Itinerant spin dynamics in iron-based superconductors and cerium-based heavy-fermion antiferromagnets [article]

Gerd Friemel, Universität Stuttgart, Universität Stuttgart
2014
This thesis contains a comprehensive study of the spin excitations by inelastic neutron scattering (INS) in two different correlated electron systems: the alkali-metal iron selenide superconductors (FeSe122) A(x)Fe(2-y)Se(2) (A=K, Rb, Cs) and the heavy-fermion antiferromagnet CeB6. Both systems exhibit intense modes in their spin-fluctuation spectrum below their respective transition temperatures that can be derived from the spin dynamics of the itinerant quasiparticles. However, the
more » ... s of these observations, presented here, are different for each particular compound. The A(x)Fe(2-y)Se(2) superconductors, with a uniform Tc of 32 K, belong to a qualitative new family of superconductors. They possess a distinctly different Fermi surface compared to the iron-arsenide-based analogues XFe(2)As(2) (X=Ca, Sr, Ba). Instead of the central hole pockets at q=0 and the electron pockets at X(1/2 0), which are nested by the Q_afm = (1/2 0) vector, there exist only large electron pockets at the X point. Therefore, the magnetic instability along Q_afm that presumably provides the pairing glue for the superconductivity in the shape of spin fluctuations is absent in the FeSe122. The search for spin fluctuations by INS was motivated by a theoretical analysis that predicted their presence at an incommensurate wave vector near Q_sf = (1/2 1/4), which results from a quasinesting between the flat parts of the electron pockets. Two samples, namely Rb(0.8)Fe(1.6)Se(2) and K(0.77)Fe(1.85)Se(2), were prepared and both showed a sizable anisotropic magnetic response at Q_sf in the normal state. Furthermore, upon entering the superconducting (SC) state a strong excitation appears at E = 14 meV in the spectrum at Q_sf, which is referred to as magnetic resonant mode. This mode is interpreted as a bound spin-1 exciton below the SC charge gap. Its presence implies an unconventional order parameter, which changes the sign between the electron pockets. Moreover, it has a two-dimensional reciprocal-space structure, with an in-plane wave ve [...]
doi:10.18419/opus-6853 fatcat:q2nhs2eji5cjxdl4fqztrwgyja