A global team of researchers have observed a new microscopic mechanism enabling precise control of the magneto-optical properties of excitons in alloys of 2D semiconductors, potentially enabling new valleytronics applications.
As part of this study, high-quality monolayers of mixed alloys MoxW1-xSe2 with precisely controlled chemical composition were investigated. The samples were synthesized in the Czech Republic and encapsulated between flakes of hexagonal boron nitride fabricated in Japan. For a series of samples with varying molybdenum and tungsten content, systematic photoluminescence measurements were carried out at a temperature of 10 kelvin and in strong magnetic fields reaching up to 30 tesla at the Laboratoire National des Champs Magnétiques Intenses in Grenoble. An analysis of the emitted light in circular polarizations enabled highly accurate determination of the neutral exciton g-factor.
The obtained results reveal a strong and nonlinear dependence of the exciton g-factor on the chemical composition of the alloy. While the g-factor in both MoSe2 and WSe2 monolayers is close to −4, it undergoes a dramatic change in the mixed material, reaching very high values of approximately −10 for alloys containing about 20% Mo. Such a wide tuning range of the exciton g-factor has not been previously observed in these monolayers. Comparable values had earlier been achieved only in complex moiré heterostructures, which require precise alignment of stacked layers. “In our publication, we demonstrated that for transition metal dichalcogenide alloys, controlling the chemical composition of a monolayer is sufficient to achieve this goal,” explains MSc Katarzyna Olkowska-Pucko, a PhD student at the Faculty of Physics of the University of Warsaw and the first author of the paper, published in Physical Review Letters.
A key element of this work is the identification of the microscopic mechanism responsible for the observed effect. The combination of magneto-optical measurements and ab initio density functional theory (DFT) calculations revealed that the nonlinear modulation of the exciton g-factor originates from mixing of conduction-band states between the K and Q valleys, induced by local alloy inhomogeneity. In addition, it was shown that mechanical strain can further enhance this effect.
The team was led by researchers at the University of Warsaw, in collaboration with teams from the Wrocław University of Science and Technology, Sapienza University of Rome, University of Central Florida, Laboratoire National des Champs Magnétiques Intenses, National University of Singapore, CNR-IFN, the University of Chemistry and Technology of Prague and the National Institute for Materials Science in Japan.