arXiv subjects

Augusto Marcelli

Publications and source records attributed to Augusto Marcelli.

24 records · Page 2Linked to original sources

arXiv2014

Percolative superconductivity in La2CuO4.06 by lattice granularity patterns with scanning micro X-ray absorption near edge structure

The simplest cuprate superconductor La2CuO4+y with mobile oxygen interstitials exhibits a clear phase separation, but only recently a bulk multiscale structural phase separation has been observed by using scanning micro X-ray diffraction. Here we get further information on the structural phase separation, using local probe X-ray absorption near edge structure. The spatial distribution of superconducting units is a key parameter controlling percolative superconductivity in complex matter with dispersed superconducting units. These oxides form super-molecular architectures made of superconducting atomic monolayers intercalated by spacers. Oxygen interstitials enter into the rocksalt La2O2+y spacer layers forming oxygen interstitials rich puddles and poor puddles. Their spatial distribution has been determined by using scanning La L3-edge micro X-ray absorption near edge structure. Percolating networks of oxygen rich puddles are observed in different micrometer size portions of the crystals. Moreover, the complex surface resistivity shows two jumps associated to the onset of intra-puddle and inter-puddles percolative superconductivity. The similarity of oxygen doped La2CuO4+y, with the well established phase separation in iron selenide superconductors is also discussed.

cond-mat.supr-con↗

arXiv2014

A LabVIEW based user-friendly X-ray phase-contrast imaging system software platform

X-ray phase-contrast imaging can provide greatly improved contrast over conventional absorption-based imaging for weakly absorbing samples, such as biological soft tissues and fibre composites. In this manuscript, we introduce an easy and fast way to develop a user-friendly software platform dedicated to the new grating-based X-ray phase-contrast imaging setup recently built at the National Synchrotron Radiation Laboratory of the University of Science and Technology of China. Unified management and control of 21 motorized positioning stages, of an ultra-precision piezoelectric translation stage and of the X-ray tube are achieved with this platform. The software package also covers the automatic image acquisition of the phase-stepping scanning with a flat panel detector. Moreover, a data post-processing module for signals retrieval and other custom features are in principle available. With a seamless integration of all necessary functions in a unique package, this software platform will greatly support the user activity during experimental runs.

cs.SE↗

arXiv2014

The AC multi-harmonic magnetic susceptibility measurement setup at the LNF-INFN

The AC magnetic susceptibility is a fundamental method in materials science, which allows to probe the dynamic magnetic response of magnetic materials and superconductors. The LAMPS laboratory at the Laboratori Nazionali di Frascati of the INFN hosts an AC multi-harmonic magnetometer that allows performing experiments with an AC magnetic field ranging from 0.1 to 20 Gauss and in the frequency range from 17 to 2070 Hz. A DC magnetic field from 0 to 8 T produced by a superconducting magnet can be applied, while data may be collected in the temperature range 4.2-300 K using a liquid He cryostat under different temperature cycles setups. The first seven AC magnetic multi-harmonic susceptibility components can be measured with a magnetic sensitivity of 1x10-6 emu and a temperature precision of 0.01 K. Here we will describe in detail about schematic of the magnetometer, special attention will be dedicated to the instruments control, data acquisition framework and the user-friendly LabVIEW-based software platform.

physics.ins-det↗

arXiv2014

A Two Rotor Model with spin for magnetic Nanoparticles

We argue that for some species of magnetic nanoparticles the macrospin can have a nonvanishing moment of inertia and then an orbital angular momentum. We represent such nanoparticles by two interacting rigid rotors one of which has a large spin attached to the body, namely by a Two Rotor Model with spin. By this model we can describe in a unified way the cases of nanoparticles free and stuck in an elastic or rigid matrix. We evaluate the magnetic susceptibilities for the latter case and under some realistic assumptions we get results in closed form. A crossover between thermal and purely quantum hopping occurs at a temperature much larger than that at which purely quantum tunneling becomes important. A comparison with some experimental data is outlined.

cond-mat.mes-hall↗

arXiv2013

The historical development of X-ray Absorption Fine Spectroscopy and of its applications to Materials Science

This essay sketches the development of X-ray Absorption Fine Spectroscopy (XAFS) ever since the second half of 20th century. At that time, synchrotrons started competing with X-ray discharge tubes as the sources of the excitation able to show the pre- and near-edge structures (XANES) and extended oscillations (EXAFS) that characterize the X-ray absorption edge of solid matter. Actually, modern XAFS began after 1975, when the hard-X-ray synchrotron radiation derived from storage rings took over. Ever since, XAFS greatly contributed to both technical refinement and to theoretical development of Materials Science. Although a unified theory of X-ray fine absorption has not been reached yet, many XAFS advancements benefited of theoretical models and complex calculations made possible by the continuous growth of the computing power, while contributing to developing new or previously never used materials.

cond-mat.mtrl-sci↗

arXiv2012

Optimum inhomogeneity of local lattice distortions in La2CuO4+y

Electronic functionalities in materials from silicon to transition metal oxides are to a large extent controlled by defects and their relative arrangement. Outstanding examples are the oxides of copper, where defect order is correlated with their high superconducting transition temperatures. The oxygen defect order can be highly inhomogeneous, even in "optimal" superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the "glue" binding the ordered regions together. Here we use scanning X-ray microdiffraction (with beam 300 nm in diameter) to show that for La2CuO4+y, the "glue" regions contain incommensurate modulated local lattice distortions, whose spatial extent is most pronounced for the best superconducting samples. For an underdoped single crystal with mobile oxygen interstitials in the spacer La2O2+y layers intercalated between the CuO2 layers, the incommensurate modulated local lattice distortions form droplets anticorrelated with the ordered oxygen interstitials, and whose spatial extent is most pronounced for the best superconducting samples. In this simplest of high temperature superconductors, there are therefore not one, but two networks of ordered defects which can be tuned to achieve optimal superconductivity. For a given stoichiometry, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form fractal patterns, as opposed to appearing in isolated spots. We speculate that the relationship between material complexity and superconducting transition temperature Tc is actually underpinned by a fundamental relation between Tc and the distribution of ordered defect networks supported by the materials.

cond-mat.supr-con↗