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Intriguing lattice dynamics has been predicted for aperiodic crystals thatcontain incommensurate substructures. Here we report inelastic neutronscattering measurements of phonon and magnon dispersions in Sr14Cu24O41, whichcontains incommensurate one-dimensional (1D) chain and two-dimensional (2D)ladder substructures. Two distinct pseudoacoustic phonon modes, correspondingto the sliding motion of one sublattice against the other, are observed foratomic motions polarized along the incommensurate axis. In the long wavelengthlimit, it is found that the sliding mode shows a remarkably small energy gap of1.7-1.9 meV, indicating very weak interactions between the two incommensuratesublattices. The measurements also reveal a gapped and steep linear magnondispersion of the ladder sublattice. The high group velocity of this magnonbranch and weak coupling with acoustic and pseudoacoustic phonons can explainthe large magnon thermal conductivity in Sr14Cu24O41 crystals. In addition, themagnon specific heat is determined from the measured total specific heat andphonon density of states, and exhibits a Schottky anomaly due to gapped magnonmodes of the spin chains. These findings offer new insights into the phonon andmagnon dynamics and thermal transport properties of incommensurate magneticcrystals that contain low-dimensional substructures.

Weak coupling of pseudoacoustic phonons and magnon dynamics in incommensurate spin ladder compound Sr14Cu24O41