Maurizio Sacchi

Unraveling the conduction mechanism of Al-doped ZnO films by valence band soft x-ray photoemission spectroscopy

We report on the correlation between the electrical behavior and valence band spectra of undoped and Al-doped ZnO films, obtained by using x-ray photoelectron spectroscopy. Although Al-doping can induce a conductivity increase of two orders of magnitude, we show that the gap persists and there is no semiconductor–metal transition upon doping. For the 3% Al-doped ZnO film, we measure a reduction in the band gap of ∼150meV with respect to the undoped and the 1% doped films. Our results suggest that the band conduction mechanism proposed for undoped ZnO at room temperature still dominates the conduction process in doped films.

Experimental estimate of absorption length and total electron yield (TEY) probing depth in dysprosium

Abstract Absorption spectroscopy has been performed at the M 5 edge of dysprosium and at the L 2,3 edges of nickel by collecting the total electron yield (TEY) in Dy layers of different thicknesses deposited on a Ni(110) substrate. From the thickness dependence of the Ni intensity and from the angular dependence of the Dy intensity we have obtained an experimental estimate of the probing depth and of the minimum absorption length. Both values are found to be very small. We discuss the implications of these results on XAS measurements in TEY mode.

Widely tunable two-colour seeded free-electron laser source for resonant-pump resonant-probe magnetic scattering

The advent of free-electron laser (FEL) sources delivering two synchronized pulses of different wavelengths (or colours) has made available a whole range of novel pump–probe experiments. This communication describes a major step forward using a new configuration of the FERMI FEL-seeded source to deliver two pulses with different wavelengths, each tunable independently over a broad spectral range with adjustable time delay. The FEL scheme makes use of two seed laser beams of different wavelengths and of a split radiator section to generate two extreme ultraviolet pulses from distinct portions of the same electron bunch. The tunability range of this new two-colour source meets the requirements of double-resonant FEL pump/FEL probe time-resolved studies. We demonstrate its performance in a proof-of-principle magnetic scattering experiment in Fe–Ni compounds, by tuning the FEL wavelengths to the Fe and Ni 3p resonances.

Plane-grating flat-field soft x-ray spectrometer

We describe a soft x-ray spectrometer covering the 120–800 eV range. It is intended for resonant inelastic x-ray scattering experiments performed at third generation synchrotron radiation (SR) facilities and has been developed with SOLEIL, the future French national SR source in mind. The Hettrick–Underwood principle is at the heart of the design using a combination of varied line-spacing plane grating and spherical-mirror to provide a flat-field image. It is slitless for optimum acceptance. This means the source size determines the resolving power. A spot size of ⩽5μm is planned at SOLEIL which, according to simulations, should ensure a resolving power ⩾1000 over the whole energy range. A 1024×1024 pixel charge-coupled device (CCD) with a 13μm×13μm pixel size is used. This is an improvement on the use of microchannel-plate detectors, both as concerns efficiency and spatial resolution. Additionally spectral line curvature is avoided by the use of a horizontal focusing mirror concentrating the beam in the ...

Design and performance of AERHA, a high acceptance high resolution soft x-ray spectrometer

A soft x-ray spectrometer based on the use of an elliptical focusing mirror and a plane varied line spacing grating is described. It achieves both high resolution and high overall efficiency while remaining relatively compact. The instrument is dedicated to resonant inelastic x-ray scattering studies. We set out how this optical arrangement was judged best able to guarantee performance for the 50 - 1000 eV range within achievable fabrication targets. The AERHA (adjustable energy resolution high acceptance) spectrometer operates with an effective angular acceptance between 100 and 250 μsr (energy dependent) and a resolving power well in excess of 5000 according to the Rayleigh criterion. The high angular acceptance is obtained by means of a collecting pre-mirror. Three scattering geometries are available to enable momentum dependent measurements with 135°, 90°, and 50° scattering angles. The instrument operates on the Synchrotron SOLEIL SEXTANTS beamline which serves as a high photon flux 2 × 200 μm(2) focal spot source with full polarization control.

The SEXTANTS beamline at SOLEIL: a new facility for elastic, inelastic and coherent scattering of soft X-rays

SEXTANTS is a new SOLEIL beamline dedicated to soft X-ray scattering techniques. The beamline, covering the 50-1700 eV energy range, features two Apple-II undulators for polarization control and a fixed-deviation monochromator. Two branch-lines host three end-stations for elastic, inelastic and coherent scattering experiments.

Ultrahigh-vacuum soft x-ray reflectometer

We have designed, built, and tested a new instrument for soft x-ray scattering experiments. The reflectometer works under ultrahigh vacuum and permits in situ preparation and characterization of the samples. In particular, deposition and sputtering operations can be performed while measuring x-ray scattering. We report the results of test measurements performed using synchrotron radiation. The precision of the combined positioning of sample and detector angles is better than 0.01°. Separately, sample and detector rotations have a repeatability that is better than 0.005°. Applications will be in the field of surface physics, with emphasis on magnetic properties of surfaces, thin films, and multilayered structures.

Magnetic-Properties of Fe and Tb in Tbxfe1-X Amorphous Films Studied with Soft-X-Ray Circular and Linear Dichroism

Abstract We have used linearly and circularly polarized X-rays to determine the magnetic properties of several Tb x Fe 1− x amorphous films. Absorption measurements on the M 4.5 edges of Tb and the L 2.3 edges of Fe allowed us to obtain information about the size and direction of local magnetic moments. Our results confirm that linear dichorism in rare earth M 4.5 edges can give useful information about both crystal field and magnetic effects.

Absorption cross sections at the M4,5 edges of rare earths: a soft X-ray transmission experiment

Abstract We have determined the absorption coefficients of several rare earths (Gd, Dy, Ho and Er) at the M4,5 edges (3d → 4f excitations) by measuring the soft X-ray transmission through layers of different thicknesses between 2 and 400 A. Our results are in good agreement with atomic calculations. We also compare transmission-derived absorption curves with spectra collected simultaneously in the standard total electron yield mode. Advantages and limitations of the transmission mode for soft X-rays and also its range of application are discussed.

Magnetic ordering in TbxFe1-x amorphous films : an application of x-ray dichroism with linearly polarized light

Magnetic x‐ray dichroism (MXD) in the 3d→4f  absorption transitions of rare‐earth ions is a new tool for the investigation of local magnetic properties. We have measured MXD in amorphous Tb‐Fe magnetic thin films, showing its potentials and advantages with respect to other techniques, especially when surface magnetic properties are investigated.