Collapse of Jahn-Teller Phonons in La$_{1-x}$Sr$_{x}$MnO$_3$ with Weak Magnetoresistance

TL;DR

This study reveals that in La$_{1-x}$Sr$_{x}$MnO$_3$, Jahn-Teller phonons collapse above the Curie temperature due to giant electron-phonon coupling, which correlates with magnetoresistance through diffusion dynamics rather than EPC strength.

Contribution

The paper demonstrates that Jahn-Teller phonons collapse above the Curie temperature in certain manganites, highlighting the role of giant electron-phonon coupling in charge and orbital dynamics.

Findings

Jahn-Teller-active oxygen vibrations collapse above Curie temperature

Neutron scattering agrees with DFT predictions for phonons and magnons

Magnetoresistance correlates with diffusion rate, not EPC strength

Abstract

Perovskite manganites are quantum materials exhibiting competing interactions inducing colossal magnetoresistance (CMR). The prevailing theory of CMR highlights the essential role of electron-phonon coupling (EPC), but mounting evidence suggests the underlying mechanism is more complicated. Here, we investigate phonons and spin-phonon coupling in ferromagnetic CMR manganites La$_{1-x}$Sr$_x$MnO$_3$ ($x$=0.2,0.3) with relatively small CMR associated with melting of the magnetic order above room temperature. High-resolution neutron scattering experiments combined with density functional theory (DFT) show that the low-temperature ferromagnetic phase is conventional: neutron scattering from phonons agrees with DFT predictions and magnons follow sinusoidal dispersions. Fluctuating magnetic moments and low-energy phonons remain conventional in the high temperature paramagnetic phase, indicating the Mn and La/Sr sublattices are not strongly perturbed by melting of ferromagnetism. In contrast, the Jahn-Teller-active optical oxygen vibrations collapse entirely above the Curie temperature, despite low CMR in these compositions, with some of the lost spectral weight reappearing as quasielastic scattering. We attribute this highly anomalous behavior to giant EPC in the charge and/or orbital channel. It drives cooperative diffusive motion of quasistatic carrier-trapping oxygen sublattice distortions once ferromagnetism disappears. We hypothesize the magnitude of magnetoresistance correlates with the rate of diffusion rather than with the strength of Jahn-Teller EPC.