Th-substituted SmFeAsO: Structural details and superconductivity withTcabove 50 K

Abstract

We report structural, magnetic, and transport properties of polycrystalline samples and single crystals of superconducting ${\text{Sm}}_{1\ensuremath{-}x}{\text{Th}}_{x}\text{FeAsO}$ with maximal ${T}_{c}$ 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 ${z}_{\text{As}}$ remains almost unchanged while that of Sm/Th ${z}_{\text{Sm}/\text{Th}}$ 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 $8\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{A}/{\text{cm}}^{2}$ for single crystals at 2 K in fields up to 7 T. The magnetic penetration depth anisotropy ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}$ increases with decreasing temperature, in a similar way to that of ${\text{SmFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{y}$ single crystals. The upper critical field estimated from resistance measurements is anisotropic with slopes of $\ensuremath{\sim}5.4\text{ }\text{T}/\text{K}$ ($H\ensuremath{\parallel}ab$ plane) and $\ensuremath{\sim}2.7\text{ }\text{T}/\text{K}$ ($H\ensuremath{\parallel}c$ axis), at temperatures sufficiently far below ${T}_{c}$. The low-temperature upper critical field anisotropy ${\ensuremath{\gamma}}_{H}$ is in the range of $\ensuremath{\sim}2$, consistent with the tendency of a decreasing ${\ensuremath{\gamma}}_{H}$ with decreasing temperature, previously reported for ${\text{SmFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{y}$ single crystals.