722 research outputs found
Nonequilibrium Green's Function Approach to Phonon Transport in Defective Carbon Nanotubes
We have developed a new theoretical formalism for phonon transport in
nanostructures using the nonequilibrium phonon Green's function technique and
have applied it to thermal conduction in defective carbon nanotubes. The
universal quantization of low-temperature thermal conductance in carbon
nanotubes can be observed even in the presence of local structural defects such
as vacancies and Stone-Wales defects, since the long wavelength acoustic
phonons are not scattered by local defects. At room temperature, however,
thermal conductance is critically affected by defect scattering since incident
phonons are scattered by localized phonons around the defects. We find a
remarkable change from quantum to classical features for the thermal transport
through defective CNTs with increasing temperature.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let
Electronic Transport in Fullerene C20 Bridge Assisted by Molecular Vibrations
The effect of molecular vibrations on electronic transport is investigated
with the smallest fullerene C20 bridge, utilizing the Keldysh nonequilibrium
Green's function techniques combined with the tight-binding molecular-dynamics
method. Large discontinuous steps appear in the differential conductance when
the applied bias-voltage matches particular vibrational energies. The magnitude
of the step is found to vary considerably with the vibrational mode and to
depend on the local electronic states besides the strength of
electron-vibration coupling. On the basis of this finding, a novel way to
control the molecular motion by adjusting the gate voltage is proposed.Comment: 9 pages, 4 figures, accepted for publication in Phys. Rev. Let
Neutron Diffraction Study of FeSn
A neutron diffraction study on the powdered sample of FeSn has been made in order to determine the magnetic structure of this compound. The magnetic unit cell is twice as large as the chemical cell, being doubled along the c-axis. The moments of iron atoms are ferromagnetically coupled within a c-plane, while they are coupled antiferromagnetically to those on the adjacent c-planes. The moments lie in the c-plane. The atomic magnetic moment of Fe is obtained to be 1.5_5±0.1μ_B at liquid nitrogen temperature
Second-order nonadiabatic couplings from time-dependent density functional theory: Evaluation in the immediate vicinity of Jahn-Teller/Renner-Teller intersections
For a rigorous quantum simulation of nonadiabatic dynamics of electrons and
nuclei, knowledge of not only first-order but also second-order nonadiabatic
couplings (NAC), is required. Here we propose a method to efficiently calculate
second-order NAC from time-dependent density functional theory (TDDFT), on the
basis of the Casida ansatz adapted for the computation of first-order NAC,
which has been justified in our previous work and can be shown to be valid for
calculating second-order NAC between ground state and singly excited states
within the Tamm-Dancoff approximation. Test calculations of second-order NAC in
the immediate vicinity of Jahn-Teller and Renner-Teller intersections show that
calculation results from TDDFT, combined with modified linear response theory,
agree well with the prediction from the Jahn-Teller / Renner-Teller models.
Contrary to the diverging behavior of first-order NAC near all types of
intersection points, the Cartesian components of second-order NAC are shown to
be negligibly small near Renner-Teller glancing intersections, while they are
significantly large near the Jahn-Teller conical intersections. Nevertheless,
the components of second-order NAC can cancel each other to a large extent in
Jahn-Teller systems, indicating the background of neglecting second-order NAC
in practical dynamics simulations. On the other hand, it is shown that such a
cancellation becomes less effective in an elliptic Jahn-Teller system and thus
the role of second-order NAC needs to be evaluated in the rigorous framework.
Our study shows that TDDFT is promising to provide accurate data of NAC for
full quantum mechanical simulation of nonadiabatic processes.Comment: 27 pages, 10 figure
Time-Dependent Multi-Component Density Functional Theory for Coupled Electron-Positron Dynamics
Electron-positron interactions have been utilized in various fields of
science. Here we develop time-dependent multi-component density functional
theory to study the coupled electron-positron dynamics from first principles.
We prove that there are coupled time-dependent single-particle equations that
can provide the electron and positron density dynamics, and derive the formally
exact expression for their effective potentials. Introducing the adiabatic
local density approximation to time dependent electron-positron correlation, we
apply the theory to the dynamics of a positronic lithium hydride molecule under
a laser field. We demonstrate the significance of electron-positron dynamical
correlation by revealing the complex positron detachment mechanism and the
suppression of electronic resonant excitation by the screening effect of the
positron.Comment: 6 pages, 3 figure
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