Variation of the Electronic Structure in Systematically SynthesizedSr2MO4(M=Ti, V, Cr, Mn, and Co)

We have systematically synthesized single-crystalline thin films of layered perovskites ${\mathrm{Sr}}_{2}M{\mathrm{O}}_{4}$ ($M=\mathrm{Ti}$, V, Cr, Mn, and Co) which cannot be obtained in a form of bulk crystal apart from $M=\mathrm{Mn}$. The two-dimensional electronic structure of these ${M}^{4+}$ oxides, ranging from a correlated insulator to a ferromagnetic metal, has been investigated by using their optical conductivity spectra with polarizations $E\ensuremath{\perp}c$ and $E\ensuremath{\parallel}c$, which reveal systematic variation of the correlated charge gap, Mott-Hubbard gap, or charge-transfer gap. Temperature dependence of the gap-transition spectra is argued in the light of possible spin and/or orbital ordering.