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PubMed2005

Quantum chemical study of the electronic structure of NiCH2 + in its ground state and low-lying electronic excited states.

The electronic structure of NiCH(2) (+), representative of transition metal carbene ions, is investigated by means of several methods of quantum chemistry. The relative stabilities of the four low-lying doublet electronic states ((2)A(1), (2)A(2), (2)B(1), and (2)B(2)) are determined at the coupled cluster singles and doubles level (CCSD) and triples level [CCSD(T) and CCSDT-3] with both a Hartree-Fock and density functional theory (Kohn-Sham) reference. The equation-of-motion coupled cluster for treatment of excited states in singles and doubles approximation (EOM-CCSD) is used to characterize the transition energies from the (2)A(1) electronic ground state to the low-lying doublet excited states. The (2)A(2) and (2)B(1) states are nearly degenerate, found to be separated by 940 cm(-1) at the EOM-CCSD level, in agreement with the CASSCF energy ordering. The (2)B(2) state is calculated to be higher in energy by more than 1.0 eV. The spin purity of the low-lying doublet and quadruplet states described by CCSD calculations based on the unrestricted open-shell Hartree-Fock reference is discussed.

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PubMed2006

A generalized quantum chemical approach for elastic and inelastic electron transports in molecular electronics devices.

A generalized quantum chemical approach for electron transport in molecular devices is developed. It allows one to treat devices where the metal electrodes and the molecule are either chemically or physically bonded on equal footing. An extension to include the vibration motions of the molecule has also been implemented which has produced the inelastic electron-tunneling spectroscopy of molecular electronics devices with unprecedented accuracy. Important information about the structure of the molecule and of metal-molecule contacts that are not accessible in the experiment are revealed. The calculated current-voltage (I-V) characteristics of different molecular devices, including benzene-1,4-dithiolate, octanemonothiolate [H(CH2)8S], and octanedithiolate [S(CH2)8S] bonded to gold electrodes, are in very good agreement with experimental measurements.

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PubMed2006

An elongation method for first principle simulations of electronic structures and electron transport properties of finite nanostructures.

An effective elongation method has been developed to study electronic structures and electron transport properties of nanoelectronic and bioelectronic devices at a hybrid density functional theory level. It enables to treat finite nanostructures consisting of as many as 28 000 electrons and has been successfully applied to sub-120-nm-long conjugated polymers, sub-60-nm-long single-walled carbon nanotubes, and 30 base-pair DNA molecules. The calculated current-voltage characteristics of different systems are found to be in good agreement with the experiments. Some unexpected behaviors of these nanosized devices have been discovered.

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PubMed1979

Integrated spatial electron populations in molecules: The electron projection function.

A "projection function," P(x,z), is defined as the partial integral of the molecular electron density, rho(x,y,z), over the region -infinity < y < +infinity. The projection provides a three-dimensional representation of molecular electron distributions. Chemically useful information can be discerned from graphical displays in either perspective plot or contour format. Numerical integration of the function gives the integrated spatial electron population for any region of interest. The use of the projection function and difference functions is exemplified by application to acetaldehyde.

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PubMed1982

Picosecond photochemistry of a cofacial diporphyrin containing iron(III) and zinc(II): Mimicking electron transfer between cytochrome c and the primary electron donor in reaction centers of photosynthetic bacteria.

Comparison of picosecond kinetic and spectroscopic data for Zn octaethylporphine and Fe(III)Cl octaethylporphine with that for Zn-Fe(III)Cl, a cofacial diporphyrin composed of a Zn porphyrin covalently bound to an Fe(III)Cl porphyrin with two chains of five atoms each, supports the assignment of a light-driven electron transfer (k > 10(11)s(-1)) within Zn-Fe(III)Cl to form [Zn(+).-Fe(II)]Cl. The kinetics (k approximately 10(10)s(-1)) and thermodynamics of the reverse electron transfer are compared to those of a similar electron transfer in bacterial photosynthesis, the reduction of an oxidized bacteriochlorophyll dimer, (BChl)(2) (+)., by Fe(II) cytochrome c.

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PubMed2002

Influence of the electron distribution function shape on nonlocal electron heat transport in laser-heated plasmas.

A nonlocal model of electron heat flow in laser-heated plasmas taking into account the super-Gaussian deformation of the electron velocity distribution function by the laser heating was developed. Based on comparisons to Fokker-Planck simulations of hot spot heating, it performs better than previous models. The growth rate of thermal filamentation is considerably changed by this thermal conductivity modification. First results from a formula describing the isotropic component, f(0)(x,v), of the electron velocity distribution function as a spatial convolution over local Maxwellians are also presented.

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PubMed2005

Stopping of relativistic electrons in a partially degenerate electron fluid.

The stopping mechanisms of relativistic electron beams in superdense and partially degenerate electron fluid targets are investigated in the framework of the fast ignitor concept for inertial confinement fusion. In order to comply with specific demands in this area, we focus attention on the target partial degeneracy parameter theta= T(e) / T(f) , in terms of the thermal to Fermi temperature ratio. The target electron fluid is thus modeled very accurately with a random phase approximation dielectric function. The stopping results are shown to be very weakly theta dependent. However, a quantum target description is needed to recover their correct increasing trend with increasing projectile energy. The ranges and effective penetration depths in precompressed thermonuclear fuels are shown to be nearly a factor of 2 shorter than earlier classical estimates in the same conditions. The overall conclusions pertaining to the feasibility of fast ignition thus remain unchanged.

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PubMed2006

Laser acceleration of electrons to giga-electron-volt energies using highly charged ions.

The recent proposal to use highly charged ions as sources of electrons for laser acceleration [S. X. Hu and A. F. Starace, Phys. Rev. Lett. 88, 245003 (2002)] is investigated here in detail by means of three-dimensional, relativistic Monte Carlo simulations for a variety of system parameters, such as laser pulse duration, ionic charge state, and laser focusing spot size. Realistic laser focusing effects--e.g., the existence of longitudinal laser field components-are taken into account. Results of spatial averaging over the laser focus are also presented. These numerical simulations show that the proposed scheme for laser acceleration of electrons from highly charged ions is feasible with current or near-future experimental conditions and that electrons with GeV energies can be obtained in such experiments.

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PubMed2001

Long-pulse improved central electron confinement in the TCV tokamak with electron cyclotron heating and current drive.

Current profile tailoring by electron cyclotron heating (ECH) and current drive (ECCD) is used to improve central electron energy confinement in the TCV tokamak. Counter-ECCD on axis alone achieves this goal in a transient manner only. A stable scenario is obtained by a two-step sequence of off-axis ECH, which stabilizes magnetohydrodynamics modes, and on-axis counter-ECCD, which generates a flat or inverted current profile. This high-confinement regime, with central temperatures up to 9 keV (at a normalized beta(N) approximately 0.6), has been sustained for the entire duration of the heating pulse, or over 200 electron energy confinement times and 5 current redistribution times.

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PubMed2001

Role of boron p-electrons and holes in superconducting MgB2, and other diborides: a fully relaxed, full-potential electronic structure study.

We present the results of fully relaxed, full-potential electronic structure calculations for the new superconductor MgB (2), and BeB (2), NaB (2), and AlB (2), using density-functional-based methods. Our results, described in terms of (i) density of states (DOS), (ii) band structure, and (iii) the DOS and the charge density around the Fermi energy E(F), clearly show the importance of B p-band for superconductivity. In particular, we show that around E(F), the charge density in MgB (2), BeB (2), and NaB (2) is planar and is associated with the B plane. For BeB (2) and NaB (2), we find significant differences in their electronic structure due to differences in the number of valence electrons and the lattice constants a and c.

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PubMed2002

Detection of spin-polarized electrons injected into a two-dimensional electron gas.

The spin-dependent mean-free path of electrons in a high-mobility InAs two-dimensional electron gas (2DEG) is measured. Ferromagnetic metal/insulator/2DEG junctions are fabricated on a common channel in a nonlocal geometry and used as spin injectors and detectors. For electrons in spin-orbit eigenstates at 4.5 K, lower bounds for the spin mean-free path and relaxation time are Lambda(S) > or = 4.6 microm and tau(s) > or = 3.8 ps, respectively. The temperature dependence is weak over the range 4.5<T<150 K.

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PubMed2002

Spin-resolved unoccupied electronic band structure from quantum size oscillations in the reflectivity of slow electrons from ultrathin ferromagnetic crystals.

The spin-dependent reflectivity of electrons with energies between 0 and 20 eV from Fe single crystals 2-8 monolayers thick on a W(110) surface is studied by spin-polarized low energy electron microscopy. The quantum size oscillations in the reflectivity are analyzed in a similar manner as in photoemission of ground state electrons, yielding the spin-resolved unoccupied state band structure of Fe in the Gamma N direction in the energy range studied.

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PubMed2003

Magnetoresistance of a 2D electron gas caused by electron interactions in the transition from the diffusive to the ballistic regime.

On a high-mobility 2D electron gas we have observed, in strong magnetic fields (omega(c)tau>1), a parabolic negative magnetoresistance caused by electron-electron interactions in the regime of k(B)Ttau/ variant Planck's over 2pi approximately 1, which is the transition from the diffusive to the ballistic regime. From the temperature dependence of this magnetoresistance the interaction correction to the conductivity deltasigma(ee)(xx)(T) is obtained in the situation of a long-range fluctuation potential and strong magnetic field. The results are compared with predictions of the new theory of interaction-induced magnetoresistance.

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PubMed2003

Atomic-resolution electron energy loss spectroscopy imaging in aberration corrected scanning transmission electron microscopy.

The "delocalization" of inelastic scattering is an important issue for the ultimate spatial resolution of innershell spectroscopy in the electron microscope. It is demonstrated in a nonlocal model for electron energy loss spectroscopy (EELS) that delocalization of scanning transmission electron microscopy (STEM) images for single, isolated atoms is primarily determined by the width of the probe, even for light atoms. We present experimental data and theoretical simulations for Ti L-shell EELS in a [100] SrTiO3 crystal showing that, in this case, delocalization is not significantly increased by dynamical propagation. Issues relating to the use of aberration correctors in the STEM geometry are discussed.

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PubMed2003

Electron-electron momentum exchange in strong field double ionization.

We have investigated the momentum balance between the two electrons from strong field double ionization of argon at 780 nm and 1.9 x 10(14) W/cm(2). Experimental data show that perpendicular to the laser polarization direction the electrons emerge preferentially in opposite directions. Results of model calculations are found to agree well with the data and reveal a dominant role of the Coulomb correlation between the two outgoing electrons in this kinematical geometry. Differences between the experimental observations and the theoretical results for the ion momentum distribution indicate the importance of the further effects during the three-body breakup.

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PubMed2003

Proposal for production and detection of entangled electron-hole pairs in a degenerate electron gas.

We demonstrate theoretically that the shot noise produced by a tunnel barrier in a two-channel conductor violates a Bell inequality. The nonlocality is shown to originate from entangled electron-hole pairs created by tunneling events-without requiring electron-electron interactions. The degree of entanglement (concurrence) equals 2(T1T2)(1/2)(T1+T2)(-1), with T1,T2<<1 the transmission eigenvalues. A pair of edge channels in the quantum Hall effect is proposed as an experimental realization.

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PubMed2003

Fluctuations of g factors in metal nanoparticles: effects of electron-electron interaction and spin-orbit scattering.

We investigate the combined effect of spin-orbit scattering and electron-electron interactions on the probability distribution of g factors of metal nanoparticles. Using random matrix theory, we find that even a relatively small interaction strength significantly increases g-factor fluctuations for not-too-strong spin-orbit scattering (ratio of spin-orbit rate and single-electron level spacing 1/tau(so)delta < or near 1), and leads to the possibility to observe g factors larger than 2.

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PubMed2004

Electronic-structure dependence of the electron-phonon interaction in Ag.

The linewidths of sp- and d-band derived electronic quantum-well states in thin films of Ag on Fe(100) are measured as a function of temperature to yield the electron-phonon coupling parameters. The results vary by a factor of up to 35 among the different states. The origin of these huge differences is traced to the decay path selection for the various initial states of the holes created by the photoemission process. The electron-phonon coupling parameter for the top d-band quantum-well state, 0.015+/-0.006, is the smallest ever reported.

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