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R.A Ewings1, T.G. Perring*1, J. Gillett2, S.D. Das2, S.E. Sebastian2, A.E. Taylor3, T. Guidi1, A.T. Boothroyd3
1 ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, United Kingdom. 2 Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom. 3 Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom. We report inelastic neutron scattering measurements of the magnetic excitations in SrFe2As2, the parent of a family of iron-based superconductors [1]. The data extend throughout the Brillouin zone and up to energies of ~260meV. We find that the spectrum calculated from the often-used local-moment J1-J2 model fails to explain the data in several key respects. At a temperate of 6 K, well below the joint magnetic and structural ordering temperature TS,N=192 K, the data cannot be explained by a single set of parameters J1a , J1b and J2 (here J1a and J1b are the two nearest-neighbour exchange parameters in the iron planes, with J1a ≫|J1b|), and above ~100meV the excitations are strongly damped. Moreover, on warming above TS,N into the tetragonal phase, one would expect J1a ≡ J1b and hence a soft mode at Q=(0.5,0.5), referenced to the Fe square lattice, due to frustration in the local-moment model. However the spectrum is largely unaltered with respect to that measured in the ordered phase. The qualitative features of the magnetic excitation spectrum that cannot be described by the J1-J2 model, in particular the lack of a soft mode at Q=(0.5,0.5), are readily explained by calculations from a 5-band itinerant mean-field model [2] without invoking additional broken symmetry. Key words: Fe-based superconductors, itinerant magnetism, nematic order [1] R. A. Ewings et al, Phys. Rev. B 83, 214519 (2010). [2] E. Kaneshita and T. Tohyama, Phys. Rev. B 82, 094441 (2010). |
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