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Spin-polarized electrons in atomic layer materials formed on solid surfaces

Prog. Surf. Sci. 97, 100665 (2022).
In this review, we summarize the recent progress in the understanding of the spin-polarized electronic states in two-dimensional (2D) atomic layer materials (ALMs) formed on solid surfaces. The spin-polarized electronic states caused by the combination of spin-orbit coupling (SOC) with broken spatial inversion symmetry along the surface normal direction is one of the most exotic phenomena that appears on ALMs formed on solid surfaces as well as clean solid surfaces. The so-called Rashba-Bychkov (RB) effect that arises from the potential gradient induced by broken inversion symmetry was believed to be the main origin of these spin-polarized electronic states. However, the spin texture of most ALMs are different from that caused by the ideal RB effect. Due to the high impact of the spin-polarized electronic states of 2D materials in not only spin-related fundamental science but also in applications since they are the key concepts to realize future semiconductor spintronics devices, much efforts have been made to elucidate the origin of these peculiar spin textures. So far, the deviations in spin texture from the ideal one have been attributed to be induced by perturbation, such as entanglement of spin and orbital momenta. In this review, we first illustrate how the symmetry of the ALM’s atomic structure can affect the spin texture, and then introduce that various spin textures, ranging from the RB-type and symmetry-induced type to spin textures that cannot be explained based on the origins proposed so far, can be simply induced by the orbital angular momentum. This review aims to provide an overview on the insights gained on the spin-polarized electronic states of ALMs and to point out opportunities for exploring exotic physical properties when combining spin and other physics, e.g. superconductivity, and to realize future spintronics-based quantum devices.

 

Distinct Tamm and Shockley surface states on Re(0001) mixed by spin-orbit interaction

Phys. Rev. B 105, L241412 (2022).
Tamm and Shockley states, these two paradigmatic concepts are used to describe surface states not only in electronic systems but also in photonic and phononic crystals. The Re(0001) surface hosts both types of electronic surface states in neighboring but qualitatively different energy gaps. Interestingly, spin-orbit interaction generates a double W-shaped energy vs k dispersion by mixing both types of states and lifting their spin degeneracy. By combining spin- and angle-resolved photoemission, tight-binding model calculations, as well as density functional theory including the photoemission process, we develop verifiable criteria to distinguish between the two types of surface states and arrive at a consistent picture of the role of spin-orbit interaction in such a scenario.

Rashba-split image-potential state and unoccupied surface electronic structure of Re(0001)

Phys. Rev. B 105, 155419 (2022).
The influence of spin-orbit interaction on the unoccupied electronic structure of the Re(0001) surface is investigated by spin- and angle-resolved inverse photoemission and density-functional theory calculations. In the two high-symmetry azimuths ΓK and ΓM, we identify transitions into d-derived bulk states as well as different types of surface states. The Rashba-type spin-split hole pocket around Γ finds continuation in empty spin-split surface states for higher k, thereby forming W-shaped states whose lower parts are partially occupied. A large energy gap below and above the vacuum energy around Γ hosts image-potential-induced surface states. The n=1 member of the Rydberg-like series exhibits a free-electron-like E(k) dispersion with an effective mass of m/me=1.2±0.1. Careful spin-resolved measurements for several angles of electron incidence allow us to detect Rashba-type spin-dependent energy splittings of this state with a Rashba parameter of αR=105±33meVÅ.
 

Rashba-split surface state and spin-dependent photon emission from Re(0001) at Γ

Phys. Rev. B 104, 205425 (2021).
The unoccupied electronic structure of the Re(0001) surface is investigated by spin- and angle-resolved inverse photoemission, experimentally and theoretically. The work is focused on the states around the center of the surface Brillouin zone Γ, where a hole pocket of a surface state with Rashba-type spin splitting is detected. Furthermore, we observe spin-dependent photon emission from unpolarized surface and bulk states at Γ. The size and sign of the spin asymmetry depends on experimental parameters such as the direction of the electron spin polarization and the photon detection angle. Maximum (zero) spin asymmetry is detected if the electron spin polarization and the plane of photon emission are perpendicular (parallel). The effect is traced back to spin-orbit-induced hybridization of the involved states.
 

Rashba-type splitting of the Au(110) surface state: A combined inverse and direct photoemission study

Phys. Rev. B 104, L161101 (2021).
The Shockley surface state located at Y on the (1×2)-reconstructed Au(110) surface is predicted to exhibit
a Rashba-type spin splitting. Previous photoemission experiments searched for this splitting but it could not be
resolved yet. In order to uncover a possible splitting, the unoccupied surface state on Au(110) is examined with
spin- and angle-resolved inverse photoemission, whereas Na-covered Au(110) allows for investigation of the
now occupied surface state by means of spin- and angle-resolved direct photoemission. Our data show clear spin
splittings in the order of 100 meV with a sign reversal at Y in the surface state’s in-plane spin components which
is characteristic for a Rashba-type behavior. Furthermore, we deduce an effective mass of m = (0.27 ± 0.02)me
and a Rashba parameter of αR = (0.46 ± 0.04) eVÅ from direct photoemission measurements.