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Studies of anisotropy of iron based superconductors [electronic resource].

Published:
Washington, D.C. : United States. Dept. of Energy. Office of Science, 2013.
Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy.
Additional Creators:
Ames Laboratory, United States. Department of Energy. Office of Science, and United States. Department of Energy. Office of Scientific and Technical Information
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Summary:
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{sub c2}(T). Study was undertaken on single crystals of Ba(Fe{sub 1-x}Coₓ)₂As₂ 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 tted with a power-law function Δλ(T) = ATⁿ. 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{sub ±} scenario for the whole doping range. Knowing that the s{sub ±} gap symmetry exists across the superconducting dome for the electron doped systems, we next looked at λ(T), in optimally - doped, SrFe₂(As{sub 1-x}Pₓ)₂, x =0.35. Both, as-grown (T{sub c} ~ 25 K) and annealed (T{sub c} ~ 35 K) single crystals of SrFe₂(As{sub 1-x}Pₓ)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 BaFe₂(As{sub 1-x}Pₓ)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{sub c2}, of two overdoped compositions of Ba(Fe{sub 1-x}Niₓ)₂As₂, 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{sub 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{sub c2} vs. θ curve clearly deviates from the Ginzburg-Landau theory.
Report Numbers:
E 1.99:is-t 3108
is-t 3108
Subject(s):
Note:
Published through SciTech Connect.
05/15/2013.
"is-t 3108"
Murphy, Jason.
Funding Information:
DE-AC02-07CH11358
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