Infrared optical excitations in $\mathrmLa_2$$\mathrmNiO_4$
Measurements are reported of the transmission of single crystals of La2NiO4 containing no excess oxygen. Several weak absorption bands are observed in the range 0.15–2 eV arising from magnetic and crystal-field excitations. The lowest-energy band, at 0.25 eV, is quantitatively described by a recent theory for phonon-assisted creation of a virtual bound state of two magnons. However, unlike La2CuO4, no higher-energy sidebands are detected, suggesting that the broad, structured absorption seen clearly in La2CuO4 between ∼0.4 and 1 eV results from sidebands to crystal-field excitons as previously proposed.