Giant Dielectric Response in the One-Dimensional Charge-Ordered Semiconductor $({\mathrm{NbSe}}_4{)}_3\mathrm{I}$

We report on broadband dielectric spectroscopy on the one-dimensional semiconductor $({\mathrm{NbSe}}_4{)}_3\mathrm{I}$. Below the structural phase transition close to 270 K, which is accompanied by complex charge-order processes, we observe colossal dielectric constants with a frequency and temperature dependence very similar to what is observed in charge-density wave systems.

Figure 1

(a) ${\epsilon }^{\prime }(\tau )$ and (b) ${\sigma }^{\prime }(\tau )$ for various frequencies. The line in (a) represents the static susceptibility $\Delta \epsilon$ as determined from an analysis of the data in Fig. 2. The upper inset shows $\Delta \epsilon (\tau )$ obtained for three samples with different geometry and contact materials. The line in (b) is the dc conductivity measured by a standard 4-point technique. The lower inset shows the relaxation time (circles; from an analysis of the frequency-dependent spectra, Fig. 2) and the dc resistivity [solid line: 4-point measurement; squares: from ${\sigma }^{\prime }(\nu )$] in an Arrhenius representation. The dashed lines demonstrate thermally activated behavior.

Figure 2

(a) ${\epsilon }^{\prime }(\nu )$ and (b) ${\epsilon }^{\prime \prime }(\nu )$ for various temperatures. The lines are fits with the empirical Cole-Cole function.

Figure 3

Schematic drawing of the chain structure of $({\mathrm{NbSe}}_4{)}_3\mathrm{I}$ above [(a) and (b)] and below (c) the structural phase transition temperature $T_c=270\mathrm{K}$. In (a), the twist between adjacent ${\mathrm{Se}}_4$ rectangles is omitted. In (b) and (c), only the Nb ions are shown. In the low-temperature phase, the two chains are shifted with respect to each other by approximately 0.4 Å. This is schematically indicated by the arrows.