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Register a new userVortex pinning : a probe for nanoscale disorder in iron-based superconductors
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Cornelis Jacominus Van Der Beek
Publication date
2011
fields
Physics
and research's language is
English
Authors
Cornelis Jacominus Van Der Beek
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The pinning of quantized flux lines, or vortices, in the mixed state is used to quantify the effect of impurities in iron-based superconductors (IBS). Disorder at two length scales is relevant in these materials. Strong flux pinning resulting from nm-scale heterogeneity of the superconducting properties leads to the very disordered vortex ensembles observed in the IBS, and to the pronounced maximum in the critical current density jc at low magnetic fields. Disorder at the atomic scale, most likely induced by the dopant atoms, leads to weak collective pinning and a magnetic field-independent contribution jcoll. The latter allows one to estimate quasi-particle scattering rates.
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We use scanning tunneling microscopy to map the surface structure, nanoscale electronic inhomogeneity, and vitreous vortex phase in the hole-doped superconductor Sr$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ with $T_c$=32 K. We find the low-$T$ cleaved surface is dominated by a half-Sr/K termination with $1times 2$ ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by $sigma/bar{Delta}$=16% on a $sim$3 nm length scale, with average $2bar{Delta}/k_B T_c=3.6$ in the weak coupling limit. The vortex core size provides a measure of the superconducting coherence length $xi$=2.3 nm. We quantify the vortex lattice correlation length at 9 T in comparison to several iron-based superconductors. The comparison leads us to suggest the importance of dopant size mismatch as a cause of dopant clustering, electronic inhomogeneity, and strong vortex pinning.
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Charge doping of iron-pnictide superconductors leads to collective pinning of flux vortices, whereas isovalent doping does not. Moreover, flux pinning in the charge-doped compounds is consistently described by the mean-free path fluctuations introduced by the dopant atoms, allowing for the extraction of the elastic quasiparticle scattering rate. The absence of scattering by dopant atoms in isovalently doped BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ is consistent with the observation of a linear temperature dependence of the low-temperature penetration depth in this material.
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