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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.

The acetone bandpass detector for inverse photoemission: operation in proportional and Geiger–Müller modes

Meas. Sci. Technol. 29, 065901 (2018).
Inverse photoemission is the most versatile experimental tool to study the unoccupied electronic structure at surfaces of solids. Typically, the experiments are performed in the isochromat mode with bandpass photon detectors. For gas-filled counters, the bandpass behavior is realized by the combination of the photoionization threshold of the counting gas as the high-pass filter and the ultraviolet transmission cutoff of an alkaline earth fluoride entrance window as the low-pass filter. The transmission characteristics of the entrance window determine the optical bandpass. The performance of the counter depends on the composition of the detection gas and the fill-gas pressure, the readout electronics and the counter geometry. For the well-known combination of acetone and CaF2, the detector can be operated in proportional and Geiger–Müller modes. In this work, we review aspects concerning the working principles, the counter construction and the read-out electronics. We identify optimum working parameters and provide a step-by-step recipe how to build, install and operate the device.

Location of the valence band maximum in the band structure of anisotropic 1T′−ReSe2

Phys. Rev. B 97, 165130 (2018).
Transition-metal dichalcogenides (TMDCs) are a focus of current research due to their fascinating optical and electronic properties with possible technical applications. ReSe2 is an interesting material of the TMDC family, with unique anisotropic properties originating from its distorted 1T structure (1T '). To develop a fundamental understanding of the optical and electric properties, we studied the underlying electronic structure with angle-resolved photoemission (ARPES) as well as band-structure calculations within the density functional theory (DFT)–local density approximation (LDA) and GdW approximations. We identified the ΓM1 direction, which is perpendicular to the a axis, as a distinct direction in k space with the smallest bandwidth of the highest valence band. Using photon-energy-dependent ARPES, two valence band maxima are identified within experimental limits of about 50 meV: one at the high-symmetry point Z, and a second one at a non-high-symmetry point in the Brillouin zone. Thus, the position in k space of the global valence band maximum is undecided experimentally. Theoretically, an indirect band gap is predicted on a DFT-LDA level, while quasiparticle corrections lead to a direct band gap at the Z point.

In or Out of Control? Electron Spin Polarization in Spin–Orbit-Influenced Systems

In: Wandelt, K., (Ed.) Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, vol.2, pp 131–137 (2018).
Angle-resolved photoemission and inverse photoemission provide direct experimental access to the occupied and unoccupied electronic structure E(k) of solid surfaces, respectively. The additional quantum number “spin” carries valuable information about exchange interaction in magnetically ordered systems and about spin–orbit interaction, which becomes especially important in systems with heavy elements. The experiments aim at determining the intrinsic spin polarization of the electronic states under investigation—but is this what is measured? While in ferromagnets the magnetization direction serves as the spin quantization axis, the situation is more complex in spin–orbit-influenced systems. First, spin–orbit interaction leads to k-dependent spin textures of the electronic states, called spin-momentum locking. Second, the experimental measurement of spin polarization by photoemission and inverse photoemission contains additional, even k-independent spin effects—depending on the orbital composition of the state under investigation in combination with the choice of experimental parameters. Is the electron spin polarization in or out of control? On the one hand, examples show that it is not straightforward to determine the spin polarization of electronic states. On the other hand, spin-resolved measurements performed with deliberately chosen geometries can provide comprehensive information about the orbital symmetries of the involved states. The (110) surface of tungsten serves as a textbook example for “controlling” the electron spin polarization.