Specific heat anomalies for [Formula: see text] in superconducting single crystal doped BaFe(2)As(2): comparison of different flux growth methods

Local inhomogeneity and filamentary superconductivity in Pr-doped CaFe2As2

We use multiscale techniques to determine the extent of local inhomogeneity and superconductivity in Ca0.86Pr0.14Fe2As2 single crystal. The inhomogeneity is manifested as a spatial variation of the praseodymium concentration, local density of states, and superconducting order parameter. We show that the high-Tc superconductivity emerges from cloverlike defects associated with Pr dopants. The highest Tc is observed in both the tetragonal and collapsed tetragonal phases, and its filamentary nature is a consequence of nonuniform Pr distribution that develops localized, isolated superconducting regions within the crystals.

Superconductivity in undoped single crystals of BaFe(2)As(2): field and current dependence

In previous work on undoped MFe(2)As(2), partial drops in the resistivity indicative of traces of superconductivity have been observed for some samples with M =  Ba (T(c)∼20 K, up to 25% drop in ρ) and M =  Ca (T(c)∼10 K, up to 45% drop in ρ). A complete drop in the resistivity to ρ = 0, along with a finite fraction of Meissner flux expulsion, has been observed for M  = Sr, T(c) = 22 K. Using In-flux grown single crystal samples of undoped BaFe(2)As(2), we find a complete drop in the resistivity to 0 for most samples beginning at T(c)(onset) = 22.5 K. However-in contrast to the SrFe(2)As(2) results-there is no measurable Meissner effect and no suppression of the resistive superconducting transition with annealing. The current sensitivity of the superconducting resistive transition in our samples of BaFe(2)As(2) is quite strong, with an increase in the current density of a factor of 15 to ∼1.5 A cm(-2) not changing T(c)(onset) but broadening the transition significantly and causing ρ to remain finite as [Formula: see text]. To investigate whether this unusually low critical current is indicative of filamentary conduction lacking the apparent anisotropy seen in the critical magnetic field, H(c2), measurements for, e.g., the bulk superconductor Co-doped BaFe(2)As(2), H(c2) was measured in both crystalline directions. These BaFe(2)As(2) samples show H(c2)(T) values in the ab-plane and along the c-axis comparable to those seen for BaFe(2-x)Co(x)As(2), which has a similar T(c). Since the lack of T(c) suppression after annealing argues against strain-induced superconductivity as proposed for the other undoped MFe(2)As(2) materials, another possible cause for the superconductivity in BaFe(2)As(2) is discussed.