University of Tsukuba
HEAD：CCS 名称／ CCS・筑波大ロゴ

Quantum Condensed Matter Physics
Time-Dependent Density Functional Theory for Ultrafast Dynamics Induced by Laser Pulses
Time-dependent density functional theory (TDDFT) is widely used to describe(a)electron dynamics in atoms, molecules, and solids induced by light fields. We havebeen developing a real-time and real-space computational code to solve thetime-dependent Kohn-Sham equation in crystalline solid, the basic equation inTDDFT. The code is efficiently parallelized in k-points and orbitals in the timeevolution calculations. The code may be used to calculate dielectric function, ultrafast
electron-phonon dynamics of femto- and attosecond time scale induced by intenseand ultrashort laser pulses, and coupled dynamics of electrons and electromagnetic
fields in the multiscale implementation of the code.

(b)

(c)

Recently we have applied the real-time TDDFT method to the coherent phonon

generation in GaAs, a well-known compound semiconductor [see Fig. (a)], induced byultrashort laser pulses. Figure (b) shows laser electric field (top) and the force(d)(bottom) acting on phonon mode induced by the laser pulse. The electron densitychange from the ground state is shown when the laser field is maximum, and when
the laser field ended, as shown in Figs. (c) and (d) respectively.

Numerical Simulation on the Multiple Exciton Generation in Single-Walled Carbon Nanotubes
The multiple exciton generation (MEG) in single-walled carbon nanotubes(e)(SWNTs) is theoretically investigated. We perform the calculation of the MEG rate byconsidering the direct photogeneration of multiple excitons. In particular, we only

consider the generation of two-exciton states. A single photon does not excite twoexcitons since it is forbidden by the selection rule. However, it is possible to consider

the situation where superposed states of multiexciton states are realized because theCoulomb interaction can resonantly couple among multiexciton states. This process
was originally proposed for the MEG in nanocrystal. The direct photogeneration

produces many excitons by a single photon absorption and thus it is the simultaneous
process [see Fig. (e)].

By solving the Bethe-Salpeter equation within the single orbital tight-binding

approximation, our calculation shows that it is possible to generate multiexcitons by a(f)single photon as a consequence of the resonant coupling between optically active
single-exciton state and the multiexciton states. Generation rates for one excitonshows the usual structure of the linear absorption spectrum with the lowest exciton

state, E11, and higher states including the continuum of excitons above the band gap[Fig. (f)]. On the other hand, the generation rate of the two-exciton abruptly begins at
the threshold excitation energy, which is two times larger than the lowest exciton

energy, i.e. 2E11. This fact corroborates the fact that the single photon can generate

multi excitons. This is a consequence of the strong Coulomb interaction between

excitons and of the van Hove singularity of the density of states for the multiexcitons.
This is very unique feature of SWNTs that have quasi-one-dimensional structures.

Furthermore, our calculation predicts the threshold energy for MEG as 2E11, which isconsistent with the recent experimental results. The present work clarifies themicroscopic process of MEG process in SWNTs and sheds light on the underlying
physics of the MEG.

FOOT：連絡先／ CCS のアドレス

http://www.ccs.tsukuba.ac.jp/