Excited-State Trions in Monolayer WS_{2}.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Mechanics

Background:

• Two-dimensional (2D) semiconductors exhibit unique excitonic properties.
• Trions, or charged excitons, are crucial quasiparticles in these materials.
• Understanding excited states of trions is essential for their technological applications.

Purpose of the Study:

• To discover and characterize the excited 2s trion state in monolayer tungsten disulfide (WS_{2}).
• To investigate the energetic position and binding energy of the 2s trion relative to excitons and other trions.
• To explore the temperature-dependent behavior of trions and their oscillator strength transfer to excitons.

Main Methods:

• Absorption spectroscopy was employed to experimentally detect the 2s trion.
• Ab initio calculations, specifically GW and Bethe-Salpeter equation (BSE) methods, were used for theoretical analysis.
• Temperature-dependent measurements were performed to study spectral evolution.

Main Results:

• An excited bound three-particle state, the 2s trion, was identified in monolayer WS_{2}.
• The 2s trion was observed energetically below the 2s exciton.
• The binding energy of the 2s trion (22 meV) was found to be smaller than that of 1s trions (37 meV intravalley, 31 meV intervalley).
• With increasing temperature, oscillator strengths transfer from 1s and 2s trions to their respective neutral excitons.

Conclusions:

• The discovery of the 2s trion expands our understanding of quasiparticle excitations in 2D materials.
• Trion-exciton resonance pairs exhibit temperature-dependent optical fingerprints.
• Trions play a significant role across the entire excitation spectrum of 2D semiconductors.