Session A1: Phase-sensitive Probes of the Pairing Symmetry in Fe-Based Superconductors

Possible suggestions for order parameter phase-sensitive experiments in the superconducting iron pnictides

The iron pnictide superconductors have undergone intensive study since the original discovery by Kamihara et al early in 2008, with maximum T$_{c}$'s exceeding 50 K. Despite this, the most basic questions such as pairing symmetry and mechanism have not been definitively settled. For the cuprates, the SQUID loop and tricrystal phase-sensitive experiments were instrumental in finally determining the d-wave gap symmetry; similar experiments were designed and implemented for triplet p-wave superconductivity. However, the main challenge in pnictides is to distinguish between two superconducting states, the sign-changing ``s$_\pm$" and single-sign ``s$_{++}$ states, which belong to the same point symmetry class. This means that while designing a Josephson loop one needs to invent a recipe to filter out different types of carriers at the two different contacts. By definition this is a {\it quantitative} rather than {\it qualitative} effect, and involves the relative amplitude of the order parameter, density-of-states and Fermi velocity, and the character of the electronic wavefunctions. Presently proposed methods either attempt to determine an optimal angle (i.e., non-90$^{\circ}$) for a SQUID junction, use different barrier characteristics in different directions, or exploit `sandwich' junctions employing two or more superconductors. In this talk, I discuss several recent proposals for phase-sensitive experiments which could help resolve the pairing symmetry controversy, as well as experimental work in this area.   

Josephson effect studies of pairing symmetry in Fe-based superconductors

To investigate the pairing symmetry in the recently discovered Fe-based superconductors, Josephson effect studies have been performed on two types of $c$-axis junctions incorporating 122-type iron pnictide superconductors: junctions between $s$-wave superconductors and iron pnictide superconductors [1] and junctions between electron-doped and hole-doped iron pnictide superconductors [2]. The ac Josephson effect was observed in the $I-V$ characteristics for both types of junctions under microwave irradiation. By applying external magnetic fields parallel to the junction interfaces, Fraunhofer-like patterns were obtained. Analysis based on the obtained modulation patterns suggests that the Josephson current is flowing along the $c$-axis direction within a typical area of 10 $\times $ 10 ($\mu $m)$^{2}$. The presence of Josephson coupling between an $s$-wave superconductor and a 122-type iron pnictide superconductor along the $c$-axis strongly supports an $s$-wave symmetry in the iron pnictide superconductor. Moreover, our observed Josephson effect in the bicrystal junctions indicates that phase coherence can be established between electron-doped and hole-doped iron pnictide superconductors. Such a phase-coherent $p-n$ structure is an important component [3] for performing definitive phase-sensitive tests for the proposed $s\pm $ symmetry in Fe-based superconductors. Progress in carrying out such tests will be discussed. Recent results on systematic measurements of the energy gap using Andreev reflection spectroscopy with highly transparent contacts will also be presented. This work is supported by the NSF (DMR-0653535) and performed in collaboration with S. R. Saha, N. P. Butch, K. Kirshenbaum, J. Paglione, R. L. Greene, I. Takeuchi at UMD, and Y. S. Oh, Y. Liu, L. Q. Yan, K. H. Kim at SNU. \\[4pt] [1] X. H. Zhang \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 147002 (2009).\\[0pt] [2] X. H. Zhang \textit{et al.}, Appl. Phy. Lett. \textbf{95}, 062510 (2009).\\[0pt] [3] D. Parker and I. I. Mazin, Phys. Rev. Lett. \textbf{102}, 227007 (2009).   

Integer and half-integer flux-quantum transitions in a niobium/iron-pnictide loop

The recent discovery of iron-based superconductors challenges the existing paradigm of high-temperature superconductivity. Owing to their unusual multi-orbital band structure, magnetism, and electron correlation, theories propose a unique sign-reversed s$\pm$-wave pairing state, with the order parameter changing sign between the electron and hole Fermi pockets. However, because of the complex Fermi surface topology and material related issues, the predicted sign reversal remains unconfirmed. Here we report a novel phase-sensitive technique for probing unconventional pairing symmetry in the polycrystalline iron-pnictides. Through the observation of both integer and half-integer flux-quantum transitions in composite niobium/iron-pnictide loops, we provide the first phase-sensitive evidence of the sign change of the order parameter in NdFeAsO$_{0.88}$F$_{0.12}$, lending strong support for unconventional s$\pm$-wave pairing symmetry. Implications on the microscopic pairing mechanism will also be discussed.   

Scanning SQUID microscopy studies of the penetration depth and pairing symmetry in Fe-pnictide superconductors

We present scanning SQUID magnetometry and susceptometry data on dense polycrystalline samples of the iron oxypnictide superconductors NdFeAsO$_{0.94}$F$_{0.06}$ and SmFeAsO$_{0.85}$, and on a single crystal of LaFePO. On LaFePO, we demonstrate, in a local measurement requiring no correction for sample geometry,that the magnetic penetration depth is linear in $T$, indicating line nodes in the superconducting order parameter. On polycrystalline NdFeAsO$_{0.94}$F$_{0.06}$ and SmFeAsO$_{0.85}$, we show that the order parameter is $s$-wave: if the order parameter were non-$s$-wave there would be direction-dependent phase shifts in the intergrain tunnelling, which in a dense polycrystalline sample would result in spontaneous orbital currents below $T_c$. Orbital currents would give a complex magnetization, and moments polarizable by a weak field during cooling. Paramagnetism against weak cooling fields is a well-established phenomenon in polycrystalline cuprate samples, consistent with $d$-wave superconductivity. In NdFeAsO$_{0.94}$F$_{0.06}$ and SmFeAsO$_{0.85}$, we observe neither the complex magnetization nor weak-field paramagnetism, ruling out non-$s$-wave order parameters.   

Andreev spectra of multiband superconductors

A theory of Andreev conductance is formulated for junctions involving normal metals (N) and multiband superconductors (S) and applied to the case of superconductors with nodeless $s_{\pm}$-wave order parameter symmetry, as possibly realized in the recently discovered pnictides. We find qualitative differences from tunneling into s-wave or d-wave superconductors that may help to identify such a state. Interband interference leads to a suppression of Andreev reflection in the case of a highly transparent N/S interface and to a current deficit in the tunneling regime. Surface bound states may appear, both at zero and at nonzero energies [1] We calculate the surface density of state (SDOS) of $s_{\pm}$-wave Cooper pair in two-band superconductor model, where gap functions have different signs between two bands. The tunneling spectroscopy of $s_{\pm}$ wave is much more complex as compared to the $d$-wave case realized in high-$T_{c}$ cuprates [2]. \\[4pt] [1] A. A. Golubov, A. Brinkman, Yukio Tanaka, I. I. Mazin, and O. V. Dolgov, Phys. Rev. Lett. \textbf{103}, 077003 (2009) \newline [2] S. Onari and Y. Tanaka, Phys. Rev. B \textbf{79}, 174526 (2009).