Direct observation of ultrafast amorphous-amorphous transitions indicated by bond stretching and angle bending in phase-change material GeTe

The intrinsic nature of glass states and glass transitions at the atomic scale remain a fundamental open question in condensed-matter physics and materials science. By combining femtosecond electron diffraction with time-dependent density-functional theory molecular dynamics simulations, we directly observe ultrafast amorphous-amorphous transitions in amorphous GeTe, manifested as rapid Ge-Te (Ge) bond stretching within 0.2 ps and subsequent angle bending of the Ge-Te (Ge)-Ge motif on a 0.5-2 ps timescale. Critically, the ultrafast bond stretching is accompanied by localized oscillation modes with the frequency of 3.10 THz, unambiguously signaling the local Peierls-like bonding structure and the flexibility of these polarized bonds. These ultrafast collective atomic motions provide a direct structural origin for the boson peak and pay the way for systematic optimization of relaxation and crystallization kinetics.

💡 Research Summary

This paper presents the first direct observation of ultrafast amorphous‑amorphous (AA) transitions in the phase‑change material GeTe by combining femtosecond electron diffraction (FED) with time‑dependent density‑functional theory molecular dynamics (TD‑DFT‑MD). The authors prepared 20 nm‑thick amorphous GeTe films, transferred them onto TEM grids, and performed pump‑probe experiments at a base temperature of 35 K using 800 nm, sub‑100 fs laser pulses (2.0–3.0 mJ cm⁻²). The FED system, with ~50 fs temporal resolution and picometer spatial sensitivity, recorded time‑resolved diffraction patterns that were subsequently converted into radial‑averaged intensity curves and pair‑distribution functions (PDFs).

Two distinct structural responses were identified. Within the first 0.2 ps after photoexcitation, the Ge–Te (Ge) bond length expands rapidly, manifested as a decrease in diffraction intensity in the 2.3–2.6 Å region and a concomitant increase in the 2.8–3.5 Å region. Simultaneously, a coherent oscillation at ~3.10 THz appears in both reciprocal and real space, matching the frequency of the Peierls‑distortion‑suppression mode previously observed in crystalline GeTe. This localized mode directly evidences a Peierls‑like, highly polarizable bonding motif persisting in the amorphous network.

Following the initial bond stretching, a slower evolution occurs over 0.5–2 ps. The PDF shows a decay of intensity around 4.0 Å (primarily Ge–Ge distances) and a sign reversal near 5.4 Å, indicating a redistribution of interatomic distances. The authors interpret this as an angle‑bending motion of the Ge–Te (Ge)–Ge three‑atom unit, i.e., a collective many‑body relaxation that moves the local structure toward a deeper basin of the potential‑energy surface (PES). The ΔPDF maps reveal that the bond‑length distribution broadens from ~2.3 to 3.5 Å, reflecting a quasi‑continuous set of shallow minima spanning ~1.2 Å. This landscape generates spatial fluctuations of force constants, providing a natural explanation for the excess vibrational density of states that constitutes the boson peak in glasses.

TD‑DFT‑MD simulations reproduce both the ultrafast bond expansion and the subsequent angle‑bending, attributing the initial response to electronic delocalization: photo‑excited electrons reduce the covalent character of Ge–Te bonds, leading to Coulomb‑driven nuclear motion. The later relaxation is driven by multi‑atom correlations that reshape the local network. Importantly, the bond‑stretching step also reduces the population of defective Ge–Ge “wrong” bonds, effectively preparing the system for nucleation. The authors propose that a double‑pulse excitation scheme could exploit this incubation period to accelerate crystallization, potentially pushing the speed limit of phase‑change memory devices.

Overall, the study demonstrates that FED can resolve sub‑picometer, sub‑picosecond structural dynamics in disordered solids, bridging the gap between static structural models (continuous random network, tetrahedral vs. octahedral motifs) and dynamic phenomena such as the boson peak, β‑relaxation, and crystallization kinetics. By directly visualizing the traversal of multiple local minima on the PES, the work provides a unified, atomistic picture of how electronic excitation reshapes the amorphous network, offering new pathways for material design in non‑volatile memory and neuromorphic computing.