Anke B. Schmidt

Valley spin polarization by using the extraordinary Rashba effect on silicon

The addition of the valley degree of freedom to a two-dimensional spin-polarized electronic system provides the opportunity to multiply the functionality of next-generation devices. So far, however, such devices have not been realized due to the difficulty to polarize the valleys, which is an indispensable step to activate this degree of freedom. Here we show the formation of 100% spin-polarized valleys by a simple and easy way using the Rashba effect on a system with C3 symmetry. This polarization, which is much higher than those in ordinary Rashba systems, results in the valleys acting as filters that can suppress the backscattering of spin-charge. The present system is formed on a silicon substrate, and therefore opens a new avenue towards the realization of silicon spintronic devices with high efficiency.

Rashba-type spin splitting at Au(111) beyond the Fermi level: the other part of the story

We present a combined experimental and theoretical study of spin–orbit-induced spin splittings in the unoccupied surface electronic structure of the prototypical Rashba system Au(111). Spin- and angle-resolved inverse-photoemission measurements reveal a Rashba-type spin splitting in the unoccupied part of the L-gap surface state. With increasing momentum parallel to the surface, the spectral intensity is lowered and the spin splitting vanishes as the surface state approaches the band-gap boundary. Furthermore, we observe significantly spin-dependent peak positions and intensities for transitions between unoccupied sp-like bulk bands. Possible reasons for this behavior are considered: initial and final-state effects as well as the transition itself, which is controlled by selection rules depending on the symmetry of the involved states. Based on model calculations, we identify the initial states as origin of the observed Rashba-type spin effects in bulk transitions.

Surface electronic structure of fcc Co films: a combined spin-resolved one- and two-photon-photoemission study

The surface electronic structure of face-centred-cubic cobalt films on Cu(0 0 1) was studied by spin-resolved one- and two-photon photoemission. A minority surface state in a Δ1-symmetry gap of the minority band-structure was identified at about 0.45 eV below the Fermi energy. This state causes a resonance-like enhancement in the population of the minority image-potential surface state in the two-photon-photoemission experiment excited by p-polarized light. Additionally, it appears as a surface-sensitive spectral feature in normal photoemission (PE) and its existence is confirmed by calculations within the one-step model of PE. The majority counterpart of the surface state is theoretically expected at a binding energy of about 1.95 eV. Due to lifetime broadening and bulk transitions in this energy range, the majority state does not appear as a pronounced feature in the spectra. Bulk-derived states close to the Fermi level exhibit shifts to higher binding energy with increasing film thickness, while the minority surface state does not change its energy as a function of Co coverage. These results provide a basis for the interpretation of time-resolved measurements concerning ultrafast magnetization dynamics, which rely on a detailed knowledge of the surface electronic structure of ultrathin films.

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

Danny Thonig, Tomáš Rauch, Hossein Mirhosseini, ∗ Jürgen Henk, Ingrid Mertig, 3 Henry Wortelen, Bernd Engelkamp, Anke B. Schmidt, and Markus Donath Department of Physics and Astronomy, Material Theory, University Uppsala, Box 516, 75120 Uppsala, Sweden Institute of Physics, Martin Luther University Halle-Wittenberg, Von-Seckendorff-Platz 1, 06120 Halle (Saale), Germany Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle (Saale), Germany Physikalisches Institut, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany (Dated: May 13, 2016)

Rotatable spin-polarized electron source for inverse-photoemission experiments.

We present a ROtatable Spin-polarized Electron source (ROSE) for the use in spin- and angle-resolved inverse-photoemission (SR-IPE) experiments. A key feature of the ROSE is a variable direction of the transversal electron beam polarization. As a result, the inverse-photoemission experiment becomes sensitive to two orthogonal in-plane polarization directions, and, for nonnormal electron incidence, to the out-of-plane polarization component. We characterize the ROSE and test its performance on the basis of SR-IPE experiments. Measurements on magnetized Ni films on W(110) serve as a reference to demonstrate the variable spin sensitivity. Moreover, investigations of the unoccupied spin-dependent surface electronic structure of Tl/Si(111) highlight the capability to analyze complex phenomena like spin rotations in momentum space. Essentially, the ROSE opens the way to further studies on complex spin-dependent effects in the field of surface magnetism and spin-orbit interaction at surfaces.

Momentum resolution in inverse photoemission

We present a method to determine the electron beam divergence, and thus the momentum resolution, of an inverse-photoemission setup directly from a series of spectra measured on Cu(111). Simulating these spectra with different beam divergences shows a distinct influence of the divergence on the appearance of the Shockley surface state. Upon crossing the Fermi level, its rise in intensity can be directly linked with the beam divergence. A comparison of measurement and simulation enables us to quantify the momentum resolution independent of surface quality, energy resolution, and experimental geometry. With spin resolution, a single spectrum taken around the Fermi momentum of a spin-split surface state, e.g., on Au(111), is sufficient to derive the momentum resolution of an inverse-photoemission setup.

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

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.