Superconductivity in Undoped Single Crystals of BaFe2As2: Field and Current Dependence

TL;DR

This study investigates superconductivity in undoped BaFe2As2 single crystals, revealing a complete resistive transition without Meissner effect, and explores its current and magnetic field dependence to understand its nature.

Contribution

It provides detailed measurements of superconductivity in undoped BaFe2As2, showing resistive transitions unaffected by annealing and analyzing its critical current and magnetic field responses.

Findings

Complete resistive transition at ~22.5 K in BaFe2As2

No measurable Meissner effect observed

Critical magnetic field values similar to doped counterparts

Abstract

In previous work in undoped MFe2As2, partial drops in the resistivity indicative of traces of superconductivity have been observed in some samples of M=Ba (Tc ~ 20 K, up to 25% drop in resistivity) and M=Ca (Tc ~ 10 K, up to 45 % drop in resistivity.) A complete drop in the resistivity to 0, along with a finite fraction of Meissner flux expulsion, has been observed in 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 in most samples beginning at Tc(onset) = 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/cm2 not changing Tc(onset) but broadening the transition significantly and causing the resistivity to remain finite as T approaches 0. To investigate if this unusually low critical current is indicative of filamentary conduction lacking the apparent anisotropy seen in the critical magnetic field, Hc2, measurements of, 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 in 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.