Resonant two-phonon Raman scattering as a probe of hole crystal formation in Sr14-x Cax Cu24 O41

The charge dynamics of the spin ladder compound Sr14-x Cax Cu24 O41 with x=0,6,13.6 has been investigated using wavelength- and temperature-dependent phonon Raman scattering on single crystals. In the unsubstituted, x=0, compound, a set of sharp two-phonon lines shows a strong increase in intensity below T∼200 K for light polarized along the ladder layer and with excitation energy close to the charge transfer gap ∼1.9 eV. The temperature dependence of the strongly enhanced two-phonon bands below 200 K closely follows the recently reported formation of a standing charge density wave in the ladders [P. Abbamonte, Nature 431, 1078 (2004)]. Upon calcium substitution the polarized resonant Raman response rapidly decreases, signaling an increase of hole mobility in the ladder units. Temperature-dependent measurements of the x=13.6 sample indicate mobility of holes down to <8 K.