Professor Stephen Blundell

Abstract:

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCa₂Fe₄As₄F₂ above and below its superconducting transition temperature T _c ≈ 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 Fe₂As₂ separated by insulating CaF₂ 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 _R of the observed spin resonance is broadly consistent with the relationship E _R = 4.9k _B T _c, 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 _R with that deduced from the total superconducting gap value integrated over the Fermi surface.

Abstract:

© 2020 American Physical Society. We performed nuclear magnetic resonance (NMR) and muon spin relaxation (μSR) experiments to identify the magnetic ground state of the frustrated quantum A-site spinel, CuAl2O4. Our results verify that the ground state does not exhibit a long-range magnetic ordering, but a glasslike transition manifests at T∗=2.3K. However, the Gaussian shape and the weak longitudinal field dependence of μSR spectra below T∗ show that the ground state has dynamic spin fluctuations, distinct from those of conventional spin glasses.