Spin-Polarized Positronium Time-of-Flight Spectroscopy for Probing Spin-Polarized Surface Electronic States

Area of Science:

• Surface science
• Condensed matter physics
• Materials science

Background:

• The energy spectrum of positronium atoms at surfaces relates to the surface's electron density of states (DOS).
• Spin-dependent surface DOS can be probed using spin-polarized positrons.

Purpose of the Study:

• To develop and apply a spin-polarized positronium time-of-flight spectroscopy technique.
• To investigate the spin-dependent surface DOS of various materials, including metals and graphene.

Main Methods:

• Development of a spin-polarized positronium time-of-flight spectroscopy apparatus.
• Utilizing a ^{22}Na positron source and electrostatic beam transportation.
• Sampling topmost surface electrons near the Γ point and Fermi level.

Main Results:

• Observed characteristic negative spin polarizations for Ni(111) and Co(0001) surfaces.
• Graphene deposition on Ni(111) and Co(0001) led to the vanishing of spin polarization.
• CFGG and CMS surfaces showed weak spin polarizations, deviating from expected bulk half-metallicity.

Conclusions:

• Graphene deposition significantly alters the spin polarization of ferromagnetic surfaces.
• The observed spin polarization changes suggest modifications at or near the Dirac points.
• Surface electronic properties of complex alloys like CFGG and CMS differ from their bulk characteristics.