spin ARPES of transition metal dichachogenide

本论文采用分子束外延（MBE）方法成功地生长了Cr_2Se_3的单层膜（ML）、多层膜，并用反射式高能电子衍射（RHEED）和低能电子衍射（LEED）对晶体结构进行了表征。为了研究Cr2 Se 3薄膜的电子和磁性，我进行了X射线吸收光谱（XAS），X射线磁圆二色性（XMCD），角分辨光电子能谱（ARPES）。XAS和XMCD结果的Cr L边缘（2 p- 3d）显示出清晰的XMCD信号，表明ML Cr2 Se 3中的铁磁性。XAS和XMCD谱的实验结果与采用Cr ~（3+）组态的电荷转移团簇模型计算结果符合得很好。ML Cr2 Se 3的ARPES结果显示出高色散带和微小的电子袋在K点，这与第一性原理计算是一致的。此外，带结构的温度依赖性表现出显着的动量依赖的带分裂和带结构的变化低于铁磁转变。令人惊讶的是，当降低T时，电子袋的强度也显示出系统性的增加。在多层Cr_2Se_3中也观察到这种带结构随居里温度（Tc）的变化，其中Tc与ML情况相比降低。不寻常的能带结构的演变不能简单地解释的交换耦合。我的结论是，考虑到强的pd杂化和自旋轨道耦合（S.O. C）以及电子载流子密度是必要的，以了解Cr2 Se 3的铁磁性薄膜。

I have succeeded to grow monolayer (ML), multilayers of Cr2Se3 by molecule beam epitaxy (MBE) method and characterized the crystal structure by reflective high energy electron diffraction (RHEED) and low energy electron diffraction (LEED). To investigate the electronic and magnetic properties of Cr2Se3 ultrathin films, I carried out x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and angle resolved photoemission spectroscopy (ARPES). The Cr L edge (2p- 3d) of XAS and XMCD results showed a clear XMCD signal, indicating the ferromagnetism in ML Cr2Se3. The experimental XAS and XMCD spectra are in good agreement with the charge transfer cluster model calculation using Cr3+ configuration. The ARPES results of ML Cr2Se3 showed highly dispersive bands and tiny electron pocket at K points, which are consistent with first principles calculations. Further, temperature-dependence of band structure showed dramatic momentum-dependent band splitting and band structure change below the ferromagnetic transition. Surprisingly, the intensities of electron pockets also showed systematic increase when reducing T. This kind of band structure changes across Curie temperature (Tc) were also observed in multiplayer of Cr2Se3 in which Tc is reduced compared to the ML case. The unusual band structure evolution cannot be simply explained in terms of the exchange coupling. I concluded that consideration of strong pd hybridization and spin-orbit coupling (S.O.C) as well as electron carrier density is necessary to understand the ferromagnetism of Cr2Se3 ultrathin films.