September 3, 2019.
After graphene was discovered in 2005 , there has been a number of studies on layered compounds, two-dimensional materials and other graphene-related materials, searching for new applications beyond the conventional ones. Theses materials possess unique quantum properties such as topological states and spin-valley coupling which can not be observed in bulk materials. From these extraordinary properties, these materials have been developed extensively aiming to fabricate the next generation devices including quantum computer and valleytronics.
In this project, we focus on the exploring and understanding the two-Dimensional Quantum States of Transition-Metal Dichalcogenides/Oxides by using Angle-Resolved Photoemission Spectroscopy. These materials have specific atomic structure where each in-layer of these materials is bonded with strong ionic bonding while each neighboring planes are interacted with weak van der Waals (vdW) force with few orders of magnitude weaker than in-plane interaction (Figure 1). These layered materials can be mechanically exfoliated to few layers similar to graphene. The unique properties in these materials were found to be originated from the spin-inversion breaking. In this report, we will present the discovery of new quantum property in Transition-Metal Dichalcogenides which are 1) discovery of topological quantum states in PdTe2 and 2) direct observation of strain-induced orbital valence band splitting in HfSe2 by sodium intercalation.
Fig. 1 Atomic structure of layered-transition metal dichalcogenides .
2. Discovery of Topological Quantum States in PdTe2
PdTe2 is the transition metal chalcogenide which shows not only interesting electronic structure but also exhibits superconductivity. The DFT calculation with orbital characters of PdTe2 is shown in Figure 2 which can be compared with the electronic structure experimentally measured by ARPES. While most of the features show similarity, the research team finds that the experimental results show additional features between 1-2 eV in binding energy. There is a flat band in out of plane direction suggesting that the bands are 2D/surface type (supported by the DFT calculation in Figures. 2c and 2d); this one shows a Rashba-type band splitting for the in-plane direction. There is also topological 3D (bulk) Dirac cone (BDP). The coexistence of these features along with superconductivity shows the richness in physics of how charge, spin and orbital interplay with each other. This provides the classification of topological quantum states which may be used in advanced electronic devices or quantum computers .
Fig. 2 a) Orbitally resolved bulk electronic structure of PdTe2. b) Our ARPES measurements reproduce the calculated out-of-plane dispersion, revealing the formation of bulk Dirac points (BDPs) and gapped crossings. c)-d) The corresponding bulk Dirac cones and topological surface states located within the inverted bandgaps are clearly observed in our ARPES measurements and supercell calculations. d) projected onto the first two unit cells along the G–M direction.
3. Direct observation of strain-induced orbital valence band splitting in HfSe2 by sodium intercalation
Discovery of the Valance Band Splitting of HfSe2 Induced by Surface Strain by using ARPES technique, the research team has investigated the change in electronic structure of HfSe2 whose surface is evaporated by Na metal as shown in Figure 3. The team finds that the valance energy bands splits with the separation of 400 meV and the band gap becomes smaller by around 280 meV. To understand this change, the team also calculated the DFT energy bands when there is strain on the surface which could be induced by the Na intercalation (see Figure 4); this strain can be experimentally detected by the deformation of Fermi surface map. This finding provides some concept of using strain to control the electronic structure in spintronics and straintronics devices .
Fig. 3 Electronic structure of HfSe2 single crystal along the M-G-K direction of (a) freshly cleaved and (b) decent amounts of sodium evaporation, clearly indicates the chemical shift and valence band splitting at the G point.
Fig. 4 (a) The calculated orbital projected band structure of HfSe2 using DFT calculation + vdW correction. (b) Schematic of armchair tensile strain caused by the addition of a sodium atom at the zone center. (c), (d) Schematic band dispersions near the valence band maximum.
4. Other Publications
4.1 Fermiology and Superconductivity of Topological Surface States in PdTe2 .
4.2 Enhanced ferromagnetism in mechanically exfoliated CVD-carbon films prepared by using adamantane as precursor .
4.3 High-Resolution Photoemission on Sr2RuO4 Reveals Correlation-Enhanced Effective Spin-Orbit Coupling and Dominantly Local Self-Energies .
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Assoc. Prof. Dr. Worawat Meevasana
School of Physics, Institute of Science, Suranaree University of Technology,
Nakhon Ratchasima - 30000, Thailand.