Excess speicific heat of the gapped sliding phonons in the incommensurate composite crystal Sr$_{14}$Cu$_{24}$O$_{41}$

TL;DR

This study reveals that in Sr$_{14}$Cu$_{24}$O$_{41}$, excess low-temperature specific heat arises from low-energy gapped phonon modes caused by the sliding of incommensurate chain and ladder layers, modifiable by doping and annealing.

Contribution

It demonstrates the link between excess specific heat and low-energy gapped phonons in an incommensurate crystal, highlighting the role of sliding modes in such systems.

Findings

Excess specific heat is suppressed by Al doping and oxygen annealing.

Low-energy gapped phonon modes (~1 meV) are associated with sliding layers.

Sliding phonons originate from incommensurate chain-ladder structure.

Abstract

We show that low temperature specific heat (C$_p$) of the incommensurate chain-ladder system Sr$_{14}$Cu$_{24}$O$_{41}$ is enriched by the presence of a rather large excess contribution of non-magnetic origin. Diluted Al doping at the Cu site or annealing the crystal in an O$_2$ atmosphere suppresses this feature considerably. Using the THz time-domain spectroscopy, we show that the occurrence of excess specific heat is associated with the presence of very low-energy ($\sim$ 1 meV) gapped phonon modes that originate due to the sliding motion of oppositely charged mutually incommensurate chain and ladder layers.