscopy and spectroscopy. A gap map of the unreconstructed surface indicates a high degree of homogeneity in this system. Spectra at 2 K show two nodeless superconducting gaps with $\Delta_1=5.3\pm0.1$ meV and $\Delta_2=2.5\pm0.2$ meV. The gaps close as the temperature is increased to the bulk $T_c$ indicating that the surface accurately represents the bulk. A dip-hump structure is observed below $T_c$ with an energy scale consistent with a magnetic resonance recently reported by inelastic neutron scattering

The High Time Resolution Universe Survey - V: Single-pulse energetics and modulation properties of 315 pulsars

We report on the pulse-to-pulse energy distributions and phase-resolved modulation properties for catalogued pulsars in the southern High Time Resolution Universe intermediate-latitude survey. We selected the 315 pulsars detected in a single-pulse search of this survey, allowing a large sample unbiased regarding any rotational parameters of neutron stars. We found that the energy distribution of many pulsars is well-described by a log-normal distribution, with few deviating from a small range in log-normal scale and location parameters. Some pulsars exhibited multiple energy states corresponding to mode changes, and implying that some observed "nulling" may actually be a mode-change effect. PSRJ1900-2600 was found to emit weakly in its previously-identified "null" state. We found evidence for another state-change effect in two pulsars, which show bimodality in their nulling time scales; that is, they switch between a continuous-emission state and a single-pulse-emitting state. Large modulation occurs in many pulsars across the full integrated profile, with increased sporadic bursts at leading and trailing sub-beam edges. Some of these high-energy outbursts may indicate the presence of "giant pulse" phenomena. We found no correlation with modulation and pulsar period, age, or other parameters. Finally, the deviation of integrated pulse energy from its average value was generally quite small, despite the significant phase-resolved modulation in some pulsars; we interpret this as tenuous evidence of energy regulation between distinct pulsar sub-beams.Comment: Before full MNRAS publication, supplementary material is available temporarily at http://dl.dropbox.com/u/22076931/supplementary_material.pd

Terahertz photoconductivity and plasmon modes in double-quantum-well field-effect transistors

Double-quantum-well field-effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity. The voltage tuned resonance is determined by the plasma oscillations of the composite structure. The resonant photoconductivity requires a double-quantum well but the mechanism whereby plasma oscillations produce changes in device conductance is not understood. The phenomenon is potentially important for fast, tunable terahertz detectors

Large-scale Breit-Pauli R-matrix calculations for transition probabilities of Fe V

Ab initio theoretical calculations are reported for the electric (E1) dipole allowed and intercombination fine structure transitions in Fe V using the Breit-Pauli R-matrix (BPRM) method. We obtain 3865 bound fine structure levels of Fe V and $1.46 x 10^6$ oscillator strengths, Einstein A-coefficients and line strengths. In addition to the relativistic effects, the intermediate coupling calculations include extensive electron correlation effects that represent the complex configuration interaction (CI). Fe V bound levels are obtained with angular and spin symmetries $SL\pi$ and $J\pi$ of the (e + Fe VI) system such that $2S+1$ = 5,3,1, $L \leq$ 10, $J \leq 8$. The bound levels are obtained as solutions of the Breit-Pauli (e + ion) Hamiltonian for each $J\pi$, and are designated according to the `collision' channel quantum numbers. A major task has been the identification of these large number of bound fine structure levels in terms of standard spectroscopic designations. A new scheme, based on the analysis of quantum defects and channel wavefunctions, has been developed. The identification scheme aims particularly to determine the completeness of the results in terms of all possible bound levels for applications to analysis of experimental measurements and plasma modeling. An uncertainty of 10-20% for most transitions is estimated.Comment: 31 pages, 1 figure, Physica Scripta (in press

Towards Many-objective Optimisation with Hyper-heuristics: Identifying Good Heuristics with Indicators

PPSN 2016: 14th International Conference on Parallel Problem Solving from Nature, 17-21 September 2016, Edinburgh, ScotlandThis is the author accepted manuscript. The final version is available from Springer Verlag via the DOI in this record.The use of hyper-heuristics is increasing in the multi-objective optimisation domain, and the next logical advance in such methods is to use them in the solution of many-objective problems. Such problems comprise four or more objectives and are known to present a significant challenge to standard dominance-based evolutionary algorithms. We in- corporate three comparison operators as alternatives to dominance and investigate their potential to optimise many-objective problems with a hyper-heuristic from the literature. We discover that the best results are obtained using either the favour relation or hypervolume, but conclude that changing the comparison operator alone will not allow for the generation of estimated Pareto fronts that are both close to and fully cover the true Pareto front.This work was funded under EPSRC grant EP/K000519/1

Time-dependent density functional theory: Past, present, and future

Time-dependent density functional theory (TDDFT) is presently enjoying enormous popularity in quantum chemistry, as a useful tool for extracting electronic excited state energies. This article discusses how TDDFT is much broader in scope, and yields predictions for many more properties. We discuss some of the challenges involved in making accurate predictions for these properties.Comment: 12 pages, 4 figure