spin ARPES of transition metal dichachogenide

用分子束外延(MBE)方法成功地生长了Cr2Se3的单层和多层，并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超薄膜的电磁性能，我进行了x射线吸收光谱(XAS)、x射线磁性圆二色谱(XMCD)和角分辨光电子能谱(ARPES)的测试。XAS和XMCD的Cr L边(2p-3d)显示出清晰的XMCD信号，表明ML Cr2Se3具有铁磁性。实验的XAS和XMCD谱与用Cr3+组态计算的电荷转移团簇模型符合得很好。ML Cr2Se3的ARPES结果显示在K点处有高度色散的能带和微小的电子口袋，这与第一性原理计算结果一致。此外，能带结构随温度的变化表现出明显的动量依赖的能带分裂和铁磁相变下的能带结构变化。令人惊讶的是，随着T的降低，电子口袋的强度也出现了系统性的增加，这种跨越居里温度(T_C)的能带结构变化也在Cr2Se_3的多层膜中观察到，其中T_c比ML情况下降低。这种反常的能带结构演化不能简单地用交换耦合来解释。我的结论是，考虑强的Pd杂化和自旋-轨道耦合(S.O.C)以及电子载流子密度对于理解Cr2Se3超薄膜的铁磁性是必要的。

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.