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Breaking time-reversal symmetry at the M point: Spin signal from a surface state on Tl/Ge(111)

Phys. Rev. B 101, 165411 (2020).
We detected and analyzed a spin-dependent intensity asymmetry for spin-degenerate surface states at the ¯¯¯M point in the unoccupied electronic structure of Tl/Ge(111). Approximating the initial state by a plane wave, we calculated the inverse-photoemission process and obtained good agreement with the experimental data. Our model reveals that this spin asymmetry at a point of time-reversal invariant momentum (TRIM) is of different origin than other effects discussed in the literature, which are based on the light detection geometry, photon energy, and experimental probing depth of the electrons. Instead, the spin asymmetry in this nonmagnetic material with spin-orbit interaction is caused by breaking the initial-state time-reversal symmetry in the experiment. The effect is of general nature and can occur in spin- and angle-resolved inverse photoemission as well as photoemission at TRIM points.

Spin Structure of K Valleys in Single-Layer WS2 on Au(111)

Phys. Rev. Lett. 121, 136402 (2018).
The spin structure of the valence and conduction bands at the K and K′ valleys of single-layer WS2 on Au(111) is determined by spin- and angle-resolved photoemission and inverse photoemission. The bands confining the direct band gap of 1.98 eV are out-of-plane spin polarized with spin-dependent energy splittings of 417 meV in the valence band and 16 meV in the conduction band. The sequence of the spin-split bands is the same in the valence and in the conduction bands and opposite at the K and the K′ high-symmetry points. The first observation explains “dark” excitons discussed in optical experiments; the latter points to coupled spin and valley physics in electron transport. The experimentally observed band dispersions are discussed along with band structure calculations for a freestanding single layer and for a single layer on Au(111).

Circular-polarized-light-induced spin polarization characterized for the Dirac-cone surface state at W(110) with C2v symmetry

Scientific Reports 8, 10440 (2018).
The C2v surface symmetry of W(110) strongly influences a spin-orbit-induced Dirac-cone-like surface state and its characterization by spin- and angle-resolved photoelectron spectroscopy. In particular, using circular polarized light, a distinctive k-dependent spin texture is observed along the ΓH direction of the surface Brillouin zone. For all spin components Px, Py, and Pz, non-zero values are detected, while the initial-state spin polarization has only a Py component due to mirror symmetry. The observed complex spin texture of the surface state is controlled by transition matrix element effects, which include orbital symmetries of the involved electron states as well as the geometry of the experimental set-up.

Retrieving the initial-state spin polarization from spin-resolved photoemission: Proposal for a case study on W(110)

Phys. Rev. B 98, 045124 (2018).
Spin- and angle-resolved photoelectron spectroscopy is commonly used to determine the spin texture of the occupied electronic states. If spin-orbit coupling is strong, the spin polarization of the photoelectrons and that of the initial states may deviate significantly. To alleviate part of this problem we propose a recipe for improved spin retrieval. The basic idea is to combine photoemission intensities from (at least) two different photoemission experiments in a way which reflects the symmetry of the photoemission setups; the procedure avoids group-theoretical analyses or relativistic photoemission calculations. In this paper we introduce the approach, motivated by the example of photoemission from W(110) illuminated by circularly polarized light. Limitations of the method are discussed.