D. Di Gioacchino

Local structural disorder in REFeAsO oxypnictides by RE L3 edge XANES

The REFeAsO (RE =xa0La, Pr, Nd and Sm) system has been studied by RE L(3) x-ray absorption near edge structurexa0(XANES) spectroscopy to explore the contribution of the REO spacers between the electronically active FeAs slabs in these materials. The XANES spectra have been simulated by full multiple scattering calculations to describe the different experimental features and their evolution with the RE size. The near edge feature just above the L(3) white line is found to be sensitive to the ordering/disordering of oxygen atoms in the REO layers. In addition, shape resonance peaks due to As and O scattering change systematically, indicating local structural changes in the FeAs slabs and the REO spacers due to RE size. The results suggest that interlayer coupling and oxygen order/disorder in the REO spacers may have an important role in the superconductivity and itinerant magnetism of the oxypnictides.

Arsenic K-edge XANES study of REFeAsO oxypnictides

Arsenic K-edge X-ray absorption near-edge structure (XANES) spectroscopy has been used to investigate the local geometry around the pnictogen site in the REFeAsO (RE=La, Pr, Nd and Sm) oxypnictides, combined with full multiple-scattering calculations to understand different XANES features. The As K-edge XANES spectrum for the LaFeAsO is found to be significantly different from the other oxypnictides. The results suggest that interlayer order/disorder has a significant effect on the XANES features with the coupling between the FeAs and the REO spacers being important in the electronic and magnetic properties of the oxypnictides.

Defective iron-oxide nanoparticles synthesised by high temperature plasma processing: a magnetic characterisation versus temperature.

Magnetic properties and phase compositions of iron-oxide nanoparticles synthesised by a high temperature arc plasma route have been investigated by Mössbauer spectroscopy and high harmonic magnetic AC susceptibility measurements, and correlated with morphological and structural properties for different synthesis conditions. The Mössbauer spectra precisely determined the presence of different iron-oxide fractions in the investigated nanoparticles, while the high harmonic magnetic susceptibility measurements revealed the occurrence of metastable magnetic phases evolving in temperature and time. This study illustrates magnetic properties and dynamics of the magnetic configurations of iron-oxide nanoparticles grown by high temperature plasma, a process less explored so far but extremely useful for synthesising large numbers of nanoparticles for industrial applications.

RAP: thermoacoustic detection at the DAΦNE beam test facility

In order to investigate the anomalous response at ultra-low temperatures of the resonant-mass gravitational wave detector NAUTILUS, the RAP experiment has been planned to measure the vibrations in a small cylindrical aluminium bar when hit by 10 5 510 MeV electrons from the DANE beam test facility, corresponding to the energies released by typical extensive air showers. The results of the measurement at low temperature and in the superconducting regime are crucial to understand the interaction of ionizing particles with bulk superconductors and to confirm the results on the thermoacoustic model of the past experiments. The first run of RAP experiment is scheduled for the end of June. The scheme of operation and the preliminary results at room temperature will be presented.

First results of the RAP experiment (acoustic detection of particles) in the low temperature regime

The results on cosmic rays detected by the gravitational wave antenna NAUTILUS have motivated an experiment (RAP) based on a suspended cylindrical bar, which is made of the same aluminum alloy as NAUTILUS and is exposed to a high energy electron beam. Mechanical vibrations originate from the local thermal expansion caused by warming up due to the energy lost by particles crossing the material. The aim of the experiment is to measure the amplitude of the fundamental longitudinal vibration at different temperatures. We report on the results obtained down to a temperature of about 4 K for an Al 5056 bar, which agree at the level of 10% with the predictions of the model describing the underlying physical process. Very preliminary results for a Niobium bar at temperatures below and above the transition temperature are also reported.

Acoustic detection of particles, the RAP experiment: present status and results

Cosmic ray events with rate and energy much higher than expected were detected by the ultracryogenic gravitational antenna Nautilus located at the Laboratori Nazionali di Frascati, when it was operating in superconducting state. Mechanisms related to the superconductivity state of the material could be involved in such a way to enhance the conversion efficiency of the particle energy into vibrational energy of the detector. The RAP experiment has the aim to study the mechanical response of a small metallic resonant bar to short pulses of high energy electron beam, investigating the response of the bar both in normal and in superconducting state. The results obtained for an Al5056 bar down to a temperature of 4 K are reported and the preliminary results for a niobium bar at temperature below and above the superconducting-normal transition are also discussed.

RAP — ACOUSTIC DETECTION OF PARTICLES: FIRST RESULTS AT 4.2 K

RAP (Rivelazione Acustica di Particelle) is a small cylindrical aluminum bar (l = 500 mm, d = 181.7 mm) placed at the DAΦNE Beam Test Facility, where it is hit by a 510 MeV electron beam, coming from the DAΦNE Linac. Aim of the experiment is to measure the mechanical vibrations of the bar caused by the interaction with the beam. On June, 2004 RAP successfully collected data for the first time at cryogenic temperature. Several runs at different temperatures (4.5, 70 and 273 K) have been performed and a number of shots in normal-conducting state of the bar were detected. The preliminary results are in good agreement with the Thermo-Acoustic Model. In the next months the mounting of a dilution refrigerator and the data taking of the bar in the superconducting state (T = 100 mK) have been planned.