56-411, Department of Physics & Astronomy, Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea
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TEAM I: Orbital Angular Momentum (OAM) / 2D Materials
The research aim of team-I is to understand novel properties stemming from orbital degrees of freedom on surfaces and interfaces. They include Rashba, Dresselhaus and catalytic reaction. We wish to understand these phenomena by investigating the electronic structures. Some of the topics under investigation are listed below.
Orbital angular momentum: Au(111)
The momentum-dependent spin splitting in two-dimensional electronic structure is known as the Rashba effect. The Rashba-type spin splitting has been experimentally observed in electronic state under inversion symmetry breaking (ISB) including surface state of Au(111), Cu(111), Ag(111) and Ir(111) , topological insulators(TI) and interface state of heterostructure. While the band dispersion and the origin of 2DEG at the surface seem to be well studied, there are still issues to be resolved. For example, the origin of the surface band splitting is not settled down. It was recently suggested that orbital angular momentum (OAM) exists in surface state of (high-Z materials) and plays the key role in the Rashba-type spin splitting. Such OAM can be studied by circular dichroism (CD) as proved by the density-functional theory (DFT) calculation. We have combined our ARPES observation with OAM model to explain the origin of the surface band splitting in the surface states of Au(111). (Figure 1)
Catalytic activity: Pt and Pt-based alloys
Polymer exchange membrane fuel cell (PEMFC) received much attention due to its low operating temperature and clean by-product. Pt-based materials have been extensively investigated as the catalyst since they show the best performance in oxygen reduction reaction (ORR) in PEMFC. Recent investigations on Pt-based alloys revealed that the ORR could be enhanced significantly by adding transition metals to Pt. We study the electronic structure of Pt and Pt-alloys to investigate the origin of catalytic activity enhancement induced by transition metal substitution. Figure 2 presents an experimentally measured Fermi surface map of Pt (111) which shows relevant electronic states for the catalytic reaction. (Figure 2)
Figure 1. ARPES data of Au(111) surface states with Circularly polarized light [Phys. Rev. B 95, 115144 (2017)]
Figure 2. Pt surface states
Two-dimensional (2D) materials, in which the low-spatial dimension forces the the wave function of the electron confined within a plane, have attracted much attention in condensed matter physics due to their interesting physical phenomena such as superconductivity, density wave, and topological states. We investigate such phenomena in quasi-two dimensional materials - layered 3D materials in which layers are weakly coupled through the weak van der Waals force. We especially investigate the electronic structures of transition metal dichalcogenides (TMDCs), possibly useful for electronic device applications. A couple of examples are as follows.
Bulk properties of TMDCs
Recent studies have been geared toward investigation of monolayer TMDCs due to usefulness for optical devices. Nevertheless, study of bulk materials can be useful because bulk systems are readily available and monolayer TMDCs retain many of bulk properties. We study the bulk forms of these TMDCs, especially group 6 MX2 (M = Mo, W; X = S, Se). (Figure 3)
Hydrogen treated MoS2
Exfoliation and thin film growth methods have been widely used to make atomically thin TMDCs. However, these methods are not quite suitable for mass production of ultra thin films. In order to obtain such systems with an easy to access method, we used a new method in which we expose bulk MX2 to high pressure H2. ARPES data from pristine and hydrogen treated MoS2 are shown in Fig. 2. The data indicate that bulk MoS2 has been transformed into stack of multilayer MoS2. (Figure 4)
Figure 3. ARPES data of MX2 (M = Mo, W; X = S, Se)
[Scientific Reports, 6, 36389 (2016)]
Figure 4. ARPES data of pristine and hydrogen treated MoS2