spin ARPES of transition metal dichachogenide

利用分子束外延法（MBE）成功地生长出了Cr2Se3的单层（ML）、多层，并利用反射高能电子衍射（RHEED）和低能电子衍射（LEED）对晶体结构进行了表征。为了研究Cr2Se3超薄膜的电子和磁性能，我进行了x射线吸收光谱（XAS）、x射线磁圆二色性（XMCD）和角分辨光电发射光谱（ARPES）。XAS和XMCD结果的Cr L边（2p- 3d）显示明显的XMCD信号，表明ML Cr2Se3具有铁磁性。实验XAS和XMCD光谱与采用Cr3+结构的电荷转移团簇模型计算结果吻合较好。ML Cr2Se3的ARPES结果在K点显示出高色散带和微小的电子口袋，这与第一性原理计算一致。此外，能带结构的温度依赖性表现为明显的动量依赖性能带分裂和铁磁跃迁下能带结构的变化。令人惊讶的是，当温度降低时，电子口袋的强度也呈现出系统的增加，这种带结构的变化也在Tc降低的Cr2Se3中观察到。这种不寻常的能带结构演变不能简单地用交换耦合来解释。本文认为，考虑强pd杂化和自旋轨道耦合（S.O.C）以及载流子密度是理解Cr2Se3超薄膜铁磁性的必要条件。

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.