C-axis lattice dynamics in Bi-based cuprate superconductors

TL;DR

This study combines experimental spectral ellipsometry and theoretical shell model calculations to analyze the c-axis lattice phonon modes in various Bi-based cuprate superconductors, providing mode assignments and explaining superconductivity-induced phonon anomalies.

Contribution

It offers a unified shell model approach for different Bi-based cuprates, accurately assigning phonon modes and explaining observed anomalies in their lattice dynamics.

Findings

Good agreement between calculated and experimental phonon frequencies.

Reliable phonon mode assignments for Bi2201, Bi2212, and Bi2223.

Explanation of superconductivity-induced phonon anomalies.

Abstract

We present results of a systematic study of the c axis lattice dynamics in single layer Bi2Sr2CuO6 (Bi2201), bilayer Bi2Sr2CaCu2O8 (Bi2212) and trilayer Bi2Sr2Ca2Cu3O10 (Bi2223) cuprate superconductors. Our study is based on both experimental data obtained by spectral ellipsometry on single crystals and theoretical calculations. The calculations are carried out within the framework of a classical shell model, which includes long-range Coulomb interactions and short-range interactions of the Buckingham form in a system of polarizable ions. Using the same set of the shell model parameters for Bi2201, Bi2212 and Bi2223, we calculate the frequencies of the Brillouin-zone center phonon modes of A2u symmetry and suggest the phonon mode eigenvector patterns. We achieve good agreement between the calculated A2u eigenfrequencies and the experimental values of the c axis TO phonon frequencies which allows us to make a reliable phonon mode assignment for all three Bi-based cuprate superconductors. We also present the results of our shell model calculations for the Gamma-point A1g symmetry modes in Bi2201, Bi2212 and Bi2223 and suggest an assignment that is based on the published experimental Raman spectra. The superconductivity-induced phonon anomalies recently observed in the c axis infrared and resonant Raman scattering spectra in trilayer Bi2223 are consistently explained with the suggested assignment.