43 research outputs found
Time-dependent density-functional theory of exciton-exciton correlations in the nonlinear optical response
We analyze possible nonlinear exciton-exciton correlation effects in the
optical response of semiconductors by using a time-dependent density-functional
theory (TDDFT) approach. For this purpose, we derive the nonlinear
(third-order) TDDFT equation for the excitonic polarization. In this equation,
the nonlinear time-dependent effects are described by the time-dependent
(non-adiabatic) part of the effective exciton-exciton interaction, which
depends on the exchange-correlation (XC) kernel. We apply the approach to study
the nonlinear optical response of a GaAs quantum well. In particular, we
calculate the 2D Fourier spectra of the system and compare it with experimental
data. We find that it is necessary to use a non-adiabatic XC kernel to describe
excitonic bound states - biexcitons, which are formed due to the retarded TDDFT
exciton-exciton interaction
Nonadiabatic Time-Dependent Spin-Density Functional Theory for strongly correlated systems
We propose a nonadiabatic time-dependent spin-density functional theory
(TDSDFT) approach for studying the single-electron excited states and the
ultrafast response of systems with strong electron correlations. The
correlations are described by the correlation part of the nonadiabatic
exchange-correlation (XC) kernel, which is constructed by using some exact
results for the Hubbard model of strongly correlated electrons. We demonstrate
that the corresponding nonadiabatic XC kernel reproduces main features of the
spectrum of the Hubbard dimer and infinite-dimensional Hubbard model, some of
which are impossible to obtain within the adiabatic approach. The theory may be
applied for DFT study of strongly correlated electron systems in- and
out-of-equilibrium, including the important case of nanostructures, for which
it leads to a dramatic reduction of necessary computational power
Magnetic anisotropy of FePt nanoparticles
We carry out a systematic theoretical investigation of Magneto Crystalline
Anisotropy (MCA) of L10 FePt clusters with alternating Fe and Pt planes along
the (001) direction. We calculate the structural relaxation and magnetic moment
of each cluster by using ab initio spin-polarized density functional theory
(DFT), and the MCA with both spin-polarized DFT (including spin-orbit coupling
self-consistently) and the torque method. We find that the MCA of any composite
structure of a given size is enhanced with respect to that of the same-sized
pure Pt or pure Fe cluster as well as to that of any pair of Fe and Pt atoms in
bulk L10 FePt. This enhancement results from the hybridization we observe
between the 3d orbital of the Fe atoms and the 5d orbital of their Pt
neighbors. This hybridization, however, affects the electronic properties of
the component atoms in significantly different ways. While it somewhat
increases the spin moment of the Fe atoms, it has little effect on their
orbital moment; at the same time, it greatly increases both the spin and
orbital moment of the Pt atoms. Given the fact that the spin-orbit coupling
(SOC) constant of Pt is about 7 times greater than that of Fe, this Fe-induced
jump in the orbital moment of the Pt atoms produces the increase in MCA of the
composite structures over that of their pure counterparts. That any composite
structure exhibits higher MCA than bulk L10 FePt results from the lower
coordination of Pt atoms in the cluster, whether Fe or Pt predominates within
it. We also find that bipyramidal clusters whose central layer is Pt have
higher MCA than their same-sized counterparts whose central layer is Fe. This
results from the fact that Pt atoms in such configurations are coordinated with
more Fe atoms than in the latter. By thus participating in more instances of
hybridization, they contribute higher orbital moments to the overall MCA of the
unit