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Existence of topological nontrivial surface states in strained transition metals: W, Ta, Mo, and Nb

Phys. Rev. B 94, 155132 (2016).
We show that a series of transition metals with strained body-centered cubic lattice—W, Ta, Nb, and Mo—hosts surface states that are topologically protected by mirror symmetry and, thus, exhibits nonzero topological invariants. These findings extend the class of topologically nontrivial systems by topological crystalline transition metals. The investigation is based on calculations of the electronic structures and of topological invariants. The signatures of a Dirac-type surface state in W(110), e.g., the linear dispersion and the spin texture, are verified. To further support our prediction, we investigate Ta(110) both theoretically and experimentally by spin-resolved inverse photoemission: unoccupied topologically nontrivial surface states are observed.

Orbital-symmetry-selective spin characterization of Dirac-cone-like state on W(110)

Phys. Rev. B 93, 161403(R) (2016).
The surface of W(110) exhibits a spin-orbit-induced Dirac-cone-like surface state, which is of mainly dz2 orbital character near Γ, although it is strongly influenced by the twofold C2v surface symmetry. Its distinctive k-dependent spin polarization along ΓH is revealed by spin- and angle-resolved photoemission excited with p and s-polarized light. The spin texture of the surface state is found to change sign upon switching from p- to s-polarized light. Based on electronic-structure calculations, this behavior is explained by the orbital composition of the Dirac-cone-like state. The dominant part of the state has even mirror symmetry and is excited by ppolarized light. A minor part with odd symmetry is excited by s-polarized light and exhibits a reversed spin polarization. Our study demonstrates in which way spin-orbit interaction combines the spin degree of freedom with the orbital degree of freedom and opens a way to manipulate the spin information gathered from the Dirac-cone-like surface state by light. Our results prove that “spin control” is not restricted to topological surface states with p-type orbital symmetry in topological insulators.

Effects of orbital composition in a pair of spin-orbit-split surface bands at Tl/Ge(111)

Phys. Rev. B 93, 085412 (2016).
The spin texture of the unoccupied surface electronic structure of the metal-semiconductor hybrid system Tl/Ge(111)−(1×1) is investigated by spin- and angle-resolved inverse photoemission as well as quasiparticle band-structure calculations. Spin-polarized surface bands with rotating spin and giant energy splitting are found along ΓK(K′), forming valleys with alternating out-of-plane spin polarization at K and K′. This behavior is known from the equivalent hybrid system on Si(111). Along ΓM, a pair of surface bands appears within a projected bulk band gap, whose equivalent on Tl/Si(111) is a surface resonance because, there, it overlaps with bulk states. Surprisingly, the spin splitting of these bands on Tl/Ge(111) is much smaller than on Tl/Si(111) despite the stronger surface localization and the heavier substrate. Our detailed analysis of the band structure and a tight-binding model including all relevant interactions show that a remarkable interplay between spin-orbit coupling and hybridization is responsible for this unexpected result. The comparison between the two similar hybrid systems demonstrates that the strength of the spin-orbit coupling alone, based on the atomic number of the respective elements, is not sufficient to estimate spin splittings of spin-orbit-influenced surface states.

Spin-orbit influence on dz2-type surface state at Ta(110)

Phys. Rev. B 92, 161408(R) (2015).
The influence of spin-orbit interaction on an occupied surface state at Ta(110) is investigated with spin- and angle-resolved photoemission and electronic structure calculations. The surface state appears in a symmetry gap at a binding energy of 0.45 eV at Γ and exhibits a free-electron-like E(k) dispersion with an effective mass m∗/me of about −1.35 along Γ H. Photoemission results for excitation with s- and p-polarized light confirm the predicted dz2-type symmetry of the state close to Γ. Spin-resolved data for finite kreveal a pure Rashba-type spin texture with a Rashba parameter of 0.063±0.007 eVÅ. These findings clearly prove a sizable impact of spin-orbit coupling on the dz2 surface state and resolve a longstanding disagreement on this issue.

Optimizing the performance of bandpass photon detectors for inverse photoemission: Transmission of alkaline earth fluoride window crystals

Rev. Sci. Instrum. 86, 085101 (2015).
Bandpass photon detectors are widely used in inverse photoemission in the isochromat mode at energies in the vacuum-ultraviolet spectral range. The energy bandpass of gas-filled counters is usually formed by the ionization threshold of the counting gas as high-pass filter and the transmission cutoff of an alkaline earth fluoride window as low-pass filter. The transmission characteristics of the window have, therefore, a crucial impact on the detector performance. We present transmission measurements in the vacuum-ultraviolet spectral range for alkaline earth fluoride window crystals in the vicinity of the transmission cutoff as a function of crystal purity, surface finish, surface contamination, temperature, and thickness. Our findings reveal that the transmission characteristics of the window crystal and, thus, the detector performance depend critically on these window parameters.