Ultrafast exciton fluid flow in an atomically thin MoS₂ semiconductor

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Optics

Background:

• Excitons, or coupled electron-hole pairs, are fundamental quasiparticles in semiconductors.
• Collective states of excitons can exhibit unique nonlinear optical properties.

Purpose of the Study:

• To experimentally demonstrate and characterize a collective fluid-like state of excitons in molybdenum disulfide (MoS2).
• To investigate the conditions under which this exciton fluid behavior emerges and its transport properties.

Main Methods:

• Utilizing photoluminescence spectroscopy to observe exciton propagation in atomically thin MoS2.
• Conducting experiments with hexagonal-boron-nitride-encapsulated MoS2 devices.
• Performing theoretical simulations to understand momentum conservation and local equilibrium.

Main Results:

• Observed exciton fluid propagation over distances of at least 60 μm at speeds of ~1.8 × 10^7 m/s.
• The collective phase emerges above a critical laser power and below a critical temperature (Tc ≈ 150 K).
• Exciton fluid behavior is independent of crystallographic defects and geometric constraints, indicating robust collective dynamics.

Conclusions:

• Experimental evidence supports the description of exciton transport as a classical fluid.
• Momentum conservation and local equilibrium are key features enabling this fluid dynamics behavior.
• The findings open new avenues for exploring quantum fluid phenomena in atomically thin materials.