title: Suppression of spin-exciton state in hole overdoped iron-based superconductors.
abstract: The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2Δs), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1-xKxFe2As2 for 0.5 < x < 0.84 studied by inelastic neutron scattering. We find that the behavior of the spin resonance dramatically changes from optimum to overdoped regions. Strong resonance peaks are observed clearly below 2Δs in the optimum doping region, while they are absent in the overdoped region. Instead, there is a transfer of spectral weight from energies below 2Δs to higher energies, peaking at values of 3Δs for x = 0.84. These results suggest a reduced impact of magnetism on Cooper pair formation in the overdoped region.
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: C H
authors.initials: CH
authors.lastname: Lee
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Kihou
authors.affiliation: Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, D-85748 Garching, Germany.
authors.forename: J T
authors.initials: JT
authors.lastname: Park
authors.affiliation: Aoyama Gakuin University, Sagamihara 252-5258, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Horigane
authors.affiliation: Aoyama Gakuin University, Sagamihara 252-5258, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Fujita
authors.affiliation: II. Physikalisches Institut, Universität zu Köln,50937 Cologne, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Waßer
authors.affiliation: II. Physikalisches Institut, Universität zu Köln,50937 Cologne, Germany.
authors.forename: N
authors.initials: N
authors.lastname: Qureshi
authors.affiliation: Laboratoire Léon Brillouin (LLB), C.E.A./C.N.R.S., F-91191 Gif-sur-Yvette Cedex, France.
authors.forename: Y
authors.initials: Y
authors.lastname: Sidis
authors.affiliation: Aoyama Gakuin University, Sagamihara 252-5258, Japan.
authors.forename: J
authors.initials: J
authors.lastname: Akimitsu
authors.affiliation: II. Physikalisches Institut, Universität zu Köln,50937 Cologne, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Braden