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

Tyler C. Sterling, A. T. Savici+5 more|arXiv (Cornell University)|Mar 5, 2026

Magnetic and transport properties of perovskites and related materials0 citations

TL;DR

The study shows a total collapse of Jahn-Teller active bond-stretching optical phonons above the Curie temperature in La1-xSrxMnO3 with x=0.2,0.3, while magnons remain conventional, challenging links between electron-phonon coupling and colossal magnetoresistance and suggesting diffusion of lattice distortions governs CMR.

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.

Motivation & Objective

• Investigate phonons and spin–phonon coupling in La1-xSr x MnO3 (x=0.2,0.3) with relatively small CMR.
• Determine whether Jahn–Teller active optical phonons collapse and how spectral weight redistributes above Tc.
• Rule out structural phase transitions, twinning, and spin-phonon coupling as causes of phonon anomalies.
• Assess how phonon behavior relates to magnetoresistance and diffusive lattice distortions.

Proposed method

• High-resolution inelastic neutron scattering (time-of-flight and triple-axis) to map phonons and magnons across the Brillouin zone.
• Density functional theory calculations (PBE+U, PAW) to predict lattice dynamics in orthorhombic and rhombohedral phases and compute inelastic neutron scattering intensities.
• Phonon calculations with Phonopy and euphonic to unfold to pseudocubic notation and compare with experiment.
• Linear spin-wave theory to model ferromagnetic magnons and extract exchange constants.
• Multizone fitting of phonon spectra to accurately track JT-active branches across temperatures.
• Assessment of spectral weight redistribution via sum rules and identification of quasielastic scattering components.

Experimental results

Research questions

• RQ1 Do Jahn–Teller active bond-stretching phonons collapse above Tc in La1-xSrxMnO3 with x=0.2,0.3?
• RQ2 Is there observable spin–phonon coupling or magnon–phonon hybridization contributing to phonon renormalization?
• RQ3 Are structural phase transitions or twinning responsible for the anomalous phonons?
• RQ4 How is spectral weight redistributed above Tc—into quasielastic scattering or elsewhere—and what does this imply about lattice dynamics and CMR?

Key findings

• Low-temperature ferromagnetic phase is conventional: phonons match DFT predictions and magnons follow sinusoidal dispersions.
• Above Tc, the Jahn–Teller–active bond-stretching optical phonon branch collapses across the entire Brillouin zone edge, with the missing weight reappearing as quasielastic scattering.
• DFT calculations show the phonon collapse is not caused by a structural phase transition or twinning; spin-phonon coupling is negligible.
• Quasielastic scattering increases above Tc, indicating diffusion of charge-trapping lattice distortions rather than coherent phonon modes.
• Magnon dispersions show no significant broadening or gaps due to spin–phonon coupling, consistent with weak spin-orbit interaction in manganites.
• The magnitude of magnetoresistance does not correlate straightforwardly with Jahn–Teller phonon renormalization; diffusion rates of distortions are proposed as controlling CMR rather than EPC strength.