D. Kutnyakhov

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.

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.

Electronic structure of large disc-type donors and acceptors

Searching for new pi-conjugated charge-transfer systems, the electronic structure of a new acceptor-donor pair derived from coronene (C(24)H(12)) was investigated by ultraviolet photoelectron spectroscopy (UPS). The acceptor coronene-hexaone (C(24)H(6)O(6), in the following abbreviated as COHON) and the donor hexamethoxycoronene (C(30)H(24)O(6), abbreviated as HMC) were adsorbed as pure and mixed phases on gold substrates. At low coverage, COHON adsorption leads to the appearance of a charge-transfer induced interface state 1.75 eV below the Fermi energy. At multilayer coverage the photoemission intensity of the interface state drops and the valence spectrum of neutral COHON appears. The sample work function decreases from 5.3 eV (clean Au) to 4.8 eV (monolayer) followed by an increase to 5.6 eV (multilayer). The formation of a significant interface dipole due to charge-transfer at the metal-organic interface is possibly accompanied by a change in molecular orientation. HMC on Au exhibits no interface state and the sample work function decreases monotonically to ca. 4.8 eV (multilayer). The UPS spectra of individual donor and acceptor multilayers show good agreement with density functional theory modeling. In donor/acceptor mixed films the photoemission signal of the donor (acceptor) shifts to higher (lower) binding energy. This trend is predicted by the calculation and is anticipated when charge is transferred from donor to acceptor. We propose that mixed films of COHON and HMC constitute a weak charge-transfer system.

Direct 3D mapping of the Fermi surface and Fermi velocity

We performed a full mapping of the bulk electronic structure including the Fermi surface and Fermi-velocity distribution vF(kF) of tungsten. The 4D spectral function ρ(EB; k) in the entire bulk Brillouin zone and 6 eV binding-energy (EB) interval was acquired in ∼3 h thanks to a new multidimensional photoemission data-recording technique (combining full-field k-microscopy with time-of-flight parallel energy recording) and the high brilliance of the soft X-rays used. A direct comparison of bulk and surface spectral functions (taken at low photon energies) reveals a time-reversal-invariant surface state in a local bandgap in the (110)-projected bulk band structure. The surface state connects hole and electron pockets that would otherwise be separated by an indirect local bandgap. We confirmed its Dirac-like spin texture by spin-filtered momentum imaging. The measured 4D data array enables extraction of the 3D dispersion of all bands, all energy isosurfaces, electron velocities, hole or electron conductivity, effective mass and inner potential by simple algorithms without approximations. The high-Z bcc metals with large spin-orbit-induced bandgaps are discussed as candidates for topologically non-trivial surface states.

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.

Multi-MHz time-of-flight electronic bandstructure imaging of graphene on Ir(111)

In the quest for detailed spectroscopic insight into the electronic structure at solid surfaces in a large momentum range, we have developed an advanced experimental approach. It combines the 3D detection scheme of a time-of-flight momentum microscope with an optimized filling pattern of the BESSY II storage ring. Here, comprehensive data sets covering the full surface Brillouin zone have been used to study faint substrate-film hybridization effects in the electronic structure of graphene on Ir(111), revealed by a pronounced linear dichroism in angular distribution. The method paves the way to 3D electronic bandmapping with unprecedented data recording efficiency.

Vectorial spin polarization detection in multichannel spin-resolved photoemission spectroscopy using an Ir(001) imaging spin filter

We report on spin- and angular-resolved photoemission spectroscopy using a high-resolution imaging spin filter based on a large Ir(001) crystal enhancing the effective figure of merit for spin detection by a factor of over 103 compared to standard single-channel detectors. Furthermore, we review the spin filter preparation and its lifetime. The spin filter efficiency is mapped on a broad range of scattering energies and azimuthal angles. Large spin filter efficiencies are observed for the spin component perpendicular as well as parallel to the scattering plane depending on the azimuthal orientation of the spin filter crystal. A spin rotator capable of manipulating the spin direction prior to detection complements the measurement of three observables, thus allowing for a derivation of all three components of the spin polarization vector in multichannel spin polarimetry. The experimental results nicely agree with spin-polarized low-energy electron diffraction calculations based on a fully relativistic multiple scattering method in the framework of spin-polarized density functional theory.

Determination of surface and interface magnetic properties for the multiferroic heterostructure Co/BaTiO3 using spleed and arpes

Co/BaTiO3(0 0 1) is one of the most interesting multiferroic heterostructures as it combines different ferroic phases, setting this way the fundamentals for innovative technical applications. Various theoretical approaches have been applied to investigate the electronic and magnetic properties of Co/BaTiO3(0 0 1). Here we determine the magnetic properties of 3 ML Co/BaTiO3 by calculating spin-polarized electron diffraction as well as angle-resolved photoemission spectra, with both methods being well established as surface sensitive techniques. Furthermore, we discuss the impact of altering the BaTiO3 polarization on the spectra and ascribe the observed changes to characteristic details of the electronic structure.