Low-energy collective dynamics of charge stripes in the doped nickelateLa2−xSrxNiO4+δobserved with optical conductivity measurements

Abstract

We have investigated charge dynamics in the static stripe ordered phase of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{4+\ensuremath{\delta}}$ at lattice temperatures below the charge ordering transition, via optical conductivity measurements at low energies $(1--10\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$. 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.