Spin-resolved Photoemission Experiments Research Articles

α -Sn phase on Si(111): Spin texture of a two-dimensional Mott state

The $\ensuremath{\alpha}$-Sn reconstruction on Si(111) is a prototype system for a two-dimensional Mott phase. In this study we performed spin-resolved photoemission experiments and analyzed in detail the spin structure of this electronically correlated surface state. The analysis of the spin-integrated bands as well as the spin texture of the surface states along different crystallographic directions provide clear evidence for the formation of collinear antiferromagnetic $(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ domains, while the Sn reconstruction reveals a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ symmetry. The Rashba splitting of the highest occupied Mott state was found to be $\mathrm{\ensuremath{\Delta}}k=0.05\phantom{\rule{0.16em}{0ex}}{\AA{}}^{\ensuremath{-}1}$, i.e., the $\ensuremath{\alpha}$-Sn phase should be termed a weakly spin-orbit coupled Mott system.

Spin-resolved photoelectron spectroscopy using femtosecond extreme ultraviolet light pulses from high-order harmonic generation.

The fundamental mechanism responsible for optically induced magnetization dynamics in ferromagnetic thin films has been under intense debate since almost two decades. Currently, numerous competing theoretical models are in strong need for a decisive experimental confirmation such as monitoring the triggered changes in the spin-dependent band structure on ultrashort time scales. Our approach explores the possibility of observing femtosecond band structure dynamics by giving access to extended parts of the Brillouin zone in a simultaneously time-, energy- and spin-resolved photoemission experiment. For this purpose, our setup uses a state-of-the-art, highly efficient spin detector and ultrashort, extreme ultraviolet light pulses created by laser-based high-order harmonic generation. In this paper, we present the setup and first spin-resolved spectra obtained with our experiment within an acquisition time short enough to allow pump-probe studies. Further, we characterize the influence of the excitation with femtosecond extreme ultraviolet pulses by comparing the results with data acquired using a continuous wave light source with similar photon energy. In addition, changes in the spectra induced by vacuum space-charge effects due to both the extreme ultraviolet probe- and near-infrared pump-pulses are studied by analyzing the resulting spectral distortions. The combination of energy resolution and electron count rate achieved in our setup confirms its suitability for spin-resolved studies of the band structure on ultrashort time scales.

The role of space charge in spin-resolved photoemission experiments

Spin-resolved photoemission is one of the most direct ways of measuring the magnetization of a ferromagnet. If all valence band electrons contribute, the measured average spin polarization is proportional to the magnetization. This is even the case if electronic excitations are present, and thus is of particular interest for studying the response of the magnetization to a pump laser pulse. Here, we demonstrate the feasibility of ultrafast spin-resolved photoemission using free electron laser (FEL) radiation and investigate the effect of space charge on the detected spin polarization. The sample is exposed to the radiation of the FEL FLASH in Hamburg. Surprisingly, the measured spin polarization depends on the fluence of the FEL radiation: a higher FEL fluence reduces the measured spin polarization. Space-charge simulations can explain this effect. These findings have consequences for future spin-polarized photoemission experiments using pulsed photon sources.

Manipulating femtosecond magnetism through pressure: First-principles calculations

Inspired by a recent pressure experiment in fcc Ni, we propose a simple method to use pressure to investigate the laser-induced femtosecond magnetism. Since the pressure effect on the electronic and magnetic properties can be well controlled experimentally, this leaves little room for ambiguity when compared with theory. Here we report our theoretical pressure results in fcc Ni: Pressure first suppresses the spin moment reduction and then completely diminishes it; further increase in pressure to 40 GPa induces a demagnetization-to-magnetization transition. To reveal its microscopic origin, we slide through the $L$-$U$ line in the Brillouin zone and find two essential transitions are responsible for this change, where the pressure lowers two valence bands, resulting in an off-resonant excitation and thus a smaller spin moment reduction. In the spin-richest $L$-$W$-${W}^{\ensuremath{'}}$ plane, two spin contours are formed; as pressure increases, the contour size retrieves and its intensity is reduced to zero eventually, fully consistent with the spin-dipole factor prediction. These striking features are detectable in time- and spin-resolved photoemission experiments.

Spin-polarization limit in Bi2Te3Dirac cone studied by angle- and spin-resolved photoemission experiments andab initiocalculations

The magnitude of electron spin polarization in topologically protected surface states is an important parameter with respect to spintronics applications. In order to analyze the warped spin texture in Bi${}_{2}$Te${}_{3}$ thin films, we combine angle- and spin-resolved photoemission experiments with theoretical ab initio calculations. We find an in-plane spin polarization of up to $\ensuremath{\sim}$45$%$ in the topologically protected Dirac cone states near the Fermi level. The Fermi surface of the Dirac cone state is warped and shows an out-of-plane spin polarization of $\ensuremath{\sim}$15$%$. These findings are in quantitative agreement with dedicated simulations which find electron density of the Dirac cone delocalized over the first quintuple layer with spin reversal occurring in the surface atomic layer.

Spin orientation and sign of the Rashba splitting in Bi/Cu(111)

Whereas the magnitude of the Rashba spin splitting at surfaces has been studied in detail, less is known about its sign which determines the precise spin orientation of the electronic states. We investigate the microscopic origin of this sign by spin-resolved photoemission experiments and first-principles calculations on the strongly spin-orbit coupled surface states in Bi/Cu(111). We conclude that the sign of the Rashba splitting is determined by the particular charge asymmetry near the atomic cores. The precise spin orientation on heavy-element surfaces thus can comprise information on wave-function localizations and related aspects otherwise hardly accessible by other experimental methods.

A possible way for removing instrumental asymmetries in spin resolved photoemission with unpolarized light

We introduce a new method, a linear subtraction, to eliminate instrumental asymmetries in spin resolved photoemission with excitation by unpolarized light. The new method is applied successfully to analyze the spin polarizations in the valence band and 4d core level photoemission, excited with unpolarized light from a nonmagnetic Pt crystal. It is also tested in the spin analysis of the 2p core level photoemission, generated with circularly polarized X-rays from magnetic Fe. Even though the new method of linear subtraction is applicable for limited cases, it is a very valuable method for removing instrumental asymmetries especially when the helicity flipping, magnetization flipping, and even geometry flipping in spin resolved photoemission experiment are not possible.

Gradient expansion approach to multiple-band Fermi liquids

Promoted by the recent progress of Berry phase physics in spin galvanomagnetic communities, we develop a systematic derivation of the reduced Keldysh equation (RKE) which captures the low-energy dynamics of quasiparticles constrained within doubly degenerate bands forming a single Fermi surface. The derivation begins with the Keldysh equation for a quite general multiple-band interacting Fermi system, which is originally an ${N}_{b}\ifmmode\times\else\texttimes\fi{}{N}_{b}$ matrix-formed integral (or infinite-order differential) equation, with ${N}_{b}$ being the total number of bands. To derive the RKE for quasiparticle on a Fermi surface in question, we project out both the fully occupied and empty band degrees of freedom perturbatively in the gradient expansion, whose coupling constant measures how a system is disequilibrated. As for the electron-electron interactions, however, we only employ the so-called adiabatic assumption of the Fermi liquid theory, so that the electron correlation effects onto the adiabatic transport of quasiparticles, i.e., the Hermitian (real) part of the self-energy, are taken into account in an unbiased manner. The RKE thus derived becomes an SU(2) covariant differential equation and treats the spin and charge degrees of freedom on an equal footing. Namely, the quasiparticle spin precessions due to the non-Abelian gauge fields are automatically encoded into its covariant derivatives. When further solved in favor of spectral functions, this covariant differential equation suggests that quasiparticles on a doubly degenerate Fermi surface acquire spin-selective Berry curvature corrections under the applied electromagnetic fields. This theoretical observation gives us some hints of possible experimental methodology for measuring the SU(2) Berry's curvatures by spin-resolved photoemission experiments. Due to the nontrivial frequency dependence of (the Hermitian part of) self-energy, our RKE is composed of Berry's curvatures in the $d+1$ dual space, i.e., $k\text{\ensuremath{-}}\ensuremath{\omega}$ space, so that the dual electric field is already introduced. To provide a simple way to understand this ``temporal'' component of the U(1) Berry's curvature, we also provide the dual analog of the Ampere's law, where the ``spatial'' rotation of the electric field in combination with the temporal derivative of the well-known magnetic component is determined by the U(1) magnetic monopole ``current.''

Spin–orbit coupling in the L-gap surface states of Au(111): spin-resolved photoemissionexperiments and first-principles calculations

The spin–orbit-split L-gap surface states on Au(111) are investigated by means of spin- and angle-resolvedphotoelectron spectroscopy and relativistic first-principles calculations, the latter includingboth electronic structure and photoemission. The dispersion, momentum distribution andspin polarization of the surface states are consistent with those of a two-dimensionalelectron gas with Rashba–Bychkov spin–orbit coupling (SOC). The surface symmetrymanifests itself in the spin-integrated photoemission intensities, thereby providing details ofthe orbital composition of the surface states. For the spin polarization, modulations whichshow up in theory are below the experimental detection limit of 5%. The dependence ofboth dispersion and spin polarization on the SOC strength are addressed theoretically. Thecombination of experiment and theory establishes a consistent picture of theL-gap surface states.

REINVESTIGATION OF THE ELECTRONIC STRUCTURE AND FERROMAGNETISM OF THE NONRECONSTRUCTED Cr(001) 1 × 1 SURFACE

In angle- and spin-resolved photoemission experiments, we have observed that the ferromagnetic order of the Cr(001) surface is strongly influenced by oxygen concentration. Oxygen atoms of a few atomic percent of the total volume have a substantial influence on the ferromagnetic order of the Cr(001) surface.

Magnetic dichroism in photoemission from oxygen atoms adsorbed on cobalt surfaces

We report on the first observation of magnetic circular dichroism in the angular distribution MCDAD from O 2p photoelectrons from oxygen adsorbed on thin epitaxial cobalt films on W(110). Up to now, MCDAD effects in photoemission have only been observed from ferromagnetic thin films and single crystals. Here, we show evidence for a small splitting in the oxygen induced 2p x and 2p z orbitals when reversing the magnetization direction. The observed effect will be discussed with respect to related spin-resolved photoemission experiments.

Spin-polarized photoemission of

The first spin-resolved photoemission experiment on an iron - boron amorphous alloy using a synchrotron source is presented. The experimental spin polarization of the d band of has been compared with three theoretical predictions and found to be in best agreement with self-consistent spin-polarized calculations based on a supercell LMTO approach. The observed average spin polarization of the valence band is approximately twice that of 10 eV secondary electrons. Hysteresis loops for determined from the 1 eV and 20 eV secondary-electron asymmetry are similar to those determined using the magnetooptic Kerr effect, but show a lower coercivity. The differences are attributed to a combination of different sampling depths of the two techniques and to the surface inhomogeneity of the sample.

Coverage dependence of the spin-resolved photoemission from epitaxially grown Ag layers on Pt(111)

Ag/Pt(111) has been studied by spin-, angle-, and energy-resolved photoemission with circularly polarized synchrotron radiation of BESSY for different Ag coverages. The prepared layers were characterized by AES and LEED and turned out to grow epitaxially. An independent layer thickness determination and some additional structure information were obtained from Rutherford backscattering studies in combination with channeling. The spin-resolved photoemission experiments were performed for normal incidence of circularly polarized light and normal emission of the photoelectrons. The spin information allowed a separation of spin-orbit split peaks of the Ag adsorbate layer and partly also a separation of Ag adsorbate peaks from the Pt substrate background intensity. This “method-induced improved resolution” was used to deduce information about the development of the Ag band structure in the Λ-direction from the coverage dependence of spin-resolved photoemission spectra. Dispersion of occupied bands was found to be already almost completely developed for a three-layer system. Information about the development of the band structure in its unoccupied part was obtained from the resonant behavior of peaks at about 6 eV below E F. The resonant behavior is already observed for three Ag layers and increases with layer thickness. We observe a peak in the photoemission spectra contributing to the resonances which is not predicted by existing theoretical models.

Characterization of Symmetry Properties of Pt(111) Electron Bands by Means of Angle-, Energy-, and Spin-Resolved Photoemission with Circularly Polarized Synchrotron Radiation

An angle-, energy-, and spin-resolved photoemission experiment has been performed for Pt(111) with the circularly polarized synchrotron radiation of the new German dedicated storage ring BESSY. The photoelectron polarization measured as function of the kinetic energy at different photon energies between 6.5 and 24 eV for normal incidence and normal emission shows a pronounced spectral variation up to \ifmmode\pm\else\textpm\fi{}55%. The electron intensities and polarizations observed allow a symmetry-resolved band characterization of Pt in the $\ensuremath{\Gamma}\ensuremath{-}L$ direction.

Correlation effects in band ferromagnets as detected by energy- and angle-resolved spin-polarized photoemission: Ni(110)

Calculated spectra of photoemission from Ni(110) with the use of a single-step model of the photoemission process and with the inclusion of self-energy corrections within the $t$-matrix approximation are compared to the results of a recent energy-, angle-, and spin-resolved photoemission experiment with linearly polarized light ($h\ensuremath{\nu}=16.85$ eV). The power of this technique to study electronic structure of correlated, magnetic solids is for the first time fully exploited. Good agreement is found not only in peak positions, but also in relative intensities as well as line shapes. In addition, evidence is provided that surface relaxation is important for the full understanding of the electronic structure of this particular crystal surface.

Spin polarized photoelectrons from atoms and molecules

This paper reviews the experimental activities since 1979 for the study of the spin polarization of photoelectrons emitted by atoms and molecules exposed to VUV radiation. Two series of studies have been performed: First, using unpolarized and linearly polarized radiation the photoionization of the rare gases Ar, Kr, and Xe has been studied in an angle-, energy-, and spin-resolved photoemission experiment. Second, the spin polarization of photoelectrons produced by circularly polarized VUV radiation has been measured for some atoms and molecules. This was performed for most targets by use of the synchrotron radiation in Bonn.