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Participant Seminar Abstracts

Miss. María Lourdes Amigó

Comisión Nacional de Energía Atómica - Centro Atómico Bariloche and Instituto Balseiro, Universidad Nacional de Cuyo, S.C. de Bariloche, Argentina

Experimental study of the multiband character of the FeSe family.

Abstract: Transport properties of the ab-plane were studied for single crystals of the superconductor ?-FeSe family with applied magnetic field up to 16 T. The crystals of ?-FeSe present a strongly anisotropic positive magnetoresistance below 90K, this temperature.

Mr.Federico Caglieris

Department of Physics, University of Genova/CNR-SPIN, Genova

Seebeck effect of iron-based materials: a sensitive probe of carrier-spin wave coupling

Abstract: Thermoelectric properties have been proven to be very sensitive probes of many aspects of interest in solid state physics. Among them, in the last years, unconventional superconductivity in iron pnictides has been a largely debated topic.

Dr. Anita Guarino

Dipartimento di Fisica 'E.R. Caianiello', Università di Salerno, Fisciano (Sa)

Pinning properties of iron chalcogenides

Abstract: We investigate flux pinning mechanisms in the Fe(Se1-xTex) compound, that is similar to the high temperature superconductors due to its stacked layers crystal structure. Compositional analysis and magneto-resistance measurements on several epitaxial thin films grown on different substrates (CaF2, LaAlO3, SrTiO3) have been performed. From the Arrhenius plots in the magnetic fields up to 9 T, the activation energy is derived as a function of magnetic field and current density, U(H, J). The influence of magnetic field orientation on the pinning energy activation mechanism is also studied to enlighten the isotropy of the superconducting properties of the material. The exponents of the power law dependence U0(H)~H-α, as well as the magnitude of the U0 values, have been evaluated. A single exponent feature is generally noticed on thin films grown on SrTiO3 substrate, in which a strong single vortex regime has been correlated to extrinsic c-axis defects. This characteristic has been also observed in thin films grown on LaAlO3, but only in the parallel configuration, thus indicating the larger intrinsic anisotropy of these films. On the contrary, the U0(H) behavior of the sample grown on CaF2 show a crossover from a strong single vortex to a collective pinning regime. At the end this overall comparison can be useful to develop a technological material able to compete with high temperature superconductors.

Dr. Antonio Leo

Dipartimento di Fisica 'E.R. Caianiello', Università di Salerno, Fisciano (Sa)

Flux flow instabilities in mesoscopic superconductors

Abstract: Conventional and quantum superconducting electronics are based on micro- and nano-scaled devices often operating in the so-called mesoscopic regime. In this regime, superconductors exhibit peculiar phenomena intimately connected to their intrinsic properties. Flux-Flow Instability (FFI) is the name used to classify a group of phenomena related to different fundamental mechanisms (mostly electronic or thermal) connected to quasiparticle energy relaxation processes. Regardless of the mechanisms, in terms of Current-Voltage Characteristic (CVC) a unique feature is observed, i.e. an abrupt jump from the flux-flow branch to the normal state. A critical value for the velocity of the Abrikosov vortex lattice is associated to the point in the CVC where the jump is observed, namely the instability point. In this work we investigate the so far totally unexplored interplay between mesoscopic regime and FFI. Here we present experimental data showing how such confinement can strongly influence the FFI behaviour as a function of the applied magnetic field. These experimental findings are supported by simulation based on Time-Dependent Ginzburg-Landau model. We find that flux-flow motion is limited by reaching an upper threshold of the vortex critical velocity at low fields.

Mr. Pasquale Marra

Dipartimento di Fisica 'E.R. Caianiello', Università di Salerno, Fisciano (Sa)

Resonant inelastic x-ray scattering as a probe of the superconducting order parameter in iron-based superconductors

Abstract: We develop a phenomenological theory to predict the characteristic features of the momentum-dependent scattering amplitude in resonant inelastic x-ray scattering (RIXS) in iron-based superconductors. Taking into account all relevant orbital states and their specific content along the Fermi surface we evaluate the dynamical structure factors of charge and spin based on tight-binding models for the compounds LaOFeAs and LiFeAs, which are fully consistent with recent angle-resolved photoemission spectroscopy (ARPES) data. While the orbital content strongly modifies the momentum dependence of RIXS intensities, we find considerable enhancement of the intensity of sign reversing excitations in the spin structure factor. Moreover, a characteristic intensity redistribution between spin and charge transfer excitations is found in case of antisymmetric momentum dependence of the superconducting order parameter. Consequently, our results show that RIXS spectra can discriminate between s± and s wave gap functions in the singlet pairing case. In addition, a possible triplet pairing based on chiral p-wave gap function affects the intensity at small momenta and can be distinguished from singlet pairing.

Mr. Pierre Massat

MPQ Laboratory, Paris 7 University, Paris

Charge nematic fluctuations and superconducting gap in Ba1?xKxFe2As2 and FeSe probed by Raman scattering

Abstract: We report an electronic Raman scattering study of two iron-based superconductors : hole doped Ba1?xKxFe2As2 (K-Ba122) and FeSe. In the tetragonal phase of both systems we study the development of charge nematic fluctuations as a function of temperature.

Mr. Pablo Villar-Arribi
ESRF, Grenoble

Enhanced thermoelectric properties on non-magnetic double-perovskites

Abstract: Efficient thermoelectric materials should present large Seebeck coefficient, high electrical conductivity and low thermal conductivity. For obtaining this, a highly degenerate band structure around the Fermi level and optimal carrier concentration are des.

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