# Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials

## TL;DR

This study reveals sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials, uncovering ultrafast, non-thermal pathways that could enhance memory technology performance.

## Contribution

It introduces a novel ultrafast x-ray technique to observe atomic-scale dynamics during phase change, revealing reversible nanoscale modifications driven by electronic states.

## Key findings

- Reversible lattice modifications occur within 4 ps without heating.
- Identification of a long-lived metastable state with muted interatomic interactions.
- Discovery of non-thermal pathways enabling faster, more efficient phase switching.

## Abstract

Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Femtosecond laser excitation and an ultrashort x-ray probe is used to show the temporal separation of electronic and thermal effects in a long-lived ($>$100 ps) transient metastable state of Ge$_{2}$Sb$_{2}$Te$_{5}$ with muted interatomic interaction induced by a weakening of resonant bonding. Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect allows the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit, and improved energy efficiency and reliability of phase-change memory technologies.