Polarization Of Photoemission Research Articles

Zen and the art of dichroic photoemission

The discovery of magnetic dichroism in photoemission is celebrating its 25th anniversary this year. Here a review of the underlying general theory for the angular and spin dependence of dichroic core-level photoemission is presented using both a single-particle model and a many-body approach. The established methods of angular momentum coupling offer an elegant and powerful way to analyse the magnetic dichroism and spin polarization in photoemission from core and localized valence levels. In the presence of core-valence interactions one can distinguish different fundamental spectra, which via sum rules are related to physical properties described by coupled tensor operators for spin and orbital moments. By separating the angular dependence from the physical information, different geometries can be distinguished to measure the magnetic circular dichroism (MCD), linear dichroism (LD), circular dichroism in the angular dependence (CDAD), and magnetic linear dichroism in the angular dependence (MLDAD). Various ways to probe the core-hole polarization are discussed, such as using the angular dependence, moment analysis of the spectral distribution, and resonant photoemission decay.

Induced spin filtering in electron transmission through chiral molecular layers adsorbed on metals with strong spin-orbit coupling

Recent observations of considerable spin polarization in photoemission from metal surfaces through monolayers of chiral molecules were followed by several efforts to rationalize the results as the effect of spin-orbit interaction that accompanies electronic motion on helical, or more generally strongly curved, potential surfaces. In this paper we (a) argue, using simple models, that motion in curved force-fields with the typical energies used and the characteristic geometry of DNA cannot account for such observations; (b) introduce the concept of induced spin filtering, whereupon selectivity in the transmission of the electron orbital angular momentum can induce spin selectivity in the transmission process provided there is strong spin-orbit coupling in the substrate; and (c) show that the spin polarization in the tunneling current as well as the photoemission current from gold covered by helical adsorbates can be of the observed order of magnitude. Our results can account for most of the published observations that involved gold and silver substrates; however, recent results obtained with an aluminum substrate can be rationalized within the present model only if strong spin-orbit coupling is caused by the built-in electric field at the molecule-metal interface.

Investigation of photoemission of polarized electrons from stressed superstructures

Photoemission of polarized electrons from heterostructures based on GaAs/AlInGaAs superstructures with minimum discontinuities of the conduction band edge and on AlGaAs/AlInGaAs superstructures with stressed quantum wells is investigated. A comparison of the experimental dependences of photoemission polarization on the excitation energy with the calculated dependences allows the polarization losses to be estimated in different photoemission stages. Record values of the polarization degree are reported.

Observation of an underlying relativistic effect in the valence bands of Pt

We have measured the photoelectron spin polarization emitted by unpolarized UV radiation from the valence-bands of the well ordered Pt(0 0 1)-(5 × 1) surface and the disordered surface destroyed by Ar ions bombardment. Almost identical spin polarizations have been observed in both cases. This observation suggests that the electron spin polarization in photoemission caused by unpolarized light is determined by a short-range order of atoms. This finding has an obvious implication that the electron spin polarization in photoemission caused by unpolarized light can be used to study the bulk electronic structure of the nonmagnetic materials.

Photoemission of polarized electrons from InAlGaAs/GaAs superlattices with minimum conduction band offsets

Photoemission of polarized electrons from heterostructures based on InAlGaAs/GaAs superlattices with minimum conduction-band offsets is investigated. The comparison of the excitation energy dependence of the photoemission polarization degree with the calculated spectra make it possible to determine the polarization losses at different stages of the photoemission. A maximum polarization of P = 91% and a quantum yield of 0.14% are close to the best results obtained for photocathodes on the basis of strained semiconductor superlattices.

Magnetic dichroism and electron spin polarization in photoemission: analytical results

For cubic (001), (110) and (111) surface systems with in-plane or perpendicular magnetization, valence-band photoemission along the surface normal is studied analytically by evaluating electric dipole transition matrix elements between half-space initial and final states of the appropriate double-group symmetry. Explicit expressions are obtained for the spin-polarization vector of the photoelectrons, and the spin-averaged intensity and its change upon reversal of the magnetization direction, i.e. magnetic dichroism, for circularly and linearly polarized incident light. These results firstly elucidate the origin of spin polarization and dichroism in terms of an interplay between spin - orbit coupling and exchange, and secondly provide a systematic overview of possible effects. In particular, we predict new types of magnetic linear dichroism for s-polarized light in the case of magnetization perpendicular to surfaces with a twofold rotation axis and in the case of in-plane magnetization of fcc (111) or hcp (0001) surfaces.

Spin polarization in Cu core-level photoemission with linearly polarized soft x rays.

We report on the first observation of electron spin polarization in photoemission from inner shells of a nonmagnetic solid with linearly polarized radiation. The effect is observed in normal emission from the $2p$ and $3p$ core levels of Cu(001) excited with $p$-polarized soft-x-ray radiation at oblique incidence. We interpret the effect in terms of spin-orbit interaction and interference of continuum partial waves. The results are relevant for the understanding of magnetic dichroism and spin polarization in core-level photoemission from ferromagnets.

Electron-spin polarization in photoemission from thin AlxGa1−xAs

The polarization of photoemitted electrons from thin AlxGa1−xAs layers grown by molecular-beam epitaxy has been studied as a function of Al concentration by varying x in steps of 0.05 from 0.0 to 0.15. As the fraction x is increased, the wavelength dependence of the polarization shifts toward shorter wavelengths, permitting wavelength tuning of the region of maximum polarization. A maximum electron polarization of 42%–43% is obtained for AlxGa1−xAs samples with x≥0.05 while the maximum polarization of GaAs (x=0) samples reaches 49%. To investigate the lower polarization of AlxGa1−xAs, additional samples have been studied, including a short-period superlattice (GaAs)7 - (AlAs)1 .

Observation of strain-enhanced electron-spin polarization in photoemission from InGaAs.

Electron-spin polarization in excess of 70% has been observed in photoemission from a 0.1-\ensuremath{\mu}m-thick epitaxial layer of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As with x\ensuremath{\approxeq}0.13 grown on a GaAs substrate. Under these conditions, the epitaxial layer is expected to be highly strained by the 0.9% lattice mismatch. The electron polarization and the quantum efficiency have been measured as a function of the excitation photon energy from 1.25 to 2.0 eV. A significant enhancement of the electron polarization occurs in the vicinity of 1.33 eV where the expected strain-induced level splitting permits optical excitation of a single-band transition.

Circular Dichroism and Spin Polarization in Photoemission from Adsorbates and Non-Magnetic Solids

Spin-resolving techniques in photoemission are presently attracting wide interest because they can provide a detailed insight into specific aspects of electronic structure and the dynamics of the photoexcitation process. Recent experimental work in the field of non-magnetic systems is reviewed with emphasis on the complementary nature of the information gained by exploiting the photon- and/or electron-spin. A very promising achievement is the verification of circular dichroism (CDAD) in the angular distribution of photoelectrons. It arises without spin-orbit interaction through an interference of final-state partial waves. Every aligned or oriented target (molecular adsorbates, single crystals) should in principle show this effect in photoemission with circularly polarized light. Electron-spin polarization (ESP) in photoemission is induced by circularly polarized light as a direct consequence of spin-orbit interaction. This type of spectroscopy thus provides a unique method for determining the symmetries of electronic bands of high-Z materials. Recent examples of both categories are discussed.

Enhanced electron spin polarization in photoemission from thin GaAs

The polarization of photoemitted electrons from thin GaAs layers grown by molecular beam epitaxy has been measured. Polarization as high as 49% was observed for a 0.2-μm-thick GaAs sample at excitation photon wavelengths longer than 750 nm. The maximum polarization is dependent on the thickness of the GaAs layer, decreasing to about 41% for a 0.9-μm-thick GaAs sample.

Theoretical study of electron-spin polarization in photoemission from Au surfaces

We have calculated the spin polarization of photoelectrons emitted by circularly polarized light from Au surfaces. Results for the electron-spin polarization (ESP) of the photoyield are presented for clean and contaminated surfaces for photon energies up to 10 eV. In addition spectra for energy resolved photoelectrons for the (111) and (110) surface are presented. To calculate the electron wavefunctions and matrix elements required for a determination of the ESP for gold, we used a relativistic version of a fcc interpolation scheme. Our calculations demonstrate that the ESP reflects the hybridization properties of the electron bands in particular in the vicinity of band crossings which have been lifted by spin-orbit interaction. The effect of an acceptance cone is discussed. A satisfying agreement is found with experimental work for the energetic positions of the extrema of the ESP.

Theoretical study of electron-spin polarization in photoemission from Au surfaces

Abstract We have calculated the spin polarization of photoelectrons emitted by circularly polarized light from Au surfaces. Results for the electron-spin polarization (ESP) of the photoyield are presented for clean and contaminated surfaces for photon energies up to 10 eV. In addition spectra for energy resolved photoelectrons for the (111) and (110) surface are presented. To calculate the electron wavefunctions and matrix elements required for a determination of the ESP for gold, we used a relativistic version of a fcc interpolation scheme. Our calculations demonstrate that the ESP reflects the hybridization properties of the electron bands in particular in the vicinity of band crossings which have been lifted by spin-orbit interaction. The effect of an acceptance cone is discussed. A satisfying agreement is found with experimental work for the energetic positions of the extrema of the ESP.

Calculation of electron-spin polarization in photoemission from Au(110)

We have calculated the spin polarization of electrons photoemitted by circularly polarized light from Au(110). Results for the spin polarization of the angular resolved photoyield are given for clean and contaminated surfaces for photon energies ℏΩ up to 10 eV. In addition we calculated the spectrum for energy resolved photoelectrons for ℏΩ = 8.5 eV. Our calculations demonstrate that from an analysis of the spin polarization of energy resolved photoelectrons detailed information on the electronic structure of solids follows.

Energy band gap and electron spin polarization in photoemission

A contribution to the electron spin polarization in photoemission is investigated for the transition from occupied states in conduction bands to the states whose energies lie in a band gap. This contribution can be either positive or negative.

Photoelectron Spin Polarization and Ferromagnetism of Crystalline and Amorphous Nickel

We observe electron spin polarization in photoemission from nickel, undetected in earlier experimental investigations. The saturation electron spin polarization for films prepared on hot substrates is higher than for films on cold substrates and the preferential direction of the magnetic moment is parallel to the magnetization even for electrons excited from energy states near the Fermi level.