Andrew Boothroyd

Abstract:

We have investigated charge dynamics in the static stripe ordered phase of La2-x Srx Ni O4+δ at lattice temperatures below the charge ordering transition, via optical conductivity measurements at low energies (1-10 meV). The thermally activated dynamic response of the charge stripes is found to be characteristic of a collective mode such as a pinned charge density wave. At incommensurate doping levels, the pinning energy is reduced, owing to the presence of real-space defects in the stripe order, and a pronounced increase in the oscillator strength is seen. The results provide compelling evidence for the existence of low-energy collective modes of the charge stripes. © 2008 The American Physical Society.

Abstract:

Polarized- and unpolarized-neutron scattering was used to study the temperature evolution of the magnetic excitations of charge ordered La5/3Sr1/3NiO4. We studied two features in detail: (i) the resonance-like scattering at 27 meV in the quasi-two-dimensional magnetic excitations from the ordered Ni2+ (S = 1) spins, and (ii) the diffuse scattering associated with quasi-one-dimensional antiferromagnetic correlations along the stripes. Although both these features persist up to at least similar to 125 K, their temperature dependences are found to be quite different. We argue from the results that the resonance- like feature observed in La2-xSrxNiO4 (x similar to 1/3) is not caused by a coupling between the spin dynamics of the two magnetic sub-systems.