Superconductivity in undoped single crystals of BaFe2As2: field and current dependence

In previous work on undoped MFe2As2, partial drops in the resistivity indicative of traces of superconductivity have been observed for some samples with M = Ba (Tc∼20 K, up to 25% drop in ρ) and M = Ca (Tc∼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, Tc = 22 K. Using In-flux grown single crystal samples of undoped BaFe2As2, we find a complete drop in the resistivity to 0 for most samples beginning at Tconset = 22.5 K. However—in contrast to the SrFe2As2 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 BaFe2As2 is quite strong, with an increase in the current density of a factor of 15 to ∼1.5 A cm−2 not changing Tconset but broadening the transition significantly and causing ρ to remain finite as . To investigate whether this unusually low critical current is indicative of filamentary conduction lacking the apparent anisotropy seen in the critical magnetic field, Hc2, measurements for, e.g., the bulk superconductor Co-doped BaFe2As2, Hc2 was measured in both crystalline directions. These BaFe2As2 samples show Hc2(T) values in the ab-plane and along the c-axis comparable to those seen for BaFe2−xCoxAs2, which has a similar Tc. Since the lack of Tc suppression after annealing argues against strain-induced superconductivity as proposed for the other undoped MFe2As2 materials, another possible cause for the superconductivity in BaFe2As2 is discussed.