We have studied the magnetic and superconducting properties of Ba(Fe 0.95 Co 0.05 ) 2 As 2 as a function of temperature and external magnetic field using neutron scattering and muon spin rotation. Below the superconducting transition temperature the magnetic and superconducting order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5 T. We perform a microscopic modelling of the data by use of a five-band Hamiltonian relevant to iron pnictides. In the superconducting 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.
Vortex molecules can form in a two component superfluid when a Rabi field drives transitions between the two components. We study the ground state of an infinite system of vortex molecules in 2D, using a numerical scheme which makes no use of the lowest Landau level approximation. We find the ground state lattice geometry for different values of inter-component interactions and strength of the Rabi field. In the limit of large field when molecules are tightly bound, we develop a complimentary analytical description. The energy governing the alignment of molecules on a triangular lattice is found to correspond to that of an infinite system of 2D quadrupoles, which may be written in terms of an elliptic function Q(zij; ω1, ω2). This allows for a numerical evaluation of the energy which enables us to find the ground state configuration of the molecules.
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