Origins of the hourglass dispersion in layered cobaltates

Abstract

The interplay between uni-directional charge and spin stripe correlations in copper oxide-based high-temperature superconductors is a matter of long-standing debate . While static magnetic stripe order has been shown to compete with superconductivity, fluctuating stripes have been argued to play an important role in the pairing mechanism of unconventional superconductors. By probing the magnetic correlations of hole-doped layered copper oxide superconductors using inelastic neutron scattering, a universal spectrum has been resolved whose shape in momentum-energy resembles an hourglass. Its origin or even if the system should be considered in the local-moment or itinerant framework has been the subject of debate for some time.

I shall present our research into the insulating, layered cobalt oxide La_2-xSr_xCoO₄—an isostructural analogue of the copper oxide superconductors. Our measurements have revealed that it also has a characteristic hourglass dispersion that we could explain using a parameter-free model based on a stripe-ordered magnetic ground state. However, direct evidence for stripe-order was missing and led to some studies to propose a stripe-free scenario to explain the dispersion. I shall present our polarised neutron diffraction results to examine the magnetic order in La_2-xSr_xCoO₄ and discuss its implications on the role of stripes forming an hourglass spectrum.