Excitonic Valley Effects in Monolayer WS₂ under High Magnetic Fields

Abstract

Transition-metal dichalcogenides can be easily produced as atomically thin sheets, exhibiting the possibility to optically polarize and read out the valley pseudospin of extremely stable excitonic quasiparticles present in these 2D semiconductors. Here, we investigate a monolayer of tungsten disulfide in high magnetic fields up to 30 T via photoluminescence spectroscopy at low temperatures. The valley degeneracy is lifted for all optical features, particularly for excitons, singlet and triplet trions, for which we determine the g factor separately. While the observation of a diamagnetic shift of the exciton and trion resonances gives us insight into the real-space extension of these quasiparticles, magnetic field-induced valley polarization effects shed light onto the exciton and trion dispersion relations in reciprocal space. The field dependence of the trion valley polarizations is in line with the predicted trion splitting into singlet and triplet configurations.

Interlayer excitons in a bulk van der Waals semiconductor

“…Strikingly, the effective g -factor of X IL is of opposite sign, with g IL  = +4 ± 0.5, indicating a different physical mechanism. Each excitonic resonance exhibits a diamagnetic shift depending on B 2 : , with e the electronic charge, the excitonic root-mean-square (RMS) radius, and the reduced mass 22 . is the deviation of the exciton’s mean transition energy for from the zero-field value.…”

Engineering Valley Polarization of Monolayer WS2: A Physical Doping Approach

“…By comparing these findings with a general doping dependent valley relaxation mechanism and previous reports on exciton dynamics, we reveal the correlation between P c and physical doping. Our results demonstrate the tunable valley polarization degree under low cost and helium‐free condition, equally effective as conducted under 10 T magnetic field . We successfully enriched the understanding of valley relaxation process by electrostatic and optical doping means, which denotes an essential step toward practical valleytronic applications.…”

“…Compared to magnetic methods, electrical/optical control of valley states are more favored for future valleytronic devices operations. As a more practical method to create population imbalance between two valleys, our approaches correspond to a 10 T magnetic field which enhances polarization degree of trion emission from negligible value to about 25% at 4.2 K . For PL without magnetic field, Cui et al reported P c = 40% at near resonance excitation (2.088 eV) and 16% at off‐resonance excitation (2.331 eV), measured at 10 K. Korn and co‐workers reported 21% for singlet trion and 34% for triplet trion with 2.15 eV laser excitation at 4.5 K .…”

“…Last, we compare our results with the previous reports on tailoring P c in exfoliated monolayer WS 2 . Recent magneto‐PL study has demonstrated valley Zeeman splitting and monotonically increasing P c of A − emission with applied magnet field in out of plane direction . Compared to magnetic methods, electrical/optical control of valley states are more favored for future valleytronic devices operations.…”

Micro-Extinction Spectroscopy (MExS): a versatile optical characterization technique

“…At monolayer thickness, TMDs can exhibit direct electronic and optical bandgaps ranging from the visible to the near-infrared. Optical spectroscopy techniques such as photoluminescence (PL) and Raman are currently the key methods used in studying TMD properties such as bandgap energy, [15,16] emission efficiency, [17][18][19] and defect density [19][20][21]. However, PL and Raman spectroscopies have limited quantum efficiency, making them unfit for high-throughput applications.…”