A comprehensive study of the vibrationally resolved S 2p(-1) Auger electron spectrum of carbonyl sulfide.

High-resolution normal Auger-electron spectra of carbonyl sulfide subsequent to S 2p(-1) photoionization at photon energies of 200, 220, and 240 eV are reported along with corresponding photoelectron spectra. In addition, theoretical results are presented that take the core-hole orientation of the various spin-orbit-split and molecular-field-split S 2p(-1) states into account. Auger transitions to eight metastable dicationic final states are observed and assigned on the basis of the theoretical results. From Franck-Condon analysis, assuming Morse potentials along the normal coordinates for seven of the observed quasi-stable dicationic final states, information on the potential-energy surfaces is derived and compared with theoretical results from the literature.

Resonant soft x-ray scattering from stepped surfaces of SrTiO3.

We studied the resonant diffraction signal from stepped surfaces of SrTiO(3) at the Ti 2p → 3d (L(2,3)) resonance in comparison with x-ray absorption (XAS) and specular reflectivity data. The steps on the surface form an artificial superstructure suitable as a model system for resonant soft x-ray diffraction. A small step density on the surface is sufficient to produce a well defined diffraction peak. We determined the optical parameters of the sample across the resonance and found that the differences between the energy dependence of the x-ray absorption signal, the specular reflectivity and the step-related peak reflect the different quantities probed in these signals. When recorded at low incidence or detection angles, XAS and specular reflectivity spectra are strongly distorted by the changes of the angle of total reflection with energy. The resonant diffraction spectrum is less affected and can be used as a spectroscopic probe even in less favorable geometries.

Metastable states in NO2+ probed with Auger spectroscopy.

High-resolution N 1s and O 1s photoelectron spectra (PES) of NO are presented together with spectra of the subsequent Auger decay. The PES are analyzed by taking spin-orbit splitting of the (2)Π ground state into account providing detailed information on equilibrium distances, vibrational energies, and lifetime widths of the core-ionized states. In the Auger electron spectra (AES) transitions to five metastable dicationic final states are observed, with two of them previously unobserved. A Franck-Condon analysis of the vibrational progressions belonging to these transitions provides detailed information on the potential-energy curves of the dicationic final states as well as on the relative Auger rates. The present calculations of the potential-energy curves of NO(2+) agree well with the experimental results and allow an assignment of the two hitherto unresolved Auger transitions to excited states of NO(2+), C(2)Σ(+)and c(4)Π.

Fermi-surface topology and helical antiferromagnetism in heavy lanthanide metals.

Detailed angle-resolved photoemission studies of Tb and Dy metal in the paramagnetic phase provide direct experimental proof of the presence of nesting features in the Fermi surfaces (FS) of these heavy lanthanide (Ln) metals. The observations clearly support the hypothesis that nesting of the FS in the paramagnetic phase is responsible for the development of helical antiferromagnetic ordering in heavy Ln metals. They also show that magnetic exchange splitting of the electronic states is responsible for the disappearance of FS nesting in the ferromagnetic phases.

Antiferromagnetic order with atomic layer resolution in EuTe(111) films.

The temperature dependences of the magnetizations of individual atomic layers across an epitaxial antiferromagnetic EuTe film were derived from virtually background-free magnetic Bragg peaks with pronounced Laue oscillations recorded with soft x rays at the Eu-M5 resonance. The magnetizations of the outermost layers decrease significantly differently from those of bulk layers, in agreement with Heisenberg-Monte Carlo calculations. The results demonstrate the applicability of the method to complex ordering phenomena at surfaces and interfaces of thin films.

Temperature-induced reversal of magnetic interlayer exchange coupling.

For epitaxial trilayers of the magnetic rare-earth metals Gd and Tb, exchange coupled through a nonmagnetic Y spacer layer, element-specific hysteresis loops were recorded by the x-ray magneto-optical Kerr effect at the rare-earth M5 thresholds. This allowed us to quantitatively determine the strength of interlayer exchange coupling (IEC). In addition to the expected oscillatory behavior as a function of spacer-layer thickness dY, a temperature-induced sign reversal of IEC was observed for constant dY, arising from magnetization-dependent electron reflectivities at the magnetic interfaces.

Direct observation of t2g orbital ordering in magnetite.

Using soft-x-ray diffraction at the site-specific resonances in the Fe L2,3 edge, we find clear evidence for orbital and charge ordering in magnetite below the Verwey transition. The spectra show directly that the (001/2) diffraction peak (in cubic notation) is caused by t2g orbital ordering at octahedral Fe2+ sites and the (001) by a spatial modulation of the t2g occupation.

Spectroscopic analysis of small organic molecules: a comprehensive near-edge x-ray-absorption fine-structure study of C6-ring-containing molecules.

We report high-resolution C 1s near-edge x-ray-absorption fine-structure (NEXAFS) spectra of the C6-ring-containing molecules benzene (C6H6), 1,3- and 1,4-cyclohexadiene (C6H8), cyclohexene (C6H10), cyclohexane (C6H12), styrene (C8H8), and ethylbenzene (C8H10) which allow us to examine the gradual development of delocalization of the corresponding pi electron systems. Due to the high experimental resolution, vibrational progressions can be partly resolved in the spectra. The experimental spectra are compared with theoretical NEXAFS spectra obtained from density-functional theory calculations where electronic final-state relaxation is accounted for. The comparison yields very good agreement between theoretical spectra and experimental results. In all cases, the spectra can be described by excitations to pi*- and sigma*-type final-state orbitals with valence character, while final-state orbitals of Rydberg character make only minor contributions. The lowest C 1s-->1pi* excitation energy is found to agree in the (experimental and theoretical) spectra of all molecules except for 1,3-cyclohexadiene (C6H8) where an energy smaller by about 0.6 eV is obtained. The theoretical analysis can explain this result by different binding properties of this molecule compared to the others.

Spectroscopy of stripe order in La1.8Sr0.2NiO4 using resonant soft x-ray diffraction.

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.

Electronic structure and dynamics of quantum-well states in thin Yb metal films.

Quantum-well states above the Fermi energy in thin Yb(111) metal films deposited on a W(110) single crystal were studied by low-temperature scanning tunneling spectroscopy. These states are laterally highly localized and give rise to sharp peaks in the tunneling spectra. A quantitative analysis of the spectra yields the bulk-band dispersion in the Gamma-L direction as well as quasiparticle lifetimes. The quadratic energy dependence of the lifetimes is in quantitative agreement with Fermi-liquid theory.

Finite-size effect on magnetic ordering temperatures in long-period antiferromagnets: holmium thin films.

The thickness dependence of the helical antiferromagnetic ordering temperature T(N) was studied for thin Ho metal films by resonant magnetic soft x-ray and neutron diffraction. In contrast with the Curie temperature of ferromagnets, T(N) was found to decrease with film thickness d according to [T(N)(infinity)-T(N)(d)]/T(N)(d) proportional variant (d-d(0))(-lambda(')), where lambda(') is a phenomenological exponent and d(0) is of the order of the bulk magnetic period L(b). These observations are reproduced by mean-field calculations that suggest a linear relationship between d(0) and L(b) in long-period antiferromagnets.

Magneto-optical near-field microscopy of ultrathin films in ultrahigh vacuum.

We demonstrate that magnetic domains of ultrathin films can be imaged in situ in ultrahigh vacuum (UHV) by scanning near-field optical microscopy in combination with a Sagnac interferometer (Sagnac-SNOM). In the present set-up, the Sagnac interferometer is not only used for measuring the magneto-optical Kerr effect but also for tip-to-sample distance control. The high sensitivity of the instrument enabled us to study the magnetization reversal processes in externally applied magnetic fields in ultrathin films of Fe and Ni grown on Cu(100) single crystals.

Statistical properties of inter-series mixing in helium: from integrability to chaos.

The photoionization spectrum of helium shows considerable complexity close to the double-ionization threshold. By analyzing the results from both our recent experiments and ab initio three- and one-dimensional calculations, we show that the statistical properties of the spacings between neighboring energy levels clearly display a transition towards quantum chaos.

X-ray magneto-optics in lanthanides.

Magneto-optical methods in the visible light regime generally lack element specificity, which has become a considerable shortcoming in research on advanced heteromagnetic systems. Using circularly polarized soft x rays tuned to a 4d-4f core-level transition of a lanthanide element, the specularly reflected x-ray intensity changes proportionally to the magnetization of this element and, e.g., hystereses are easily measured element specifically. In contrast to the case of visible light, temperature dependent 4d-4f magneto-optical signals are not influenced by the thermal lattice expansion.

Magnetic splitting of valence states in ferromagnetic and antiferromagnetic lanthanide metals

The magnetic splitting of Delta(2) valence states in the heavy lanthanide metals Gd, Tb, Dy, and Ho was studied in epitaxial films by angle-resolved photoemission, revealing an essentially Stoner-like temperature dependence in all cases. It scales linearly with the 4f spin moment, even in the case of the helical antiferromagnet Ho. Such a behavior can be explained by a substantial localization of the corresponding wave function in the c direction. The helical magnetic structure was confirmed for the thin Ho films by in situ resonant magnetic x-ray diffraction.

Magnetic domain imaging with a scanning near-field optical microscope using a modified Sagnac interferometer.

We report on the combination of a scanning near-field optical microscope and a modified Sagnac interferometer for magnetic-domain imaging in the reflection mode. The Sagnac interferometer is used for detection of the magneto-optical Kerr effect. Since the interferometer is inherently insensitive to polarization changes caused by topography effects, magnetic-domain imaging is not limited to samples with flat surfaces. In this way, it is possible to image magnetic bits written on the tracks of a magneto-optical disc that has a rather pronounced surface profile.