The nature of the magnetic ground state giving rise to the hourglass-shaped magnetic spectrum common among high-temperature superconductors is a matter of debate.
© H. Rønnow (EPFL)
A collaboration between EPFL and the University of Oxford have detected the presence of stripe charge order accompanied by quasi-one-dimensional antiferromagnetic order in La
5/3Sr 1/3CoO 4, providing a natural explanation for this characteristic spectrum.
Stripe phases are a form of complex matter involving coupled spin-and-charge order, observed in certain copper oxide superconductors. Stripe fluctuations have been proposed to be important to the mechanism of superconductivity. A key piece of evidence for this idea is the universal form of the magnetic spectrum of hole-doped copper oxide superconductors, which is in the shape of an hourglass. This type of spectrum emerges naturally from a stripe-ordered ground state. The magnetic ground state of La
5/3Sr 1/3CoO 4 has been controversial since it was also found to display an hourglass dispersion spectrum. In the work published in Nature Communications, scientists at EPFL and the University of Oxford have used polarized neutron diffraction at the Institut Laue–Langevin to show that Co 2+ and Co 3+ order themselves in unidirectional charge- and spin-stripes.
At the time of the initial discovery of the hourglass magnetic spectrum in insulating cobalt oxide La
5/3Sr 1/3CoO 4, there was indirect evidence for stripe order in this material, and the result was significant because it provided an experimental demonstration of how an hourglass spectrum could arise from a stripe-ordered ground state. However, direct evidence for the charge-stripe order has been missing until now. The apparent absence of stripes led to an alternative, stripe-free, model to explain the hourglass spectrum based on a nanoscopic phase separated regions of checkerboard and Néel magnetic order.
In this study, Peter Babkevich from Henrik Rønnow’s lab at EPFL, working with colleagues at the Universities of Oxford and Central Lancashire, have detected a signature of charge stripes in La
5/3Sr 1/3CoO 4. The results show that the magnetic ground state is more complicated than initially thought, containing macroscopic regions of spin and charge-stripe order, as well as charge-checkerboard order.
However, the measurements reinforce the stripe-model as the underlying mechanism for the hourglass magnetic spectrum in the cobaltates. The present results provide an experimental basis for theories that assume a ground state with static or slowly fluctuating stripes in order to explain the hourglass spectrum in cuprates.