Long-wavelength phonon dynamics in incommensurate Bi2Sr2CaCu2O8+δ crystals by Bril

Influence of incommensurate structure on the elastic constants of crystalline Bi2Sr2CaCu2O8+δ by Brillouin light scattering spectroscopy

“…The incommensurate structure is not manifested in the surface modes for which only the R and LR peaks were observed. Moreover, the surface mode velocities V R are nearly independent of direction of propagation in the ab-crystallographic plane, while the bulk mode velocities showed little dependence on propagation direction over the range probed (as measured by angle from the crystallographic c-axis) [8]. These velocities are presented in table 1.…”

“…Figure 1 shows representative spectra for each of the three samples, with additional spectra being presented in figure 1 of [8]. In total, Brillouin peaks due to six distinct bulk acoustic phonon modes and two surface acoustic modes were observed.…”

“…In total, Brillouin peaks due to six distinct bulk acoustic phonon modes and two surface acoustic modes were observed. These peaks have previously been assigned to four quasi-transverse sublattice modes (QT i , i = 1 − 4), two quasi-longitudinal sublattice modes (QL 1 and QL 2 ), the Rayleigh surface mode (R) and the longitudinal surface resonance (LR), the number and character of the bulk modes reflecting the incommensurate structure of Bi-2212 [8]. The incommensurate structure is not manifested in the surface modes for which only the R and LR peaks were observed.…”

“…Some elastic constants, however, remain unmeasured even under ambient conditions at room temperature and the values of others uncertain due to factors such as the nature of approximations employed, sub-optimal crystal quality, and, in the case of Brillouin studies, extremely weak signals and the apparent lack of a reliable refractive index value from which to determine bulk acoustic phonon velocities from spectral peak frequency shifts [6,7]. Moreover, recent studies have uncovered interesting new acoustic phonon physics associated with the incommensurate structure of Bi-2212 [8][9][10][11], the investigation and understanding of which is in its infancy. In particular, Brillouin [8] and neutron scattering [9] studies revealed acoustic modes in excess of those expected for a typical commensurate crystal, calling into question mode assignments and consequently, elastic constants, reported in previous studies in which the incommensurability of Bi-2212 was not considered.…”

“…Moreover, recent studies have uncovered interesting new acoustic phonon physics associated with the incommensurate structure of Bi-2212 [8][9][10][11], the investigation and understanding of which is in its infancy. In particular, Brillouin [8] and neutron scattering [9] studies revealed acoustic modes in excess of those expected for a typical commensurate crystal, calling into question mode assignments and consequently, elastic constants, reported in previous studies in which the incommensurability of Bi-2212 was not considered. Furthermore, while both of these studies find that Bi-2212 is better classed as a composite rather than a modulated incommensurate crystal, they differ in constituent sublattice assignments, the former suggesting Bi 2 Sr 2 O 4 and CaCu 2 O 4 [8] and the latter, Bi 2 SrO 2+δ and SrCaCu 2 O 6 [9].…”