Superconducting Properties of Pr4Fe2As2Te1−xO4 and Sm4Fe2As2Te1-xO4-yFy
Recently a new family of FeAs-based superconductors, L4Fe2As2Te1-xO4 (L=Pr, Sm, Gd), has been synthetized by high pressure methods in our laboratories. The crystal structure consists of layers of L2O2 tetrahedra separated by alternating layers of Te atoms chains and of Fe2As2 tetrahedra. It has some resemblances with the known 1111 phase except for the considerably larger c lattice constant and the intercalated tellurium atoms. Critical temperatures (Tc) up to 46 K can be achieved upon substitution of the lanthanide element (L) and optimum F-doping. We present an electrical transport study of two members of this family: Pr4Fe2As2Te1-xO4 and Sm4Fe2As2Te1-xO4-yFy single crystals. Upper critical fields along c-axis and ab plane are measured in magnetic fields up to 16 T, shedding light on the pair-breaking mechanism and the vortex phase diagram. High values of magnetic field anisotropy reflect the unprecedented structural anisotropy of these compounds (c/a~7.4). Resistivity measurements under high pressure show an increase of Tc in Pr4Fe2As2Te0.88O4 and a decrease in Sm4Fe2As2Te0.72O2.8F1.2. Focused Ion Beam technology is employed to contact and shape the samples at the micro-scale for a direct measurement of electronic anisotropy.