Spin-polarized electrons in monolayer MoS₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Mechanics

Background:

• Coulomb interactions are critical for the ground state of two-dimensional electron gases (2DEGs) at low densities.
• In conventional materials like silicon and gallium arsenide, electrons localize at low densities.
• Transition-metal dichalcogenides (TMDs) offer unique properties where Coulomb correlations persist at experimentally relevant densities.

Purpose of the Study:

• Investigate the ground state of a 2DEG in a gated monolayer of molybdenum disulfide (a TMD).
• Explore the role of Coulomb interactions in spin polarization and band occupation.
• Determine if spin polarization occurs even in the absence of an external magnetic field.

Main Methods:

• Utilized optical susceptibility measurements, a local, minimally invasive, and spin-selective probe.
• Conducted experiments on a gated monolayer of molybdenum disulfide.
• Applied a magnetic field of 9.0 T and varied electron concentrations up to 5 × 10^12 cm^-2.

Main Results:

• Presented evidence of a spin-polarized ground state in the 2DEG.
• Observed occupation of only two out of four available conduction bands.
• These occupied bands share the same spin but differ in valley quantum numbers.
• Spin polarization intensified with decreasing 2DEG density.

Conclusions:

• Coulomb interactions are key to symmetry breaking and spin polarization in TMD 2DEGs.
• The ground state is likely occupied by only two bands with identical spin, irrespective of magnetic field.
• Exchange couplings are proposed as the mechanism for spin alignment in these systems.