Phonon-assisted formation of an itinerant electronic density wave

TL;DR

This study demonstrates that phonons play a crucial role in the formation of spin density waves by dynamically pinning the order parameter, revealing a non-thermal formation pathway influenced by lattice vibrations.

Contribution

It provides experimental evidence that phonons influence the formation path of SDWs, showing a direct phonon-assisted transition bypassing thermal states.

Findings

SDW forms with low-temperature period within 500 fs after quench

Phonons dynamically pin the SDW order parameter

SDW formation bypasses thermal high-temperature state

Abstract

Electronic instabilities drive ordering transitions in condensed matter. Despite many advances in the microscopic understanding of the ordered states, a more nuanced and profound question often remains unanswered: how do the collective excitations influence the electronic order formation? Here, we experimentally show that a phonon affects the spin density wave (SDW) formation after an SDW-quench by femtosecond laser pulses. In a thin film, the temperature-dependent SDW period is quantized, allowing us to track the out-of-equilibrium formation path of the SDW precisely. By exploiting its persistent coupling to the lattice, we probe the SDW through the transient lattice distortion, measured by femtosecond X-ray diffraction. We find that within 500 femtoseconds after a complete quench, the SDW forms with the low-temperature period, directly bypassing a thermal state with the high-temperature period. We argue that a momentum-matched phonon launched by the quench changes the formation path of the SDW through the dynamic pinning of the order parameter.