Variation of the Electronic Structure in Systematically Synthesized ${\mathrm{Sr}}_{2}M{\mathrm{O}}_4$ ($M=\mathrm{Ti}$, 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 $E\perp c$ and $E\parallel c$, 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.

Figure 1

Reciprocal-space mapping of the (a) (310) and (b) (303) x-ray diffraction peaks for a ${\mathrm{Sr}}_{2}M{\mathrm{O}}_4$ film grown on ${\mathrm{LaSrAlO}}_4$ (LSAO) (100) substrate. Crosses denote the corresponding peak positions for the bulk compounds [2, 3, 4, 5]. (c) Definition of each axis.

Figure 2

Optical conductivity spectra $\sigma (\omega )$ in single-crystal thin films of ${\mathrm{Sr}}_{2}M{\mathrm{O}}_4$ ($M=\mathrm{Ti}$, V, Cr, Mn, and Co) with $E\perp c$ (solid lines) and $E\parallel c$ (broken lines) measured at 7 K, except for $M=\mathrm{Ti}$ measured at room temperature. Dotted straight lines represent the extrapolation to deduce the optical gap values.

Figure 3

Temperature dependence of the in-$M{\mathrm{O}}_2$-plane ($E\perp c$) optical conductivity spectra for (a) ${\mathrm{Sr}}_2{\mathrm{VO}}_4$ and (b) ${\mathrm{Sr}}_2{\mathrm{CrO}}_4$. Inset shows the possible initial and final states of $t_{2g}$ orbitals for the optical transitions on the neighboring $M$ ($M=\mathrm{V},\mathrm{Cr}$) sites within the $M{\mathrm{O}}_2$ plane.

Figure 4

CT type and Mott-Hubbard (MH) type optical gap against the $3d$ transition-metal elements in ${\mathrm{Sr}}_{2}M{\mathrm{O}}_4$ ($M=\mathrm{Ti}$, V, Cr, Mn and Co) thin films. Solid and broken lines are merely a guide to the eye. The data for $M=\mathrm{Fe}$ (indicated by a shaded bar) is taken from Ref. 19.