Long-wavelength phonon dynamics in incommensurate Bi2Sr2CaCu2O(8+delta) crystals by Brillouin light scattering spectroscopy

TL;DR

This study used Brillouin light scattering spectroscopy to investigate phonon dynamics in Bi2Sr2CaCu2O(8+delta) crystals, revealing multiple acoustic modes and insights into their incommensurate structure and sublattice interactions.

Contribution

It provides the first detailed characterization of phonon modes in incommensurate Bi2Sr2CaCu2O(8+delta) using Brillouin spectroscopy, linking phonon behavior to crystal structure.

Findings

Eight distinct acoustic modes identified

Existence of a ~95 GHz optic-like phonon peak

Determined sublattice mass ratio and phonon crossover frequency

Abstract

Room temperature phonon dynamics in crystals of the high-Tc superconductor Bi2Sr2CaCu2O(8+delta) were probed using Brillouin light scattering spectroscopy. Eight distinct acoustic modes were observed and identified, including two quasi-longitudinal bulk modes and four quasi-transverse bulk modes. A peak at a frequency shift of ~95 GHz with behaviour reminiscent of an optic phonon was also observed in the spectra. The existence and nature of these modes is a manifestation of the incommensurate structure of Bi2Sr2CaCu2O(8+delta) and suggests that it may be categorized as a so-called composite incommensurate crystal comprised of two weakly interacting sublattices. A mass ratio of m1/m2=0.36 obtained from the two measured quasi-longitudinal acoustic velocities led to sublattice assignments of Bi2Sr2O4 and CaCu2O4. The Brillouin data also places an upper limit of ~10 GHz on the crossover frequency between commensurate and incommensurate phonon dynamics.