In unconventional superconductors, the major challenge in mechanistic research is to reveal how electrons form into Cooper pairs and collectively condense into a superconducting state under both charge and spin interactions. Iron-based superconductors are similar to copper-oxide and heavy fermion-based superconductors, and they also exhibit strong spin fluctuations, which likely promote superconducting pairing by acting as the bosonic “pairing glue” . Such an argument is supported by a spin resonance mode with a universally linear peak energy scale with Tvs. However, it is still unclear whether in such multi-orbital systems the spin system can have preferred fluctuating directions that are coupled to the orbital degree of freedom.
Neutron scattering is a direct probe for measuring spin fluctuations in materials, and therefore a powerful tool in investigating the mechanisms of unconventional superconductivity. With spatial resolution in polarized neutron scattering, it will give us detailed information on spin-orbit coupling and spin anisotropy in iron-based superconductors.
So far, there are three confirmed magnetic orders in iron-pnictide superconductors: collinear band-like order with in-plane moments called band spin density wave (SSDW); colinear biaxial order with c-axis polarized moments, called charge spin density wave (CSDW); and non-collinear coplanar order with in-plane moments called spin vortex crystal (SVC) phase. Accumulating evidence suggests that spin resonance is preferentially polarized along the c-axis in the superconducting state coexisting with SSDW or CSDW orders.
Recently, Liu Chang et al. in the professors. The group of Luo Huiqian and Li Shiliang from the Institute of Physics of the Chinese Academy of Sciences (CAS), in collaboration with Bourges Philippe, Sidis Yvan from the University of Paris-Saclay, He Guanghong and Li Yuan from the University of Beijing and other colleagues, revealed the resonant spin mode and spin anisotropy in the SVC ordered superconductor CaK(Fe0.96Neither0.04)4Like4.
The researchers discovered two spin resonance modes with odd and even L symmetries with respect to the reduced distance within the Fe-As bilayer unit. Polarization analysis suggests that the odd mode is highly anisotropic, manifesting as a strong c-axis component and two weakly anisotropic in-plane components. Such c-axis spin-preferred excitations appear already in the SVC phase and continue even to the paramagnetic phase until the spin anisotropy finally disappears at high temperature.
These results provide the missing piece of the puzzle on the spin-orbit coupling effect in iron-pnictide superconductors, and suggest that c-axis magnetic excitations are universally preferred by presumably orbital-selective superconductor pairing.
Meanwhile, the shape of the magnetic order depends on material-specific symmetry characteristics in addition to spin-orbit coupling, leading to a rich variety of interactions between superconductivity and magnetism in iron-based superconductors.
This study was published in Physical examination letters.
Connecting two classes of unconventional superconductors
Chang Liu et al, Preferred spin excitations in the bilayer iron-based superconductor CaK(Fe0.96Neither0.04)4Like4 with the Spin-Vortex Crystal Order, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.128.137003
Quote: Polarized neutron scattering reveals spin-preferred excitations in an iron-based bilayer superconductor (April 6, 2022) Retrieved April 7, 2022 from https://phys.org/news/2022-04-polarized-neutron-reveals -bilayer-iron-based .html
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