DOE OSTI2013
To study the electronic anisotropy in iron based superconductors, the temperature dependent London penetration depth, Δλ (T), have been measured in several compounds, along with the angular dependent upper critical field, H c2 (T). Study was undertaken on single crystals of Ba(Fe 1-x Co x ) 2 As 2 with x=0.108 and x=0.127, in the overdoped range of the doping phase diagram, characterized by notable modulation of the superconducting gap. Heavy ion irradiation with matching field doses of 6 T and 6.5 T respectively, were used to create columnar defects and to study their effect on the temperature Δλ (T). The variation of the low-temperature penetration depth in both pristine and irradiated samples was fitted with a power-law function Δλ (T) = AT n . Irradiation increases the magnitude of the pre-factor A and decreases the exponent n, similar to the effect on the optimally doped samples. This finding supports the universal s ± scenario for the whole doping range. Knowing that the s ± gap symmetry exists across the superconducting dome for the electron doped systems, we next looked at λ (T), in optimally - doped, SrFe 2 (As 1-x P x ) 2 , x =0.35. Both, as-grown (T c ~ 25 K) and annealed (T c ~ 35 K) single crystals of SrFe 2 (As 1-x P x ) 2 were measured. Annealing decreases the absolute value of the London penetration depth from λ(0) = 300 ± 10 nm in as-grown samples to λ (0) = 275±10 nm. At low temperatures, λ (T) ~ T indicates a superconducting gap with line nodes. Analysis of the full-temperature range superfluid density is consistent with the line nodes, but differs from the simple single-gap d-wave. The observed behavior is very similar to that of BaFe 2 (As 1-x P x ) 2 , showing that isovalently substituted pnictides are inherently different from the charge-doped materials. In-plane resistivity measurements as a function of temperature, magnetic field, and its orientation with respect to the crystallographic ab-plane were used to study the upper critical field, H c2 , of two overdoped compositions of Ba(Fe 1-x Ni x ) 2 As 2 , x=0.054 and x=0.072. Measurements were performed using precise alignment (with accuracy less than 0.1°) of the magnetic field with respect to the Fe-As plane. The dependence of the H c2 on angle θ between the field and the ab- plane was measured in isothermal conditions in a broad temperature range. We found that the shape of the H c2 vs. θ curve clearly deviates from the Ginzburg-Landau theory.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗