Kazuhiro Yabana

Attosecond band-gap dynamics in silicon

Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling. Excited electrons in semiconducting silicon are tracked on a time scale faster than the lattice vibrations. [Also see Perspective by Spielmann] Watching electrons dart through silicon The ultimate speed limit in electronic circuitry is set by the motion of the electrons themselves. Schultze et al. applied attosecond spectroscopy to glimpse this motion in a sample of silicon, the semiconducting building block of modern integrated circuits (see the Perspective by Spielmann). The technique distinguished the electron dynamics—which proceed faster than a quadrillionth of a second after laser excitation—from the comparatively slower lattice motion of the silicon atomic nuclei. Science, this issue p. 1348; see also p. 1293

Real-space, real-time method for the dielectric function

We present an algorithm to calculate the linear response of periodic systems in the time-dependent density functional theory, using a real-space representation of the electron wave functions and calculating the dynamics in real time. The real-space formulation increases the efficiency for calculating the interaction, and the real-time treatment decreases storage requirements and allows the entire frequency-dependent dielectric function to be calculated at once. We give as examples the dielectric functions of a simple metal, lithium, and an elemental insulator, diamond.

Optical response of small silver clusters

The time-dependent local-density approximation is applied to the optical response of the silver clusters, Ag2 ,A g 3 ,A g 8, and Ag 9 . The calculation includes all the electrons beyond the closed-shell Ag 111 ionic core, thus including explicitly the filled d shell in the response. The excitation energy of the strong surface plasmon near 4 eV agrees well with experiment. The theoretical transition strength is quenched by a factor of 4 with respect to the pure s-electron sum rule in Ag8 due to the d electrons. Also, in this system the theoretical integrated strength function is remarkably well described by the Mie theory using the bulk dielectric constant. However, the strength is somewhat less than reported experimentally. @S1050-2947~99!04511-4#

Glauber model analysis of the fragmentation reaction cross sections of 11Li

Abstract A theory to evaluate the reaction, interaction, and two-neutron removal cross sections of 11 Li is presented in the framework of the Glauber theory in order to elucidate the role of the halo-neutron density distribution. The nuclear parts of the cross sections can be calculated with the neutron densities as input. Two kinds of the densities are compared with experiment at 800 MeV/nucleon. A good agreement with experiment is obtained for light targets. It is emphasized that the optical-limit approximation is not good for calculating the reaction cross section of a nucleus with halo structure. The reaction mechanism underlying the two-neutron removal process is discussed in some detail. A relation of meson-production cross sections with the property of the profile function is also discussed.

Application of the time-dependent local density approximation to optical activity

As part of a general study of the time-dependent local density approximation (TDLDA), we here report calculations of optical activity of chiral molecules. The theory automatically satisfies sum rules and the Kramers-Kronig relation between circular dichroism and optical rotatory power. We find that the theory describes the measured circular dichroism of the lowest states in methyloxirane with an accuracy of about a factor of two. In the chiral fullerene C_76 the TDLDA provides a consistent description of the optical absorption spectrum, the circular dichroism spectrum, and the optical rotatory power, except for an overall shift of the theoretical spectrum.

Time-dependent density functional theory for strong electromagnetic fields in crystalline solids

We apply the coupled dynamics of time-dependent density functional theory and Maxwell equations to the interaction of intense laser pulses with crystalline silicon. As a function of electromagnetic field intensity, we see several regions in the response. At the lowest intensities, the pulse is reflected and transmitted in accord with the dielectric response, and the characteristics of the energy deposition is consistent with two-photon absorption. The absorption process begins to deviate from that at laser intensities ~ 10^13 W/cm^2, where the energy deposited is of the order of 1 eV per atom. Changes in the reflectivity are seen as a function of intensity. When it passes a threshold of about 3 \times 1012 W/cm2, there is a small decrease. At higher intensities, above 2 \times 10^13 W/cm^2, the reflectivity increases strongly. This behavior can be understood qualitatively in a model treating the excited electron-hole pairs as a plasma.

Photoabsorption spectra in the continuum of molecules and atomic clusters

We present linear response theories in the continuum capable of describing photoionization spectra and dynamic polarizabilities of finite systems with no spatial symmetry. Our formulations are based on the time-dependent local density approximation with uniform grid representation in the three-dimensional Cartesian coordinate. Effects of the continuum are taken into account either with a Green’s function method or with a complex absorbing potential in a real-time method. The two methods are applied to a negatively charged cluster in the spherical jellium model and to some small molecules (silane, acetylene and ethylene).

A massively-parallel electronic-structure calculations based on real-space density functional theory

Abstract Based on the real-space finite-difference method, we have developed a first-principles density functional program that efficiently performs large-scale calculations on massively-parallel computers. In addition to efficient parallel implementation, we also implemented several computational improvements, substantially reducing the computational costs of O ( N 3 ) operations such as the Gram–Schmidt procedure and subspace diagonalization. Using the program on a massively-parallel computer cluster with a theoretical peak performance of several TFLOPS, we perform electronic-structure calculations for a system consisting of over 10,000 Si atoms, and obtain a self-consistent electronic-structure in a few hundred hours. We analyze in detail the costs of the program in terms of computation and of inter-node communications to clarify the efficiency, the applicability, and the possibility for further improvements.

Linear response theory in the continuum for deformed nuclei: Green's function vs time-dependent Hartree-Fock with the absorbing boundary condition

The continuum random-phase approximation is extended to the one applicable to deformed nuclei. We propose two different approaches. One is based on the use of the three-dimensional (3D) Greens function, and the other is the small-amplitude TDHF with the absorbing boundary condition. Both methods are based on the 3D Cartesian grid representation and applicable to systems with no symmetry on nuclear shape. The accuracy and identity of these two methods are examined with the BKN interaction. Using the full Skyrme energy functional in the small-amplitude TDHF approach, we study the isovector giant dipole states in the continuum for {sup 16}O and for even-even Be isotopes.

Reaction mechanism of 11Li at intermediate energy

Abstract The reaction of 11 Li, a nucleus characterized by a halo structure, is analyzed by the ( 9 Li + n + n)-target four-body model in the intermediate energy region. The eikonal approximation and adiabatic approximation are employed not only to make the microscopic calculation feasible but to get intuitive pictures for the reaction mechanism. To test the accuracy of the eikonal approximation in the intermediate energy region, the deuteroon reaction is also analyzed by the (p + n)-target three-body model and compared with the coupled discretized continuum-channels analysis. It is found that the eikonal approximation works quite successfully for various reaction mechanisms including the break-up process. The total reaction cross section and the two-neutron removal cross section of the 11 Li + 12 C reaction are analyzed. The measured interaction cross section is nicely reproduced in our calculation. It is found that the elastic break-up process plays an important role in the intermediate energy region.