GW Level Research Articles

Benchmarking GW against exact diagonalization for semiempirical models

We calculate groundstate total energies and single-particle excitation energies of seven pi conjugated molecules described with the semi-empirical Pariser-Parr-Pople (PPP) model using self-consistent many-body perturbation theory at the GW level and exact diagonalization. For the total energies GW captures around 65% of the groundstate correlation energy. The lowest lying excitations are overscreened by GW leading to an underestimation of electron affinities and ionization potentials by approximately 0.15 eV corresponding to 2.5%. One-shot G_0W_0 calculations starting from Hartree-Fock reduce the screening and improve the low-lying excitation energies. The effect of the GW self-energy on the molecular excitation energies is shown to be similar to the inclusion of final state relaxations in Hartree-Fock theory. We discuss the break down of the GW approximation in systems with short range interactions (Hubbard models) where correlation effects dominate over screening/relaxation effects. Finally we illustrate the important role of the derivative discontinuity of the true exchange-correlation functional by computing the exact Kohn-Sham levels of benzene.

Fully self-consistent GW calculations for molecules

We calculate single-particle excitation energies for a series of 33 molecules using fully selfconsistent GW, one-shot G$_0$W$_0$, Hartree-Fock (HF), and hybrid density functional theory (DFT). All calculations are performed within the projector augmented wave (PAW) method using a basis set of Wannier functions augmented by numerical atomic orbitals. The GW self-energy is calculated on the real frequency axis including its full frequency dependence and off-diagonal matrix elements. The mean absolute error of the ionization potential (IP) with respect to experiment is found to be 4.4, 2.6, 0.8, 0.4, and 0.5 eV for DFT-PBE, DFT-PBE0, HF, G$_0$W$_0$[HF], and selfconsistent GW, respectively. This shows that although electronic screening is weak in molecular systems its inclusion at the GW level reduces the error in the IP by up to 50% relative to unscreened HF. In general GW overscreens the HF energies leading to underestimation of the IPs. The best IPs are obtained from one-shot G$_0$W$_0$ calculations based on HF since this reduces the overscreening. Finally, we find that the inclusion of core-valence exchange is important and can affect the excitation energies by as much as 1 eV.

Evolutionary structure prediction and electronic properties of indium oxide nanoclusters

Indium sesquioxide is widely used as a transparent conducting oxide in modern optoelectronic devices; the rising cost of indium has generated interest in the nanoscale properties of In(2)O(3), and questions arise as to the nature of its physicochemical properties below the bulk regime. We report the stable and metastable stoichiometric clusters of (In(2)O(3))(n), where n = 1-10, as predicted from an evolutionary search within the classical interatomic potential and quantum density functional energy landscapes. In contrast to the paradigm set by ZnO, which favours high symmetry bubble-like structures, the In(2)O(3) nanoclusters are found to tend towards dense, low symmetry structures approaching the bulk system at remarkably small molecular masses. Electronic characterisation is performed at the hybrid density functional and many-body GW levels to obtain accurate predictions of the spectroscopic properties, with mean values of the ionisation potentials and electron affinities calculated as 7.7 and 1.7 eV, respectively.

Kadanoff-Baym approach to quantum transport through interacting nanoscale systems: From the transient to the steady-state regime

We propose a time-dependent many-body approach to study the short-time dynamics of correlated electrons in quantum transport through nanoscale systems contacted to metallic leads. This approach is based on the time-propagation of the Kadanoff-Baym equations for the nonequilibrium many-body Green's function of open and interacting systems out of equilibrium. An important feature of the method is that it takes full account of electronic correlations and embedding effects in the presence of time-dependent external fields, while at the same time satisfying the charge conservation law. The method further extends the Meir-Wingreen formula to the time domain for initially correlated states. We study the electron dynamics of a correlated quantum wire attached to two-dimensional leads exposed to a sudden switch-on of a bias voltage using conserving many-body approximations at Hartree-Fock, second Born and GW level. We obtain detailed results for the transient currents, dipole moments, spectral functions, charging times, and the many-body screening of the quantum wire as well as for the time-dependent density pattern in the leads, and we show how the time-dependence of these observables provides a wealth of information on the level structure of the quantum wire out of equilibrium. For moderate interaction strenghts the 2B and GW results are in excellent agreement at all times. We find that many-body effects beyond the Hartree-Fock approximation have a large effect on the qualitative behavior of the system and lead to a bias dependent gap closing and quasiparticle broadening, shortening of the transient times and washing out of the step features in the current-voltage curves.

Tight-binding description of the quasiparticle dispersion of graphite and few-layer graphene

A universal set of third-nearest-neighbor tight-binding (TB) parameters is presented for calculation of the quasiparticle (QP) dispersion of $N$ stacked $s{p}^{2}$ graphene layers $(N=1\dots{}\ensuremath{\infty})$ with $AB$ stacking sequence. The present TB parameters are fit to ab initio calculations on the GW level and are universal, allowing to describe the whole $\ensuremath{\pi}$ ``experimental'' band structure with one set of parameters. This is important for describing both low-energy electronic transport and high-energy optical properties of graphene layers. The QP bands are strongly renormalized by electron-electron interactions, which results in a 20% increase in the nearest-neighbor in-plane and out-of-plane TB parameters when compared to band structure from density-functional theory. With the new set of TB parameters we determine the Fermi surface and evaluate exciton energies, charge carrier plasmon frequencies, and the conductivities which are relevant for recent angle-resolved photoemission, optical, electron energy loss, and transport measurements. A comparision of these quantitities to experiments yields an excellent agreement. Furthermore we discuss the transition from few-layer graphene to graphite and a semimetal to metal transition in a TB framework.

A GW Level High-Power Radial Line Helical Array Antenna

High-power radial line helical array antenna is a novel antenna developed to realize directional radiation of high-power microwaves. This antenna directly accepts an azimuthally symmetric output of a high-power microwave source and radiates circularly polarized microwave with a boresight peak. It has a high efficiency, a compact structure, and a notable power-handling capacity. Starting from its fundamental design theory, an antenna prototype consisting of 48 elements is proposed. Numerical simulation and experimental measurement are accomplished for the prototype. In the range of 3.7-4.1 GHz, the experimental result shows an antenna VSWR of below 1.2, an antenna gain of over 23.2 dB, an aperture efficiency of more than 65%, and an antenna axial ratio of below 1.6 dB. In addition, the high-power experiment proves that this antenna has a power-handling capacity of more than 1 GW.

Gigawatt power microwave devices and vacuum

Low pressure breakdown is a major concern for developing high power microwave (HPM) devices, especially when going to more compact designs and to extreme powers, typically cm-size gaps in the tube and GW level output. In connection with the vacuum quality, we will consider the general aspects which may contribute to limit the operation of a HPM device: high voltage arcing, electrode erosion and degassing, rf breakdown in the cavities, surface breakdown at the windows, repetition rate. Vacuum measurement and electromagnetic compatibility with vacuum equipment will be treated too. As an example, we will consider in details the development of a compact magnetically insulated line oscillator (MILO) operating successfully at the Gigawatt level in the S-band.

Investigation of Laser Shock Peening on Aero-Engine Compressor Rotor Blade

Compressor rotor blade is one of the critical parts of aero-engine. Because of the complex structure and bad work environment, it often went wrong. More over, the trouble of blade can result in serious accident of engine even the aircraft. But the conventional shot peening strengthen method couldn’t take ideal effect, so a new method, laser shock peening on aero-engine compressor rotor blade was studied in this paper. The principle of laser shock peening strengthening was presented first, and the appropriate shock parameters of a certain type blade are fixed on by modeling. The parameters were as follows: laser pulse width is 20ns, output energy is 25J, the spot diameter is 8mm, and peak value is at GW level, and the laser peak value power is bigger than material dynamic yield intensity. Based on these parameters, an experiment was performed by shocking the blade through a repeated Nd: glass LASER supplied by Jiangsu University. Through contrastive performance test, an analysis on the high frequency fatigue life and surface rigidity of the blade fore and after laser shock was performed. The result was concluded that laser shock peening can significantly increase the high frequency fatigue life of aero-engine compressor rotor blade, and enhance the surface rigidity to increase the performance of contuse resistance. At last the reasons were analyzed from two aspects: laser shock can impart compressive residual stress on blade and reduce the size of crystal grain. Surface compressive residual stress can change the blade distribution of load when working; and reduce the size of crystal grain can bring in more crystal interface, for the more interface the crystal has the higher dislocation moving resistance and yield stress is produced.

Physics of Superpulses in Storage Ring Free‐Electron Lasers

Contradictory to the widespread perception, storage ring free‐electron lasers with substantial net gain can generate peak lasing power reaching GW levels in the so‐called superpulse mode. This power level is sufficient for studies of nonlinear processes and efficient intracavity harmonic generation. This letter describes the physics of the superpulses and a phase‐space refreshment in the electron beam responsible for this phenomenon.

Technical requirements for economical viability of electricity generation in stabilized wind parks

Wind generation is substantially increasing its share in the electricity generation portfolio in many countries. In the case of Spain, it can more than double in the next 4 years, reaching a 20 GW level. To become competitive in a liberalized market, wind energy reliability requirements should be guaranteed, so storage systems are being considered to avoid the impact of the lack of adequate winds on the electricity generation process. This paper, assuming different scenarios for the electricity markets and the wind farm operation, analyzes the minimum requirements on energy efficiency of a storage system based on the use of hydrogen as energy vector to become economically competitive for each of those scenarios. Simulation of a wind park with such a buffer system for different operation scenarios shows a direct dependence of the economical profit on the energy efficiency of this hydrogen backup system. A direct conclusion of the study is the necessity for an improvement of the currently available values of the efficiency of the subsystems that compose the hydrogen buffer. These improvements seem to be possible in the short term when attending to the current research efforts.

Consideration on blanket structure for fusion DEMO plant at JAERI

Our idea on the DEMO plant is that it must demonstrate (1) an electric power generation of one GW level, (2) self-sufficiency of tritium fuel (TBR is more than 1.05), (3) year-long continuous operation, etc. At the same time, DEMO is expected to use technologies to be proven by 2020 and present an economical prospect of fusion energy in the operational time of the reactor. The design guidelines for the blanket are defined in order to meet the mission of the DEMO plant as mentioned above. Major design conditions are surface heat flux of 0.5 MW/m 2 with peaking factor of 2, a neutron wall load of 3.5 MW/m 2 with peaking factor of 1.5 and a neutron fluence of about 10 MW/m 2. To moderate the technological extrapolation, reduced activation ferritic steel (F82H) structural material, Li 2TiO 3 and Be neutron multiplier are considered. To improve the economical aspect, supercritical water with inlet/outlet temperatures of 280/510 °C is chosen as coolant material, with coolant pressure of 25 MPa. As a result, a thermal efficiency of 41% is achieved. To obtain higher plasma performance, MHD instabilities suppressing shell structure is adopted with structural compatibility to the blanket structure. To meet higher plant availability requirements (more than 75%), the hot cell maintenance approach is selected for the replaceable power core components.

Concept of core and divertor plasma for fusion DEMO plant at JAERI

Fusion DEMO plant design at JAERI directs toward compactness and economy of the reactor with a fusion output at 3 GW level. The smaller size of the center solenoid (CS) has a large impact on reducing the reactor size and weight. Three candidates for CS, namely “-less”, “slim” and “full” CS, are considered according to the role of CS coil that is plasma ramp up and plasma shaping. From points of views of technical feasibility, obtained triangularity ( δ x ) and weight of the reactor, the slim-CS is selected as the candidate for DEMO at JAERI. For slim-CS, high δ x (≥0.43), which is enough to suppress giant ELMs, is obtained, and highly radiative outer divertor is required in steady state power handling. The consistent equilibrium was found to satisfy the divertor requirements and radial build. During the plasma current ramp up phase, the acceleration voltage of neutral beam injection (NBI) has to be changed more than two times. In order to inject sufficient power during the low-energy beam injection, the present technology must be replaced by a new constant-current, variable-voltage technology.

Ab initiostudy ofMg(AlH4)2

Magnesium alanate Mg(AlH4)2 has recently raised interest as a potential material for hydrogen storage. We apply ab initio calculations to characterize structural, electronic and energetic properties of Mg(AlH4)2. Density functional theory calculations within the generalized gradient approximation (GGA) are used to optimize the geometry and obtain the electronic structure. The latter is also studied by quasi-particle calculations at the GW level. Mg(AlH4)2 is a large band gap insulator with a fundamental band gap of 6.5 eV. The hydrogen atoms are bonded in AlH4 complexes, whose states dominate both the valence and the conduction bands. On the basis of total energies, the formation enthalpy of Mg(AlH4)2 with respect to bulk magnesium, bulk aluminum and hydrogen gas is 0.17 eV/H2 (at T = 0). Including corrections due to the zero point vibrations of the hydrogen atoms this number decreases to 0.10 eV/H2. The enthalpy of the dehydrogenation reaction Mg(AlH4)2 -> MgH2 +2Al+3H2(g) is close to zero, which impairs the potential usefulness of magnesium alanate as a hydrogen storage material.

Using VUV high-order harmonics generated in gas as a seed for single pass FEL

In this paper we illustrate the main features of a new scheme which aims at obtaining coherent radiation at very short wavelengths (down to 1.5 nm) by using the Harmonics of an intense laser, generated in a gas, as a seed to inject in a long undulator, either in the pure SASE or in the HGHG configuration. The unique spatial and temporal coherence properties of the High-Order Harmonics are then conserved, and the High Gain FEL amplifier system saturates up to the GW level. An experiment is here proposed, to be performed on the new compact LINAC-based source SCSS, currently under development in SPring-8.

New results and prospects for harmonic generation in storage ring FELs

In this paper we review new results and prospects for harmonic generation in storage ring (SR) FELs. Traditionally, high harmonics were generated in SR FELs using conventional (IR or visible) pulsed lasers as a driver. Discovery of super-pulses with GW levels of intra-cavity peak power provided for new ways of generating coherent harmonics in the VUV and X-ray ranges. Super-pulses are generated in the OK-4/Duke SR FEL using gain modulation technique. As we reported at the previous FEL conference, the intra-cavity power on the OK-4 FEL was sufficient for effective generation of third and higher harmonics in the fundamental FEL wavelength. Recently, the OK-4/Duke SR FEL generated second, third, fourth, fifth and seventh coherent harmonics in the range from 37 to 135nm. The tunability of the FEL wavelength provided for natural wavelength tunability of the harmonic radiation.Recent experimental results of the UV and VUV harmonic generation in the OK-4/Duke SR FEL demonstrated peak spectral brightness comparable with VUV SASE FELs. We discuss the prospects of the intra-cavity short-wavelength harmonic generation in SR FEL.

Generation of GW radiation pulses from a VUV free-electron laser operating in the femtosecond regime.

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30-100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95-105 nm.

Lightning control system using high power microwave FEL

Abstract A research project for developing a thunder lightning control system using an induction linac based high power microwave free electron laser (FEL) was started at JAERI. The system will produce a weakly ionized plasma rod in the atmosphere by high power microwaves and control a lightning path, away from, e.g., nuclear power stations and rocket launchers. It has been known that about 1 MW/cm2 power density is enough for the atmospheric breakdown in the microwave region, which means that a high power microwave FEL with GW level output power is feasible for atmospheric breakdown, and accordingly is feasible for a thunder lightning control tool by making a conductive plasma channel in the atmosphere. From the microwave attenuation consideration in the atmosphere, FELs of 35 GHz (0.13 dB/km), 90 GHz (0.35 dB/km), 140 GHz (1.7 dB/km), and of 270 GHz (4.5 dB/km) are the best candidates for the system. Compared with other proposed lightning control systems using a visible or an ultraviolet laser, the system using microwave has the advantage that microwave has a smaller attenuation with rain or snow which always exists in the real atmospheric circumstances when lightning occurs. A detailed description and feasibility study of the system will be given.

Simulation of tapered FEL amplifiers in millimeter and infrared regions

Numerical calculations are performed based on the one-dimensional model for a tapered FEL amplifier with an axial guide field operating in the Raman regime and an IR-FEL amplifier based on a test linac of the Pohang Accelerator Laboratory (PAL). The longitudinal electric field can be formulated either by solving the coupled Maxwell and linear fluid equations or by solving the coupled kinetic and longitudinal wave equations. Both formulations are incorporated into the numerical code and the simulation results are compared. The criterion whether the space charge effect should be considered or not can be described in terms of the electron beam parameter ξ(ξ 2 = e 2n b ϵ 0mγ 0c 2k w 2) . We have also theoretically investigated the possibility of a tunable IR-FEL with a test linac of energy 20–60 MeV of PAL with effects of energy spread and space charge. The current density of the test linac, if properly enhanced, makes it feasible to amplify a 14 kW signal of 10.6 μm radiation to a 2 GW level.

High-power magnetic-compression heating of field-reversed configurations

Magnetic compression heating experiments at the 1 GW level on field-reversed configuration (FRC) compact toroid plasmas are reported. FRC’s formed in a tapered theta-pinch coil have been translated into a single-turn compression coil, where the external magnetic field is slowly raised up to seven times its initial value. Significant electron and ion heating consistent with the expected B4/5 adiabatic scaling is observed, despite significant particle diffusion, which is enhanced during compression. The n=2 rotational instability is enhanced during compression, but has been controlled to an extent by the application of an external quadrupole field. The particle and flux confinement times, τN and τφ, remain approximately equal and decrease roughly with the square of the plasma radius R during compression, implying a constant nonclassical field-null resistivity. The observed τN and τφ magnitudes and scalings are compared with classical and anomalous transport theories, and existing empirical models. Particle diffusion dominates the energy confinement, accounting for three-fourths of the total losses. Upper bounds on the electron thermal diffusivities are estimated.

Relations between Precipitation and Shallow Groundwater in Illinois

Abstract The statistical relationships between monthly precipitation (P) and shallow groundwater levels (GW) in 20 wells scattered across Illinois with data for 1960–84 were defined using autoregressive integrated moving average (ARIMA) modeling. A lag of 1 month between P to GW was the strongest temporal relationship found across Illinois, followed by no (0) lag in the northern two-thirds of Illinois where mollisols predominate, and a lag of 2 months in the alfisols of southern Illinois. Spatial comparison of the 20 P-GW correlations with several physical conditions (aquifer types, soils, and physiography) revealed that the parent soil materials of outwash alluvium, glacial till, thick loess (≥2.1 m), and thin loess (>2.1) best defined regional relationships for drought assessment. Equations developed from ARTMA using 1960–79 data for each region were used to estimate GW levels during the 1980–81 drought, and estimates averaged between 25 to 45 cm of actual levels. These estimates are considered adequate...