Persistent low-energy phonon broadening near the charge order $q$-vector in bilayer cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$

TL;DR

This study reveals persistent low-energy phonon broadening near a specific wavevector in Bi-2212 cuprates, occurring regardless of charge density wave presence, suggesting a possible precursor to CDW formation.

Contribution

It demonstrates that phonon broadening exists both inside and outside the CDW phase, challenging the notion that it is exclusive to CDW states in cuprates.

Findings

Phonon broadening observed at q_B ~ 0.28 r.l.u. in Bi-2212.

Broadening persists across doping levels, independent of CDW presence.

Excludes CDW as the cause of broadening in heavily overdoped samples.

Abstract

We report a persistent low-energy phonon broadening around $q_{B} \sim 0.28$ r.l.u. along the Cu-O bond direction in the high-$T_c$ cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212). We show that such broadening exists both inside and outside the conventional charge density wave (CDW) phase, via temperature dependent measurements in both underdoped and heavily overdoped samples. Combining inelastic hard x-ray scattering, diffuse scattering, angle-resolved photoemission spectroscopy, and resonant soft x-ray scattering at the Cu $L_3$-edge, we exclude the presence of a CDW in the heavily overdoped Bi-2212 similar to that observed in the underdoped systems. Finally, we discuss the origin of such anisotropic low-energy phonon broadening, and its potential precursory role to the CDW phase in the underdoped region.