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Register a new userZeeman Splitting and Inverted Polarization of Biexciton Emission in Monolayer WS2
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We investigate the magnetic-field-induced splitting of biexcitons in monolayer WS$_2$ using polarization-resolved photoluminescence spectroscopy in out-of-plane magnetic fields up to 30 T. The observed $g$ factor of the biexciton amounts to $-3.89$, closely matching the $g$ factor of the neutral exciton. The biexciton emission shows an inverted circular field-induced polarization upon linearly polarized excitation, i.e. it exhibits preferential emission from the high-energy peak in a magnetic field. This phenomenon is explained by taking into account the configuration of the biexciton constituents in momentum space and their respective energetic behavior in magnetic fields. Our findings reveal the critical role of dark excitons in the composition of this many-body state.
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We have measured circularly polarized photoluminescence in monolayer MoSe2 under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of $mp$ 0.12 meV/T for emission arising, respectively, from the K and K valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley degeneracy. The magnitude of the Zeeman shift agrees with predicted magnetic moments for carriers in the conduction and valence bands. The relative intensity of neutral and charged exciton emission is modified by the magnetic field, reflecting the creation of field-induced valley polarization. At high doping levels, the Zeeman shift of the charged exciton increases to $mp$ 0.18 meV/T. This enhancement is attributed to many-body effects on the binding energy of the charged excitons.
Lifting the valley degeneracy of monolayer transition metal dichalcogenides (TMD) would allow versatile control of the valley degree of freedom. We report a giant valley exciton splitting of 18 meV/T for monolayer WS2, using the proximity effect from a ferromagnetic EuS substrate, which is enhanced by nearly two orders of magnitude from the 0.2 meV/T obtained by an external magnetic field. More interestingly, a sign reversal of the valley exciton splitting is observed as compared to that of WSe2 on EuS. Using first principles calculations, we investigate the complex behavior of exchange interactions between TMDs and EuS, that is qualitatively different from the Zeeman effect. The sign reversal is attributed to competing ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions for Eu- and S- terminated EuS surface sites. They act differently on the conduction and valence bands of WS2 compared to WSe2. Tuning the sign and magnitude of the valley exciton splitting offers opportunities for versatile control of valley pseudospin for quantum information processing.
We present Raman measurements of mono- and few-layer WS2. We study the monolayer A1 mode around 420 cm(-1) and its evolution with the number of layers. We show that with increasing layer number there is an increasing number of possible vibrational patterns for the out-of-plane Raman mode: in N-layer WS2 there are N Gamma-point phonons evolving from the A1 monolayer mode. For an excitation energy close to resonance with the excitonic transition energy we were able to observe all of these N components, irrespective of their Raman activity. Density functional theory calculations support the experimental findings and make it possible to attribute the modes to their respective symmetries. The findings described here are of general importance for all other phonon modes in WS2 and other layered transition metal dichalcogenide systems in the few layer regime.
Optical quantum emitters are a key component of quantum devices for metrology and information processing. In particular, atomic defects in 2D materials can operate as optical quantum emitters that overcome current limitations of conventional bulk emitters, such as yielding a high single-photon generation rate and offering surface accessibility for excitation and photon extraction. Here we demonstrate electrically stimulated photon emission from individual point defects in a 2D material. Specifically, by bringing a metallic tip into close proximity to a discrete defect state in the band gap of WS2, we induce inelastic tip-to-defect electron tunneling with an excess of transition energy carried by the emitted photons. We gain atomic spatial control over the emission through the position of the tip, while the spectral characteristics are highly customizable by varying the applied tip-sample voltage. Atomically resolved emission maps of individual sulfur vacancies and chromium substituent defects are in excellent agreement with the electron density of their respective defect orbitals as imaged via conventional elastic scanning tunneling microscopy. Inelastic charge-carrier injection into localized defect states of 2D materials thus provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources.
Atomically thin crystals of transition metal dichalcogenides are ideally suited to study the interplay of light-matter coupling, polarization and magnetic field effects. In this work, we investiagte the formation of exciton-polaritons in a MoSe2 monolayer, which is integrated in a fully-grown, monolithic microcavity. Due to the narrow linewidth of the polaritonic resonances, we are able to directly investigate the emerging valley Zeeman splitting of the hybrid light-matter resonances in the presence of a magnetic field. At a detuning of -54.5 meV (13.5 % matter constituent of the lower polariton branch), we find a Zeeman splitting of the lower polariton branch of 0.36 meV, which can be directly associated with an excitonic g factor of 3.94pm0.13. Remarkably, we find that a magnetic field of 6T is sufficient to induce a notable valley polarization of 15 % in our polariton system, which approaches 30% at 9T. Strikingly, this circular polarization degree of the polariton (ground) state exceeds the polarization of the exciton reservoir for equal magnetic field magnitudes by approximately 50%, as a consequence of enhanced relaxation of bosons in our monolayer-based system.
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