We analyze antiferromagnetism and superconductivity in novel F e−based superconductors within the itinerant model of small electron and hole pockets near (0, 0) and (π, π). We argue that the effective interactions in both channels logarithmically flow towards the same values at low energies, i.e., antiferromagnetism and superconductivity must be treated on equal footings. The magnetic instability comes first for equal sizes of the two pockets, but looses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces (extended s-wave symmetry). We argue that the T dependencies of the spin susceptibility and NMR relaxation rate for such state are exponential only at very low T , and can be well fitted by power-laws over a wide T range below Tc.

Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin-orbit interaction and the spin-momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.

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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...