36,400 research outputs found
Monte-Carlo approach to calculate the proton stopping in warm dense matter within particle-in-cell simulations
A Monte-Carlo approach to proton stopping in warm dense matter is implemented
into an existing particle-in-cell code. The model is based on multiple
binary-collisions among electron-electron, electron-ion and ion-ion, taking
into account contributions from both free and bound electrons, and allows to
calculate particle stopping in much more natural manner. At low temperature
limit, when ``all'' electron are bounded at the nucleus, the stopping power
converges to the predictions of Bethe-Bloch theory, which shows good
consistency with data provided by the NIST. With the rising of temperatures,
more and more bound electron are ionized, thus giving rise to an increased
stopping power to cold matter, which is consistent with the report of a
recently experimental measurement [Phys. Rev. Lett. 114, 215002 (2015)]. When
temperature is further increased, with ionizations reaching the maximum,
lowered stopping power is observed, which is due to the suppression of
collision frequency between projected proton beam and hot plasmas in the
target.Comment: 6 pages, 4 figure
Monte-Carlo approach to calculate the ionization of warm dense matter within particle-in-cell simulations
A physical model based on a Monte-Carlo approach is proposed to calculate the
ionization dynam- ics of warm dense matters (WDM) within particle-in-cell
simulations, and where the impact (col- lision) ionization (CI), electron-ion
recombination (RE) and ionization potential depression (IPD) by surrounding
plasmas are taken into consideration self-consistently. When compared with
other models, which are applied in the literature for plasmas near thermal
equilibrium, the temporal re- laxation of ionization dynamics can also be
simulated by the proposed model. Besides, this model is general and can be
applied for both single elements and alloys with quite different composi-
tions. The proposed model is implemented into a particle-in-cell (PIC) code,
with (final) ionization equilibriums sustained by competitions between CI and
its inverse process (i.e., RE). Comparisons between the full model and model
without IPD or RE are performed. Our results indicate that for bulk aluminium
in the WDM regime, i) the averaged ionization degree increases by including
IPD; while ii) the averaged ionization degree is significantly over estimated
when the RE is neglected. A direct comparison from the PIC code is made with
the existing models for the dependence of averaged ionization degree on thermal
equilibrium temperatures, and shows good agreements with that generated from
Saha-Boltzmann model or/and FLYCHK code.Comment: 7 pages, 4 figure
Scanning tunneling microscopy and spectroscopy of nanoscale twisted bilayer graphene
Nanoscale twisted bilayer graphene (TBG) is quite instable and will change
its structure to Bernal (or AB-stacking) bilayer with a much lower energy.
Therefore, the lack of nanoscale TBG makes its electronic properties not
accessible in experiment up to now. In this work, a special confined TBG is
obtained in the overlaid area of two continuous misoriented graphene sheets.
The width of the confined region of the TBG changes gradually from about 22 nm
to 0 nm. By using scanning tunnelling microscopy, we studied carefully the
structure and the electronic properties of the nanoscale TBG. Our results
indicate that the low-energy electronic properties, including twist-induced van
Hove singularities (VHSs) and spatial modulation of local density-of-state, are
strongly affected by the translational symmetry breaking of the nanoscale TBG.
Whereas, the electronic properties above the energy of the VHSs are almost not
influenced by the quantum confinement even when the width of the TBG is reduced
to only a single moire spot.Comment: 4 Figure
Iron-boron pair dissociation in silicon under strong illumination
The dissociation of iron-boron pairs (FeB) in Czochralski silicon under strong illumination was investigated. It is found that the dissociation process shows a double exponential dependence on time. The first fast process is suggested to be caused by a positive Fe in FeB capturing two electrons and diffusion triggered by the electron-phonon interactions, while the second slow one would involve the capturing of one electron followed by temperature dependent dissociation with an activation energy of (0.21 +/- 0.03) eV. The results are important for understanding and controlling the behavior of FeB in concentrator solar cells
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