Within the last decade powerful methods have been developed to study surfaces using bright low-energy positron beams. These novel analysis tools exploit the unique properties of positron interaction with surfaces, which comprise the absence of exchange interaction, repulsive crystal potential and positron trapping in delocalized surface states at low energies. By applying reflection high-energy positron diffraction (RHEPD) one can benefit from the phenomenon of total reflection below a critical angle that is not present in electron surface diffraction. Therefore, RHEPD allows the determination of the atom positions of (reconstructed) surfaces with outstanding accuracy. The main advantages of positron annihilation induced Auger-electron spectroscopy (PAES) are the missing secondary electron background in the energy region of Auger-transitions and its topmost layer sensitivity for elemental analysis. In order to enable the investigation of the electron polarization at surfaces low-energy spin-polarized positrons are used to probe the outermost electrons of the surface. Furthermore, in fundamental research the preparation of well defined surfaces tailored for the production of bound leptonic systems plays an outstanding role. In this report, it is envisaged to cover both, the fundamental aspects of positron surface interaction and the present status of surface studies using modern positron beam techniques.
Bulk Fermi surface and momentum density in heavily doped La$_{2-x}$Sr$_x$CuO$_4$ using high resolution Compton scattering and positron annihilation spectroscopies
本文用康普顿散射法观察了La $_{2-x}$ Sr $_x$ CuO $_4$的过掺杂（$x$ =0.3）单晶体费米面（FS）。根据测量的康普顿剖面进行的二维（2D）动量密度重建在沿[100]的第三布里渊区产生了清晰的FS特征。密度泛函理论（DFT）计算与动量密度实验之间的定量一致表明，费米液体物理恢复在过掺杂制度。特别是预测的FS拓扑被发现是在良好的符合相应的实验数据。我们发现测量的正电子湮没辐射（2D-ACAR）谱的二维角相关和基于DFT的计算之间类似的定量协议。然而，无论是在理论上还是在实验上，2D-ACAR都没有给出扩展动量空间中FS的清晰特征。
Data-driven sensitivity analysis in a total-reflection high-energy positron diffraction (TRHEPD) experiment of the Si$_4$O$_5$N$_3$ / 6H-SiC (0001)-($\sqrt{3} \times \sqrt{3}$) R30$^\circ$
本文提出了一种实验测量的数据分析方法，它包括一个自动优化过程和灵敏度分析。该方法被施加到一个全反射高能正电子衍射（TRHEPD）实验，一种新的技术，确定表面结构或材料表面附近的原子的位置的结果。该方法数值求解了具有多个试表面结构的全动态量子衍射理论中的偏微分方程。在灵敏度分析中，我们着重于实验的不确定性和个别拟合参数的变化，这是通过解决方差-协方差矩阵的特征值问题进行分析。一台现代大规模并行超级计算机被用来在适度的计算时间内完成分析。敏感性分析为数据分析中变量的选择提供了依据，以保证实际的可靠性。在a Si $_4$ O $_5$ N $_3$ /6 H-SiC（0001）-（$\sqrt{3} \times \sqrt{3}$）R30 $^\circ$表面的结构测定中证明了本分析方法的有效性。此外，该分析方法适用于TRHEPD以外的许多实验。
