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Séminaire “chaire d’excellence LANEF”

Vendredi 22 juin à 14h00,
Salle Nevill Mott (D420)

Orateur : A. Dimoulas (Institute of Nanoscience and Nanotechnology, National Center for Scientific Research DEMOKRITOS, Greece)
“Epitaxial 2D Topological Semimetals"


Topological Dirac and Weyl semimetals known as the “new 3D graphene”, are a new state of matter that has brought excitement in the scientific community. Discovering and engineering topological semimetals from the family of 2D Tansition Metal Dichalcogenide materials could open the way for exploitation of their unique topological properties by fabricating thin epitaxial films and devices on suitable crystalline substrates.
HfTe2, ZrTe2, TiTe2 and MoTe2 semimetal thin films are grown by Molecular Beam Epitaxy (MBE) on technologically important AlN/Si and InAs/Si substrates. ESRF synchrotron GIXD and STEM show that the materials are rotationally aligned with the InAs substrates having low in-plane mosaicity (lowest reported so far) and a clear quasi van der Waals (vdW) gap with the substrate indicating high quality vdW epitaxy. Imaging of electronic band structure by in-situ ARPES provides for the first time evidence that 1T-HfTe2 and 1T-ZrTe2 are Dirac semimetals. The 2D Dirac cones persist down to the ultimate 2D limit of a single layer indicating that ZrTe2 could be regarded as electronic analogue to graphene. Moreover, using STEM, we make the first direct observation at room temperature of the non-centrosymmetric orthorhombic (Td), type-II Weyl semimetal phase in epitaxial MoTe2. Its stabilty at room temperature is considered to be a result of enlarged lattice parameters imposed by the substrate which stabilizes an elongated interlayer antibonding state characteristic of Td-MoTe2. In 1 ML and thick 1T-TiTe2, using in-situ STM, STEM and GIXD we obtained evidence for a charge density wave instability at room temperature and correlate it with notable modifications in the electronic bandstructure imaged by in-situ ARPES.

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