Th-substituted SmFeAsO: Structural details and superconductivity with Tc above 50 K

Abstract

We report structural, magnetic, and transport properties of polycrystalline samples and single crystals of superconducting Sm1−xThxFeAsO with maximal Tc above 50 K, prepared under high pressure. Bulk superconducting samples do not undergo a structural phase transition from tetragonal to orthorhombic symmetry at low temperatures. The unit-cell parameters a and c shrink with Th substitution and the fractional atomic coordinate of the As site zAs remains almost unchanged while that of Sm/Th zSm/Th increases. Upon warming from 5 to 295 K the increase in the FeAs layer thickness is dominant, while the changes in the other structural building blocks are minor, and they compensate each other, since the As-Sm/Th distance contracts by about the same amount as the O-Sm/Th expands. The polycrystalline and single-crystalline samples are characterized by a full diamagnetic response in low magnetic field, by a high intergrain critical current density for polycrystalline samples, and by a critical current density on the order of 8×105 A/cm2 for single crystals at 2 K in fields up to 7 T. The magnetic penetration depth anisotropy γλ increases with decreasing temperature, in a similar way to that of SmFeAsO1−xFy single crystals. The upper critical field estimated from resistance measurements is anisotropic with slopes of ∼5.4 T/K (H∥ab plane) and ∼2.7 T/K (H∥c axis), at temperatures sufficiently far below Tc. The low-temperature upper critical field anisotropy γH is in the range of ∼2, consistent with the tendency of a decreasing γH with decreasing temperature, previously reported for SmFeAsO1−xFy single crystals.

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