Effects of electron irradiation on resistivity and London penetration depth ofBa1−xKxFe2As2

Abstract

Irradiation with 2.5 MeV electrons at doses up to 5.2 ×10 19 electrons/cm 2 was used to introduce point-like defects in single crystals of Ba1−xKxFe2As2 with x = 0.19 (Tc = 14 K), x = 0.26 (Tc = 32 K) and 0.34 (Tc = 39 K) to study the superconducting gap structure by probing the effect of non-magnetic scattering on electrical resistivity, ρ(T ), and London penetration depth, λ(T ). For all compositions, the irradiation suppressed the superconducting transition temperature, Tc and increased resistivity. The low -temperature behavior of λ(T ) is best described by the power -law function, ∆λ(T ) = A(T /Tc) n . While substantial suppression of Tc supports s± pairing mechanism, in samples close to the optimal doping, x = 0.26 and 0.34, the exponent n remained high (n ≥ 3) indicating robust full superconducting gaps. For the x = 0.19 composition, exhibiting coexistence of superconductivity and long -range magnetism, the suppression of Tc 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, Tsm, 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 Ba1−xKxFe2As2 superconductors, the ratio of the inter-band to intra-band pairing strength, and associated gap anisotropy, increases upon the departure from the optimal doping.

Energy gap evolution across the superconductivity dome in single crystals of (Ba 1− x K x )Fe 2 As 2

“…In the optimally doped region, x ≈ 0.35 to 0.4, two effective isotropic superconducting gap scales (roughly with a 2:1 magnitude ratio) were identified in many experiments, for example, thermal conductivity ( 6 ), London penetration depth ( 7 , 8 ), and angle-resolved photoemission spectroscopy (ARPES) ( 8 – 12 ). In the heavily overdoped region, x ≥ 0.8, a gap with line nodes was identified by thermal conductivity ( 13 – 15 ), London penetration depth ( 16 ), and ARPES ( 9 , 11 ).…”

Magnetic penetration depth in disordered iron-based superconductors

“…Since these initial reports, the London penetration depth has been measured systematically in a variety of families of iron-pnictides and ironchalcogenides. [21][22][23][24][25][26][27][28][29][30][31] Perhaps the most striking recent ob-servation is a disorder-induced topological change of the superconducting gap structure, as revealed from the low-T behavior of δλ L in BaFe 2 (As 1−x P x ) 2 . 32 Nonmagnetic defects were controllably introduced by electron irradiation, and it was found that the nodal state of P-doped BaFe 2 As 2 changes to a nodeless state with increasing disorder.…”

Enhancement of superconducting transition temperature by pointlike disorder and anisotropic energy gap in FeSe single crystals

“…Therefore, simultaneous measurement of another disorder-sensitive parameter, for example, London penetration depth, λ(T ), can be used to impose additional constraints on the possible pairing models. Measurements of the low-temperature variation, ∆λ(T ) = λ(T ) − λ(0), can be used to study the gap anisotropy [7,8] and to distinguish between s ± and s ++ pairing [5]. The latter was successfully used to study nodal BaFe 2 (As,P) 2 [9] and SrFe 2 (As,P) 2 [7] where potential scattering lifted the nodes proving them accidental, therefore strongly supporting s ± pairing.…”