Valley Phonons and Exciton Complexes in a Monolayer Semiconductor (2001.01769v1)

Abstract: The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconducting transitional metal dichalcogenides have emerged as an outstanding platform for studying these coupling effects because they possess unique spin-valley locking physics for hosting rich excitonic species and the reduced screening for strong Coulomb interactions. Here, we report the observation of multiple valley phonons, phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone, and the resulting exciton complexes in the monolayer semiconductor WSe2. From Lande g-factor and polarization analyses of photoluminescence peaks, we find that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation. This leads to a series of photoluminescence peaks as valley phonon replicas of dark trions. Using identified valley phonons, we also uncovered an intervalley exciton near charge neutrality, and extract its short-range electron-hole exchange interaction to be about 10 meV. Our work not only identifies a number of previously unknown 2D excitonic species, but also shows that monolayer WSe2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons.

Analysis of Valley Phonons and Exciton Complexes in a Monolayer Semiconductor

The paper "Valley Phonons and Exciton Complexes in a Monolayer Semiconductor" presents a comprehensive paper of the interactions between spin, charge, and phonon dynamics in monolayer semiconductor WSe₂. This research focuses on exploring the phenomena arising from valley phonons and their impact on exciton complexes, providing detailed insights into intervalley scattering mechanisms and their implications for understanding two-dimensional semiconducting materials.

Key Findings

The authors report the identification of multiple valley phonons in WSe₂, which facilitate efficient spin-conserving intervalley scattering processes. These phonons manifest during the formation and relaxation of excitons, contributing to the photoluminescence peaks associated with dark trion states. Notably, three distinct valley phonons were characterized, revealing a spin-preserving intervalley scattering mechanism that supersedes intravalley spin-flip processes during exciton formation.

A novel exciton near charge neutrality was uncovered, displaying an electron-hole exchange interaction of approximately 10 meV. Furthermore, the researchers identified several previously unknown excitonic species, indicating the potential of WSe₂ as a prime candidate for investigating complex spin, pseudospin, and phonon interactions.

Experimental Methodology

The method involved exfoliating WSe₂ monolayers and employing hexagonal boron nitride encapsulation to maintain stability. Photoluminescence spectroscopy under varying gate voltages and polarization conditions allowed detailed characterization of excitonic states and phonon-assisted scattering processes. The use of magneto-PL provided insights into valley index assignments via effective Landé g-factor extraction, aiding in the identification of valley phonon-assisted recombination processes.

Numerical and Spectroscopic Insights

The paper presents substantial numerical results, including specific g-factors associated with identified excitonic states. The findings indicate that K₃ valley phonons lead to recombination states with recognizable spectral and polarization characteristics, underscoring their significance in intervalley scattering. The energy differences between dark trion states and their phonon replicas were found to correlate with theoretical predictions, highlighting the reliability of the experimental approach and its alignment with quantum mechanical models.

Theoretical Implications

This work emphasizes the foundational role of valley phonons in exciton dynamics, bridging theoretical predictions and experimental observations. The findings challenge existing paradigms, suggesting that phonon-assisted processes are pivotal in the electron relaxation pathways within monolayer semiconductors. The identification of spin-conserved processes facilitated by K-point phonons enriches the understanding of pseudospin behavior and potentially offers pathways for controlling spin-valley states through optical methods.

Future Directions

Future research may prioritize quantifying electron-phonon coupling strengths to enhance theoretical models of monolayer WSe₂. There is potential for utilizing coherent control techniques to manage excitonic populations, exploiting stimulated Raman interactions in pursuit of advanced optoelectronic applications.

Overall, the paper provides valuable contributions to the field of semiconductor physics, offering insights that may stimulate further investigations into phonon-related phenomena and their implications for device technology and quantum materials research.