Christian Tusche

Spin resolved photoelectron microscopy using a two-dimensional spin-polarizing electron mirror

We report on an imaging spin-filter for electrons. The specular reflection of low-energy electrons at the surface of a tungsten single crystal is used to project a spin-filtered two-dimensional image onto a position sensitive detector. Spin-filtering is based on the spin-dependent reflectivity of electrons due to spin-orbit coupling in the scattering target, while a two-dimensional field of view, encoded in the angle of incidence, is conserved in the outgoing beam. We characterize the efficiency of the spin-filter by recording photoelectron emission microscopy images of the magnetic domain structure of 8 monolayers cobalt grown on copper (100).

Quantitative spin polarization analysis in photoelectron emission microscopy with an imaging spin filter

Using a photoelectron emission microscope (PEEM), we demonstrate spin-resolved electron spectroscopic imaging of ultrathin magnetic Co films grown on Cu(100). The spin-filter, based on the spin-dependent reflection of low energy electrons from a W(100) crystal, is attached to an aberration corrected electrostatic energy analyzer coupled to an electrostatic PEEM column. We present a method for the quantitative measurement of the electron spin polarization at 4 × 10³ points of the PEEM image, simultaneously. This approach uses the subsequent acquisition of two images with different scattering energies of the electrons at the W(100) target to directly derive the spin polarization without the need of magnetization reversal of the sample.

Correction of the deterministic part of space–charge interaction in momentum microscopy of charged particles

Ultrahigh spectral brightness femtosecond XUV and X-ray sources like free electron lasers (FEL) and table-top high harmonics sources (HHG) offer fascinating experimental possibilities for analysis of transient states and ultrafast electron dynamics. For electron spectroscopy experiments using illumination from such sources, the ultrashort high-charge electron bunches experience strong space-charge interactions. The Coulomb interactions between emitted electrons results in large energy shifts and severe broadening of photoemission signals. We propose a method for a substantial reduction of the effect by exploiting the deterministic nature of space-charge interaction. The interaction of a given electron with the average charge density of all surrounding electrons leads to a rotation of the electron distribution in 6D phase space. Momentum microscopy gives direct access to the three momentum coordinates, opening a path for a correction of an essential part of space-charge interaction. In a first experiment with a time-of-flight momentum microscope using synchrotron radiation at BESSY, the rotation in phase space became directly visible. In a separate experiment conducted at FLASH (DESY), the energy shift and broadening of the photoemission signals were quantified. Finally, simulations of a realistic photoemission experiment including space-charge interaction reveals that a gain of an order of magnitude in resolution is possible using the correction technique presented here.

Spin Mapping of Surface and Bulk Rashba States in Ferroelectric a-GeTe(111) Films

The breaking of bulk inversion symmetry in ferroelectric semiconductors causes a Rashba-type spin splitting of electronic bulk bands. This is shown by a comprehensive mapping of the spin polarization of the electronic bands in ferroelectric α- GeTe(111) films using a time-of-flight momentum microscope equipped with an imaging spin filter that enables a simultaneous measurement of more than 10 000 data points. The experiment reveals an opposite spin helicity of the inner and outer Rashba bands with a different spin polarization in agreement with theoretical predictions, confirming a complex spin texture of bulk Rashba states. The outer band has about twice larger spin polarization than the inner one, giving evidence of a spin-orbit effect being related to the orbital composition of the band states. The switchable inner electric field of GeTe implies new functionalities for spintronic devices.

Anomalous d-like surface resonances on Mo(110) analyzed by time-of-flight momentum microscopy.

The electronic surface states on Mo(110) have been investigated using time-of-flight momentum microscopy with synchrotron radiation (hν=35 eV). This novel angle-resolved photoemission approach yields a simultaneous acquisition of the E-vs-k spectral function in the full surface Brillouin zone and several eV energy interval. (kx,ky,EB)-maps with 3.4 Å(-1) diameter reveal a rich structure of d-like surface resonances in the spin-orbit induced partial band gap. Calculations using the one-step model in its density matrix formulation predict an anomalous state with Dirac-like signature and Rashba spin texture crossing the bandgap at Γ¯ and EB=1.2 eV. The experiment shows that the linear dispersion persists away from the Γ¯-point in an extended energy- and k∥-range. Analogously to a similar state previously found on W(110) the dispersion is linear along H¯-Γ¯-H¯ and almost zero along N¯-Γ¯-N¯. The similarity is surprising since the spin-orbit interaction is 5 times smaller in Mo. A second point with unusual topology is found midway between Γ¯ and N¯. Band symmetries are probed by linear dichroism.

Analysis of the electronic structure of copper via two-dimensional photoelectron momentum distribution patterns

Using a unique momentum microscope, we measured energy- resolved momentum distributions of valence-band electrons photoemitted into the whole half-space above the Cu(111) and Cu(001) surfaces. The experimental results are compared to one-step photoemission calculations. Convincing agreement between theoretical and experimental photoelectron momentum patterns can only be achieved by orbital-dependent corrections which emulate many-body self-energy effects in the electronic structure of Cu (Strocov et al 2002 Phys. Rev. B 66 195104). By the analysis of the Shockley surface state of Cu(111), we show that these self-energy corrections also affect the surface electronic structure in specific ways. We find that the Shockley surface state of Cu(111) is shifted differently in energy than the bulk states. As a consequence, the agreement between the theoretically calculated and the experimentally measured binding energy of this surface state is improved. Energy-resolved two- dimensional valence-band photoelectron mapping provides an alternative means of determining self-energy values experimentally.

Spin-texture inversion in the giant Rashba semiconductor BiTeI

Semiconductors with strong spin–orbit interaction as the underlying mechanism for the generation of spin-polarized electrons are showing potential for applications in spintronic devices. Unveiling the full spin texture in momentum space for such materials and its relation to the microscopic structure of the electronic wave functions is experimentally challenging and yet essential for exploiting spin–orbit effects for spin manipulation. Here we employ a state-of-the-art photoelectron momentum microscope with a multichannel spin filter to directly image the spin texture of the layered polar semiconductor BiTeI within the full two-dimensional momentum plane. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the valence and conduction band electrons in BiTeI have spin textures of opposite chirality and of pronounced orbital dependence beyond the standard Rashba model, the latter giving rise to strong optical selection-rule effects on the photoelectron spin polarization. These observations open avenues for spin-texture manipulation by atomic-layer and charge carrier control in polar semiconductors.

Electron-Vibration Coupling in Molecular Materials: Assignment of Vibronic Modes from Photoelectron Momentum Mapping.

Electron-phonon coupling is one of the most fundamental effects in condensed matter physics. We here demonstrate that photoelectron momentum mapping can reveal and visualize the coupling between specific vibrational modes and electronic excitations. When imaging molecular orbitals with high energy resolution, the intensity patterns of photoelectrons of the vibronic sidebands of molecular states show characteristic changes due to the distortion of the molecular frame in the vibronically excited state. By comparison to simulations, an assignment of specific vibronic modes is possible, thus providing unique information on the coupling between electronic and vibronic excitation.

Direct k-space imaging of Mahan cones at clean and Bi-covered Cu(111) surfaces

Using a specifically tailored experimental approach, we revisit the exemplary effect of photoemission from quasi-free electronic states in crystals. Applying a momentum microscope, we measure photoelectron momentum patterns emitted into the complete half-space above the sample after excitation from a linearly polarized laser light source. By the application of a fully three-dimensional (3D) geometrical model of direct optical transitions, we explain the characteristic intensity distributions that are formed by the photoelectrons in k-space under the combination of energy conservation and crystal momentum conservation in the 3D bulk as well as at the two-dimensional (2D) surface. For bismuth surface alloys on Cu(111), the energy-resolved photoelectron momentum patterns allow us to identify specific emission processes in which bulk excited electrons are subsequently diffracted by an atomic 2D surface grating. The polarization dependence of the observed intensity features in momentum space is explained based on the different relative orientations of characteristic reciprocal space directions with respect to the electric field vector of the incident light.

Spin texture of time-reversal symmetry invariant surface states on W(110)

We find in the case of W(110) previously overlooked anomalous surface states having their spin locked at right angle to their momentum using spin-resolved momentum microscopy. In addition to the well known Dirac-like surface state with Rashba spin texture near the -point, we observe a tilted Dirac cone with circularly shaped cross section and a Dirac crossing at 0.28 ×   within the projected bulk band gap of tungsten. This state has eye-catching similarities to the spin-locked surface state of a topological insulator. The experiments are fortified by a one-step photoemission calculation in its density-matrix formulation.