Photoexcitation provides a versatile route to drive quantum materials into nonequilibrium states, opening opportunities for phase engineering beyond conventional tuning parameters such as temperature, magnetic field, pressure, or chemical doping/substitution. VO2, a prototypical correlated oxide, has long served as a model system for understanding photoinduced insulator-metal transitions, yet the sequence of structural and electronic transitions remains intensely debated. Here, we uncover a hidden photoinduced transition pathway in epitaxially strained VO2 thin films, in which the structural transition precedes the electronic insulator-metal transition, reversing the canonical temporal order. Femtosecond X-ray diffraction reveals a transient structural state characterized by the disappearance of vanadium dimers generating dynamic tensile strain, while time-resolved terahertz spectroscopy shows that the electronic gap closes only after the strain relaxation. This lattice-driven transition highlights the pivotal role of Mott correlations in dictating electronic properties under nonequilibrium conditions. Our findings establish strain-light coupling as a design principle for ultrafast control of phase transitions, offering new avenues for reconfigurable electronic and photonic devices based on correlated oxides.

Nanostructures formed by spontaneously broken symmetry have provided new ways to manipulate quantum states. Specifically, topological structures with periodic spatial ordering, such as polar vortices and skyrmions, can be ideal hosts for creating engineered responses in both spatial and frequency domains. So far, however, only a few examples of such hierarchical engineering have been reported in the literature. Here we demonstrate that the spatially modulated piezoelectric response of a polar vortex structure can create strain waves with a characteristic nanoscale wavefront. Using time-resolved pump-probe resonant X-ray scattering and diffraction measurements, coupled with dynamical phase-field simulations, we show that the piezoelectric modulation of the spontaneously formed polar vortex crystal functions as an acoustic diffraction grating. This system converts incoming laterally uniform strain waves into outgoing waves with a characteristic sub-terahertz frequency, driven by an intrinsic excitation of the polar vortex crystal. Moreover, our phase-field simulations suggest that the dynamic mechanical displacements exhibiting vortex textures are generated from both space- and time-varying piezoelectric responses. Our findings illustrate a new method for generating nanoscale strain waves with unique spatial textures by tuning the hierarchical order of polar topologies to engineer new collective modes, allowing for a wide range of control through the topological lattice.

The search for the liquid-liquid critical point in supercooled water is challenging owing to rapid crystallization. We studied supercooled water at timescales before ice formation by heating high- and low-density amorphous ices using infrared ultrafast laser pulses, followed by x-ray scattering. By varying the pump laser fluence, we accessed liquid states straddling the predicted critical point. We observed a crossover from a discontinuous to a continuous transition at which broad and slow structural variations occurred, consistent with critical fluctuations and slowing down. We also observed a rapid increase in the heat capacity indicating a critical divergence at 210 ± 8 K coincident with enhanced density fluctuations. These results suggest that our experiments have directly probed the vicinity of a critical point in supercooled water.

Magnetoelastic coupling in van der Waals (vdW) magnetic materials enables a unique interplay between the spin and lattice degrees of freedom. Characterizing the elastic responses with atomic and femtosecond resolution across the magnetic transition is essential for guiding the design of magnetically tunable actuators and strain-mediated spintronic devices. Here, ultrafast X-ray diffraction employed at a free-electron laser reveals that the atomic displacements, wave vectors, and dispersion relations of acoustic phonon modes in a vdW antiferromagnet FePS3 are coupled with the magnetic order, by tracking both in-plane and out-of-plane Bragg peaks upon optical excitation across the Néel temperature (TN). One transverse mode shows that a quasi-out-of-plane atomic displacement undergoes a significant directional change across TN. Its quasi-in-plane wave vector is derived by comparing the measured sound velocity and the first-principles calculations. The other transverse mode is an interlayer shear acoustic mode whose amplitude is strongly enhanced in the antiferromagnetic phase, exhibiting eight times stronger amplitude than the longitudinal acoustic mode below TN. The atomically resolved characterization of acoustic phonon dynamics that couple with magnetic ordering opens opportunities for harnessing unique magnetoelastic coupling in vdW magnets on ultrafast timescales.

Photoexcitation provides a versatile route to drive quantum materials into nonequilibrium states, opening opportunities for phase engineering beyond conventional tuning parameters such as temperature, magnetic field, pressure, or chemical doping/substitution. VO 2 , a prototypical correlated oxide, has long served as a model system for understanding photoinduced insulator–metal transitions, yet the sequence of structural and electronic transitions remains intensely debated. Here, we uncover a hidden photoinduced transition pathway in epitaxially strained VO 2 thin films, in which the structural transition precedes the electronic insulator–metal transition, reversing the canonical temporal order. Femtosecond X‐ray diffraction reveals a transient structural state characterized by the disappearance of vanadium dimers generating dynamic tensile strain, while time‐resolved terahertz spectroscopy shows that the electronic gap closes only after the strain relaxation. This lattice‐driven transition highlights the pivotal role of Mott correlations in dictating electronic properties under nonequilibrium conditions. Our findings establish strain–light coupling as a design principle for ultrafast control of phase transitions, offering new avenues for reconfigurable electronic and photonic devices based on correlated oxides.

We present a Data Acquisition (DAQ) system architecture specifically designed for time-resolved X-ray scattering and spectroscopy experiments at the Femtosecond X-ray Scattering (FXS) endstation at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). This system addresses the critical challenges associated with high data production rates, alignment of data from various devices, the demands for efficient data handling, and scalability of entire system. To enable rapid preliminary analysis and data visualization, we have implemented a real-time analysis approach prior to data saving. In addition, the system provides user-friendly data access and analysis capabilities, ensuring efficient exploration and utilization of the collected data.

The hard X-ray undulator line at Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) provides a wide range of photon energies encompassing both the tender (2–5 keV) and hard (5–15 keV) X-ray regimes. Its Femtosecond X-ray Scattering (FXS) endstation is dedicated to research in condensed matter physics and materials science, and supports various time-resolved X-ray experiments, such as diffraction, spectroscopy and resonant X-ray scattering. We report the development of a multi-functional chamber at the FXS endstation to support the experiments in the tender X-ray regime where significant air scattering and absorption pose challenges. This chamber enables optical-pump/X-ray-probe experiments in a vacuum environment, with its functionality demonstrated through time-resolved resonant elastic X-ray scattering experiments on a ruthenate Li2RuO3 near the Ru L3 (∼2.84 keV) absorption edge. Designed to readily accommodate modular instruments, the chamber offers flexibility to provide diverse experiment conditions requested by users at the FXS endstation.

A transmissive single-shot spectrometer has been developed to monitor shot-to-shot spectral structures in the hard X-ray beamline of the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). The established spectrometer comprises 10 µm-thick Si crystals bent to a radius of curvature of 100 mm. Depending on the photon energy range, either the Si (111) or Si (110) crystal can be selected for spectral analysis. Especially in the energy range 4.5–17 keV, the spectrometer is designed to cover a spectral range wider than the full free-electron laser bandwidth and to guarantee a high resolution sufficient for resolving each spectral spike. This paper presents the design specifications, instruments and performance of this spectrometer, which has also been applied to demonstrate the spectral properties of various XFEL sources, such as self-amplified spontaneous emission, monochromatic and seeded beams.

Despite the biological relevance of the disulfide bond as the motive, which stabilizes the tertiary structure of many proteins, its photostability, UV-induced bond cleavage mechanisms and secondary photochemistry are still contested after decades of research. In this study, we employed femtosecond X-ray absorption spectroscopy to unravel the photochemistry of the aliphatic disulfide bridge of the semi-essential proteinogenic amino acid L-cysteine (L-cystine) in aqueous solution. We observe homolytic bond cleavage upon UV irradiation and the emergence of thiyl radicals as the single primary photoproduct and its ultrafast decay due to geminate recombination at remarkably high quantum yield in excess of 80% within 20 ps. These dynamics coincide with the emergence of a secondary product, attributed to the perthiyl radical. More than 70% of broken disulfide bridges form within the first nanosecond after bond cleavage. From these observations, we establish a dynamic photostability of the disulfide bridge and a mechanism of perthiyl radical formation from a 'hot' ground-state parent molecule that asymmetrically fragments along a carbon-sulfur bond, resolving long-standing questions in the photochemistry of disulfide bridges in condensed phase.

We carried out a short beamtime at the Pohang Accelerator Laboratory x-ray Free Electron Laser to perform a pump-probe (PP) laser excitation diffraction experiment on the silicon (222) forbidden Bragg peak. To limit the x-ray penetration, we used a "device layer" silicon film wafer bonded to a silicon substrate. The sample, specially fabricated by MEMC Electronic Materials, had a Si(100) substrate bonded to a 170 nm Si(100) film rotated at 45 degrees for crystallographic isolation. A second sample was reactive-ion-etched down to 52 nm thickness. In the silicon lattice, the covalent bonds are seen exclusively at the 222 reflection. Upon laser excitation, these electrons are expected to be excited to the valence band on femtosecond electronic time scales. The Si(222) reflection is therefore expected to be extinguished on this fast time scale, while the electron-phonon coupled acoustic response is determined by the lattice dynamics. The latter is determined by the speed of sound over the device thickness, which is in the mid-picosecond range.

Disordering atomic structures offers a functionality hardly expected in ordered states, including phase-change memory and photonic computing, offering the potential to renovate von Neumann architecture for neuromorphic engineering with low latency. However, significant energy consumption during the disordering compromises the data reliability and integration efficiency, which is traditionally regarded to take place after melting. Here, we investigate time for disordering in isochronal and isochoric manners, challenging the conventional melt-quenching theory. The disordering times of pure Sb, Ag-In-Sb-Te, and In surpass that of InSb by over 50 times, despite a higher melting point and a lower laser absorption rate of Sb compared to InSb. This nontrivial contrast is elucidated by theoretical calculation that delocalized electrons enable flexible modification of bond lengths even below the melting points where undermined bond directionality provides room for atoms to depart from their original positions. Facilitated by delocalized electrons, specifically through metavalent and metallic bonding rather than covalent bonding, atoms can be disordered without undergoing melting, which aligns with the rapid disordering of Sb compared to that of InSb. The results bridge the unaddressed gap between chemical interaction and kinetic behaviors during the disordering and suggest design rules highlighting electron-delocalization rather than solely relying on melting points to improve energy efficiency.

Two-dimensional (2D) materials and their heterostructures enable unconventional electronic properties and functionalities not accessible in their bulk counterparts. This approach is now being extended to magnetic materials to engineer their spin structures and magnetic fields produced by them. However, spin dynamics of 2D magnetic heterostructures remain largely unexplored. Here, we demonstrate that heterointerfacing Heisenberg square-lattice antiferromagnet (AF) Sr2IrO4 with its bilayer variant Ising AF Sr3Ir2O7 in a superlattice leads to liquid-like spin dynamics in the former, characterized by slow recovery of the AF order after its transient suppression by an optical pump, and complete absence of spin waves except in an immediate vicinity of the ordering wavevector. Instead, the spin excitation spectra are dominated by isotropic continua, which in previous works have been interpreted as fractional spin excitations, or spinons, that extends to unprecedentedly low energies. Thus, our results provide a pathway to frustrated magnetism in square lattices by heterointerfacing two distinct types of AFs.

Novel crystalline materials consisting of organic, inorganic or organometallic building blocks have emerged as promising materials with wide applications [1-5]. Of great importance is to characterize not only the static structures of those materials grown in single crystals but also the conversion of their structures in response to external stimuli [6, 7]. Femtosecond time-resolved crystallography has the potential to probe real-time dynamics of such structural conversions but has not yet been realized for chemical reactions in crystals made of small-molecule building blocks. Here, we apply time-resolved serial femtosecond crystallography (TR-SFX), a powerful technique for visualizing protein structural dynamics, to a metal–organic framework consisting of Fe-porphyrins and Zr6 nodes and elucidate the real-time structural dynamics initiated by the ligand photodissociation. The time-resolved electron density maps from the TR-SFX data unveil trifurcating structural pathways: (i) coherent oscillatory movements of Zr and Fe atoms, assigned to an optical phonon mode of 0.18 THz, (ii) a transient structure with Fe-porphyrin and Zr6 node undergoing doming and disordering movements, respectively, and (iii) a vibrationally hot structure with isotropic structural disorder, which is rarely observed in protein crystals. These findings demonstrate the feasibility of TR-SFX measurement on chemical systems.

A Nanobeam X-ray Experiments (NXE) instrument was developed and installed at the hard X-ray beamline of the Pohang Accelerator Laboratory X-ray Free Electron Laser. This instrument consists of a diagnostic system, focusing optics, an X-ray diffraction endstation and a femtosecond laser delivery system. The NXE instrument enables sophisticated X-ray experiments using nanofocused X-rays. At a 9.5 keV X-ray energy, the beam was successfully focused to 390 nm × 230 nm at the focal plane using Kirkpatrick-Baez mirrors. Following the successful commissioning experiments in December 2021 and April 2022, the instrument became available for regular user experiments in January 2023. The first user experiment was conducted in January 2024. This article provides detailed information on the beamline optics, the NXE instrument, and its performance and capabilities.

Crystalline systems consisting of small-molecule building blocks have emerged as promising materials with diverse applications. It is of great importance to characterize not only their static structures but also the conversion of their structures in response to external stimuli. Femtosecond time-resolved crystallography has the potential to probe the real-time dynamics of structural transitions, but, thus far, this has not been realized for chemical reactions in non-biological crystals. In this study, we applied time-resolved serial femtosecond crystallography (TR-SFX), a powerful technique for visualizing protein structural dynamics, to a metal–organic framework, consisting of Fe porphyrins and hexazirconium nodes, and elucidated its structural dynamics. The time-resolved electron density maps derived from the TR-SFX data unveil trifurcating structural pathways: coherent oscillatory movements of Zr and Fe atoms, a transient structure with the Fe porphyrins and Zr 6 nodes undergoing doming and disordering movements, respectively, and a vibrationally hot structure with isotropic structural disorder. These findings demonstrate the feasibility of using TR-SFX to study chemical systems.

Ultrafast photoinduced melting provides an essential platform for studying nonequilibrium phase transitions by linking the kinetics of electron dynamics to ionic motions. Knowledge of dynamic balance in their energetics is essential to understanding how the ionic reaction is influenced by femtosecond photoexcited electrons with notable time lag depending on reaction mechanisms. Here, by directly imaging fluctuating density distributions and evaluating the ionic pressure and Gibbs free energy from two-temperature molecular dynamics that verified experimental results, we uncovered that transient ionic pressure, triggered by photoexcited electrons, controls the overall melting kinetics. In particular, ultrafast nonequilibrium melting can be described by the reverse nucleation process with voids as nucleation seeds. The strongly driven solid-to-liquid transition of metallic gold is successfully explained by void nucleation facilitated by photoexcited electron–initiated ionic pressure, establishing a solid knowledge base for understanding ultrafast nonequilibrium kinetics.

The combination of X-ray free-electron lasers (XFELs) with serial femtosecond crystallography represents cutting-edge technology in structural biology, allowing the study of enzyme reactions and dynamics in real time through the generation of `molecular movies'. This technology combines short and precise high-energy X-ray exposure to a stream of protein microcrystals. Here, the XFEL structure of carbonic anhydrase II, a ubiquitous enzyme responsible for the interconversion of CO2 and bicarbonate, is reported, and is compared with previously reported NMR and synchrotron X-ray and neutron single-crystal structures.

Here, we investigate the hypothesis that despite the existence of at least two high-density amorphous ices, only one high-density liquid state exists in water. We prepared a very-high-density amorphous ice (VHDA) sample and rapidly increased its temperature to around 205 ± 10 K using laser-induced isochoric heating. This temperature falls within the so-called “no-man’s land” well above the glass-liquid transition, wherein the IR laser pulse creates a metastable liquid state. Subsequently, this high-density liquid (HDL) state of water decompresses over time, and we examined the time-dependent structural changes using short x-ray pulses from a free electron laser. We observed a liquid–liquid transition to low-density liquid water (LDL) over time scales ranging from 20 ns to 3 μs, consistent with previous experimental results using expanded high-density amorphous ice (eHDA) as the initial state. In addition, the resulting LDL derived both from VHDA and eHDA displays similar density and degree of inhomogeneity. Our observation supports the idea that regardless of the initial annealing states of the high-density amorphous ices, the same HDL and final LDL states are reached at temperatures around 205 K.