Spatio-temporal Probing of a Photoexcited Jointly Commensurate Charge Density Wave

Commensuration, the condition in which the ratio between two physical quantities is a rational number, can give rise to various exotic phenomena that affect both electrons and the underlying lattice in quantum materials. This concept is particularly important in charge density waves (CDWs), which are periodic modulations of electron density resulting from the interplay between electronic and lattice degrees of freedom. Beyond the conventional incommensurate and commensurate CDWs, a new type called a jointly commensurate CDW (JC-CDW) was discovered in the layered material EuTe₄, where multiple coexisting incommensurate CDWs collectively exhibit commensurability. However, the collective response of the individual CDWs that form a JC-CDW to external perturbations, such as light irradiation, remains unknown. In this work, we address this challenge by employing a combination of time-resolved techniques to directly probe the dynamics of CDWs in EuTe₄ upon photoexcitation. Time-resolved X-ray diffraction is used to simultaneously probe both the amplitude and phase dynamics of the JC-CDW in a time domain, while complementary ultrafast electron diffraction further reveals that the real-space shape of the topological defects seeded in the photoexcited JC-CDW is a shear. These findings not only establish the unique persistence of JC-CDWs out of equilibrium but also present a new platform for the optical manipulation of topological defects.