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Magneto-optical spectroscopy of semiconducting transition metal dichalcogenides

Subject Area Experimental Condensed Matter Physics
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 336082288
 
Final Report Year 2023

Final Report Abstract

The proposed project involved optical and magneto-optical investigation of spin-valleylayer physics of van der Waals semiconductors such as MoS2, MoSe2, WS2, WSe2 and MoTe2. We are able to fulfil most of the objectives (approximately 80%) in the proposed project. Additionally, we find new surprises in our data which open new areas of research. For example, we measured valley Zeeman splitting of A and B excitons in monolayer and bulk of these materials under high magnetic fields of up to 30 T using micro reflectance and micro photoluminescence (PL) spectroscopy. We find that the g-factors of B excitons are comparable to that of A excitons in all of these materials. We also measured valley Zeeman splitting of interlayer excitons in bilayer and/or bulk MoS2, MoSe2, and MoTe2. The g-factors of interlayer excitons are of the opposite sign compared to the intralayer A and B excitons, which sets an optical fingerprint of the interlayer excitons. From a measurement of the diamagnetic shifts of the excitons, we calculate exciton Bohr radii in these materials as well. In the course of our magneto-optical investigations, we developed a new high-performance Faraday rotation spectroscopy method for performing spatiallyresolved Zeeman spectroscopy of excitons under low magnetic fields, and as a function of temperature. From our Faraday spectroscopy investigations on monolayer WS2 at liquid helium and room temperatures, we find that the exciton gfactors are identical within the experimental error. This behavior in van dar Waals semiconductors is drastically different from the III-V semiconductor quantum wells, where g-factors of excitons vary strongly with temperature. Using high-quality micro absorption measurements on hBN-encapsulated monolayer WS2, WSe2, MoS2 and MoSe2 layers, we find the interplay of dark excitons and trions with their bright counterparts and their role in oscillator strengths and line widths of the bright states. Our data suggest that dark and bright trions follow a Fermi- Dirac distribution as a function of temperature. These results are also in agreement with our high-field magneto reflectance data where trions are strongly polarized under the presence of a magnetic field. We deduce energies of the dark trions relative to bright trions in these samples. Our results indicate that the lowest energy transition in doped MoSe2 is a bright trion. However, in doped WSe2 and WS2, it is a dark trion. In doped MoS2, the lowest energy transition is a mixed state of bright and dark trions. In another work, we surprisingly discover a new excited bound state of a trion, the 2s trion, which appears lower in energy compared to the 2s exciton. We find that the exciton-trion pairs display a transfer of oscillator strength from exciton to trion, when temperature is lowered from room temperature to liquid helium temperature. This constitutes a fingerprint of the occurrence of the exciton-trion pair and was vital in our discovery of the 2s trion. Our work has opened a new area of research of excited-state trions which is gaining momentum from both experimental and theoretical sides.

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