Dao Xiang

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.

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.

Superionic conductors (SICs) exhibit liquidlike ionic diffusivities while maintaining a periodic crystalline lattice, making them promising candidates for applications in fuel cells, solid-state electrolytes, and thermoelectric materials. The transient local structures of mobile ions and their cooperative interactions with the host lattice are of pivotal importance regarding both the ion and heat conduction in SICs. However, accurately capturing the structural evolution of mobile ions remains a significant challenge due to the inherent complexities involved. Here, we employed ultrafast electron diffraction (UED) with femtosecond-temporal and angstrom-spatial resolution to resolve the structural evolution of mobile Ag + ions in AgCrSe 2 . Our experiments identify a critical process originating from localized Ag + vibrations at the long-range-ordered lattice sites to a formation of short-range-correlated Ag + trimer structures through a drastic contraction of the Ag + ─ Ag + bond from approximately 3.68 to approximately 3.00 Å in 1.97 ps. Combining real-time time-dependent density-functional-theory and molecular dynamics simulation, we further reveal the crucial role of these contracted trimer structures in enabling fast Ag + diffusion by opening up excess free volume and reducing local energy barriers. Such an intimate relation between fast diffusion and local lattice variations is not exclusive and can be extended beyond AgCrSe 2 to materials such as Li 10 X P 2 S 12 ( X = Si , Ge, Sn). The ability to track ion diffusion with UED also provides new avenues for exploring the atomistic mechanisms of fast ion diffusion in next-generation solid-state electrolytes, fuel cells, and ion transport membranes.

Inspired by noncollinear magnetic dipole order, noncollinear electric dipole order is anticipated to open up rich ferroelectric physics and new opportunities for device applications. However, establishing such order in single crystals remains challenging, as electric polarization is typically locked to crystallographic axes. Here we report noncollinear ferrielectricity in the van der Waals crystal WO₂Br₂, arising from competition between ferroelectric and antiferroelectric phonon modes. The noncollinear dipole order of WO₂Br₂ is revealed by decoupling the antipolar and the polar displacement components, both of which are directly visualized by scanning transmission electron microscopy. This noncollinear dipole order enables 90° polarization flip under hydrostatic pressure through two energetically degenerate transition pathways. Moreover, ultrafast electron diffraction measurements show that optical excitation drives two distinct coherent phonon modes associated with the ferroelectric and antiferroelectric orders. This work enriches the physics of noncollinear electric dipole order for emergent ferroelectric device applications.

Inspired by noncollinear magnetic dipole order, the noncollinear electric dipole order is anticipated to induce rich ferroelectric physics for fascinating device applications. However, establishing noncollinear electric dipole ordering in single crystals is challenging due to electric polarization locking to the crystallographic axes. Here, we report the noncollinear ferrielectricity in a van der Waals crystal WO2Br2 by introducing the competition of ferroelectric and antiferroelectric modes, and uncover its critical role on the polarization switching under hydrostatic pressure and coherent phonon dynamics. The noncollinear dipole order of WO2Br2 was revealed by decoupling the antipolar displacement component along the b-axis and the polar component along the c-axis, both of which were directly visualized by scanning transmission electron microscopy (STEM). Moreover, this noncollinear dipole order not only drives the robust in-plane uniaxial macroscopic ferroelectric behavior, but also enables 90° polarization flip under hydrostatic pressure with two energetically degenerate transition pathways: one via an antipolar intermediate phase, the other through a 45° polar phase respectively. Specially, ultrafast electron diffraction measurements confirm that optical excitation drives two distinct coherent phonon modes associated with the ferroelectric and antiferroelectric orders respectively, shedding light on ultrafast control of the exotic noncollinear ferroelectric states. Our work definitely unlocks the underlying physics of noncollinear ferroelectricity for emergent ferroelectric device applications.

A key challenge in ultrafast science has been to directly track the coupled motions of electrons and nuclei in real space and real time. This Letter presents a significant step toward this goal by demonstrating the feasibility of time-resolved real-space tracking of valence electron and hydrogen dynamics during the photodissociation of ammonia (NH_{3}) using MeV ultrafast electron diffraction. It is demonstrated that the enhanced temporal resolution, in conjunction with the analysis of the charge-pair distribution function, enables the disentanglement of the correlated motion of valence electrons and hydrogens in a photoexcited ammonia molecule. The methodology employed in this Letter, which utilizes the charge-pair distribution function from ultrafast electron scattering to retrieve intertwined electron and nucleus dynamics, may open up new opportunities in the study of quantum dynamics for a wide range of molecules.

The Ultrafast Transient Experimental Facility (UTEF) at Chongqing University is constructing a next-generation angle-resolved photoemission spectroscopy (ARPES) beamline designed to simultaneously achieve sub-meV energy resolution, continuous photon energy tunability (10–40 eV), high photon flux (>10 ¹² photons s ⁻¹ at the sample position), full polarization control, and ultra-low-temperature sample environments (13 photons s ⁻¹ . A dual-endstation layout enables flexible operation for both ultra-high-resolution measurements at ultra-low temperatures and large-angle, high-flux spin-resolved experiments. Through comprehensive optical optimization—including mirror coatings, customized grating groove profiles and precision focusing geometry—the system can deliver photon flux exceeding 10 ¹⁴ photons s ⁻¹ (0.1% bandwidth) ⁻¹ with a spatial beam spot size of approximately 30 µm, while maintaining sub-0.4 meV energy resolution. This work presents the optical design and projected performance of the UTEF ARPES beamline.

An excitonic insulator is a material that hosts an exotic ground state, where an energy gap opens due to spontaneous condensation of bound electron-hole pairs. Ta_{2}NiSe_{5} is a promising candidate for this type of material, but the coexistence of a structural phase transition with the gap opening has led to a long-standing debate regarding the origin of the insulating gap. Here we employ MeV ultrafast electron diffraction to obtain quantitative insights into the atomic displacements in Ta_{2}NiSe_{5} following photoexcitation, which has been overlooked in previous time-resolved spectroscopy studies. In conjunction with first-principles calculations using the measured atomic displacements, we find that the structural change can largely account for the photoinduced reduction in the energy gap without considering excitonic effects. Our Letter illustrates the importance of a quantitative reconstruction of individual atomic pathways during nonequilibrium phase transitions, paving the way for a mechanistic understanding of a diverse array of phase transitions in correlated materials where lattice dynamics can play a pivotal role.

An excitonic insulator is a material that hosts an exotic ground state, where an energy gap opens due to spontaneous condensation of bound electron-hole pairs. Ta$_2$NiSe$_5$ is a promising candidate for this type of material, but the coexistence of a structural phase transition with the gap opening has led to a long-standing debate regarding the origin of the insulating gap. Here we employ MeV ultrafast electron diffraction to obtain quantitative insights into the atomic displacements in Ta$_2$NiSe$_5$ following photoexcitation, which has been overlooked in previous time-resolved spectroscopy studies. In conjunction with first-principles calculations using the measured atomic displacements, we find that the structural change can largely account for the photoinduced reduction in the energy gap without considering excitonic effects. Our work illustrates the importance of a quantitative reconstruction of individual atomic pathways during nonequilibrium phase transitions, paving the way for a mechanistic understanding of a diverse array of phase transitions in correlated materials where lattice dynamics can play a pivotal role.

Conical intersections play a pivotal role in excited-state quantum dynamics. Capturing transient molecular structures near conical intersections remains challenging due to the rapid timescales and subtle structural changes involved. We overcome this by combining the enhanced temporal resolution of mega-electron-volt ultrafast electron diffraction with a super-resolution real-space inversion algorithm, enabling visualization of nuclear and electronic motions at conical intersections with sub-angstrom resolution, surpassing the diffraction limit. We apply this technique to the textbook example of the ring-opening reaction of 1,3-cyclohexadiene, which proceeds through two conical intersections within 100 femtoseconds. The super-resolved transient structures near conical intersections reveal a C-C bond length difference of less than 0.4 angstrom and an approximately 30-femtosecond traversal time of the nuclear wave packet between them. These findings establish super-resolution ultrafast scattering as a transformative tool for uncovering quantum dynamics in molecules and open new avenues for studying light-matter interactions at the most fundamental level.