Strong interlayer magnetic exchange coupling in La3Ni2O7-δ revealed by inelastic neutron scattering

Recently, an inelastic neutron scattering work on the bilayer nickelate La₃Ni₂O_7-δ polycrystal finished by a team from Sun Yat-sen University was published in Science Bulletin. The team employed neutron spectroscopy to study the magnetic ground state and spin dynamics of La₃Ni₂O_7-δ at ambient pressure.

The neutron diffraction results show that there is no magnetic order down to 10 K. In the inelastic channel, they observed some weak spin excitations, which contain low-energy spin excitations at several millielectronvolts and almost non-dispersive high-energy spin excitations around 45 meV. These results can be explained to be from strong interlayer and weak intralayer magnetic couplings of stripe-type antiferromagnetic orders. The corresponding interlayer and intralayer magnetic couplings could be around 60 meV and 3~4 meV, respectively.

These observations are quite different from that in cuprate and iron-based superconductors, which have dominant intralayer exchange couplings. To date, there are still a lot of debates about the high-T_c mechanism in nickelates, but the consensus of most existed theories is that interlayer magnetic couplings play a key role. The super exchanges between two adjacent nickel layers are mediated by the apical oxygens, whose vacancies can break the interlayer couplings directly, and further break the pressure-induced high-T_c superconductivity.

Thus, this work not only studies the spin excitations of La₃Ni₂O_7-δ at ambient pressure, determines the unique magnetic couplings in bilayer nickelates, but also provides crucial experimental evidences for understanding the high-T_c mechanism and the role of apical oxygens.