Competing magnetic and superconducting order and the role of vortices in iron-based superconductors

We discuss recent neutron and muSR measurements of the magnetic and superconducting (SC) properties of Co-doped Ba-122 as a function of temperature and external magnetic field [1]. Below the T$_{\mathrm{c}}$, the magnetic and SC order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the SC state [1]. We perform a microscopic modeling of the data by use of a five-band Hamiltonian relevant to iron pnictides. In the SC state, vortices can slow down and freeze spin fluctuations locally. When such regions couple they result in a long-range ordered antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement with experiments [1]. Lastly, we also study the low energy bound states in the vortex core of LiFeAs, where the quasiparticle states in the vortex core can provide useful information about the gap structure [2]. \\[4pt] [1] J. Larsen, G. Stieber, S. L. Holm, K. Lefmann, C. Niedermayer, T. Wolf, preprint 2014\\[0pt] [2] B. Mencia Uranga, B. M. Andersen, preprint 2014