Double-step excitation in spin polarized photoemission from NEA GaAs(100)

Spin polarized photoelectrons emitted from a NEA GaAs(100)-photocathode, which is activated in a special way with oxygen and cesium, can have a broad energy distribution with a maximum energy higher than expected for an one-photon excitation process within the bulk from the valence band edge into the conduction band. Also the shape of the energy distribution curves of the emitted electrons depends on the intensity of the incident light. This double-step excitation process is observed for aged and/or oxidized activation layers, especially for surfaces with a strongly developed, negative electron affinity. To give an explanation of these effects, it is suggested that excited conduction electrons absorb a further photon within the band bending region. This second photoemission excitation step seems to be enhanced by an interfacial barrier between the band bending region and the cesium-oxygen activation layer as well as by the existence of two-dimensional surface subbands.     

Magnetism of surfaces by spin polarized photoemission

Spin polarized photoemission is used as a method to study the magnetic peoperties of solid surfaces. Here we consider those systems where the spin polarization is believed to be conserved during the photoemission process. The surface sensitivity of the experiment will be therefore mainly determined by the relative magnitudes of the escape depth of the photoelectrons and of the magnetic coherence length. We analyze both the temperature and magnetic-field dependence of the degree of spin polarizationP of the electrons emitted from single crystals of magnetite, Fe₃O₄, and thin films (D=2–3000 Å) of Fe, Co and Ni. These data yield information on 1) the temperature dependence of the magnetization of the surface, 2) the relative sizes of and, 3) changes of the magnetic moment of atoms in the surface layers, and 4) the surface magnetic anisotropy. We show that this technique emerges as a novel tool to study magnetic systems with free surfaces and to test the predictions of recent calculations of the magnetic properties of surfaces. Emphasis is given to the fact that only relative changes ofP and not its absolute values are relevant for an analysis of the data.     

Doping-dependent electronic structure of cuprates studied using angle-scanned photoemission

Full k -maps of the electronic structure near the Fermi level of differently doped cuprates measured with angle-scanned photoelectron spectroscopy are presented. The valence band maximum of the antiferromagnetic insulator Sr₂CuO₂Cl₂, which is taken as a representative of an undoped cuprate, and the Fermi surfaces of overdoped, optimally doped and underdoped Bi₂Sr₂CaCu₂O_8+δ high-temperature superconductors are mapped in the normal state. The results confirm the existence of large Luttinger Fermi surfaces at high doping with a Fermi surface volume proportional to (1+x), where x is the hole concentration. At very low doping, however, we find that this assumption based on Luttinger's theorem is not fulfilled. This implies a change in the topology of the Fermi surface. Furthermore the intensity of the shadow bands observed on the Fermi surface of Bi₂Sr₂CaCu₂O_8+δ as a function of the doping is discussed. Received 12 October 1999 and Received in final form 12 April 2000     

Spin polarized photoemission from molecular beam epitaxy-grown be-doped GaAs

We have made a comparative study of the performance of spin polarized photocathodes based on molecular beam epitaxy (MBE) grown GaAs with Be-doping and on liquid phase epitaxy (LPE) grown GaAs with Zn-doping. The experiments were performed on GaAs (100), (110), and (111) surfaces atT-300 K. The photoelectron spin polarization (P) of the MBE-photocathodes is face dependent reaching 49% for the (111) surface, a value close to P=50% predicted by theory. In contrast for the LPE-photocathodes P is significantly lower (19P36%). Possible causes for the higher polarization of the MBE-photocathodes are investigated. The influence of the Zn- and Be-dopant is elucidated by theoretical model calculations which shows that replacing the Zn-dopant by Be reduces significantly the depolarization at very low temperatures, but not at room temperature. It is therefore concluded that theinterface region between the GaAs substrate and the MBE-GaAs layer grown on top of it strongly influences the performance of the source.On leave from the Istituto di Fisica Universitá di Modena I-41100 Modena Italy.     

Direct observation of strain-induced change in surface electronic structure

We have observed a novel modification of a surface state due to a local strain field induced by a nanopattern formation. N adsorption on the Cu(100) surface induces a nanoscale grid pattern, where the clean Cu regions remain periodically. The lattice is contracted on the clean region by adjacent c(2 x 2)N domains, which have a larger lattice constant. On this patterned surface, we have investigated the Tamm-type surface state at M by means of angle-resolved ultraviolet photoelectron spectroscopy. The binding energy of the Tamm state shifts toward the Fermi level continuously with increasing N coverage, i.e., the intensity of the strain field. This behavior due to the strain field is completely different from that caused by electron confinement observed on vicinal surfaces. The Brillouin zone extension corresponding to the lattice contraction was also detected.     

Direct observation of unconventional topological spin structure in coupled magnetic discs

Confined magnetic thin films are known to exhibit a variety of fascinating topological spin states such as Skyrmions, vortices, and antivortices. Such topological excitations are fundamentally important to our understanding of quantum critical phenomenon and related phase transitions. Here we report on the direct observation of an unconventional topological spin state and its behavior in antiferromagnetically coupled NiFe discs at room temperature. The observed spin structure is similar to the theoretically predicted merons which have not yet been observed directly. We have used in situ Lorentz microscopy magnetizing experiments combined with micromagnetic simulations to follow the stability and the behavior of the meron state. The work presented in this paper will open new opportunities for direct experimental investigation of various topological states that can provide insights into the fundamental physics of their interactions.     

Visualizing electronic chirality and Berry phases in graphene systems using photoemission with circularly polarized light

Electronic chirality near the Dirac point is a key property of graphene systems, which is revealed by the spectral intensity patterns as measured by angle-resolved photoemission spectroscopy under various polarization conditions. Specifically, the strongly modulated circular patterns for monolayer (bilayer) graphene rotate by ±90° (±45°) in changing from linearly to circularly polarized light; these angles are directly related to the phases of the wave functions and thus visually confirm the Berry's phase of π (2π) around the Dirac point. The details are verified by calculations.     

The electronic structure of gold studied by photoemission

Measurements of the photoemission spectra from gold films of different workfunction are reported. Clear evidence is seen for the importance ofk conservation in transitions from the 5d-bands. Comparison with the band structure calculated by Christensen suggests that all the features in the spectrum could be explained by direct transitions.     

Direct STM observation of electronic structure modification of naphthacenequinone molecules due to photoisomerization

Light-induced conformational transformations of the naphthacenequinone (NQ) molecules are observed by scanning tunneling microscopy (STM). NQ molecules packed in a Langmuir-Blodgett (LB) film are shown to form stable ordered structures on a surface of highly oriented pyrolytic graphite (HOPG). The local density of electronic states is found to exhibit the distinct peak which is characteristic of two-dimensional conductivity. An additional subband of empty electronic states is found for NQ molecules in form A but not in form B. The constant-height STM images of individual molecules in form A demonstrate an additional structure that is indicative of a conformational transition of the NQ molecules. This transition involves the transfer of the phenoxy group from one oxygen to another. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 486–490 (25 September 1998)     

Electron spin polarization in the photoemission of NEA GaAs1−x P x

Negative electron affinity GaAs_1–x P _x -photocathodes emit spin-polarized electrons if irradiated with circularly polarized light. The spectra of spin polarization of photoelectrons emitted from crystals with different phosphorus contentx resemble each other in shape but shift to shorter wavelengths with increasingx. Polarization values up to 40% are observed for electrons from crystals withx=0 andx=0.38. Cathodes with higher phosphorus contents increasing fromx=0.65 tox=0.87 and tox = 1.0 deliver photoelectrons with decreasing degrees of spin polarization of 17%, 15%, and 9%, respectively, at maximum.     

The electronic structure of KMnO4 investigated by resonant photoemission

The valence band of potassium permanganate (KMnO₄) was investigated by means of resonant photoemission spectroscopy (ResPES) at the Mn2p, Mn3p and O1s edge. These data confirm the previous conclusions of a strong deviation from a purely ionic charge distribution in this compound. The ResPES data are in agreement with previous results about the character of the individual valence band states. A simple cluster model is used to explain qualitatively the different structures seen in the valence band spectra and their dependence on the photon energy.     

The electronic structure of KMnO4 investigated by resonant photoemission

The valence band of potassium permanganate (KMnO₄) was investigated by means of resonant photoemission spectroscopy (ResPES) at the Mn2p, Mn3p and O1s edge. These data confirm the previous conclusions of a strong deviation from a purely ionic charge distribution in this compound. The ResPES data are in agreement with previous results about the character of the individual valence band states. A simple cluster model is used to explain qualitatively the different structures seen in the valence band spectra and their dependence on the photon energy.     

Direct observation of ferromagnetic spin polarization in gold nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.     

Direct observation of domain-wall configurations transformed by spin currents

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.     

Direct observation of nuclear spin diffusion in real space

Images directly visualizing the spatial spin-diffusion process are reported. The measurements were performed using a magnetic resonance force microscope. The field gradient associated with the force-detection experiment is large enough to affect the spin dynamics and a modified kinetics of the spin-diffusion process is observed. The effects of the gradient were compensated for by a pulse scheme and a pure Zeeman diffusion rate constant of D=(6.2+/-0.7)x10{-12} cm{2}/s in CaF2 was observed.