Phononic gaps in the charged incommensurate planes of Sr14Cu24O41

The terahertz (THz) excitations in the quantum spin-ladder system Sr14Cu24O41 have been determined along the c-axis using THz time-domain, Raman and infrared spectroscopy. Low-frequency infrared and Raman active modes are observed above and below the charge-ordering temperature T_{co} ~ 200 K over a narrow interval of 1 - 2 meV . A new infrared mode around 1 meV develops below ~ 100 K. The temperature dependence of these modes shows that they are coupled to the charge- and spin-density-wave correlations in this system. These low-energy features are conjectured to originate in the gapped sliding-motion of the chain and ladder sub-systems, which are both incommensurate and charged.