Publications associated with Quantum Matter in High Magnetic Fields

Observation of a neutron spin resonance in the bilayered superconductor CsCa<sub>2</sub>Fe<sub>4</sub>As<sub>4</sub>F<sub>2</sub>.

Journal of physics. Condensed matter : an Institute of Physics journal 32 (2020) 435603-

DT Adroja, SJ Blundell, F Lang, H Luo, Z-C Wang, G-H Cao

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCa<sub>2</sub>Fe<sub>4</sub>As<sub>4</sub>F<sub>2</sub> above and below its superconducting transition temperature T <sub>c</sub> ≈ 28.9 K to investigate the presence of a neutron spin resonance. This compound crystallises in a body-centred tetragonal lattice containing asymmetric double layers of Fe<sub>2</sub>As<sub>2</sub> separated by insulating CaF<sub>2</sub> layers and is known to be highly anisotropic. Our INS study clearly reveals the presence of a neutron spin resonance that exhibits higher intensity at lower momentum transfer (Q) at 5 K compared to 54 K, at an energy of 15 meV. The energy E <sub>R</sub> of the observed spin resonance is broadly consistent with the relationship E <sub>R</sub> = 4.9k <sub>B</sub> T <sub>c</sub>, but is slightly enhanced compared to the values observed in other Fe-based superconductors. We discuss the nature of the electron pairing symmetry by comparing the value of E <sub>R</sub> with that deduced from the total superconducting gap value integrated over the Fermi surface.

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