Enhanced charge gap in the bilayer manganite La2-2xSr1+2xMn2O7 near x=0.4

We have investigated the optical conductivity spectra of La2-2xSr1+2xMn2O7 (0.3≤x≤0.5) systematically and found that for x near 0.4 the charge gap shows enhancement of up to ∼0.3eV just above the long-range magnetic ordering temperature. From comparison of this charge gap with other experimental results, we conclude that the peculiar x dependence of the charge gap cannot be understood in terms of the charge and lattice correlations only. We suggest that the unusual enhancement originates from the cooperative coupling between the short-range charge or lattice correlations and the quasi-one-dimensional energy band near the Fermi level.