Mendis B G
Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK.
Ultramicroscopy. 2015 Oct;157:1-11. doi: 10.1016/j.ultramic.2015.05.004. Epub 2015 May 6.
Electron vortex beams are distorted by scattering within a crystal, so that the wavefunction can effectively be decomposed into many vortex components. Using a Bloch wave approach equations are derived for vortex beam decomposition at any given depth and with respect to any frame of reference. In the kinematic limit (small specimen thickness) scattering largely takes place at the neighbouring atom columns with a local phase change of π/2rad. When viewed along the beam propagation direction only one vortex component is present at the specimen entrance surface (i.e. the 'free space' vortex in vacuum), but at larger depths the probe is in a mixed state due to Bragg scattering. Simulations show that there is no direct correlation between vortex components and the 〈Lz〉 pendellösung, i.e. at a given depth probes with relatively constant 〈Lz〉 can be in a more mixed state compared to those with more rapidly varying 〈Lz〉. This suggests that minimising oscillations in the 〈Lz〉 pendellösung by probe channelling is not the only criterion for generating a strong electron energy loss magnetic circular dichroism (EMCD) signal.
电子涡旋束在晶体内会因散射而发生畸变，从而使波函数能够有效地分解为多个涡旋分量。采用布洛赫波方法，推导了在任意给定深度以及相对于任意参考系的涡旋束分解方程。在运动学极限（小样品厚度）下，散射主要发生在相邻原子列处，局部相位变化为π/2弧度。当沿束传播方向观察时，在样品入射表面仅存在一个涡旋分量（即真空中的“自由空间”涡旋），但在较大深度处，由于布拉格散射，探针处于混合态。模拟表明，涡旋分量与〈Lz〉 Pendellösung之间没有直接关联，即与〈Lz〉变化较快的探针相比，在给定深度处〈Lz〉相对恒定的探针可能处于更混合的状态。这表明，通过探针通道化使〈Lz〉 Pendellösung中的振荡最小化并非产生强电子能量损失磁圆二色性（EMCD）信号的唯一标准。
