Low temperature dynamic polaron liquid in a manganite exhibiting colossal magnetoresistance

TL;DR

This study uses resonant inelastic x-ray scattering to investigate how electron-lattice interactions evolve in a manganite during its transition from insulator to metal, revealing a dynamic polaron liquid state at low temperatures.

Contribution

It provides direct experimental evidence of a shift from static to dynamic lattice distortions, unveiling a novel low-temperature polaronic state in a colossal magnetoresistive material.

Findings

Spectral weight shifts from harmonic phonon emissions to a broad high-energy continuum upon cooling.

Identification of a dynamic polaron liquid state in the metallic phase.

Correlation between electron-lattice coupling evolution and the metal-insulator transition.

Abstract

Polarons - fermionic charge carriers bearing a strong companion lattice deformation - exhibit a natural tendency for self-localization due to the recursive interaction between electrons and the lattice. While polarons are ubiquitous in insulators, how they evolve in transitions to metallic and superconducting states in quantum materials remains an open question. Here, we use resonant inelastic x-ray scattering (RIXS) to track the electron-lattice coupling in the colossal magneto-resistive bi-layer manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ across its metal-to-insulator transition. The response in the insulating high-temperature state features harmonic emissions of a dispersionless oxygen phonon at small energy transfer. Upon cooling into the metallic state, we observe a drastic redistribution of spectral weight from the region of these harmonic emissions to a broad high energy continuum. In concert with theoretical calculations, we show that this evolution implies a shift in electron-lattice coupling from static to dynamic lattice distortions that leads to a distinct polaronic ground state in the low temperature metallic phase - a dynamic polaron liquid.