Spin-resolved inverse photoemission of ferromagnetic surfaces

Experimental Evidences on Magnetism-Covalent Bonding Interplay in Structural Properties of Solids and during Chemisorption

Valence electrons are one of the main players in solid catalysts and in catalytic reactions, since they are involved in several correlated phenomena like chemical bonding, magnetism, chemisorption, and bond activation. This is particularly true in the case of solid catalysts containing d-transition metals, which exhibit a wide range of magnetic phenomena, from paramagnetism to collective behaviour. Indeed, the electrons of the outer d-shells are, on one hand, involved in the formation of bonds within the structure of a catalyst and on its surface, and, on the other, they are accountable for the magnetic properties of the material. For this reason, the relationship between magnetism and heterogeneous catalysis has been a source of great interest since the mid-20th century. The subject has gained a lot of attention in the last decade, thanks to the orbital engineering of quantum spin-exchange interactions and to the widespread application of external magnetic fields as boosting tools in several catalytic reactions. The topic is discussed here through experimental examples and evidences of the interplay between magnetism and covalent bonding in the structure of solids and during the chemisorption process. Covalent bonding is discussed since it represents one of the strongest contributions to bonds encountered in materials.

Spin- and angle-resolved inverse photoemission setup with spin orientation independent from electron incidence angle

A new spin- and angle-resolved inverse photoemission setup with a low-energy electron source is presented. The spin-polarized electron source, with a compact design, can decouple the spin polarization vector from the electron beam propagation vector, allowing one to explore any spin orientation at any wavevector in angle-resolved inverse photoemission. The beam polarization can be tuned to any preferred direction with a shielded electron optical system, preserving the parallel beam condition. We demonstrate the performances of the setup by measurements on Cu(001) and Au(111). We estimate the energy resolution of the overall system at room temperature to be ∼170 meV from k_BT_eff of a Cu(001) Fermi level, allowing a direct comparison to photoemission. The spin-resolved operation of the setup has been demonstrated by measuring the Rashba splitting of the Au(111) Shockley surface state. The effective polarization of the electron beam is P = 30% ± 3%, and the wavevector resolution is Δk_F ≲ 0.06 Å^-1. Measurements on the Au(111) surface state demonstrate how the electron beam polarization direction can be tuned in the three spatial dimensions. The maximum of the spin asymmetry is reached when the electron beam polarization is aligned with the in-plane spin polarization of the Au(111) surface state.

On the energy resolution of a GaAs-based electron source for spin-resolved inverse photoemission

The spin resolution in inverse photoemission spectroscopy is achieved by injecting spin-polarized electrons, usually produced by GaAs-based cold cathodes that replace hot-filament electron guns of spin-integrated setups. The overall energy resolution of the system can be enhanced by adjusting either the optical bandpass of the photon detector or the energy distribution of the electron beam. Here we discuss the influence of the photocurrent and the photocathode temperature on the energy broadening of the electron beam through the inverse photoemission spectra of the spin-splitted Shockley surface state of Au(111). First, we find that cooling down the GaAs photocathode to 77 K increases the band gap and reduces the number of allowed vertical transitions, monochromatizing the electron beam with an enhancement of about 30 meV for the energy resolution. Second, we observe a correlation between the generated photocurrent at the electron source, and the space-charge effects at the sample as a reduction of lifetime and spin asymmetry of a polarized bulk state. These observations allow defining a threshold of current density for the optimum acquisition in the measurements of spin-resolved inverse photoemission in Au.

Spin Structure of K Valleys in Single-Layer WS_{2} on Au(111)

The spin structure of the valence and conduction bands at the K[over ¯] and K[over ¯]^{'} valleys of single-layer WS_{2} on Au(111) is determined by spin- and angle-resolved photoemission and inverse photoemission. The bands confining the direct band gap of 1.98 eV are out-of-plane spin polarized with spin-dependent energy splittings of 417 meV in the valence band and 16 meV in the conduction band. The sequence of the spin-split bands is the same in the valence and in the conduction bands and opposite at the K[over ¯] and the K[over ¯]^{'} high-symmetry points. The first observation explains "dark" excitons discussed in optical experiments; the latter points to coupled spin and valley physics in electron transport. The experimentally observed band dispersions are discussed along with band structure calculations for a freestanding single layer and for a single layer on Au(111).

Spin Polarized Low Energy Electron Microscopy of Surface Magnetic Structure

Spin polarized low energy electron microscopy (SPLEEM) has been developed for the high resolution imaging of surface magnetic structure. The existing LEEM ha.s been modified by the incorporation of a. GaAs-type spin polarized electron gun. Large image contrast arises due to the spin-dependent exchange scattering, whifle the st.in-orbit contribution vanishes uniquely for the normal incidence/exit geometry used here. Pixel by pixel image subtraction for incident electron beams of opposite polarization yields precisely the spatially resolved Bragg reflection asymmetry observed in spin polarized low energy electron diffraction. The shallow electron penetration depth arising from the strong coulombic interaction is advantageous for separating surface behavior from the normally overwhelning bulk. Therefore, the use of transversally polarizedI electron beams allows the determination of in-plane surface magnetization directions. Fnrthermore, the parallel illumination and detection of SPLEEM makes it possible to image quickly with a. resolution better than 500 Å in the present configuration. A useful and direct. comparison between surface magnetic, structural, and topological features is made possible by the augmentation of the unique imaging capabilities of conventional LEEM with the magnetic sensitivity of SPLEEM. In this manner, the magnetic domain structure of a Co (0001) surface and in-situ grown Co filmns on Mo(110) have been determined.