Signature of checkerboard fluctuations in the phonon spectra of a possible polaronic metal La1.2Sr1.8Mn2O7

TL;DR

This study reveals that polarons persist as fluctuations within the metallic phase of La1.2Sr1.8Mn2O7, affecting phonon spectra and suggesting a link to competing charge-ordered phases, relevant to understanding pseudogaps in cuprates.

Contribution

It demonstrates that polaronic fluctuations are present in the metallic phase, evidenced by phonon softening and broadening near charge order wave vectors, indicating a complex interplay between metallicity and charge order.

Findings

Polaronic fluctuations persist in the metallic phase.

Phonons near charge order wave vectors are softened and broadened.

Polaronic signatures may originate from competing insulating phases.

Abstract

Charge carriers in low-doped semiconductors may distort the atomic lattice and trap themselves forming so-called small polarons. High carrier concentrations can lead to short range ordered polarons (large polarons) and even to long range charge and orbital order. Both systems should be insulating with a large electrical resistivity, which decreases with increasing temperature. However, photoemission measurements recently found a polaronic pseudogap, in a metallic phase of La2-2xSr1+2xMn2O7. This layered manganite is famous for colossal magnetoresistance (CMR) associated with a phase transition from this low-temperature metallic phase to a high temperature insulating phase. Broad charge order peaks due to large polarons observed by neutron and x-ray scattering in the insulating phase disappear when La2-2xSr1+2xMn2O7 becomes metallic. We report results of inelastic neutron scattering measurements showing that polarons remain inside the metallic phase as fluctuations that strongly broaden and soften certain phonons near the wave vectors where the charge order peaks appeared in the insulating phase. Our findings imply that polaronic signatures in metals may generally come from a competing insulating charge-ordered phase. It is highly relevant to cuprate superconductors with both a pseudogap, and a similar phonon effect associated with a competing stripe order.