M. Ferretti

Glassy dynamics of the inhomogeneous metallic phase in La 1 − x Ca x MnO 3

The anelastic spectra (elastic energy loss and dynamic modulus versus temperature) of La 1 - x Ca x MnO 3 have been measured for 0.33<x<0.5. A peak in the imaginary part of the elastic susceptibility is found slightly belowthe Curie temperature, whose temperature and frequency dependences are typical of the dielectric and magnetic susceptibilities of relaxor ferroelectrics, spin glasses and other inhomogeneous frustrated systems. The fluctuations can be associated with the magnetic clusters which are observed by several techniques or with charge-ordered insulating domains in the low temperature inhomogeneous phase. The temperature dependent distribution function of the fluctuation energy barriers has been extracted by an analysis of the experimental curves.

Synthesis and characterisation of superconducting RuSr2GdCu2O8

Abstract We report on the structural, electrical and magnetic properties of RuSr2GdCu2O8 samples made by systematic synthesis work so that they differ from one another in number of annealings and sintering temperature. Our aim is to highlight the different behaviours as a consequence of the thermal treatments. In particular, we deal with the onset of superconductivity in relation to the homogenisation process, and its influence on resistivity, structural and magnetic ordering. Finally, the problems related to the magnetic measurements due to the superimposition of different magnetic states are examined.

Upper critical field and fluctuation conductivity in the critical regime of doped SmFeAsO

We measure magnetotransport of F doped SmFeAsO samples up to 28T and we extract the upper critical fields, using different criteria. In order to circumvent the problem of criterion-dependence Hc2 values, we suggest a thermodynamic estimation of the upper critical field slope dHc2/dT based on the analysis of conductivity fluctuations in the critical regime. A high field slope as large as -12T/K is thus extracted for the optimally doped sample. We find evidence of a two-dimensional lowest Landau level (LLL) scaling for applied fields larger than mu_0H_LLL=8T. Finally, we estimate the coherence length values and we observe that they progressively increase with decreasing Tc. In all cases, the coherence length values along the c axis are smaller than the interplanar distance, confirming the two-dimensional nature of superconductivity in this compound.

Fast oxygen mobility in tetragonal YBa2Cu3O7-x by anelastic relaxation measurements

Abstract Evidence of a very high mobility of the isolated O atoms in the Cu(1)-O basal planes of tetragonal YBa2Cu3O6.2 is reported. The jumps of the isolated O atoms between the O(4) and O(5) positions are expected to give rise to a well detectable anelastic relaxation process, and a recently reported elastic energy dissipation peak in YBa2Cu3O6.2 (63 K, 1.1 kHz) was attributed to this mechanism, resulting in an O diffusion coefficient in the basal plane of 4 × 10 −4 exp (−0.11 eV kT ) cm2/sec. Measurements are produced here which show that all anelastic processes above room temperature in orthorhombic YBa2Cu3O7-x disappear in YBa2Cu3O6.2, confirming that the low temperature peak is indeed due to the diffusion of the free oxygen.

From CsPbBr3 Nano-Inks to Sintered CsPbBr3–CsPb2Br5 Films via Thermal Annealing: Implications on Optoelectronic Properties

CsPbBr3 nanocrystals passivated with short molecular ligands and deposited on a substrate were annealed from room temperature to 400 °C in inert atmosphere. Chemical, structural, and morphological transformations were monitored in situ and ex situ by different techniques, while optoelectronic properties of the film were also assessed. Annealing at 100 °C resulted in a 1 order of magnitude increase in photocurrent and photoresponse as a result of partial sintering of the NCs and residual solvent evaporation. Beyond 150 °C the original orthorhombic NCs were partially transformed into tetragonal CsPb2Br5 crystals, due to the desorption of weakly bound propionic acid ligands. The photocurrent increased moderately until 300 °C although the photoresponse became slower as a result of the formation of surface trap states. Eventually, annealing beyond 350 °C removed the strongly bound butylamine ligands and reversed the transition to the original orthorhombic phase, with a loss of photocurrent due to the numerous defects induced by the stripping of the passivating butylamine.

On the physico-chemical characterization of high Tc superconducting defect-perovskite YBa2Cu3O7−x

Abstract A systematic investigation on the influence of the starting materials on the reaction yield in the synthesis of the YBa2Cu3O7−x high Tc superconductor showed that an improvement of the superconducting properties of the samples is obtained if an excess of BaCO3 and CuO in respect to the stoichiometric composition is used and, especially, if BaO2 instead of BaCO3 is employed. The structural and magnetic properties are referred and the valency of Cu is tentatively inferred from the effective magnetic moment, deduced from the thermal behaviour of the inverse paramagnetic susceptibility in the 100 – 600K temperature range.

Influence of TiO2 Nanoparticles on Growth and Phenolic Compounds Production in Photosynthetic Microorganisms

The influence of titanium dioxide nanoparticles (pure anatase and 15% N doped anatase) on the growth of Chlorella vulgaris, Haematococcus pluvialis, and Arthrospira platensis was investigated. Results showed that pure anatase can lead to a significant growth inhibition of C. vulgaris and A. platensis (17.0 and 74.1%, resp.), while for H. pluvialis the nanoparticles do not cause a significant inhibition. Since in these stress conditions photosynthetic microorganisms can produce antioxidant compounds in order to prevent cell damages, we evaluated the polyphenols content either inside the cells or released in the medium. Although results did not show a significant difference in C. vulgaris, the phenolic concentrations of two other microorganisms were statistically affected by the presence of titanium dioxide. In particular, 15% N doped anatase resulted in a higher production of extracellular antioxidant compounds, reaching the concentration of 65.2 and 68.0 mg gDB −1 for H. pluvialis and A. platensis, respectively.

Crystal and magnetic structure of Cr- and Ni-substituted(La0.50Ca0.50)MnO3

The crystal and magnetic structure of (La0.50Ca0.50)(Mn1−xBx)O3 (x = 0.00, 0.03, 0.08; B = Cr, Ni) has been investigated between 5 and 300 K by means of dc magnetic measurements and neutron powder diffraction followed by Rietveld refinement. In the pristine compound an orthorhombic to monoclinic phase transition is detected on cooling, accompanied by a CE-type antiferromagnetic (AFM) ordering arising. Ni2+ and Cr3+ substitutions have similar effects on the structural and magnetic properties of (La0.50Ca0.50)MnO3, despite the fact that these ions are characterized by different external electronic configurations. After substitution, the orthorhombic to monoclinic phase transition is hindered. As a consequence, charge and orbital orderings are suppressed, as is the superexchange; double exchange takes place inducing ferromagnetic (FM) interactions. No evidence for stable magnetic interaction between Cr3+ or Ni2+ and the neighbouring Mn ions was detected. Nevertheless, in the Ni-substituted samples a detectable quantity of monoclinic phase forms during cooling, inside which AFM interactions take place. The amount of this secondary monoclinic phase decreases on increasing Ni substitution; the global FM magnetic moment decreases as well, due to a spin-cluster glass-type state arising. As a result a FM state is found to coexist with a spin-cluster glass-type state.

Formation and mobility of oxygen vacancies inRuSr2GdCu2O8

Oxygen vacancies are introduced in the RuO 2 and possibly CuO 2 planes of RaSr 2 GdCu 2 O 8 by annealing in vacuum above 600 K. The diffusive jumps of the O vacancies are accompanied by a reorientation of the local distortion, and are probed by measuring the elastic energy loss and modulus versus temperature at 1-10 kHz. An intense acoustic absorption peak develops near 670 K at 1 kHz and finally stabilizes after heating up to 920 K in vacuum. The analysis of the peak shows a barrier for the O diffusion of ≃ 1.4 eV, and a slowing down of Curie-Weiss type, with T C = 400 - 470 K, due to the interaction among the O vacancies. A secondary peak is attributed to O vacancies trapped at defects in the RuO 2 planes, or to vacancies in the CuO 2 planes. No sign of structural phase transformation is found up to 920 K.

Postsynthesis Transformation of Insulating Cs4PbBr6 Nanocrystals into Bright Perovskite CsPbBr3 through Physical and Chemical Extraction of CsBr

Perovskite-related Cs4PbBr6 nanocrystals present a “zero-dimensional” crystalline structure where adjacent [PbBr6]4– octahedra do not share any corners. We show in this work that these nanocrystals can be converted into “three-dimensional” CsPbBr3 perovskites by extraction of CsBr. This conversion drastically changes the optoelectronic properties of the nanocrystals that become highly photoluminescent. The extraction of CsBr can be achieved either by thermal annealing (physical approach) or by chemical reaction with Prussian Blue (chemical approach). The former approach can be simply carried out on a dried film without addition of any chemicals but does not yield a full transformation. Instead, reaction with Prussian Blue in solution achieves a full transformation into the perovskite phase. This transformation was also verified on the iodide counterpart (Cs4PbI6).