Final PhD Oral Examination (Thesis Title: “Study of Several Two Dimensional Materials”)

Speaker: 
RUI YANG
Event Date and Time: 
Thu, 2017-12-14 09:00 - 11:00
Location: 
Room 200, Graduate Student Centre
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 
Public

Abstract:
The properties of several two-dimensional (2d) materials are studied. The main content is the study of magnetic impurities in graphene through phase coherent transport phenomena, including weak localization (WL) and universal conductance fluctuations (UCF). Magnetic impurities manifest themselves in the in-plane magnetic field and temperature dependence of the dephasing (phase breaking) rate. Our experiments unambiguously reveal the existence of magnetic impurities through the in-plane magnetic field dependence of WL and UCF. The properties of the magnetic impurities are further studied through the dephasing rate as a function of magnetic field and temperature. The WL dephasing rate as a function of in-plane field shows a non-monotonic behavior, which is rooted in the existence of magnetic impurities with a Landé g factor different from that of the free electron. The collapse dephasing rate as a function of temperature is a sign of quenching magnetic impurities, which could come from Kondo coupling between magnetic impurities and conduction electrons or Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling between magnetic impurities. The implications of our graphene experiments are two-fold: on the one hand, they provide new knowledge about the interplay among magnetic moments and electrons in graphene; on the other hand, they also established an effective tool to reveal the existence of magnetic impurities in 2d systems.

In addition to graphene, we also studied the exfoliation and stability of thin sheets of FeSe — a member of the Fe-based superconductors family. Our Raman Spectroscopy, Atomic Force Microscopy, Optical Microscopy and Time-of-Flight-Secondary-Ion-Mass-Spectroscopy experiments show that FeSe nanosheets decay in air, precipitation of Se and oxidation likely occurring during the decay process. Our transport measurement shows that FeSe nanosheets exposed briefly to air can still retain superconductivity.

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