Charge density waves in ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$: Electron correlations versus structural effects

We report inelastic light scattering experiments in the spin-chain and ladder system ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$. A depletion of an electronic background is observed for $0⩽x⩽5$ at low temperatures and with light polarization both parallel to the leg and rung directions. This points to the formation of two-dimensional charge density wave in the ladders. Furthermore, the Ca-substitution dependence of the 305 and $249{\mathrm{cm}}^{-1}$ modes, sensitive to the lattice modulations of the respective chain and ladder system, suggests that the charge dynamics in the ladders is strongly influenced by the structural incommensurability.

Figure 1

(Color online) Raman spectra of ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ in $\left(cc\right)$ and $\left(aa\right)$ polarizations at $10\mathrm{K}$ and $350\mathrm{K}$, respectively. A strong renormalization is observed in the electronic background for frequencies below $400{\mathrm{cm}}^{-1}$ at low temperatures.

Figure 2

(Color online) (left panel) Temperature dependence of the 305 and $249{\mathrm{cm}}^{-1}$ modes which correspond to vibrations of the Cu chain and ladder atoms, respectively. (right panel) Substitution dependence of the 305 and $249{\mathrm{cm}}^{-1}$ modes at $10\mathrm{K}$. All spectra were measured in $\left(aa\right)$ polarization.

Figure 3

(Color online) (a) Temperature and substitution dependence of the peak position of the $312{\mathrm{cm}}^{-1}$ mode. The solid lines represent a fitting by Eq. (1). The characteristic temperature, $T^{*}$, is indicated by an arrow. (b) $T^{*}$ as a function of Ca content. (c) The onset frequency of the pseudogap as a function of Ca content.

Figure 4

(Color online) Low temperature Raman spectra of Ca substituted ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ with $0⩽x⩽12$ for intraladder $\left(cc\right)$ polarization. The dashed lines correspond to a reference point with respect to the unrenormalized electronic continuum of each spectrum and the arrows mark the onset of spectral weight depletion, i.e., opening of the pseudo gap.