Evidence for the weakly coupled electron mechanism in an Anderson-Blount polar metal

TL;DR

This study provides experimental evidence for the weakly coupled electron mechanism in a polar metal, LiOsO3, showing that electrons can be decoupled from certain phonons during a polar phase transition, supporting theoretical predictions.

Contribution

First experimental observation of the decoupled electron mechanism in a polar metal, confirming theoretical models of electron-phonon decoupling in such systems.

Findings

Electrons selectively couple to a subset of phonons during relaxation.

Up to 60% of the lattice heat capacity is decoupled from electronic excitations.

Evidence of a partially displacive transverse optical polar mode in LiOsO3.

Abstract

Over 50 years ago, Anderson and Blount proposed that ferroelectric-like structural phase transitions may occur in metals, despite the expected screening of the Coulomb interactions that often drive polar transitions. Recently, theoretical treatments have suggested that such transitions require the itinerant electrons be decoupled from the soft transverse optical phonons responsible for polar order. However, this decoupled electron mechanism (DEM) has yet to be experimentally observed. Here we utilize ultrafast spectroscopy to uncover evidence of the DEM in LiOsO$_3$, the first known band metal to undergo a thermally driven polar phase transition ($T_c$ =140 K). We demonstrate that intra-band photo-carriers relax by selectively coupling to only a subset of the phonon spectrum, leaving as much as 60 % of the lattice heat capacity decoupled. This decoupled heat capacity is shown to be consistent with a previously undetected and partially displacive TO polar mode, indicating the DEM in LiOsO$_3$.