Formation of hollow atoms at metal- and insulator surfaces

Received: 27 March 1998/Accepted: 23 October 1998     

Electron spin polarization esp at surfaces of ferromagnetic metals

Fundamental information on surface magnetic order (SMO) of ferromagnetic metals can be obtained from electron-capture, photoemission, fieldemission, spin-dependent tunneling and spin-polarized LEED experiments. The different techniques, new experimental advances and developments are discussed with particular emphasis given to electron-capture spectroscopy. This review will focus on new experimental and theoretical results (long-range and “local” SMO of ferro- and antiferromagnetic metals, surface states, SMO of thin films, new magnetic surface phases, magnetic surface reconstruction, chemisorption) obtained in the years past which have brought outstanding progress towards a deeper comprehension of the physics of ferromagnetism and towards the unravelling of the physical processes inherently involved in the various methods for spin spectroscopy. Recent data on the SMO received from experiments performed at surfaces of single crystals of 3d-TM and 4f-RE metals reveal new scientific insights and perspectives for the theoretical analysis of experimental results within the framework of the currently refined knowledge about ferromagnetism.     

Investigation of solid surfaces by nuclear spin polarized alkali atoms

Nuclear spin polarized alkali atom beams are used to investigate metal surfaces. The surface diffusion of the alkali atoms on the surface results in a randomly fluctuating electric field gradient. The relaxation is measured in dependence of the surface temperature. Using an additional external RF field, also NMR measurement can be performed. Besides some fluctuating components of the EFG which cause the relaxation process, there is also a static part of the EFG. This results in an energy splitting of the nuclear spin states and can be detected by the NMR experiments.Supported by the Deutsche Forschungsgemeinschaft, Bonn FRGA.v. Humboldt fellow, on leave from Los Alamos Nat. Lab., USA     

Local spin moment distribution in antiferromagnetic molecular rings probed by NMR

The NMR spectra of 19F and 53Cr have been obtained at low temperatures in a heterometallic substituted antiferromagnetic (AF) ring Cr7Cd with an S=3/2 ground state and compared with the spectra in a homometallic Cr8 AF ring with an S=0 ground state. From the analysis of the spectra one can derive directly model independent values of the staggered nonuniform distribution of the local moment in the heterometallic ring Cr7Cd. The experimental values are found to be in excellent agreement with the theoretical values calculated on the basis of an effective spin Hamiltonian which includes crystal field effects.     

Determining the spin polarization of surfaces by spin-polarized scanning tunneling spectroscopy

During the last few years, spin-polarized scanning tunneling microscopy and spectroscopy has been developed as a reliable tool to image surface magnetic domain structures of bulk materials as well as thin films and nanostructured systems. In principle, this technique also allows for the determination of the energy-resolved spin polarization of the sample P_S(E) with nanometer resolution, information which might play a crucial role in understanding systems like, for example, non-magnetic adatoms on magnetic surfaces. A main problem in quantifying P_S(E), however, arises from the fact that, in contrast to planar junctions, the tip–sample distance generally varies with the magnitude and direction of the surface magnetization, since the distance is controlled indirectly by the tunneling current that is itself spin-polarized. We employ a simple model of the tunneling process to investigate this issue and show that a normalization of the dI/dU spectra with the total conductance I/U is insufficient to correct for their distance dependence. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-5311, E-mail: kubetzka@physnet.uni-hamburg.de     

Element-selective spin polarization analysis of surfaces by spin-polarized ion scattering spectroscopy

We discuss the validity of surface spin polarization analysis with element selectivity using spin-polarized ion scattering spectroscopy (SP-ISS). We examined the control of the incident ⁴He⁺ spins and successfully conducted magnetic hysteresis measurement on an Fe(100) surface. The spin polarization of the Fe(100) surface exposed to O₂ atmosphere measured by spin-polarized ion neutralization spectroscopy was consistent with that reported by spin-polarized metastable de-excitation spectroscopy. The element selectivity of SP-ISS is discussed in terms of ion neutralization, re-ionization, and multiple scattering.     

Optical polarization response at gold nanosheet edges probed by scanning near-field optical microscopy

Optical properties of metallic edge-like structures known as knife-edges are a topic of interest and possess potential applications in enhanced Raman scattering, optical trapping, etc. In this work, we investigate the near-field optical polarization response at the edge of a triangular gold nanosheet, which is synthesized by a wet chemical method. A homemade scanning near-field optical microscope(SNOM) in collection mode is adopted, which is able to accurately locate its probe at the edge during experiments. An uncoated straight fiber probe is used in the SNOM, because it still preserves the property of light polarization though it has the depolarization to some extent. By comparing near-field intensities at the edge and glass substrate, detected in different polarization directions of incident light, the edge-induced depolarization is found,which is supported by the finite differential time domain(FDTD) simulated results. The depolarized phenomenon in the near-field is similar to that in the far-field.     

Single crystal surfaces probed by polarized nuclei

Nuclear magnetic resonance has developed into a very powerful technique to study the structure and dynamics of atomic and molecular systems, both in liquid and solid phase. However the investigation of single crystal surfaces with “conventional” NMR methods is essentially impossible due to the small sample size of less than 10¹⁵ sites on a cm². To overcome this for the important class of alkali adsorbates on metals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin polarized atomic beam of ⁶Li in the one case and ⁸Li in the other. The latter isotope is radioactive and undergoes a \beta‐decay with a halflife of 0.84 s. Li adsorbed on the close packed Ru(001) surface is investigated. The T{in1} relaxation rate is the main observable and is used to deduce the local electronic density of states (LDOS(E_F,r=0)) and the Li diffusion barriers at low and high adsorbate coverage. The second experiment uses ⁶Li as an adsorbate, also studied on Ru(001). The nuclear polarization is measured by beam foil spectroscopy. A novel particle detected (photon counting) Fourier‐Transform NMR technique is demonstrated, by observing the time dependent flux of circularly polarized light emitted behind the foil after a 90^\circ‐pulse has been employed at the surface. Development and prospects of the latter technique are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Under-coordinated atoms induced local strain, quantum trap depression and valence charge polarization at W stepped surfaces

We have explored the effects of atoms under-coordination on surface structure relaxation, binding energy shift of W stepped surfaces and valence charge polarization by the method of incorporating bond order-length-strength (BOLS) correlation mechanism into high-resolution X-ray photoluminescence spectra (XPS) measurements as well as density functional theory (DFT) calculations. Results show that the 4f_7/2 energy levels of bulk, surface skin and step edge W atoms shift deeper from 2.17 to 2.69 eV with respect to that of the isolated W (28.91±0.01 eV) atoms, while the valence charge energy shift upper from inner to outer layer and from bulk to stepped edge. The surface bond contraction occurs around under-coordinated atoms after geometry relaxation calculation. Consistency among BOLS calculations, DFT calculation and experimental measurements clarifies that the surface bond contraction and consolidation due to the effects of under-coordination atoms induce potential trap depression, which provides perturbation to the Hamiltonian and hence contributes to the surface core level shift deeper, and that the surface valence charge are polarized by the densely trapped core-level electrons to upper energy.     

Properties of hollow molecules probed by single-photon double ionization

The formation of hollow molecules (with a completely empty K shell in one constituent atom) through single-photon core double ionization has been demonstrated using a sensitive magnetic bottle experimental technique combined with synchrotron radiation. Detailed properties are presented such as the spectroscopy, formation, and decay dynamics of the N(2)(2+) K(-2) main and satellite states and the strong chemical shifts of double K holes on an oxygen atom in CO, CO2, and O2 molecules.     

Observation of the spin polarization of Cs atoms during optical orientation of metastable helium atoms in alkali-helium plasma

We are the first to experimentally observe a magnetic-resonance signal of 6² S _1/2 Cs atoms by absorption of light from a helium lamp that was used to optically orient metastable 2³ S ₁ He atoms. The amplitude of the cesium signal proved to be almost three orders of magnitude lower than the amplitude of the magnetic resonance of 2³ S ₁ He atoms. Particular features of the creation and observation of the collisional polarization of cesium under conditions of alkali-helium plasma have been discussed.     

Complete spin polarization of 0.5-to 500-eV electrons elastically scattered by argon atoms: Theoretical consideration

Within the framework of a phenomenological real optical potential, the elastic scattering of 0.5-to 500-eV electrons by argon atoms is studied in view of spin-orbit interaction. The energy dependences of the angular positions of minima in differential cross sections are calculated and compared with those obtained in experiments. The energies and angular positions of four critical points where the cross sections reach their minimal values are determined. It is found that the deepest minimum of three high-angle ones (with angles >90°) is located at (38.25 eV, 140.67°). The energy and angular neighborhoods of these minima where the parameter of spin polarization, the so-called Sherman function, reaches its extreme values (+1 and ?1) are determined.     

Femtosecond electron and spin dynamics probed by nonlinear optics

Received: 20 September 1998     

Chiral symmetry probed with pionic atoms

Study of pionic atom has been revisited by a recent discovery of deeply bound pionic states in the context of possible sensitivity to the partial chiral restoration in the nuclear medium. Precisely measured binding energies and widths of the deeply bound π^? Is states in ^115,119,123Sn in ^116,120,124Sn(d,³He) reactions at recoil-free conditions are used to deduce the medium-modified isovector scattering amplitude as b₁ = 0.115 ± 0.007m _π ^?1. The observed enhancement of b ₁ over the free πN value infers 34% reduction of $\left\langle {\bar qq} \right\rangle $ at the normal nuclear density, ρ = 0.17 fm^?3.     

Dynamics of polarization buildup by spin filtering

There has been much recent research into polarizing an antiproton beam, instigated by the recent proposal from the PAX (Polarized Antiproton eXperiment) project at GSI Darmstadt. It plans to polarize an antiproton beam by repeated interaction with a polarized internal target in a storage ring. The method of polarization by spin filtering requires many of the beam particles to remain within the ring after scattering off the polarized internal target via electromagnetic and hadronic interactions. We present and solve sets of differential equations which describe the buildup of polarization by spin filtering in many different scenarios of interest to projects planning to produce high-intensity polarized beams. These scenarios are: 1) spin filtering of a fully stored beam; 2) spin filtering while the beam is being accumulated, i.e. unpolarized particles are continuously being fed into the beam; 3) the particle input rate is equal to the rate at which particles are being lost due to scattering beyond the ring acceptance angle, the beam intensity remaining constant; 4) increasing the initial polarization of a stored beam by spin filtering; 5) the input of particles into the beam is stopped after a certain amount of time, but spin filtering continues. The rate of depolarization of a stored polarized beam on passing through an electron cooler is also shown to be negligible.