Extremely Small Energy Gap in the Quasi-One-Dimensional Conducting Chain Compound ${\mathrm{S}\mathrm{r}\mathrm{N}\mathrm{b}\mathrm{O}}_{3.41}$

Resistivity, optical, and angle-resolved photoemission experiments reveal unusual one-dimensional electronic properties of highly anisotropic ${\mathrm{S}\mathrm{r}\mathrm{N}\mathrm{b}\mathrm{O}}_{3.41}$. Along the conducting chain direction, we find an extremely small energy gap of only a few meV at the Fermi level. A discussion in terms of typical 1D instabilities (Peierls, Mott-Hubbard) shows that neither seems to provide a satisfactory explanation for the unique properties of ${\mathrm{S}\mathrm{r}\mathrm{N}\mathrm{b}\mathrm{O}}_{3.41}$.

Figure 1

(a) Projection along the $b$ axis of the ${\mathrm{S}\mathrm{r}\mathrm{N}\mathrm{b}\mathrm{O}}_{3.4}$ crystal structure; Nb atoms are hidden within the ${\mathrm{N}\mathrm{b}\mathrm{O}}_6$ octahedra (light grey). White (grey) circles: Sr (O) atoms. (b) dc resistivity $\rho$ versus temperature of ${\mathrm{S}\mathrm{r}\mathrm{N}\mathrm{b}\mathrm{O}}_{3.41}$ along the three crystal axes, with the respective measurement geometry. In the range 20–40 K (grey bar), one finds an activated behavior $\rho \propto \exp \{E_a/k_{B}T\}$ with $E_a$ as indicated.

Figure 2

Frequency dependent conductivity of ${\mathrm{S}\mathrm{r}\mathrm{N}\mathrm{b}\mathrm{O}}_{3.41}$ for different temperatures and the polarization E set parallel to the $a$ and $b$ axes (insets: reflectivity $R$ used for Kramers-Kronig analysis). For $\mathbf{E}||a$, the low-temperature peak around $40{\mathrm{c}\mathrm{m}}^{-1}$ indicates excitations across a gap $2\Delta \approx 5\mathrm{m}\mathrm{e}\mathrm{V}$.

Figure 3

ARPES spectra of ${\mathrm{S}\mathrm{r}\mathrm{N}\mathrm{b}\mathrm{O}}_{3.41}$ along $\overline{\Gamma }\mathrm{\text{−}}\overline{X}$ near $k_F$ at $T=25\mathrm{K}$. The angle $\Delta \theta$ is the emission angle with respect to $k_F$ ($\Delta \theta \equiv 0°$). (a) Spectra between the two “crossing points” ($k_F$, $k_F^{*}$) of the strongly dispersing band; peak positions are marked by dots. (b) Spectra around $k_F$ in $0.25°$ steps. A gap $\Delta \approx 5\mathrm{m}\mathrm{e}\mathrm{V}$ is seen when compared to the Fermi cutoff of a gold film (thin solid line). (c) Intensity plot of spectra, taken between $k_F$ and $k_F^{*}$ (step size $0.5°$) and normalized to constant total intensity; a weak, dispersing shadow band can be identified.