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.

We show, using a simple model, how ordinary disorder can gap an extended-s (A1g) symmetry superconducting state with nodes. The concommitant crossover of thermodynamic properties, particularly the T -dependence of the superfluid density, from pure power law behavior to an activated one is exhibited. We discuss applications of this scenario to experiments on the ferropnictide superconductors.

We reconsider the effect of disorder on the properties of a superconductor characterized by a signchanging order parameter appropriate for Fe-based materials. Within a simple two band model, we calculate simultaneously Tc, the change in residual resistivity ∆ρ0, and the zero-energy density of states, and show how these results change for various types of gap structure and assumptions regarding the impurity scattering. The rate of Tc suppression is shown to vary dramatically according to details of the impurity model considered. We search therefore for a practical, experimentally oriented signature of a gap of the s± type, and propose that observation of a particular evolution of the penetration depth, nuclear magnetic resonance relaxation rate, or thermal conductivity temperature dependence with disorder would suffice.

A single crystal of isovalently substituted BaðFe 1−x Ru x Þ 2 As 2 (x ¼ 0.24) is sequentially irradiated with 2.5 MeV electrons up to a maximum dose of 2.1 × 10 19 e − =cm 2 . The electrical resistivity is measured in situ at T ¼ 22 K during the irradiation and ex situ as a function of temperature between subsequent irradiation runs. Upon irradiation, the superconducting transition temperature T c decreases and the residual resistivity ρ 0 increases. We find that electron irradiation leads to the fastest suppression of T c compared to other types of artificially introduced disorder, probably due to the strong short-range potential of the pointlike irradiation defects. A more detailed analysis within a multiband scenario with variable scattering potential strength shows that the observed T c versus ρ 0 is fully compatible with s AE pairing, in contrast to earlier claims that this model leads to a too rapid suppression of T c with scattering. There are several experimental approaches to study the energy gap structure in superconductors. One of them is to measure the change of the superconducting transition temperature T c with artificially introduced disorder. Since impurity scattering mixes the superconducting order parameter at different points on the Fermi surface, controlled potential disorder may be considered a phase-sensitive probe of gap symmetry. It is well known that while the gap and critical temperature of an isotropic s-wave superconductor are insensitive to nonmagnetic disorder [1,2], superconducting states with different gap symmetries and structures may be extremely sensitive [3][4][5][6][7][8][9]. In the case of iron-based superconductors, the predictions for the effect of disorder differ for various possible pairing states and depend on details of the model. In particular, models involving repulsive interactions, including popular spin fluctuation models (for a review, see Ref.[10]). predict states where the order parameter changes signs between sheets of the Fermi surface, generically called s AE here, whereas models involving orbital fluctuations [5,6] and attractive interactions predict no sign change (s þþ ). The effect of disorder has also been studied in the coexisting superconducting and long-range magnetic order phase [8]. These different approaches can be studied within a phenomenological multiband theory that for some parameters predicts a crossover from the s AE to the s þþ state [7].

One of the most fundamental properties of an interacting electron system is its frequency-and wave-vector-dependent density response function, χ(q, ω). The imaginary part, χ (q, ω), defines the fundamental bosonic charge excitations of the system, exhibiting peaks wherever collective modes are present. χ quantifies the electronic compressibility of a material, its response to external fields, its ability to screen charge, and its tendency to form charge density waves. Unfortunately, there has never been a fully momentum-resolved means to measure χ(q, ω) at the meV energy scale relevant to modern electronic materials. Here, we demonstrate a way to measure χ with quantitative momentum resolution by applying alignment techniques from x-ray and neutron scattering to surface high-resolution electron energy-loss spectroscopy (HR-EELS). This approach, which we refer to here as "M-EELS", allows direct measurement of χ (q, ω) with meV resolution while controlling the momentum with an accuracy better than a percent of a typical Brillouin zone. We apply this technique to finite-q excitations in the optimally-doped high temperature superconductor, Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212), which exhibits several phonons potentially relevant to dispersion anomalies observed in ARPES and STM experiments. Our study defines a path to studying the long-sought collective charge modes in quantum materials at the meV scale and with full momentum control.