5月17日 美国University of Massachusetts Amherst 闫骏教授学术报告：Optical spectroscopy of layered transition metal dichalcogenides

报告题目：Optical spectroscopy of layered transition metal dichalcogenides

报告时间：2017年 5月 17日(周三)下午3：00-4：00
报告地点：唐仲英楼A-313

摘要：

Molybdenum and tungsten based transition metal dichalcogenides (TMDCs) with polymorphs in the hexagonal (H) and distorted octahedral (T’) phases are a class of layered materials with fascinating electronic properties.  H-TMDCs host massive Dirac fermions and have become a material system of choice for realizing valleytronics; bulk and monolayer T’-TMDCs are predicted to possess topologically nontrivial electronic bands.  In this talk I will discuss our recent studies using optical spectroscopy as a powerful non-destructive tool to probe the excitonic, vibrational and symmetry properties of H- and T’- TMDCs.  Using helicity-resolved measurements, we show that in H-(Mo,W)(S,Se)₂ atomic layers, due to conservation of pseudo-angular momentum,  the in-plane zone-center optical phonons switch the circular polarization of incident photons completely; in drastic contrast to valley-polarized luminescence that maintains the circular polarization of incident photons. The strong Coulomb interaction in reduced dimensions makes the Rydberg series of the tightly bound excitons well-separated in energy, and offers an intriguing platform for manipulating valley polarization and coherence. In T’-MoTe₂, we found the complex-looking phonon structures can be classified into zigzag modes and mirror modes respectively. This is confirmed by our measurements in bulk and monolayer samples. We further discovered that the low energy shear modes in bulk T’-MoTe₂ provide a sensitive tool for probing the breaking of inversion symmetry in the crystal, which is key to the existence of Weyl fermions in non-magnetic systems.