Growth methods of c-axis oriented MgB2 thin films by pulsed laser deposition

High quality MgB2 thin films have been obtained by pulsed laser deposition both on MgO and on Al2O3 substrates using different methods. In the standard two-step procedure, an amorphous precursor layer is deposited at room temperature starting both from stoichiometric target and from boron target: after this first step, it is annealed in magnesium atmosphere in order to crystallize the superconducting phase. The so obtained films show a strong c-axis orientation, evidenced by XRD analysis, a critical temperature up to 38 K and very high critical fields along the basal planes, up to 22T at 15K. Also an in situ one step technique for the realization of superconducting MgB2 thin films has been developed. In this case, the presence of an argon buffer gas during deposition is crucial and we observe a strong dependence of the quality of the deposited film on the background gas pressure. The influence of the Ar atmosphere has been confirmed by time and space-resolved spectroscopy measurements on the emission spectrum of the plume. The Ar pressure modifies strongly the plasma kinetics by promoting excitation and ionization of the plume species, especially of the most volatile Mg atoms, increasing their internal energy.Comment: Paper presented at Boromag Workshop, Genoa 17-19 June 2002, in press on SUS

Tetragonal to orthorhombic phase transition in SmFeAsO: a synchrotron powder diffraction investigation

The crystal structure of SmFeAsO has been investigated by means of Rietveld refinement of high resolution synchrotron powder diffraction data collected at 300 K and 100 K. The compound crystallizes in the tetragonal P4/nmm space group at 300 K and in the orthorhombic Cmma space group at 100 K; attempts to refine the low temperature data in the monoclinic P112/n space group diverged. On the basis of both resistive and magnetic analyses the tetragonal to orthorhombic phase transition can be located at T about 140 K.Comment: Submitted to: Superconductor Science and Technology PACS: 61.05.cp, 61.66.Fn, 74.10.+v, 74.62.Dh, 74.70.D

Optical emission investigation of laser-produced MgB2 plume expanding in an Ar buffer gas

Optical emission spectroscopy is used to study the dynamics of the plasma generated by pulsed-laser irradiation of a MgB2 target, both in vacuum and at different Ar buffer gas pressures. The analysis of the time-resolved emission of selected species shows that the Ar background gas strongly influences the plasma dynamics. Above a fixed pressure, plasma propagation into Ar leads to the formation of blast waves causing both a considerable increase of the fraction of excited Mg atoms and a simultaneous reduction of their kinetic flux energy. These results can be particularly useful for optimizing MgB2 thin film deposition processes.Comment: 11 pages,4 figures, Applied Physics Letters in pres

Fault tectonics of the Tuscan Nappe in the eastern sector of the Apuan Alps (Italy)

We present the geological-structural map of the Tuscan Nappe exposed on the eastern border of the Apuan Alps metamorphic dome (Tuscany, Italy). The 1:6,500 scaled Main Map covers an area of about 10 km(2). It contains the first detailed overview of the fault tectonics affecting the Tuscan Nappe during the exhumation and uplift of the Tuscan Metamorphic Units. We documented a polyphase fault tectonics that initially produced low-angle extensional faults and later high-angle faults. The latter started within a transtensional tectonic regime that produced left-lateral strike-slip faults. Lately a pure extensional tensor, indicating a switch of the maximum compression sigma 1 axis from sub-horizontal to sub-vertical, produced faults with a dominant dip-slip component. In our reconstruction the lateral thickness variations documented in several formations of the Tuscan Nappe is mainly controlled by tectonics and not by stratigraphy, as previously suggested