Disorder-induced topological change of the superconducting gap structure in iron pnictides

Abstract

In superconductors with unconventional pairing mechanisms, the energy gap in the excitation spectrum often has nodes, which allow quasiparticle excitations at low energies. In many cases, such as in d-wave cuprate superconductors, the position and topology of nodes are imposed by the symmetry, and thus the presence of gapless excitations is protected against disorder. Here we report on the observation of distinct changes in the gap structure of ironpnictide superconductors with increasing impurity scattering. By the successive introduction of nonmagnetic point defects into BaFe 2 (As 1 À x P x ) 2 crystals via electron irradiation, we find from the low-temperature penetration depth measurements that the nodal state changes to a nodeless state with fully gapped excitations. Moreover, under further irradiation the gapped state evolves into another gapless state, providing bulk evidence of unconventional sign-changing s-wave superconductivity. This demonstrates that the topology of the superconducting gap can be controlled by disorder, which is a strikingly unique feature of iron pnictides.

Impact of Disorder on the Superconducting Phase Diagram in BaFe2(As1−xPx)2

“…Such T -dependent impurity scattering was also observed with increasing irradiation in the previous α-particle irradiation experiment on iron-based superconductor NdFeAs(O,F), 21) and can be understood with the assumption that the magnetic moments of the defects induce Kondolike scattering, similar to the case of heavy-fermion materials. Although no discernible magnetic moment was observed after irradiation in the paramagnetic state down to 0.1 K in our system, 16) this observation indicates that non-magnetic holes created in the AFM networks induce a T -dependent scattering process that deserves further investigation to elucidate its origin. For x = 0.16( Figure 1(c)), and 0.24( Figure 1(d)), ρ(T ) exhibits a reduction at low temperatures due to the onset of superconductivity.…”

“…Through the use of successive electron irradiation, we can perform systematic measurements on a given sample with a gradual introduction of impurity scattering induced by point defects, and without changing the carrier concentration or band width. 15,16) In general, impurity scattering reduces the T c in unconventional superconductors, where the suppression rate depends on the gap structures. 17) Indeed, the superconducting dome shrinks with the introduction of scattering via chemical substitution in cuprates and heavy-fermion superconductors.…”

“…11) In order to introduce uniform point defects into the BaFe 2 (As 1−x P x ) 2 single crystals, we irradiated the sample with an electron beam with an incident energy of 2.5 MeV, which is far above the threshold energy required for the formation of vacancy-interstitial (Frenkel) pairs. 16) The sample was kept at 20 K to prevent defect migration and clustering effects during the irradiation. In order to evaluate the change in ρ(T ) with irradiation accurately, we repeated the process of ρ(T ) measurement and irradiation on the same crystal without removing the electrodes for each composition.…”

Superconducting order from disorder in 2H-TaSe 2− x S x

“…The additional complexity of competing orders such superconductivity with charge density wave (CDW) or magnetism makes this problem one of the most challenging frontiers in physics. [2][3][4][5] A large body of literature is devoted to this interplay in nearly magnetic materials. 6 The interplay of disorder and superconductivity in CDW materials have been less explored than its magnetic analog.…”

Effects of electron irradiation on resistivity and London penetration depth ofBa1−xKxFe2As2Cho1, Kończykowski2, Murphy3 et al. 2014Phys. Rev. B3811360View full textAdd to dashboardCite

“…Studying the effects of a controlled point -like disorder on superconducting properties is a powerful tool to understand the mechanisms of superconductivity [1][2][3][4][5][6][7][8]. According to the Anderson's theorem, conventional isotropic s−wave superconductors are not affected by the potential (non-magnetic) scattering, but are sensitive to a spin -flip scattering on magnetic impurities [1].…”