The energy spectrum of positronium atoms generated at a solid surface reflects the electron density of states (DOS) associated solely with the first surface layer. Using spin-polarized positrons, the spin-dependent surface DOS can be studied. For this purpose, we have developed a spin-polarized positronium time-of-flight spectroscopy apparatus based on a ${}^{22}\mathrm{Na}$ positron source and an electrostatic beam transportation system, which enables the sampling of topmost surface electrons around the $\mathrm{\Gamma }$ point and near the Fermi level. We applied this technique to nonmagnetic Pt(111) and W(001), ferromagnetic Ni(111), Co(0001) and graphene on them, ${\mathrm{Co}}_2{\mathrm{FeGa}}_{0.5}{\mathrm{Ge}}_{0.5}$ (CFGG) and ${\mathrm{Co}}_2\mathrm{MnSi}$ (CMS). The results showed that the electrons of Ni(111) and Co(0001) surfaces have characteristic negative spin polarizations, while these spin polarizations vanished upon graphene deposition, suggesting that the spin polarizations of graphene on Ni(111) and Co(0001) were mainly induced at the Dirac points that were out of range in the present measurement. The CFGG and CMS surfaces also exhibited only weak spin polarizations suggesting that the half-metallicity expected for these bulk states was not maintained at the surfaces.

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自旋极化的正电子飞行时间光谱用于探测自旋极化的表面电子态

在固体表面产生的正电子原子的能谱反映了仅与第一表面层相关的态的电子密度（DOS）。使用自旋极化的正电子，可以研究自旋相关的表面DOS。为此，我们开发了一种基于自旋极化的正电子飞行时间光谱仪${}^{22}娜$ 正电子源和静电束传输系统，可对周围的最表面电子进行采样 $\mathrm{\Gamma }$点和费米能级附近。我们将此技术应用于非磁性Pt（111）和W（001），铁磁性Ni（111），Co（0001）和石墨烯，${\mathrm{有限公司}}_{\mathrm{2个}}{\mathrm{铁镓}}_{0.5}{\mathrm{通用电器}}_{0.5}$ （CFGG）和 ${\mathrm{有限公司}}_{\mathrm{2个}}\mathrm{锰硅}$（CMS）。结果表明，Ni（111）和Co（0001）表面的电子具有特征性的负自旋极化，而这些自旋极化在石墨烯沉积时消失，表明Ni（111）和Co（0001）上的石墨烯自旋极化为主要是在当前测量范围之外的狄拉克点上引起的。CFGG和CMS表面也仅表现出弱的自旋极化，表明在这些表面上不能保持这些体态所期望的半金属性。