Abstract:

A superconducting state is described by a complex order parameter. In an unconventional superconductor, the order parameter may exhibit a sign change in both momentum and real spaces. Normally, a sign change order parameter, such as the d-wave pairing in cuprates, leads to gapless nodes or nodal lines in the superconducting state which can be detected by many experimental techniques.
Iron-based superconductors , which was discovered six years ago, exhibit a nodeless full superconducting gap on Fermi surfaces. Nevertheless, there are strong indirect experimental evidence that indicates the existence of sign change on the superconducting order parameters. How to compromise the sign change and full gap becomes an essential question. Determining the sign change pattern and understanding the origin of the sign change are also critical to solve pairing mechanism.
In this talk, I will review both experimental evidence and predictions or explanations from different theories on pairing symmetries and sign change issues on iron-based superconductors. I will discuss a new solution which can provide an unified understanding for sign changes on all iron-based superconductors.
 
 
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