Excess specific heat from the gapped sliding phonon modes in the incommensurate composite crystal Sr14Cu24O41

Abstract

We show that the low-temperature specific heat ( C p ) of the incommensurate chain-ladder system { CuO 2 } α { Sr 2 Cu 2 O 3 } = Sr 14 Cu 24 O 41 (for α = 10 / 7 ) shows a rather large excess contribution of nonmagnetic origin. Diluted Al doping at the Cu site or annealing the crystal in an O 2 atmosphere suppresses this feature considerably. Using THz time-domain spectroscopy, we relate the occurrence of excess specific heat to the presence of very low-energy ( ∼ 1 meV ) gapped phonon modes that originate from the sliding motion of oppositely charged { CuO 2 } chain and { Sr 2 Cu 2 O 3 } ladder sheets past each other.