MASAP: A package for atomic scattering amplitude in solids

We present the MsSpec Atomic Scattering Amplitude Package (MASAP), composed of a computation program and a graphical interface to generate atomic scattering amplitude (ASA) of an atom, either isolated or embedded in an environment, at any chosen energy of the impinging electron up to ≈15 KeV. The ASA is calculated using an effective, complex optical potential which provides damping effects in the scattering process in a fully relativistic framework. Optionally, scalar relativistic and non-relativistic approximations are also available to assess their applicability to a given problem. In order to describe electron propagation in solids we suggest to replace ASA's based on Plane Waves (PW) scattering with effective ASA's based on curved Spherical Waves (SW) using truncated-overlapped potentials of the Muffin-Tin (MT) type constructed according to the Mattheiss prescription. The graphical user interface generates not only ASA data files providing atomic Differential Cross Sections (DCS) but also files of related quantities such as total Cross Section (CS), both elastic and inelastic, atomic t_l-matrices and phase shifts. We found in general that the imaginary part of the optical potential enhances the calculated elastic DCSs in the forward direction compared to the same potential without the imaginary part, a feature related to the optical theorem, but gives rise to a lower intensity at all other directions as expected due to the damping effect of the complex part of the potential. We show calculated differential and transport Cross Sections for aluminum and gold atoms both in isolation and in crystals with the Face-Centered-Cubic (FCC) structure.

本报告呈现了MsSpec原子散射振幅包（MASAP），该包包含一个计算程序及一个图形界面，旨在生成原子散射振幅（ASA）。此ASA可用于描述单个原子或嵌入环境中的原子的散射振幅，适用于撞击电子能量选择范围约为15 KeV。计算ASA时，采用了一种有效的复数光学势，该势在完全相对论框架内提供散射过程中的阻尼效应。此外，为评估特定问题的适用性，还提供了标量相对论和非相对论近似。为描述固体中的电子传播，建议将基于平面波（PW）散射的ASA替换为基于曲面球面波（SW）的有效ASA，使用截断重叠势构建的Muffin-Tin（MT）类型势，并遵循Mattheiss规定。图形用户界面不仅生成提供原子微分截面（DCS）数据的ASA数据文件，还生成相关数量文件，如总截面（CS），包括弹性和非弹性截面，原子t矩阵和相移。研究发现，光学势的虚部相对于无虚部的同一势，在正向计算中提高了弹性DCS，这与光学定理相关，但正如预期的那样，由于势的复数部分的阻尼效应，在所有其他方向上均导致强度降低。我们还展示了铝和金原子的计算微分和传输截面，包括孤立原子和具有面心立方（FCC）结构的晶体。