Session H39: Focus Session: Iron Based Superconductors: Pairing Symmetry

Experimental and computational evidence for the $s_{\pm}$ pairing symmetry in Fe-based superconductors

I will first review the basic theoretical arguments that led to the so-called $s_{\pm}$ symmetry being predicted well before any experimental indications. I will discuss possible roles of phonons and of spin fluctuations, including possibility of electron-phonon coupling enhancement through magnetoelastic effects. Next I will list $negative$ experimental evidence and argue that triplet pairing and d- wave pairing can be with a good degree of confidence excluded. Lastly, I will address the issue of distingushing between the $s_{\pm}$ and a conventional $s_{++}$ states, in terms of already existing and potential experiments. While the question of nodal $vs.$ nodeless superconductivity is not immediately related to the $s_{\pm}$ $vs.$ $s_{++}$ choice, I will briefly discuss the fact that gap nodes seem to exist in some, but not the others, Fe-based superconductors, and possible theoretical mechanisms for that.   

Momentum dependence and nodes of the superconducting gap in the iron pnictides

Using general symmetry arguments and model calculations we analyze the superconducting gap in materials with multiple Fermi-surface pockets, with applications to iron pnictides. We show that the gap in the pnictides has an extended s-wave symmetry but is either nodeless or has nodes, depending on the interplay between intraband and interband interactions. We argue that the nodes in the gap emerge without a phase transition as the tendency toward a spin-density-wave order gets weaker. These findings provide a way to reconcile seemingly conflicting results of numerical and experimental studies of the pnictides. In particular, we argue that isovalent-P doped BaFe$_2$As$_{2}$ superconductors likely has an extended s-wave gap with nodes. On the other hand, electron or hole doped BaFe$_2$As$_{2}$ superconductors have a nodeless gap. Still even these materials exhibit a gapless behavior at strong disorder.   

Quasi-degenerate superconductivity in the `122' iron-pnictide superconductors

In iron-based superconductors, especially so-called `122' systems, the superconducting gap symmetry is still a debated issue both theoretically and experimentally. Most early experimental studies in BaK-122 and BaCo-122 families including ARPES, thermal conductivity, and penetration depth measurements indicate a fully-gapped superconducting state, although the consensus for the gap structure is still lacking. Here we report the magnetic penetration depth measured down to 0.01$T$/$T_{c}$ clearly shows the presence of line nodes on the superconducting gap structure in some `122' iron-pnictide superconductors. This result indicates that non-universal gap symmetry is realized in the `122' systems, depending on the substituting materials and/or its doping levels.   

$\mu$SR Studies of Electron-doped 122 Pnictide Superconductors

Single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ were studied using Transverse field (TF)-$\mu$SR. We measured the penetration depth in the superconducting state by fitting the $\mu$SR spectra to a microscopic model for a range of applied magnetic fields and dopings. We have fit the temperature dependence of the superfluid density to a phenomenological two-gap model. We find that the low-temperature superfluid density varies roughly linearly with superconducting T$_C$, both of which decrease with increasing Co substitution above x = 0.06.   

Quasiparticle dynamics across the multiple superconducting gaps in the electron doped BaFe1.85Co0.15As2

Understanding the nature of the low-lying electronic structures is extremely important in establishing the microscopic origin of superconductivity in iron pnictides. In this work, we used the time-resolved optical spectroscopy to study the quasiparticle dynamics in high-quality single crystals of electron-doped superconductor, BaFe1.85Co0.15As2 (BFCA). We show that the electron-doped BFCA is a multi-gap s-wave superconductor. Moreover, we reveal that the electron-phonon interaction alone is not strong enough to induce a superconducting phase transition in BFCA, and a new attractive pairing mechanism is necessary. Our results support a picture of electron pairing via AF fluctuations and point to the possibility that the FeAs-family of superconductors and the high-Tc cuprates may share a similar spin mediated pairing mechanism   

Nodal gap symmetry in Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ from NMR measurement in high magnetic field

There is conflicting evidence in pnictide superconductors for fully gapped s$_\pm$ wave symmetry or gap nodes at the Fermi surface. Here we report measurements of the $^{75}$As NMR Knight shift that suggest existence of gap nodes on an optimally doped single crystal of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. The measurements were done from 2 K to 300 K with external magnetic fields from 6.4 T to 16.8 T. The spin part of the Knight shift at low temperature($ [Preview Abstract]

Accidental nodes in the order parameter of the iron superconductors due to Coulomb repulsion

The nature of the order parameter in the iron superconductors is not fully elucidated. There is a general agreement that the order parameter is a spin singlet state. There is evidence for a fully gapped sign changing $s_{\pm}$ order parameter in some iron superconductors. However, there is also evidence for line nodes in some samples that might indicate a different symmetry. In this paper, we present a model that shows accidental nodes can appear within the $s_{\pm}$ order parameter scenario due to a competition between the pairing interaction and Coulomb repulsion.   

Theory of ac Josephson effects in multiple tunneling junctions: A probe of $\pm s$-wave in iron-based superconductors

An enormous amount of studies has been devoted to the identification of the pairing symmetry in iron-based superconductors. We show a theory of Josephson junctions with multi-gap superconductors. We focus on a heterotic (multi-band)superconductor- insulator-(one-band)superconductor junction. We derive the Ambegaokar-Baratoff relation. We evaluate a lower bound of $J_{\rm c}R_{\rm n}$ for $s$-wave without sign change, which may correspond to a upper bound for $\pm s$-wave. Next, we discuss how a fluctuation of relative phases between the gaps and the symmetry modify the Shapiro step. Thus, we propose a direct method to identify $\pm s$-wave.   

Vortex as a probe - suggested measurement of the order parameter structure in the iron-based superconductors

Much like an impurity, a vortex in a superconductor induces scattering processes. We suggest to use these vortex induced transitions as a probe for the order parameter structure in the iron-based superconductors. Impurities, inevitably present, induce transitions between states on the contours of constant energy which may be seen in Fourier transformed scanning tunneling spectroscopy (FT STS) . When a magnetic field is applied, vortices act as additional sources of scattering in the particle-particle channel. Whether transitions are enhanced by this channel crucially depends on the sign of the order parameter. In this work we show that while in a simple s-wave superconductor all transitions are enhanced by vortex scattering, in an s+/- superconductor only intra-pocket transitions are affected. We suggest this effect as a way to distinguish between s-wave and s+/- order parameter.   

Theory of thermal conductivity in a multi-band superconductor : Application to pnictides

We calculate low temperature thermal conductivity within a two band model for newly discovered ferro-pnictide superconductors. We consider three different cases, sign changing s-wave state, highly anisotropic s-wave state and a state with order parameter nodes on one band. To include the effect of disorder, we have performed fully self-consistent T-matrix approximation including both intraband and interband impurity scatterings. We also study the behavior of the low temperature thermal conductivity under applied magnetic field using a recently developed variant of the Brandt-Pesch-Tewordt approximation, and compare our results with latest experimental data.