Dual CDW dynamics unveiled in quantum material via ultrafast pump-probe imaging

Physicists at MIT have directly observed, in real time, how two distinct charge density waves (CDWs) re-form inside erbium tritelluride following intentional disruption — a breakthrough that reveals fundamentally different dynamics between coexisting quantum electronic phases.

Two coexisting electronic patterns in one material

Erbium tritelluride hosts two charge density waves that emerge at different temperatures. The dominant CDW appears when the material is cooled to approximately −8°C and extends primarily along one spatial direction. A second CDW forms only at much lower temperatures — around −113°C — and aligns perpendicular to the first. Together, they produce an electronic checkerboard: two ordered electron patterns coexisting within the same atomic lattice.

Under normal conditions, electrons in erbium tritelluride are distributed uniformly. Upon cooling, however, large ensembles of electrons coordinate to form repeating wave-like structures — regions of higher and lower electron concentration — known as charge density waves. This collective behavior constitutes an ordered electronic state, not governed by individual particle motion but by emergent, large-scale organization.

The coexistence of two such CDWs makes erbium tritelluride especially valuable for probing how complex quantum phases interact. For decades, scientists have sought to determine how the second phase emerges: whether it appears simultaneously across the entire sample, grows incrementally from isolated nucleation sites, or spreads continuously from boundaries. Until now, no experimental method had resolved this question.

Disrupting order to study recovery

To track the formation process, the MIT team cooled atomically thin samples of erbium tritelluride to roughly −230°C — a temperature low enough to stabilize both CDWs simultaneously. They then applied a pump-probe laser technique. A first ultrashort laser pulse acted as a controlled “shake,” temporarily disrupting the electronic checkerboard pattern. After a precisely timed delay, a second probe pulse ejected electrons from the material.

By measuring the energy and momentum of those emitted electrons, the researchers reconstructed time-resolved snapshots of the internal electronic structure as order recovered. As physicist Nuh Gedik explained, the method involves disturbing the system and then “listening” to its response — effectively converting electron emission data into a dynamic map of how electronic coherence rebuilds.

Divergent pathways of phase recovery

The experiment revealed starkly contrasting recovery behaviors. The dominant CDW returned smoothly and uniformly across the entire sample, even after strong perturbation. Its reassembly followed a continuous, gradual progression — characteristic of a second-order phase transition, analogous to the progressive loss of magnetic order upon heating.

In contrast, the subdominant CDW did not re-form uniformly. Instead, it appeared first in isolated, localized regions and expanded outward over time. This nucleation-and-growth pattern indicates a fundamentally different thermodynamic and kinetic pathway — one not previously resolved in real time for coexisting quantum electronic phases.

The findings were published in Nature Physics and offer new empirical constraints for theoretical models of quantum phase competition, including those relevant to superconductivity and magnetism in correlated electron systems.