Iron-based superconductivity


Iron-based superconductivity has been one of focus research direction in condensed matter physics. We have been working on exploring the pairing symmetry, and nematic states in this class of superconductors.

Time-reversal symmetry breaking pairing states

We proposed the complex Cooper pairing of the $s\pm id$ type based on the symmetry analysis and the competing nature between the $s$ and $d$-wave pairings [Ref. 1] . It breaks time-reversal symmetry spontaneously and could occur at low temperatures in some compounds in the iron-pnictide family. We predicted various experimentally testable signatures, including spatial inhomogeneity induced supercurrents, and a novel collective mode measurable in the B_1g-Raman mode.

Time-reversal symmetry breaking is research focus in the field of unconventional superconductivity including cuprates (e.g.YBa2Cu3O6+x) and ruthenates (e.g. Sr2RuO4). The impact of our proposal has appeared: The possibility of time-reversal symmetry breaking superconductivity in iron-pnictides has attracted attentions by several groups. Such states in iron-pnictide systems would add a new member to this exotic family if it could be verified by experiments.

Nematicity in FeSe

In collaboration with Q. K. Xue's experimental group, we proposed the mechanism of orbital ordering in FeSe superconductors to explain their scanning tunneling spectroscopy experiment [Ref. 2] . Their data revealed the evidence for nodal superconductivity, and also strong anisotropy with 4-fold symmetry breaking to the 2-fold. Different from other iron-based superconductors, FeSe has no magnetic long-range order at ambient pressure, thus this anisotropy is not directly related to antiferromagnetism. We proposed the orbital ordering between $d_{xz}$ and $d_{yz}$-orbital bands to account for this anisotropy, which breaks the tetragonal symmetry to orthorombic. Furthermore, the gap nodes can be explained in terms of the mixed $s_{x^2+y^2}$ and $s_{x^2y^2}$ pairing symmetries.

We also perform a theoretical study for the above STM experiment spectroscopy [Ref. 3] , which shows nodal superconductivity and strong anisotropy. The nodal structure can be explained with the extended s-wave pairing structure with the mixture of the sx2+y2 and sx2y2 pairing symmetries. We calculate the anisotropic vortex structure by using the self-consistent Bogoliubov–de Gennes mean-field theory. In considering the absence of magnetic ordering in the FeSe at ambient pressure, orbital ordering is introduced, which breaks the C4 lattice symmetry down to C2, to explain the anisotropy in the vortex tunneling spectra.


References and talks

  • 1. Wei-cheng Lee, Shou-cheng Zhang, and Congjun Wu , "Pairing State with a Time-Reversal Symmetry Breaking in FeAs-Based Superconductors", Phys. Rev. Lett. 102, 217002 (2009), see pdf file .

  • 2. Can-Li Song, Yi-Lin Wang, Peng Cheng, Ye-Ping Jiang,Wei Li,Tong Zhang, Zhi Li,Ke He,Lili Wang, Jin-Feng Jia, Hsiang-Hsuan Hung, Congjun Wu, Xucun Ma, Xi Chen, and Qi-Kun Xue, "Direct Observation of Nodes and Twofold Symmetry in FeSe Superconductor", Science 332, 1410 (2011), See pdf file , and supplementary material .

  • 3. Hsiang-Hsuan Hung, Can-Li Song, Xi Chen, Xucun Ma, Qi-kun Xue, Congjun Wu, "Anisotropic vortex lattice structures in the FeSe superconductor", Phys. Rev. B 85, 104510 (2012) , see pdf file.

    Back to home
    Last modified: July 15, 2007.