In-plane conduction and $c$-axis polarization in the misfit-layered oxide ${\left[{\text{Bi}}_2{\text{Ca}}_2{\text{O}}_4\right]}_q{\text{CoO}}_2$

We have used low-frequency dielectric measurements to study the evolution of anisotropic charge transport in misfit-layered oxides ${\left[{\text{Bi}}_2{\text{Ca}}_2{\text{O}}_4\right]}_q{\text{CoO}}_2$. Above $\sim 60\text{K}$, the in-plane dc conductivity obeys the Arrhenius law and the dielectric polarization occurs along the $c$ direction. We point out that this $c$-axis polarization is attributed to the activated carrier which is confined to $ab$ plane.

Figure 1

X-ray (0,0,$l$) diffraction patterns measured using Cu $K\alpha$ radiation.

Figure 2

(a) Temperature dependence of ${\rho }_{ab}$. The same data are plotted against $1/T$ in (b) and $T^{-1/4}$ in (c). (d) Temperature dependence of ${\rho }_c$. The ratio of ${\rho }_c/{\rho }_{ab}$ is plotted in (e).

Figure 3

(Color online) (a) Frequency dependence of the dielectric constant ${ϵ}_c^{\prime }$ and (b) the corresponding loss $\left({ϵ}_c^{″}\right)$ spectra. $\omega =2\pi f$ is the angular frequency. The applied ac field is $20{\text{V}}_{\text{rms}}/\text{mm}$.

Figure 4

(Color online) ${ϵ}_c^{\prime }$ spectra taken under the voltage gradient of (a) 20 ${\text{V}}_{\text{rms}}/\text{mm}$ and (b) 200 ${\text{V}}_{\text{rms}}/\text{mm}$.

Figure 5

(Color online) The dielectric and conducting properties in ${\left[{\text{Bi}}_{2+0.52}{\text{Ca}}_{2-0.52}{\text{O}}_4\right]}_{0.627}{\text{CoO}}_2$. $T^{\ast }$ is the crossover temperature. ${\sigma }_c$ is obtained from ${\rho }_c$. ${\sigma }_{c,\omega =0}$ is estimated by extrapolation of ${ϵ}_c^{″}$ spectrum to zero frequency.