Dependence of optical conductivity with δ in La2NiO4+δ single crystals

Optical conductivity and electrical resistivity of La2NiO4+δ (δ=0.11 and 0.14) single crystals have been investigated for the (a,b) plane. Results show several changes of regime that are connected with the onset of spin and charge order, static or dynamic. A surprising contrast of data for both samples, in optical and electrical conductivity, is observed at lowest temperatures. In particular, a pronounced spin glass behavior is observed for δ=0.11. In the δ=0.14 sample, the progressive melting of the charge order scales with the temperature dependence of the oscillator strength of a “forbidden” mode related to folding to the Brillouin zone center of a mode for a wavevector q=(0.28,0.28,0) associated with the modulation of stripes.