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

TL;DR

This study investigates phonon and magnon behaviors in Sr14Cu24O41, revealing weak coupling between incommensurate sublattices and high magnon thermal conductivity, providing insights into low-dimensional incommensurate magnetic crystals.

Contribution

It provides the first detailed inelastic neutron scattering analysis of phonon and magnon dispersions in Sr14Cu24O41, highlighting weak inter-sublattice coupling and its impact on thermal transport.

Findings

Observation of two pseudoacoustic phonon modes with small energy gap

Identification of a steep, high-velocity magnon branch

Weak coupling explains high magnon thermal conductivity

Abstract

Intriguing lattice dynamics has been predicted for aperiodic crystals that contain incommensurate substructures. Here we report inelastic neutron scattering measurements of phonon and magnon dispersions in Sr14Cu24O41, which contains incommensurate one-dimensional (1D) chain and two-dimensional (2D) ladder substructures. Two distinct pseudoacoustic phonon modes, corresponding to the sliding motion of one sublattice against the other, are observed for atomic motions polarized along the incommensurate axis. In the long wavelength limit, it is found that the sliding mode shows a remarkably small energy gap of 1.7-1.9 meV, indicating very weak interactions between the two incommensurate sublattices. The measurements also reveal a gapped and steep linear magnon dispersion of the ladder sublattice. The high group velocity of this magnon branch and weak coupling with acoustic and pseudoacoustic phonons can explain the large magnon thermal conductivity in Sr14Cu24O41 crystals. In addition, the magnon specific heat is determined from the measured total specific heat and phonon density of states, and exhibits a Schottky anomaly due to gapped magnon modes of the spin chains. These findings offer new insights into the phonon and magnon dynamics and thermal transport properties of incommensurate magnetic crystals that contain low-dimensional substructures.