Alfred Zong

Excitons—bound states of electrons and holes—can spontaneously condense into a quantum coherent ground state. 1 T -TiSe ₂ , a layered transition metal dichalcogenide, is a candidate for realizing this phase in a bulk material, but its putative excitonic condensation is accompanied by charge-density-wave formation that obscures signatures of excitonic instability and fluctuations in this structural transition. Here we clarify the mechanism of exciton condensation in quasi-low-dimensional systems by tracking exciton dissociation as a function of photoexcitation fluence and temperature using few-femtosecond broadband extreme-ultraviolet absorption spectroscopy. We find that short-range excitonic fluctuations emerge before long-range-order formation in the ground state. We also observe enhanced excitonic susceptibility, identified by faster exciton dissociation near the phase transition temperature, consistent with excitonic instability in this material. These results provide a framework for understanding other excitonic insulator candidates proposed in quasi-one- or quasi-two-dimensional crystals.

We present a table-top cryogenic ultrafast broadband extreme-ultraviolet absorption spectroscopy beamline designed for temperature-dependent and time-resolved investigations of quantum materials. The instrument combines a broadband high-harmonic generation source spanning 22–73 eV with automated image registration and cryogenic sample control down to 20 K, enabling element-specific measurements under both equilibrium and nonequilibrium conditions. The beamline provides sub-50-meV energy resolution and a routine 10-fs instrument response function, while maintaining long-term stability suitable for extended, ultrasensitive measurements. Benchmark experiments on NiI2, a van der Waals multiferroic, reveal temperature-dependent spectral evolution across its magnetic phase transitions, demonstrating the ability to identify element-specific contributions to the ground state and its transient response in the femtosecond regime. This instrument introduces the opportunity to unravel the very first response of a solid-state material during light–matter interaction, paving the way for understanding the complex phase space of nonequilibrium dynamics and emergent states in strongly correlated systems where the few-femtosecond electronic response has eluded most other types of time-resolved techniques.

The advent of two-dimensional moiré systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moiré superstructure formed by two coexisting charge density waves with slightly mismatched wavevectors has been realized. These incommensurate charge density waves can collectively exhibit commensurability, forming a jointly commensurate charge density wave that provides a paradigm for controlling moiré potential and periodicity. Here, we use time-resolved diffraction to probe the light-induced dynamics of the jointly commensurate charge density wave in EuTe ₄ . Our measurements distinguish the instantaneous quenching of its amplitude, as verified by time-resolved photoemission spectroscopy, from the slower evolution of phase fluctuations. Moreover, while the wavevector remains locked upon photoexcitation, the correlation length decreases exclusively perpendicular to the wavevector, revealing the formation of shear-type defects. Together, our multimodal approach reconstructs the temporal evolution of the jointly commensurate charge density wave, highlighting its robustness out of equilibrium and providing insights into the optical manipulation of moiré materials through defect control.

A number of intriguing phenomena, including exciton condensation, orbital ordering, and emergence of chirality, have been proposed to accompany charge-density-wave (CDW) formation in the layered transition metal dichalcogenide 1T-TiSe_{2}. Explaining these effects relies on knowledge of the atomic displacement pattern underlying the CDW, yet structural proposals based on spatially averaging bulk crystal diffraction and surface-dependent scanning tunneling microscopy have remained inconsistent. Here, we revisit the CDW superlattice structure of 1T-TiSe_{2} with selected-area electron diffraction, a bulk-sensitive probe capable of capturing submicrometer spatial variations while maintaining high momentum resolution. We observed superlattice peak extinction rules that are incompatible with previously reported atomic displacement patterns, and we found two categories of CDW phases characterized by distinct interlayer orderings. Our analysis identifies a set of possible superlattice structures consistent with the extinction rules, all featuring a one-dimensional displacement pattern for each atomic type and possessing a large number of nearly degenerate CDW domains. These findings shed light on the longstanding discrepancy in the low-temperature space group of 1T-TiSe_{2} and provide a new basis for understanding the gyrotropic electronic order and metastability in this material. Our results further underscore the importance of bulk-sensitive mesoscopic techniques in investigating materials that host unconventional superlattices.

The deterministic control of quantum phases via ultrafast light-matter interaction lies at the frontier of modern physics. While light can trigger hidden phases inaccessible through equilibrium pathways, these photo-induced metastable states are typically transient or highly volatile, limiting their practical utility. Here, we demonstrate a hybrid optical-thermal strategy that utilizes photo-induced topological defects as kinetic gateways to unlock a robust, hidden charge density wave (CDW) phase in the van der Waals antiferroelectric (AFE) In2Se3. Using complementary ultrafast electron diffraction and microscopy, we show that an initial optical quench induces a metastable state characterized by long-lived topological defects via the Kibble-Zurek mechanism; upon thermal cooling, these defects act as heterogeneous nucleation seeds, lowering the kinetic barrier for the system to evolve thermodynamically into a hidden CDW state. Remarkably, this phase transition is fully reversible: a single laser pulse resets the system to a defect-engineered intermediate phase, preserving the topological landscape for subsequent switching. Our results establish a paradigm for using photo-induced topological defects to imprint kinetic pathways, providing a blueprint for the discovery of hidden quantum landscapes and the development of high-speed, non-volatile memory technologies.

Photoexcitation by ultrashort laser pulses plays a crucial role in controlling reaction pathways, creating nonequilibrium material properties, and probing complex molecular dynamics. The photoresponse following a laser pulse is generally nonidentical between exposures due to spatiotemporal fluctuations or the stochastic nature of dynamical pathways. However, most ultrafast pump-probe experiments struggle to distinguish intrinsic sample fluctuations from extrinsic apparatus noise, often missing deviations from the averaged response. Leveraging the stability and high photon flux of time-resolved X-ray micro-diffraction at a synchrotron, we characterized stochastic photoinduced dynamics in a solid-state ionic conductor. By analyzing temporal evolutions of the lattice parameter of a single grain, we found that shot-to-shot fluctuations are not independent. Instead, correlations exist between nonequilibrium lattice trajectories following adjacent shots, with a characteristic correlation length of approximately 1500 shots, corresponding to an energy barrier of 0.4 ± 0.1 eV, close to the activation energy of lithium-ion diffusion.