Pair breaking versus symmetry breaking: Origin of the Raman modes in superconducting cuprates

TL;DR

This study uses Raman spectroscopy on cuprate superconductors to analyze spectral features across doping levels, revealing universal behaviors and suggesting a symmetry-breaking transition related to superconductivity onset.

Contribution

It provides a systematic analysis of Raman spectral features in cuprates, identifying a possible symmetry-breaking transition at the onset of superconductivity.

Findings

Universal spectra in B2g symmetry correlate with Tc.

Anomalous intensity increase in B1g spectra suggests a symmetry-breaking transition.

Spectral features indicate a transition at doping level p_sc2 ≈ 0.27.

Abstract

We performed Raman experiments on superconducting ${\rm Bi_2 Sr_2 (Ca_{1-x} Y_x) Cu_2 O_{8+\delta}}$ (Bi-2212) and ${\rm YBa_{2} Cu_{3}O_{6+x}}$ (Y-123) single crystals. These results in combination with earlier ones enable us to analyze systematically the spectral features in the doping range $0.07 \le p \le 0.23$. In $B_{2g}$ ($xy$) symmetry we find universal spectra and the maximal gap energy $\Delta_0$ to follow the superconducting transition temperature $T_c$. The $B_{1g}$ ($x^2-y^2$) spectra in Bi-2212 show an anomalous increase of the intensity towards overdoping, indicating that the corresponding energy scale is neither related to the pairing energy nor to the pseudogap, but possibly stems from a symmetry breaking transition at the onset point of superconductivity at $p_{\rm sc2} \simeq 0.27$.