We study the influence of quantum fluctuations on the electron self energy in the normal state of iron-pnictide superconductors using a five orbital tight binding model with generalized Hubbard on-site interactions. Within a one-loop treatment, we find that an overdamped collective mode develops at low frequency in channels associated with quasi-1D dxz and dyz bands. When the critical point for the C4 symmetry-broken phase (structural phase transition) is approached, the overdamped collective modes soften, and acquire increased spectral weight, resulting in non-Fermi liquid behaviour at the Fermi surface characterized by a frequency dependence of the imaginary part of electron self energy of the form ω λ , 0 < λ < 1. We argue that this non-Fermi liquid behaviour is responsible for the recently observed zero-bias enhancement in the tunneling signal in point contact spectroscopy. A key experimental test of this proposal is the absence of non-Fermi liquid behaviour in the hole-doped materials. Our result suggests that quantum criticality plays an important role in understanding the normal state properties of iron-pnictide superconductors.
We suggest a mechanism which promotes the existence of a phase soliton -topological defect formed in the relative phase of superconducting gaps of a two-band superconductor with s+− type of pairing. This mechanism exploits the proximity effect with a conventional s-wave superconductor which favors the alignment of the phases of the two-band superconductor which, in the case of s+− pairing, are π-shifted in the absence of proximity. In the case of a strong proximity such effect can be used to reduce soliton's energy below the energy of a soliton-free state thus making the soliton thermodynamically stable. Based on this observation we consider an experimental setup, applicable both for stable and metastable solitons, which can be used to distinguish between s+− and s++ types of pairing in the iron-based multiband superconductors.
We develop here a general formalism for multi-orbital Mott systems which can be used to understand dynamical and static spectral weight transfer. We find that the spectral weight transferred from the high energy scales is greatly increased as a result of the multi-orbital structure. As a consequence certainly dynamically generated symmetries obtain at lower values of doping than in the single-band Hubbard model. For example, in the atomic limit, the particle-hole symmetric condition in the lower band shifts from the one-band result of x = 1/3 to x = 1/(2no + 1), where no is the number of orbitals with an unpaired spin. Transport properties computed from effective low-energy theories which forbid double occupancy of bare electrons, such as the multi-orbital t-J generalization, should all be be sensitive to this particle-hole symmetric condition. Away from the atomic limit, the dynamical contributions increase the transferred spectral weight. Consequently, any phenomena which are sensitive to an emergent particle-hole symmetry should obtain at x < (1/(2no + 1).
We show here that orbital degrees of freedom produce a distinct signature in the magnetic excitation spectrum of iron-based superconductors above the magnetic ordering temperature. Because dxz and dyz orbitals are strongly connected with the Fermi surface topology, the nature of magnetic excitations can be modified significantly due to the presence of either static or fluctuating orbital correlations. Within a five-orbital itinerant model, we show that static orbital order generally leads to an enhancement of commensurate magnetic excitations even when the original Fermi surface lacks nesting at (π, 0) or (0, π). When long-range orbital order is absent, Gaussian fluctuations beyond the standard random-phase approximation (RPA) capture the effects of fluctuating orbital correlations on the magnetic excitations. We find that commensurate magnetic excitations can also be enhanced if the orbital correlations are strong. Our results offer a natural explanation for the incommensurate-to-commensurate transformation observed in a recent neutron scattering measurement (Z. Xu, et. al., arXiv:1201.4404), and we propose that this unusual transformation is an important signature to distinguish orbital from spin physics in the normal state of the pnictides. Implications for the magnetic and superconducting states are discussed.
We investigate the effects of the √ 5× √ 5 Fe vacancy ordering on the orbital and magnetic order in (K,Tl,Cs)yFe2−xSe2 using a three-orbital (t2g) tight-binding Hamiltonian with generalized Hubbard interactions. We find that vacancy order enhances electron correlations, resulting in the onset of a block antiferromagnetic phase with large moments at smaller interaction strengths. In addition, vacancy ordering modulates the kinetic energy differently for the three t2g orbitals. This results in a breaking of the degeneracy between the dxz and dyz orbitals on each Fe site, and the onset of orbital order. Consequently, we obtain a novel inverse relation between orbital polarization and the magnetic moment. We predict that a transition from high-spin to low-spin states accompanied by a crossover from orbitally-disordered to orbitally-ordered states will be driven by doping the parent compound with electrons, which can be verified by neutron scattering and soft X-ray measurements.
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