Unveiling the Unconventional Pairing in Iron-based Superconductor: Direct Observation of the Nodal Gap Structure in Ferropnictide Superconductor

Yan Zhang (left) and Zirong Ye (right), leading authors of the Nature Physics paper





Authors: Yan Zhang, Dong-Lai Feng

Affiliation: State Key Laboratory of Surface Physics, Advanced Materials Laboratory, and Department of Physics, Fudan University, Shanghai 200433, China

Link to Feng Group: Research Group of Complex Quantum Systems >>

Pairing symmetry is a pivotal characteristic of a superconductor. In the conventional BCS superconductors, the formation of Cooper pairs is due to the attractive interaction between electrons mediated by the electron-phonon interaction. Such pairing interaction results in an isotropic s-wave pairing symmetry, which is manifested as finite-sized energy gap called superconducting gap in single particle excitations throughout the entire Fermi surface. However, for many unconventional superconductors, since Coulomb repulsive interaction between electrons is often rather strong, Cooper pairs favor a non-zero angular momentum to minimize the total energy. For example, the cuprate high temperature superconductors take the d-wave pairing symmetry, which would cause superconducting gap diminishes at certain locations called nodes on the Fermi surface. Such gap nodes will have significant effects on the low temperature properties.

Four years after the discovery of the iron-based superconductors in 2008, the mechanism of this new class of high temperature superconductors is still under debate, because of their diversified structure, composition, and electronic structure. Scientists are still struggling to construct a unified picture for the basic phenomenology of different iron-based superconductors. One central issue is the exact nature of the superconducting gap. For example, in the superconducting state, some iron-based superconductors, such as Ba_1-xKFe₂As₂, BaFe_2-xCo_xAs₂, K_xFe_2-ySe₂, FeTe_1-xSe_x, etc., exhibit a nodeless behavior [1, 2], while others like LaOFeP, LiFeP, KFe₂As₂, BaFe₂(As_1-xP_x)₂, BaFe_2-xRu_xAs₂, and FeSe exhibit a nodal behavior with zero energy excitations [2].

This discrepancy on superconducting gap raises serious challenges, questions and debates. For example, one could ask whether the nodal behavior is due to d-wave pairing; and if so, why there are two types pairing symmetries or mechanisms in iron-based superconductors? Many theories have been proposed to address these fundamental questions, but no consensus has been reached. The main obstacle is that all the previous measurements do not provide detailed information of the gap structure in the momentum space, and are somewhat indirect. We have no knowledge on the location of the nodes, as to which band does it belong, and where is it in the Brillouin zone, etc.

Figure 1. (a) The three-dimensional Fermi surface of BaFe₂(As_0.7P_0.3)₂. (b) k_z dependence of the symmetrized spectra measured on the α hole FSs. (c) The superconducting gaps on the α FSs as a function of k_z.

Recently, these mysteries regarding the nature of the superconducting gap in iron-based superconductors have been resolved by our angle resolved photoemission spectroscopy (ARPES) study [3]. We have successfully determined the nodal gap structure of BaFe₂(As_1-xP_x)₂, which is a prototypical iron-based superconductor with nodal behaviors established by many transport studies. As shown in Fig. 1a, the Fermi surface of BaFe₂(As_0.7P_0.3)₂ consists of three hole Fermi surface sheets (FSs) (α, β and γ) surrounding the central Γ–Z axis of the Brillouin zone, and two electron FSs (δ and η) around the corner. Detailed survey on the electron FSs found a nodeless superconducting gap with little k_z dependence. However, for the α hole FSs, the experimental data clearly showed a zero superconducting gap or nodes located around the Z point (Fig. 1b and 1c).
























Figure 2. False-color plots of the gap distribution on the Fermi surface of BaFe₂(As_0.7P_0.3)₂.

The gap distribution of BaFe₂(As_0.7P_0.3)₂ is summarized in Fig. 2. The node is located on a ring around Z, which immediately rules out the d-wave pairing symmetry, since it would give four vertical line nodes in the diagonal directions (θ = ± 45°, ± 135°) as in the cuprates. The horizontal ring node around Z is not forced by symmetry, as it is fully symmetric with respect to the point group. Therefore, the node is an “accidental” one under the s-wave pairing symmetry, which is likely induced by the strong three-dimensional nature of the α band, and its sizable d_3z2−r2 orbital character near Z. This finding provides a general explanation as to why the gap is nodal for certain compounds and nodeless for others, and thus helps build a universal picture of the pairing symmetry in iron-based superconductors.

References:
[1] Y. Zhang, L. X. Yang, M. Xu, Z. R. Ye, F. Chen, C. He, H. C. Xu, J. Jiang, B. P. Xie, J. J. Ying, X. F. Wang, X. H. Chen, J. P. Hu, M. Matsunami, S. Kimura, and D. L. Feng, "Nodeless superconducting gap in A_xFe₂Se₂ (A=K,Cs) revealed by angle-resolved photoemission spectroscopy". Nature Materials, 10, 273–277 (2011). Abstract.
[2] J. Hirschfeld, M. M. Korshunov, and I. I. Mazin, "Gap symmetry and structure of Fe-based superconductors". Reports on Progress in Physics, 74, 124508 (2011). Abstract.
[3] Y. Zhang, Z. R. Ye, Q. Q. Ge, F. Chen, Juan Jiang, M. Xu, B. P. Xie and D. L. Feng, "Nodal superconducting-gap structure in ferropnictide superconductor BaFe₂(As_0.7P_0.3)₂". Nature Physics, doi:10.1038/nphys2248 (Published online Mar 04, 2012). Abstract.