Holstein polaron in a valley-degenerate two-dimensional semiconductor

Area of Science:

• Condensed Matter Physics
• Materials Science

Background:

• Two-dimensional (2D) crystals offer tunable carrier density for exploring many-body interactions and novel composite particles.
• Holstein polarons, electron-phonon quasiparticles, are theorized to be key in high-temperature superconductivity and carrier mobility.
• Layered semiconductor MoS2 exhibits a 2D superconducting dome with a critical temperature of 12 K, the origin of which is puzzling.

Purpose of the Study:

• To investigate the nature of charge carriers in surface-doped MoS2.
• To identify the underlying mechanism responsible for the observed 2D superconductivity in MoS2.
• To explore the role of electron-phonon coupling in the superconducting properties of 2D materials.

Main Methods:

• High-resolution band mapping of charge carriers in MoS2.
• Analysis of spectral functions to identify quasiparticle properties.
• Theoretical explanation of electron-phonon coupling based on material properties like valley degeneracy.

Main Results:

• Discovery of Holstein polarons in surface-doped MoS2, evidenced by strong band renormalizations.
• Observation of a spectral function characteristic of Holstein polarons, previously unobserved.
• Electron-phonon coupling strength increases with doping along the superconducting dome, suggesting bipolaronic pairing.

Conclusions:

• Holstein polarons are identified as the charge carriers responsible for the 2D superconductivity in MoS2.
• The valley degeneracy of MoS2 enables strong intervalley coupling via acoustic phonons, leading to short-range electron-phonon coupling.
• The findings suggest bipolaronic pairing as the mechanism for superconductivity in this 2D material.