Stripe conductivity in La1.775Sr.0225NiO4
The problem of whether or not stripes in cuprates and nickilates are insulating or metallic is fundamental to the physics of the stripe state. One can expect two possible scenarios which could lead to conductivity in the stripe state. According to the first one the stripes themselves are insulating but the system can be metallic due to fluctuations and motion of stripes. Alternatively, metallic conductivity may exist along the charge threads without a violation of stripe ordering as a whole. In the latter case, Coulomb interactions between neighboring stripes should lead to charge-density-wave order along the stripes at sufficiently low temperatures and in the absence of stripe fluctuations. We have reported Raman light-scattering and optical conductivity (Fig. 2.5) measurements on a single crystal of the title compound, which exhibits incommensurate charge-stripe order. The extra phonon peaks induced by stripe order can be understood in terms of the energies of phonons that occur at the charge-order wave vector. A strong antiresonance for a Ni-O bond-stretching mode provides clear evidence for finite dynamical conductivity within the charge stripes at energies well below the pseudogap, which is presumably associated with charge motion transverse to the stripes. The continued existence of the antiresonance at room temperature, well above the charge ordering temperature Tc, suggests that stripe correlation do not disappear above Tc. Such a result should indicate the continuing importance of strong correlations. The occurrence of stripe correlations without static order has significant implications for understanding the cuprates.
(This work was part of an international collaboration within the INTAS program of the European Community).