Electronic and vibrational properties of the low-dimensional perovskites ${\mathrm{Sr}}_{1-y}{\mathrm{La}}_y\mathrm{Nb}{\mathrm{O}}_{3.5-x}$

By angle-resolved photoemission spectroscopy and polarization-dependent infrared reflectivity measurements the electronic and vibrational properties of the low-dimensional perovskites ${\mathrm{Sr}}_{1-y}{\mathrm{La}}_y\mathrm{Nb}{\mathrm{O}}_{3.5-x}$ were studied along different crystal directions. The electronic behavior strongly depends on the oxygen and La content, including quasi-one-dimensional metallic and ferroelectric insulating behavior. An extremely small energy gap at the Fermi level is found for $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.41}$ and $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.45}$ along the conducting direction at low temperature, suggestive for a Peierls-type instability. Despite the similar nominal carrier density, for ${\mathrm{Sr}}_{0.8}{\mathrm{La}}_{0.2}\mathrm{Nb}{\mathrm{O}}_{3.50}$ the quasi-one-dimensional metallic character is suppressed and no gap opening is observed, which can be explained by differences in the crystal structure. Electron-phonon interaction appears to play an important role in this series of compounds.

Figure 1

Schematic drawing of the idealized, i.e., nondistorted, crystal structures of three compounds of the series ${\mathrm{Sr}}_n{\mathrm{Nb}}_n{\mathrm{O}}_{3n+2}$. The $\mathrm{Nb}{\mathrm{O}}_6$ units in the middle of the slabs, which presumably are the conducting channels of the lattice, are marked by thick lines.

Figure 2

ARPES spectra of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.41}$, normalized to the same total intensity, along (a) $\overline{\Gamma }-\overline{X}$ and (b) $\overline{\Gamma }-\overline{Y}$ at $75\mathrm{K}$ with $\Delta E=30\mathrm{meV}$.

Figure 3

Reflectivity $R$ of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.41}$ at different temperatures for $\mathbf{E}\Vert a$ and $\mathbf{E}\Vert b$, respectively. Inset: Low-frequency reflectivity for $\mathbf{E}\Vert a$; open (closed) circles: submm reflectivity data at $300\mathrm{K}$ $\left(5\mathrm{K}\right)$.

Figure 4

Optical conductivity ${\sigma }_1$ of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.41}$ at different temperatures for $\mathbf{E}\Vert a$ and $\mathbf{E}\Vert b$, respectively. For $\mathbf{E}\Vert a$ the low-temperature peak around $40{\mathrm{cm}}^{-1}$ indicates excitations across a gap $2\Delta \approx 5\mathrm{meV}$. Also shown are the dc conductivity values for the $a$ axis at $300\mathrm{K}$ (filled triangle), $200\mathrm{K}$ (open square), $100\mathrm{K}$ (filled square), $50\mathrm{K}$ (open circle), and $5\mathrm{K}$ (filled circle).

Figure 5

Dielectric constant ${ϵ}_1$ of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.41}$ and $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.45}$ at $300\mathrm{K}$ (thick lines) and $5\mathrm{K}$ (thin lines) for $\mathbf{E}\Vert a$ and $\mathbf{E}\Vert b$, respectively; open (closed) circles: data points at $300\mathrm{K}$ $\left(5\mathrm{K}\right)$ directly determined by transmission measurements on a thin platelet.

Figure 6

Optical conductivity ${\sigma }_1$ of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.50}$ at 300 and $5\mathrm{K}$ for $\mathbf{E}\Vert a$ and $\mathbf{E}\Vert b$, respectively. Insets: Reflectivity $R$ used for Kramers-Kronig analysis.

Figure 7

Optical conductivity ${\sigma }_1$ of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.45}$ at different temperatures for $\mathbf{E}\Vert a$ and $\mathbf{E}\Vert b$, respectively; closed circles: data points directly determined by transmission measurements on a thin platelet. For $\mathbf{E}\Vert a$ the low-temperature peak in ${\sigma }_1$ with its maximum around $60{\mathrm{cm}}^{-1}$ indicates excitations across a gap $2\Delta \approx 7\mathrm{meV}$. Insets: Reflectivity $R$ used for Kramers-Kronig analysis; open (closed) circles: submm reflectivity data at $300\mathrm{K}$ $\left(5\mathrm{K}\right)$.

Figure 8

ARPES spectra of ${\mathrm{Sr}}_{0.8}{\mathrm{La}}_{0.2}\mathrm{Nb}{\mathrm{O}}_{3.50}$ along (a) $\overline{\Gamma }-\overline{X}$ and (b) $\overline{\Gamma }-\overline{Y}$ at $75\mathrm{K}$ with $\Delta E=30\mathrm{meV}$.

Figure 9

Optical conductivity ${\sigma }_1$ of ${\mathrm{Sr}}_{0.8}{\mathrm{La}}_{0.2}\mathrm{Nb}{\mathrm{O}}_{3.50}$ at different temperatures for $\mathbf{E}\Vert a$ and $\mathbf{E}\Vert b$, respectively. Also shown are the dc conductivity values for the $a$ axis at $300\mathrm{K}$ (filled triangle), $200\mathrm{K}$ (open square), $100\mathrm{K}$ (filled square), $50\mathrm{K}$ (open circle), and $5\mathrm{K}$ (filled circle). Insets: Reflectivity $R$ used for Kramers-Kronig analysis; open (closed) circles: submm reflectivity data at $300\mathrm{K}$ $\left(5\mathrm{K}\right)$.

Figure 10

Optical conductivity ${\sigma }_1$ of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.50}$, $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.45}$, and $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.41}$ at RT for $\mathbf{E}\Vert a$ and $\mathbf{E}\Vert b$, respectively. The asterisks mark the phonon lines of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.50}$ for $\mathbf{E}\Vert a$ which survive in the conducting compounds. The arrow indicates the ferroelectric soft mode of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.50}$.

Figure 11

Optical conductivity ${\sigma }_1$ of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.41}$, $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.45}$, and ${\mathrm{Sr}}_{0.8}{\mathrm{La}}_{0.2}\mathrm{Nb}{\mathrm{O}}_{3.50}$ at $5\mathrm{K}$ for $\mathbf{E}\Vert a$.

Figure 12

Optical conductivity ${\sigma }_1$ of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.50}$ and ${\mathrm{Sr}}_{0.8}{\mathrm{La}}_{0.2}\mathrm{Nb}{\mathrm{O}}_{3.50}$ at RT for $\mathbf{E}\Vert a$ and $\mathbf{E}\Vert b$, respectively. The asterisks mark the phonon lines of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.50}$ for $\mathbf{E}\Vert a$ which survive in the conducting compound. The arrow indicates the ferroelectric soft mode of $\mathrm{Sr}\mathrm{Nb}{\mathrm{O}}_{3.50}$.