Molecular effects in the ionization of N-2, O-2, and F-2 by intense laser fields

In this paper we study the response in time of N2, O2, and F2 to laser pulses having a wavelength of 390 nm. We find single-ionization suppression in O2 and its absence in F2, in accordance with experimental results at lambda= 800 nm. Within our framework of time-dependent density functional theory we are able to explain deviations from the predictions of intense-field many-body S-matrix theory (IMST). We confirm the connection of ionization suppression with destructive interference of outgoing electron waves from the ionized electron orbital. However, the prediction of ionization suppression, justified within the IMST approach through the symmetry of the highest occupied molecular orbital (HOMO), is not reliable since it turns out that—e.g., in the case of F2—the electronic response to the laser pulse is rather complicated and does not lead to dominant depletion of the HOMO. Therefore, the symmetry of the HOMO is not sufficient to predict ionization suppression. However, at least for F2, the symmetry of the dominantly ionized orbital is consistent with the nonsuppression of ionization.

Optimal basis set for electronic structure calculations in periodic systems

An efficient method for calculating the electronic structure of systems that need a very fine sampling of the Brillouin zone is presented. The method is based on the variational optimization of a single (i.e., common to all points in the Brillouin zone) basis set for the expansion of the electronic orbitals. Considerations from k.p-approximation theory help to understand the efficiency of the method. The accuracy and the convergence properties of the method as a function of the optimal basis set size are analyzed for a test calculation on a 16-atom Na supercell.

Correlation Effects in the Compton Profile of Silicon

Ab initio nonlocal pseudopotential variational quantum Monte Carlo techniques are used to compute the correlation effects on the valence momentum density and Compton profile of silicon. Our results for this case are in excellent agreement with the Lam-Platzman correction computed within the local density approximation. Within the approximations used, we rule out valence electron correlations as the dominant source of discrepancies between calculated and measured Compton profiles of silicon.

Compton profiles of Si: Pseudopotential calculation and reconstruction effects

Using an ab initio pseudopotential calculation, we compute Compton profiles of silicon along the (100), (110), and (111) directions, and then reconstruct the pseudo-wave-functions to regain the oscillatory behavior of the all-electron valence wave functions near the atomic cores. We study the effect that this reconstruction has on the Compton profiles and their anisotropies. We find a decrease in the magnitude of the profiles at small wave vectors and in their anisotropies. These changes bring the theoretical predictions closer to experimental results.

Broken symmetry and pseudogaps in ropes of carbon nanotubes

We investigate the influence of tube-tube interactions in ropes of (10,10) carbon nanotubes, and find that these effects induce a pseudogap in the density of state (DOS) of the rope of width 0.1 eV at the Fermi level. In an isolated (n,n) carbon nanotube there are two bands that cross in a linear fashion at the Fermi level, making the nanotube metallic with a DOS that is constant in a 1.5 eV wide window around the Fermi energy. The presence of the neighbouring tubes causes these two bands to repel, opening up a band gap that can be as large as 0.3 eV. The small dispersion in the plane perpendicular to the rope smears out this gap for a rope with a large cross-sectional area, and we see a pseudogap at the Fermi energy in the DOS where the DOS falls to one third of its value for the isolated tube. This phenomenon should affect many properties of the behavior of ropes of (n,n) nanotubes, which should display a more semimetallic character than expected in transport and doping experiments, with the existence of both hole and electron carriers leading to qualitatively different thermopower and Hall-effect behaviors from those expected for a normal metal. Band repulsion like this can be expected to occur for any tube perturbed by a sufficiently strong interaction, for example, from contact with a surface or with other tubes.

Macroscopic evidence of soliton formation in multiterawatt laser-plasma interaction

A novel physical phenomenon has been observed following the interaction of an intense (10(19) W/cm(2)) laser pulse with an underdense plasma. Long-lived, macroscopic bubblelike structures have been detected through the deflection that the associated electric charge separation causes in a proton probe beam. These structures are interpreted as the remnants of a cloud of relativistic solitons generated in the plasma by the ultraintense laser pulse. This interpretation is supported by an analytical study of the soliton cloud evolution, by particle-in-cell simulations, and by a reconstruction of the proton-beam deflection.

Multi-MeV proton source investigations in ultraintense laser- foil interactions

A study of the properties of multi-MeV proton emission from thin foils following ultraintense laser irradiation has been carried out. It has been shown that the protons are emitted, in a quasilaminar fashion, from a region of transverse size of the order of 100-200 mum. The imaging properties of the proton source are equivalent to those of a much smaller source located several hundred mum in front of the foil. This finding has been obtained by analyzing proton radiographs of periodically structured test objects, and is corroborated by observations of proton emission from laser-heated thick targets.

Dynamics of electric fields driving the laser acceleration of multi-MeV protons

The acceleration of multi-MeV protons from the rear surface of thin solid foils irradiated by an intense (similar to 10(18) W/cm(2)) and short (similar to 1.5 ps) laser pulse has been investigated using transverse proton probing. The structure of the electric field driving the expansion of the proton beam has been resolved with high spatial and temporal resolution. The main features of the experimental observations, namely, an initial intense sheath field and a late time field peaking at the beam front, are consistent with the results from particle-in-cell and fluid simulations of thin plasma expansion into a vacuum.

Proton Radiography of a Laser-Driven Implosion

Protons accelerated by a picosecond laser pulse have been used to radiograph a 500 mu m diameter capsule, imploded with 300 J of laser light in 6 symmetrically incident beams of wavelength 1.054 mu m and pulse length 1 ns. Point projection proton backlighting was used to characterize the density gradients at discrete times through the implosion. Asymmetries were diagnosed both during the early and stagnation stages of the implosion. Comparison with analytic scattering theory and simple Monte Carlo simulations were consistent with a 3 +/- 1 g/cm(3) core with diameter 85 +/- 10 mu m. Scaling simulations show that protons > 50 MeV are required to diagnose asymmetry in ignition scale conditions.

Current-driven magnetic rearrangements in spin-polarized point contacts

A method for investigating the dynamics of atomic magnetic moments in current-carrying magnetic point contacts under bias is presented. This combines the nonequilibrium Green's function (NEGF) method for evaluating the current and the charge density with a description of the dynamics of the magnetization in terms of quasistatic thermally activated transitions between stationary configurations. This method is then implemented in a tight-binding (TB) model with parameters chosen to simulate the main features of the electronic structures of magnetic transition metals. We investigate the domain wall (DW) migration in magnetic monoatomic chains sandwiched between magnetic leads, and for realistic parameters find that collinear arrangement of the magnetic moments of the chain is always favorable. Several stationary magnetic configurations are identified, corresponding to a different number of Bloch walls in the chain and to a different current. The relative stability of these configurations depends on the geometrical details of the junction and on the bias; however, we predict transitions between different configurations with activation barriers of the order of a few tens of meV. Since different magnetic configurations are associated with different resistances, this suggests an intrinsic random telegraph noise at microwave frequencies in the I-V curves of magnetic atomic point contacts at room temperature. Finally, we investigate whether or not current-induced torques are conservative.

Current-induced embrittlement of atomic wires

Recent experiments suggest that gold single-atom contacts and atomic chains break at applied voltages of 1 to 2 V. In order to understand why current flow affects these defect-free conductors, we have calculated the current-induced forces on atoms in a Au chain between two Au electrodes. These forces are not by themselves sufficient to rupture the chain. However, the current reduces the work to break the chain, which results in a dramatic increase in the probability of thermally activated spontaneous fracture of the chain. This current-induced embrittlement poses a fundamental limit to the current-carrying capacity of atomic wires.

Electromigration of vacancies in copper

The total current-induced force on atoms in a Cu wire containing a vacancy are calculated using the self consistent one-electron density matrix in the presence of an electric current, without separation into electron-wind and direct forces. By integrating the total current-induced force, the change in vacancy migration energy due to the current is calculated. We use the change in migration energy with current to infer an effective electromigration driving force F-e. Finally, we calculate the proportionality constant rho* between F-e and the current density in the wire.

Magnetomechanical interplay in spin-polarized point contacts

We investigate the interplay between magnetic and structural dynamics in ferromagnetic atomic point contacts. In particular, we look at the effect of the atomic relaxation on the energy barrier for magnetic domain wall migration and, reversely, at the effect of the magnetic state on the mechanical forces and structural relaxation. We observe changes of the barrier height due to the atomic relaxation up to 200%, suggesting a very strong coupling between the structural and the magnetic degrees of freedom. The reverse interplay is weak; i.e., the magnetic state has little effect on the structural relaxation at equilibrium or under nonequilibrium, current-carrying conditions.

Semiclassical complex angular momentum theory and Pade reconstruction for resonances, rainbows, and reaction thresholds

A semiclassical complex angular momentum theory, used to analyze atom-diatom reactive angular distributions, is applied to several well-known potential (one-particle) problems. Examples include resonance scattering, rainbow scattering, and the Eckart threshold model. Pade reconstruction of the corresponding matrix elements from the values at physical (integral) angular momenta and properties of the Pade approximants are discussed in detail.

'Superluminal' tunnelling as a weak measurement effect

We exploit the analogy between the transfer of a pulse across a scattering medium and Aharonov's weak measurements to resolve the long standing paradox between the impossibility to exceed the speed of light and the seemingly "superluminal" behavior of a tunneling particle in the barrier or a photon in a "fast-light" medium. We demonstrate that superluminality occurs when the value of the duration tau spent in the barrier is uncertain, whereas when tau is known accurately, no superluminal behavior is observed. In all cases only subluminal durations contribute to the transmission which precludes faster-than-light information transfer, as observed in a recent experiment.