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The Strange Negative Magnetoresistance of MoSe2

​A team from INAC has produced and characterized, for the first time, a layer of molybdenum diselenide (MoSe2) with a triatomic thickness on an extended surface of about 1 cm². Electric conduction is more elaborate than in a classic semiconductor ("Mott gaps") and decreases in the presence of a magnetic field. These properties provide the compounds of the MoSe2 family with a high potential for electronics, optoelectronics, and valleytronics (named after the property of electrons called the "valley index").

Published on 14 March 2017

The physicists from INAC have succeeded in controlling the homogeneous deposition of one or several layers of MoSe2 with molecular thickness across large areas (1 cm²).

While the prior techniques, based on exfoliation using adhesive tape, only allowed for the obtaining of a "flake" 10 microns long, this time the scientists succeeded in completely covering a sapphire substrate with a layer of the thickness of 3 atoms (selenium, molybdenum, selenium), using van der Waals-type molecular beam epitaxy. By doing so, they were able to verify, through various analyses, that the structural properties of the ultra-thin layer were identical to those of the bulk material, and were able to study its electric conductivity by Mott gaps.

This conduction is carried out through successive trappings on defects. This very unique mode is thermally activated and is marked by a typical exponential dependence on the temperature.

The researchers from INAC have provided the first precise evidence for this relation between 160 K and room temperature. They also observed that ambient temperature conductivity increases when a magnetic field is applied. This phenomenon is rare in non-magnetic materials and may be interpreted in a way that is similar to the "weak localization" (of electrons) observed in disordered systems at low temperatures, and which also exhibits negative magnetoresistance.

Similar observations recently reported by other scientists on exfoliated tungsten disulfide layers WS2 indicate that this property of magneto-transport would be intrinsic to transition metal dichalcogenides.

XPS measurements (X ray-Photoelectronic Spectroscopy) and STEM observations (Scanning Transmission Electron Microscopy) were carried out at the nano-characterization platform of CEA Grenoble.

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