Spin-polarized photoemission

Keywords Surface reconstructions surface states angular resolved photoemission scanning tunneling spectroscopy spin polarized spectroscopy self-organized nucleation. 

Abstract Understanding the origin of chirality in nature has been an active area of research since the time of Pasteur. In this chapter we examine one possible route by which this asymmetry could have arisen, namely chiral-specific chemistry induced by spin-polarized electrons. The various sources of spin-polarized electrons (parity violation, photoemission, and secondary processes) are discussed. Experiments aimed at exploring these interactions are reviewed starting with those based on the Vester-Ulbricht hypothesis through recent studies of spin polarized secondary electrons from a magnetic substrate. We will conclude with a discussion of possible new avenues of research that could impact this area. 

The following examples focus on valence level ionization by ultraviolet light, since such processes would produce low energy electrons, which are the most effective at inducing reactions in an adsorbed molecule. There are also many examples of spin-polarized photoemission and Auger electron emission from core levels of both magnetic and nonmagnetic materials [29-35]. 

Johnson PD (1997) Spin-polarized photoemission. Rep Prog Phys 1217... 

Osterwalder J (2006) Spin-polarized photoemission. In Beaurepaire E, Bulou H, Scheurer F, Kappler JP (eds) Magnetism a synchrotron radiation approach. Springer, Berlin, pp 95-120... 

De Nadai C, van der Laan G, Dhesi SS et al (2003) Spin-polarized magnetic circular dichroism in Ni 2p core-level photoemission. Phys Rev B 68 212401... 

Menchero JG (1996) Spin polarization and magnetic circular dichroism in photoemission from the 2p core level of ferromagnetic Ni. Phys Rev Lett 76 3208... 

Thole BT, van der Laan G (1991) Origin of spin polarization and magnetic dichroism in corelevel photoemission. Phys Rev Lett 67 3306... 

Matthew JAD, Seddon EA, Xu YBO (1998) Spin polarized photoemission from amorphous alloy surfaces. J Electron Spectrosc Relat Phenomena 88-91 171-177... 

Pierce DT, Meier F (1976) Photoemission of spin-polarized electrons from GaAs. Phys Rev B 13 5484... 

Meier F, Pescia D (1981) Band-structure investigation of gold by spin-polarized photoemission. Phys Rev Lett 47 374... 

Schmiedeskamp B, Vogt B, Heinzmann U (1988) Experimental verification of a new spin-polarization effect in photoemission polarized photoelectrons from Pt(l 11) with linearly polarized radiation in normal incidence and normal emission. Phys Rev Lett 60 651... 

Cls photoemission shakeup satellites for the CO molecule were calculated with the spin-polarized discrete variational Xa method. The transition state method was applied to the estimation of multiplet peak positions for the shakeup transitions and the results are in reasonable agreement with the experimental values. 

To deal with the electronic structure of surfaces within the framework of the spin-polarized relativistic KKR formalism, the standard layer techniques used for LEED and photoemission investigations (Pendiy 1974) have been generalized by several authors (Fluchtmann et al. 1995 Scheunemann et al. 1994). As an alternative to this, Szunyogh and co-workers introduced the so-called screened version of the KKR method (Szunyogh et al. 1994, 1995). A firm basis for this approach has been supplied by the tight-binding (TB) KKR scheme introduced by Zeller etal. (1995). The corresponding spin-polarized relativistic version has been applied by various authors to multilayer and surface-layer systems (Nonas et al. 2001). 

Another consequence of spin-orbit coupling is the occurrence of the Fano effect in photoemission (Fano 1969a,b). This term denotes the phenomenon where we obtain a spin-polarized photoelectron current even for a paramagnetic solid if circularly polarized light is used for excitation (Heinzmann et al. 1972). Due to time-reversal... 

Due to time-reversal symmetry the spin-polarization from a paramagnetic solid using circularly polarized radiation is just reversed if the helicity of the radiation is reversed. This feature is of course removed if the solid is magnetically ordered, giving rise to magnetic circular dichroism in valence-band photoemission (Schneider et al. 1991). In a corresponding experiment we have in general the emission direction. 

Meier, F. (1985) Polarized electrons in surface physics. In Spin Polarized Photoemission by Optical Spin Orientation in Semiconductors (ed. R. Feder). World Scientific, Singapore. Mendelsohn, L. B., Biggs, F. and Mann, J. B. (1970) Phys. Rev. A2, 1130. 

Tamura, E., Piepke, W. and Feder, R. (1987) New spin-polarization effect in photoemission from nonmagnetic surfaces. Phys. Rev. Lett. 59,9347. 

For magnetically ordered materials, photoemitted electrons have a characteristic spin polarization that reflects the electron spin orientation occurring in the sample before the photoemission process. Recently, techniques have been developed to measure this photoelectron spin polarization (photo ESP) (21). 

We report on the use of spin polarized electron beams in the study of electronic states in solids, referring in particular to the Inverse Photoemission spectroscopy. In this technique the empty electron states are investigated, and the spin resolution allows to study their spin character, yielding valuable information in magnetic systems. Examples of application to layered magnetic nanostructures are given in particular we present data on Fe/Cr/Fe(001) multilayers, ultrathin Fe films grown on ZnSe(OOl), and LaSrMnO/SnTiO junctions. 

For the spin resolving photoemission and spin polarized metastable de-excitation spectroscopy measurements the films were magnetized by a current pulse through a coil close to the sample along the [110] direction of the tungsten substrate. 

Information on the spin resolved band structure of ferromagnetic materials can directly be obtained from spin resolving photoelectron spectroscopy. Using polarized radiation spin integrating photoemission techniques already enable to have access to magnetic properties. An enhancement of the surface sensitivity can be achieved using neutral excited spin polarized atoms which move towards the sample and are de-excited by tunneling electrons from the surface with a subsequent emission of electrons. 

Fig. 5.36 a Tunneling spectra as measured with a Fe covered probe tip above adjacent domains. An asymmetry of the dlIdU signal between the empty and filled part of the surface state can clearly be recognized. In contrast, variations in the dl/dU signal when measured with a pure W tip are always symmetrie inset), b Spin polarization of the tunneling current between an Fe covered probe tip and the Gd(0001) surface at T = 70 K filled circle) compared to spin-polarized inverse photoemission data of GdfOOOl) measured at T = liOK (asterisk) by Donath et al. [103] (reprinted with permission Irom [131]. Copyright 1999, American Institute of Physics)... 

---