Effects of electron irradiation on resistivity and London penetration depth of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ ($x\le$ 0.34) iron - pnictide superconductor

Abstract

Irradiation with 2.5 MeV electrons at doses up to 5.2 $\times$10${}^{19}$ electrons/cm${}^2$ was used to introduce point-like defects in single crystals of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ with $x=$ 0.19 ($T_c=$ 14 K), $x=$ 0.26 ($T_c=$ 32 K) and 0.34 ($T_c=$ 39 K) to study the superconducting gap structure by probing the effect of non-magnetic scattering on electrical resistivity, $\rho (T)$, and London penetration depth, $\lambda (T)$. For all compositions, the irradiation suppressed the superconducting transition temperature, $T_c$ and increased resistivity. The low - temperature behavior of $\lambda (T)$ is best described by the power - law function, $\Delta \lambda (T)=A(T/T_c{)}^n$. While substantial suppression of $T_c$ supports $s_{\pm }$ pairing mechanism, in samples close to the optimal doping, $x=$ 0.26 and 0.34, the exponent $n$ remained high ($n\ge 3$) indicating robust full superconducting gaps. For the $x=$ 0.19 composition, exhibiting coexistence of superconductivity and long - range magnetism, the suppression of $T_c$ was much more rapid and the exponent $n$ decreased toward dirty limit of $n$ = 2. In this sample, the irradiation also suppressed the temperature of structural/magnetic transition, $T_{sm}$, from 103 K to 98 K consistent with the itinerant nature of the magnetic order. Our results suggest that underdoped compositions, especially in the coexisting regime are most susceptible to non-magnetic scattering and imply that in multi-band Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ superconductors, the ratio of the inter-band to intra-band pairing strength, and associated gap anisotropy, increases upon the departure from the optimal doping.

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• Effect of proton irradiation on the normal state low-energy excitations of Ba(Fe${}_{1-x}$Rh${}_x$)${}_2$As${}_2$ superconductors
• Unusual Sensitivity of Superconductivity to Strain in Iron-Based 122 Superconductors
• Infrared probe of the gap evolution across the phase diagram of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$
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