Universal phonon softening in the pseudogap state of Tl2Ba2Can−1CunO2n+4+δ

Abstract

Exploring the origin of the pseudogap is important for the understanding of superconductivity in cuprates. Here we report a systematical experimental study on the phonon vibrational properties of ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{Ca}}_{n\ensuremath{-}1}{\mathrm{Cu}}_{n}{\mathrm{O}}_{2n+4+\ensuremath{\delta}}$ ($n=1,2,3$) single crystals based on the Raman scattering measurements over the temperature range from 10--300 K. The temperature evolution of the frequency and linewidth of the observed phonon modes in each member of this family does not follow the expected self-energy effect when entering the superconducting state. The anomalies are observed for the phonon modes involving the vibrations of the atoms in the Tl-O layer and the apical oxygen at the temperature around 150 K, which is higher above the superconducting transition. The phonon mode of the apical oxygen exhibits a pronounced universal softening behavior. From the comparison with the existing experimental data for various orders, we find that the observed starting temperature for the phonon softening corresponds to the onset opening temperature of the pseudogap. This finding indicates a large lattice effect in the pseudogap state and the non-negligible spin-phonon coupling for such a phonon softening.