Dogeun Jang

Ultrafast laser excitation drives crystalline materials into extreme nonequilibrium states, yet microstructural features governing energy redistribution remain poorly understood. Here we use time-resolved dark-field X-ray microscopy at an X-ray free-electron laser (XFEL) to show that tilted twin boundaries (TTBs) with misorientations of only 1–2° redirect the ultrafast response of Cu(111) thin films. In films with low TTB density, femtosecond optical laser excitation induces near-surface lattice disordering within 2–5 ps via electron–lattice-coupling. Increasing TTB density suppresses this pathway and instead generates a supersonic elastic shock wave propagating radially at ~6 km/s along , exceeding the longitudinal sound velocity of Cu. The shock velocity is independent of both laser fluence and TTB density, whereas its occurrence probability scales linearly with TTB density, identifying TTBs as preferential sites for elastic energy localization. We attribute this to Fermi-surface discontinuities at TTBs that disrupt carrier transport and locally concentrate laser-deposited energy; complementary measurements in TiO2 — a semiconductor in which carrier generation can be switched on and off via the band gap — establish photoexcited carriers as the necessary ingredient and TTBs as the sufficient localization sites. These results establish sub-degree crystallographic misorientation as a route for controlling ultrafast energy flow in laser-excited solids.

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.

The infinite-layer compound CaCoO 2 provides a compelling platform to explore how local orbital distortions evolve into collective electronic order. Using resonant soft x-ray scattering (RSXS) and its time-resolved counterpart (tr-RSXS), we reveal a cooperative coupling between two intertwined orders at the same in-plane wave vector, q ≈ ( 1 / 4 , 1 / 4 , 0 ) , comprising a quasi-two-dimensional orbital order associated with the distorted Co ( 1 ) sites and a quasi-three-dimensional trimerlike charge order stabilized through Co-O-Ca hybridization. The charge-sensitive component exhibits a uniform relaxation time of ∼ 400 fs ( 391 ± 5 fs ) across multiple Co resonances, evidencing a collective charge response. A comparable timescale observed in the orbital order associated with the highly distorted Co ( 1 ) sites indicates that this collective behavior naturally originates from the same orbital framework that mediates both charge and orbital degrees of freedom. Together, these findings establish CaCoO 2 as a model system in which Jahn-Teller distortions, orbital hybridization, and geometric frustration cooperate to form molecular trimer units that dictate the macroscopic electronic behavior.

Exploring the photoinduced dynamics of chiral states offers promising avenues for advanced control of condensed matter systems. Photoinduced or photoenhanced chirality in 1T-TiSe_{2} has been suggested as a fascinating platform for optical manipulation of chiral states. However, the mechanisms underlying chirality training and its interplay with the charge density wave (CDW) phase remain elusive. Here, we use time-resolved x-ray diffraction (trXRD) with circularly polarized pump lasers to probe the photoinduced dynamics of chirality in 1T-TiSe_{2}. We observe a notable (∼20%) difference in CDW intensity suppression between left and right circularly polarized pumps. Additionally, we reveal momentum-resolved circular dichroism arising from domains of different chirality, providing a direct link between CDW and chirality. An immediate increase in CDW correlation length upon laser pumping is detected, suggesting the photoinduced expansion of chiral domains. These results both advance the potential of light-driven chirality by elucidating the mechanism driving chirality manipulation in TiSe_{2}, and they demonstrate that trXRD with circularly polarized pumps is an effective tool for chirality detection in condensed matter systems.

Time-resolved x-ray liquidography (TRXL) is a powerful technique for directly tracking ultrafast structural dynamics in real space. However, resolving the motion of vibrational wavepackets generated by femtosecond laser pulses remains challenging due to the limited temporal resolution and signal-to-noise ratio (SNR) of experimental data. This study addresses these challenges by introducing singular spectrum analysis (SSA) as an efficient method for extracting oscillatory signals associated with vibrational wavepackets from TRXL data. To evaluate its performance, we conducted a comparative study using simulated TRXL data, demonstrating that SSA outperforms conventional analysis methods such as the Fourier transform of temporal profiles and singular value decomposition, particularly under low SNR conditions. We further applied SSA to experimental TRXL data on the photodissociation of triiodide (I3−) in methanol, successfully isolating oscillatory signals arising from wavepacket dynamics in ground-state I3− and excited-state I2−, which had been challenging to resolve in previous TRXL studies. These results establish SSA as a highly effective tool for analyzing ultrafast structural dynamics in time-resolved experiments and open new opportunities for studying wavepacket dynamics in a wide range of photoinduced reactions.

We present the first circularly polarized non-resonant pump time-resolved resonant inelastic X-ray scattering (tr-RIXS) experiment in H₃LiIr₂O₆, an iridium-based Kitaev system. Our calculations and experimental results are consistent with the modification of the low-energy magnetic excitations in H₃LiIr₂O₆ only during illumination by the laser pulse. We discuss these results in a cooperative framework between the Floquet engineering of the exchange interactions and the dynamic renormalization of electron-electron correlations. However, the penetration length mismatch between the X-ray probe and laser pump and the intrinsic complexity of Kitaev magnets prevents us from unequivocally extracting towards which ground state H₃LiIr₂O₆ is driven. We outline possible solutions to these challenges for light-driven stabilization and observation of the Kitaev quantum spin liquid limit by RIXS.

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 L ₃ (∼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 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.