Figure 1

(Color online) Broad-band spectra of conductivity and complex dielectric function (real ${\epsilon }^{\prime }$ and imaginary ${\epsilon }^{″}$ parts) of ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ along the $a$ axis for Ca contents $x=0$, 3, and 9 at a few selected temperatures. Strong $T$-dependent relaxationlike dispersion of ${\epsilon }^{\prime }$ and ${\epsilon }^{″}$ (insets, the full lines are from fits to the generalized Debye expression, see text), seen also as a smooth increase in the conductivity spectra, is a fingerprint of the screened CDW collective response. This response is observed at all $T<T_c$ for $x=0$ and 3, but not for $x=9$. Decrease of the infrared conductivity at low $T$ corresponds to the opening of an energy gap. At the lowest $T$ only the lowest-frequency phonon tail is seen and represented with the dash-dot ${\nu }^2$ line. The arrows denote the dc conductivity. Dotted lines are guides to the eye.Reuse & Permissions

Figure 2

Representative Cole-Cole plots of the dielectric dispersion, connected to the anisotropic collective CDW response of ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ for $x=0$ (left panel) and $x=3$ (right panel) with the $E\Vert c$ (along legs) and $E\Vert a$ (along rungs). Note that plots for the response along the $a$ axis are blown up 10 $(x=0)$ and 5 $(x=3)$ times. Full lines are from fits to the generalized Debye expression. The intersections of the arcs with ${\epsilon }^{\prime }-{\epsilon }_{\mathrm{HF}}$ axes indicate the values of $\Delta \epsilon$.Reuse & Permissions

Figure 3

(Color online) Upper and middle panels: dc resistivity logarithmic derivatives of ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ for different Ca contents $x$ along the $c$ and $a$ axes; $x=8$ is multiplied by 2, for clarity. Full lines are guides for the eye in the transition range, while above and below they are based on the fits to $\ln \rho$ vs $T^{-1}$. The arrows indicate the CDW phase transition temperature $T_c$. Lower panel: dc resistivity (inset) and corresponding logarithmic derivatives of ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ along $c$ (dashed), $a$ (solid), and $b$ (dash-dot line) axes. The peak in $d\left(\ln \rho \right)∕d\left(T^{-1}\right)$ as a signature of the phase transition is observed in the $(c,a)$ ladder plane, but not perpendicular to them (along the $b$ axis).Reuse & Permissions