We report on inelastic neutron scattering measurements that find incommensurate itinerant like magnetic excitations in the normal state of superconducting FeTe0.6Se0.4 (Tc=14K) at wave-vector Qinc = (1/2 ± , 1/2 ∓ ) with =0.09(1). In the superconducting state only the lower energy part of the spectrum shows significant changes by the formation of a gap and a magnetic resonance that follows the dispersion of the normal state excitations. We use a four band model to describe the Fermi surface topology of iron-based superconductors with the extended s(±) symmetry and find that it qualitatively captures the salient features of these data.PACS numbers: 61.05. 75.25.+z,75.30.Et,77.80.Fm,77.84.Bw,75.80.+q Magnetism is a key ingredient in the formation of Cooper pairs in unconventional high temperature superconductors 1 . A consequence of magnetism and the symmetry of the superconducting order parameter is a magnetic resonance that has been detected through inelastic neutron scattering for a wide range of magnetic superconductors ranging from the heavy fermion systems 2,3,4 , to cuprates 5,6,7,8,9 , and more recently in the iron based superconductors 10,11,12 . In each class of materials the features of the resonance in S(Q, ω) are different and are directly related to the electronic degrees of freedom, providing vital clues to the role of magnetic fluctuations in each case. We report inelastic neutron scattering measurements showing that the magnetic excitations in FeTe 0.6 Se 0.4 are incommensurate and itinerantlike, and that upon entering the superconducting state only the lower energy part of the spectrum shows significant changes by the formation of a gap and a magnetic resonance that follows the dispersion of the normal state excitations. Using a four band model that describes the Fermi surface topology of iron-based superconductors and the extended s(±) symmetry, we can qualitatively describe the salient features of the data. The good agreement between theory and experiment found here may provide clues to a better understanding of the magnetic resonance in high-T c cuprate superconductors.In cuprate superconductors a magnetic resonance, whose energy scales with the superconducting energy gap, is a saddle point of a broader spectrum of magnetic excitations 6,7,8,9 . The interpretation of the magnetic res-onance and its relationship to superconductivity is complicated in the cuprates due to the occurrence of charge stripes and the pseudogap phase that lead to hotly debated models such as quantum excitations from charge stripes 9 , or spin excitons from a particle-hole bound state of a d-wave superconductor 13,14,15 .A far clearer picture has emerged in the iron based superconductors partly due to their more itinerant nature. It was realized early on that the nesting between hole and electron Fermi surfaces related by the antiferromagnetic wavevector Q AF = (π, π) in the undoped iron superconductors, might also be responsible for unconventional superconductivity of the so-called extended s(±)wave symmetry 16,17,18...
We study the magnetic order and excitations in strong spin-orbit coupled, Van Vleck-type, d4 Mott insulators on a square lattice. Extending the previous work, we include the tetragonal crystal field splitting and explore its effects on magnetic phase diagram and magnon spectra. Two different ordered phases, with in-plane and out-of-plane orientation of the staggered moments, are found for the higher and lower values of the crystal field splitting, respectively. The magnetic excitation spectra for paramagnetic and magnetically ordered phases are calculated and discussed in the context of a candidate spin-orbit d 4 Mott insulator Ca2RuO4.
We investigate the quasiparticle interference in the heavy fermion superconductor CeCoIn 5 as a direct method to confirm the d-wave gap symmetry. The ambiguity between d xy and d x 2 −y 2 symmetry remaining from earlier specific heat and thermal transport investigations has been resolved in favor of the latter by the observation of a spin resonance that can occur only in d x 2 −y 2 symmetry. However, these methods are all indirect and depend considerably on theoretical interpretation. Here we propose that quasiparticle interference (QPI) spectroscopy by scanning tunneling microscopy (STM) can give a direct fingerprint of the superconducting gap in real space that may lead to a definite conclusion on its symmetry for CeCoIn 5 and related 115 compounds. The QPI pattern for both magnetic and nonmagnetic impurities is calculated for the possible d-wave symmetries and characteristic differences are found that may be identified by use of the STM method.
We propose an explanation for the electronic nematic state observed recently in parent iron-based superconductors [T.-M. Chuang et al., Science 327, 181 (2010)]. We argue that the quasi-onedimensional nanostructure identified in the quasiparticle interference (QPI) is a consequence of the interplay of the magnetic (π, 0) spin-density wave (SDW) order with the underlying electronic structure. We show that the evolution of the QPI peaks largely reflects quasiparticle scattering between electronic bands involved in the SDW formation. Because of the ellipticity of the electron pocket and the fact that only one of the electron pockets is involved in the SDW, the resulting QPI has a pronounced one-dimensional structure. We further predict that the QPI crosses over to two-dimensionality on an energy scale, set by the SDW gap, which we estimate from neutron scattering data to be around 90 meV.
We systematically calculate quasiparticle interference (QPI) signatures for the whole phase diagram of iron-based superconductors. Impurities inherent in the sample together with ordered phases lead to distinct features in the QPI images that are believed to be measured in spectroscopic imaging-scanning tunneling microscopy (SI-STM). In the spin-density wave phase the rotational symmetry of the electronic structure is broken, signatures of which are also seen in the coexistence regime with both superconducting and magnetic order. In the superconducting regime we show how the different scattering behavior for magnetic and non-magnetic impurities allows to verify the $s^{+-}$ symmetry of the order parameter. The effect of possible gap minima or nodes is discussed.Comment: 19 pages, 7 figure
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