Theory of magnetic excitations in iron-based layered superconductors

Abstract

Based on the effective four-band model we analyze the spin response in the normal and superconducting states of the Fe-pnictide superconductors. While the normal state spin excitations are dominated by the continuum of the interorbital antiferromagnetic fluctuations and the intraband spin density wave fluctuations, the unconventional superconductivity yields different feedback. The resonance peak in form of the well-defined spin exciton occurs only for the interband scattering at the antiferromagnetic momentum Q AF M for the s± (extended s-wave) superconducting order parameter and it disappears rapidly for q < QAF M . The resonance feature is extremely weak for the d x 2 −y 2 -wave order parameter due to specific Fermi surface topology of these compounds. The essential difference between s±-wave and d x 2 −y 2 -wave symmetries for the magnetic excitations can be used for experimental determination of the superconducting wave function symmetry.PACS numbers: 74.20.Mn, 74.20.Rp, 74.25.Ha, 74.25.Jb The relation between unconventional superconductivity and magnetism is one of the most interesting topics in the condensed matter physics. In contrast to the usual electron-phonon mediated superconductors where the paramagnetic spin excitations are suppressed below superconducting transition temperature due to the formation of the Cooper pairs with total spin S = 0, in unconventional superconductors, such as layered cuprates or heavy fermion superconductors, a bound state (spin resonance) with a high intensity forms below T c 1,2,3 . The fact that the superconducting gap is changing sign at a different parts of the Fermi surface together with a presence of the strong electronic correlations yields such an enhancement of the spin response 4 . Most interestingly, an observation of the resonance peak indicates not only that Cooper-pairing is unconventional but also that the magnetic fluctuations are most relevant for superconductivity 5 .Since the discovery of superconductivity in the quaternary oxypnictides LaFePO 6 and LaNiPO 7 , a new class of high-T c materials with Fe-based layered structure is emerging 8,9,10,11,12,13,14 . Although the microscopic nature of superconductivity in these compounds remains unclear at present, certain aspects have been already discussed 15,16,17,18,19,20,21,22,23,24,25,26,27 . In particular, ab initio band structure calculations 15,16,17,18,19,20 have shown that the conductivity and superconductivity in these systems are associated with the Fe-pnictide layer, and the electronic density of states (DOS) near the Fermi level shows maximum contribution from the Fe-3d orbitals. The resulting Fermi surface consists of two hole (h) and two electron (e) pockets. The normal state magnetic spin susceptibility determined from these bands 22 exhibits both small q ∼ 0 fluctuations and antiferromagnetic commensurate spin density wave (SDW) peaks.In this Rapid Communication, using the four-band tight-binding model we study theoretically the spin response in the normal and superconducting states of Febas...

Anisotropic neutron spin resonance in superconductingBaFe1.9Ni0.1As2

“…In the iron arsenide superconductors, this condition is satisfied by an order parameter whose sign reverses between the electron and hole pockets. [16][17][18][19][20] If this picture is correct, one would expect that the triplet would be degenerate, and thus directionally isotropic in space. For the optimal hole-doped high-T c cuprate superconductor YBa 2 Cu 3 O 6+x , polarized inelastic neutronscattering experiments suggest that this is indeed the case.…”

Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFe1.85Ni0.15As

“…Understanding the role of spin excitations in the superconductivity of iron arsenides 1-3 is important for developing a microscopic theory of superconductivity in these materials [4][5][6][7][8] . Like copper oxide superconductors, superconductivity in iron pnictides arises when electrons or holes are doped into their antiferromagnetically-ordered parent compounds 9 .…”

s-wave superconductivity with orbital-dependent sign change in checkerboard models of iron-based superconductors

“…It is highly debated that whether the iron-based superconductors belong to the same category of strongly correlated electron systems as the cuprates. Models based on both strong coupling 2-9 and weak coupling [10][11][12][13][14][15][16] approaches have been applied to understand the properties of these materials and their relation to the curpates.…”