Low-energy collective dynamics of charge stripes in the doped nickelate La2-xSrxNiO4+δ observed with optical conductivity measurements

We have investigated charge dynamics in the static stripe ordered phase of La2-xSrxNiO4+delta 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.