A Combined Experimental/Computational Study of Dicationic Ionic Liquids with Bromide and Tungstate Anions.

A panel of dicationic ionic liquids (DILs) with different rigid xylyl (ortho, meta, para) spacers and different anions (bromide and tungstate) has been synthetised and characterised through different experimental and computational techniques. Differences and analogies between the systems are analysed using information derived from their DFT structures, semiempirical dynamics, thermal behaviour, and catalytic properties versus the well-known reaction of CO added to epichlorohydrin. A comparison between the proposed systems and some analogues that present non-rigid spacers shows the key effect displayed by structure rigidity on their characteristics.

The Structure-Property Relationship of Pyrrolidinium and Piperidinium-Based Bromide Organic Materials.

Two couples of dicationic ionic liquids, featuring pyrrolidinium and piperidinium cations and different linker chains, were prepared and characterized. 1,1'-(propane-1,3-diyl)bis(1-methylpyrrolidinium) bromide, 1,1'-(octane-1,8-diyl)bis(1-methylpyrrolidinium) bromide, 1,1'-(propane-1,3-diyl)bis(1-methylpiperidinium) bromide, and 1,1'-(octane-1,8-diyl)bis(1-methylpiperidinium) bromide were synthesized in quantitative yields and high purity and thermally characterized through TGA and DSC analysis. In this study, we propose a preliminary comparative evaluation of the effect of the linker chain length and of the size of the aliphatic ammonium ring on the thermal and solubility properties of bromide dicationic ionic liquids.

Nanoscale analysis of superconducting Fe(Se,Te) epitaxial thin films and relationship with pinning properties.

The process of developing superconducting materials for large scale applications is mainly oriented to optimize flux pinning and the current carrying capability. A powerful approach to investigate pinning properties is to combine high resolution imaging with transport measurements as a function of the magnetic field orientation, supported by a pinning modelling. We carry out Transmission Electron Microscopy, Electron Energy Loss Spectroscopy and critical current measurements in fields up to 16 T varying the angle between the field and c-axis of Fe(Se,Te) epitaxial thin films deposited on CaF substrates.

The huge effect of Mn substitution on the structural and magnetic properties of LaFeAsO: the La(Fe,Mn)AsO system.

The substitution of Mn for Fe on the sub-lattice in LaFeAsO has a remarkable impact on both structural and magnetic properties; for example, the structural and magnetic transition temperatures decrease of ~20 K in samples with a Mn-content as low as x  =  0.01. Such a dramatic effect results from the high stability of the substituting Mn ion (3d ) in its high-spin state, which opposes any variation to its electronic state (configuration), perturbing thereby interactions within the Fe sub-lattice between the Fe ions surrounding the substituent.

Angular dependence of vortex instability in a layered superconductor: the case study of Fe(Se,Te) material.

Anisotropy effects on flux pinning and flux flow are strongly effective in cuprate as well as iron-based superconductors due to their intrinsically layered crystallographic structure. However Fe(Se,Te) thin films grown on CaF substrate result less anisotropic with respect to all the other iron based superconductors. We present the first study on the angular dependence of the flux flow instability, which occurs in the flux flow regime as a current driven transition to the normal state at the instability point (I*, V*) in the current-voltage characteristics.

Current Induced Resistive State in Fe(Se,Te) Superconducting Nanostrips.

We study the current-voltage characteristics of Fe(Se,Te) thin films deposited on CaF substrates in form of nanostrips (width w ~ λ, λ the London penetration length). In view of a possible application of these materials to superconductive electronics and micro-electronics we focus on transport properties in small magnetic field, the one generated by the bias current. From the characteristics taken at different temperatures we derive estimates for the pinning potential U and the pinning potential range δ for the magnetic flux lines (vortices).

An automatic method for atom identification in scanning tunnelling microscopy images of Fe-chalcogenide superconductors.

We describe a computational approach for the automatic recognition and classification of atomic species in scanning tunnelling microscopy images. The approach is based on a pipeline of image processing methods in which the classification step is performed by means of a Fuzzy Clustering algorithm. As a representative example, we use the computational tool to characterize the nanoscale phase separation in thin films of the Fe-chalcogenide superconductor FeSex Te1-x , starting from synthetic data sets and experimental topographies.

Structural properties and phase diagram of the La(Fe₁-xRux)AsO system.

Structural refinement, lattice micro-strain and spontaneous strain analyses have been carried out on selected members of the La(Fe1-xRux)AsO system using high-resolution neutron and synchrotron powder diffraction data. The obtained results indicate that the character of the tetragonal to orthorhombic structural transition changes from first order for x = 0.10, possibly to tricritical for x = 0.

Retention of the tetragonal to orthorhombic structural transition in F-substituted SmFeAsO: a new phase diagram for SmFeAs(O(1-x)F(x)).

In this Letter we propose a new phase diagram for the SmFeAs(O(1-x)F(x)) system, based on careful analysis of synchrotron powder diffraction data, SQUID, and muon spin rotation measurements. The tetragonal to orthorhombic structural transition is slightly affected by F content and is retained for the superconducting samples, even at optimal doping. These findings relate the AFM transition on a different ground with respect to the structural one and suggests that orbital ordering could be the driving force for symmetry breaking.

Observation of the crossover from two-gap to single-gap superconductivity through specific heat measurements in neutron-irradiated MgB2.

We report specific heat measurements on neutron-irradiated MgB2 samples, for which the critical temperature is lowered to 8.7 K, but the superconducting transition remains extremely sharp, indicative of a defect structure extremely homogeneous. Our results evidence the presence of two superconducting gaps in the temperature range above 21 K, while single-gap superconductivity is well established as a bulk property, not associated with local disorder fluctuations, when Tc decreases to 11 K.

Jahn-teller dynamics in charge-ordered manganites from raman spectroscopy.

The Raman spectra of the charge-ordered manganite Pr0.65Ca0.35MnO3 were studied as functions of temperature and excitation energy and compared to magnetic moment and electrical conductivity behaviors.