Collective density wave excitations in two-leg Sr_{14-x}Ca_xCu_{24}O_{41} ladders

TL;DR

This study uses Raman spectroscopy to investigate collective density wave excitations in Sr_{14-x}Ca_xCu_{24}O_{41} ladders, revealing a softening quasielastic peak indicative of pinned density waves influencing transport properties.

Contribution

It provides the first direct Raman evidence of collective density wave excitations in two-leg ladder compounds, linking these excitations to transport behavior.

Findings

Observation of a softening quasielastic peak with cooling

Identification of pinned density wave excitations

Connection between collective modes and metallic transport

Abstract

Raman measurements in the 1.5 - 20 cm-1 energy range were performed on single crystals of Sr_{14-x}Ca_xCu_{24}O_{41}. A quasielastic scattering peak (QEP) which softens with cooling is observed in the polarization parallel to the ladder direction for samples with x = 0, 8 and 12. The QEP is a Raman fingerprint of pinned collective density wave excitations screened by uncondensed carriers. Our results suggest that transport in metallic samples, which is similar to transport in underdoped high-T_c cuprates, is driven by a collective electronic response.