Low-energy excitations in type-II Weyl semimetal Td-MoTe2 evidenced through optical conductivity

By Ana Akrap, Université de Fribourg
Based on article published in Physical Review Materials

Transition metal dichalcogenides are a rich and diverse family of compounds. In both bulk and few-layer form, transition metal dichalcogenides are intensely studied for many of their interesting properties: excitons, superconductivity, and band-gap tuning by thickness. They are also pursued for their potential applications in electronics, optoelectronics, spintronics and valleytronics.

Molybdenum ditelluride (MoTe2) is a versatile transition metal dichalcogenide which showcases an abundance of exciting physics.  In it, a topological Weyl-II phase arises. This topological phase can be switched on and off by manipulating the associated structural phase transition. For instance, this is done by changing the temperature, applying strain, or hitting the sample with a light pulse. Furthermore, MoTe2 is a superconductor whose critical temperature becomes strongly enhanced as the crystal is thinned down to a monolayer. The superconducting transition in the monolayer sets in at 8 K, a temperature sixty times higher than that of the bulk compound. To understand these and many other effects in MoTe2, it is important to know this compound’s detailed band structure. Yet up to now, probing the low-energy band structure directly by experiments has proven difficult because the material has many bands crossing the Fermi level.

Through a combined use of detailed infrared spectroscopy and effective modeling, we have shown that the low-energy dynamical conductivity in MoTe2 is dominated by complex interband transitions due to a rich band structure at the Fermi level. We observe a narrow low-energy interband transition, whose unusually intense temperature dependence points to a strong thermal shift of the chemical potential in MoTe2. Our measurements imply that the tilted quasilinear bands, and an associated quickly dispersing band, are responsible for much of the low-energy interband transitions MoTe2. Finally, we detected the subtle signature of a tilted conical dispersion. Our findings help illuminate the low-energy behavior of this multifaceted compound.

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