Transport and Magnetic Properties of Single Crystalline FeSe1-xTex Superconductors

FRANCO D. G.; NIEVA G.; LUZURIAGA J.; GUIMPEL J.; CORREA V. F.

Valparaíso, Chile

Congreso; Sólidos 2009; 2009

The recent discovery of superconductivity in the oxypnictide LaFeAsO1-xFx (Tc =26K)[1] has motivated the search for other Fe based superconducting compounds. The FeX (X= chalcogenide) system was also found to be a superconductor (Tc=9K for FeSe and up to Tc=16K for FeTexSe1-x)[2]. One of the interesting aspects of these materials is their similarity with the Cu based high temperature superconductors, namely the layered structure, the magnetic interactions and the doping effects. We have synthesized FeSe1-xTex single crystals (x=0, 0.5 and 1) using a NaCl/KCl flux.  A compositional analysis shows that  FeTe and FeSe crystals have the right stoichiometry, while crystals containing Se and Te have Fe deficiency.  The structure was found to have a mixture of a tetragonal and an hexagonal phase. Only for some FeSe crystals we found a pure hexagonal structure. Low field magnetization on several crystals shows that the critical temperature is Tc~9K for x=0 and Tc~13K for x =0.5. The crystals with x=1 do not show a superconducting transition down to 2K, but a magnetic anomaly at TN~50K. Using electrical transport experiments, we have measured the transition to the superconducting state in FeSe single crystals. The measurements were performed in magnetic fields H up to 16 T, using several configurations: a) with H parallel and perpendicular to the planes of the laminar structure, b) with H perpendicular to the applied current and collinear to it.  We have constructed the magnetic phase diagrams ( H-T) for the different configurations and determined the anisotropy. We have found evidence of a vortex liquid in the mixed state.  We have also performed transport experiments tilting the angle of the magnetic field with respect to the FeSe layers of the material. In such experiments, the angular dependence of the dissipation in the mixed state indicates the possible presence of correlated defects acting as pinning centers. Magnetization hysteresis loops, i.e. M vs H curves in FeSe and FeSe1-xTex, indicate the presence of a magnetic phase in addition to the superconducting characteristic behaviour.  We have performed magnetization experiments rotating the sample in a fixed applied field, at a fixed temperature to look for the presence of correlated defects. At low fields (up to 1000 Oe) no signal of correlated defects was found.