Ultrafast Optically Induced Perturbation of Oxygen Octahedral Rotations in Multiferroic BiFeO3 Thin Films.

The functional properties of complex oxides, including magnetism and ferroelectricity, are closely linked to subtle structural distortions. Ultrafast optical excitations provide the means to manipulate structural features and ultimately to affect the functional properties of complex oxides with picosecond-scale precision. We report that the lattice expansion of multiferroic BiFeO3 following above-bandgap optical excitation leads to distortion of the oxygen octahedral rotation (OOR) pattern. The continuous coupling between OOR and strain was probed using time-resolved X-ray free-electron laser diffraction with femtosecond time resolution. Density functional theory calculations predict a relationship between the OOR and the elastic strain consistent with the experiment, demonstrating a route to employing this approach in a wider range of systems. Ultrafast control of the functional properties of BiFeO3 thin films is enabled by this approach because the OOR phenomena are related to ferroelectricity, and via the Fe-O-Fe bond angles, the superexchange interaction between Fe atoms.

High-energy-resolution off-resonant spectroscopy with self-seeded x-ray free-electron laser pulses.

This paper presents the implementation of high-energy-resolution off-resonant spectroscopy (HEROS) measurements using self-seeded x-ray free-electron laser (XFEL) pulses. This study systematically investigated XFEL conditions, including photon energy and accumulated shot numbers, to optimize the measurement efficiency for copper foil samples near the K-edge. The x-ray absorption spectra reconstructed using HEROS were compared with those derived from fluorescence-yield measurements. The HEROS-based spectra exhibited consistent line shapes independent of the sample thickness. The potential application of HEROS to high-temperature copper was also explored. HEROS offers distinct advantages including scan-free measurement of x-ray absorption spectra with reduced core-hole lifetime broadening and self-absorption effects. Using self-seeded XFEL pulses, HEROS facilitates single-shot-based pump-probe measurements to investigate the ultrafast dynamics in various materials and diverse conditions.

Emergence of liquid following laser melting of gold thin films.

X-ray structural science is undergoing a revolution driven by the emergence of X-ray Free-electron Laser (XFEL) facilities. The structures of crystalline solids can now be studied on the picosecond time scale relevant to phonons, atomic vibrations which travel at acoustic velocities. In the work presented here, X-ray diffuse scattering is employed to characterize the time dependence of the liquid phase emerging from femtosecond laser-induced melting of polycrystalline gold thin films using an XFEL. In a previous analysis of Bragg peak profiles, we showed the supersonic disappearance of the solid phase and presented a model of pumped hot electrons carrying energy from the gold surface to scatter at internal grain boundaries. This generates melt fronts propagating relatively slowly into the crystal grains. By conversion of diffuse scattering to a partial X-ray pair distribution function, we demonstrate that it has the characteristic shape obtained by Fourier transformation of the measured F(Q). The diffuse signal fraction increases with a characteristic rise-time of 13 ps, roughly independent of the incident pump fluence and consequent final liquid fraction. This suggests the role of further melt-front nucleation processes beyond grain boundaries.

Resonant X-ray emission spectroscopy using self-seeded hard X-ray pulses at PAL-XFEL.

Self-seeded hard X-ray pulses at PAL-XFEL were used to commission a resonant X-ray emission spectroscopy experiment with a von Hamos spectrometer. The self-seeded beam, generated through forward Bragg diffraction of the [202] peak in a 100 µm-thick diamond crystal, exhibited an average bandwidth of 0.54 eV at 11.223 keV. A coordinated scanning scheme of electron bunch energy, diamond crystal angle and silicon monochromator allowed us to map the Ir Lß2 X-ray emission lines of IrO2 powder across the Ir L3-absorption edge, from 11.212 to 11.242 keV with an energy step of 0.3 eV. This work provides a reference for hard X-ray emission spectroscopy experiments utilizing self-seeded pulses with a narrow bandwidth, eventually applicable for pump-probe studies in solid-state and diluted systems.

Observing femtosecond orbital dynamics in ultrafast Ge melting with time-resolved resonant X-ray scattering.

Photoinduced nonequilibrium phase transitions have stimulated interest in the dynamic interactions between electrons and crystalline ions, which have long been overlooked within the Born-Oppenheimer approximation. Ultrafast melting before lattice thermalization prompted researchers to revisit this issue to understand ultrafast photoinduced weakening of the crystal bonding. However, the absence of direct evidence demonstrating the role of orbital dynamics in lattice disorder leaves it elusive. By performing time-resolved resonant X-ray scattering with an X-ray free-electron laser, we directly monitored the ultrafast dynamics of bonding orbitals of Ge to drive photoinduced melting. Increased photoexcitation of bonding electrons amplifies the orbital disturbance to expedite the lattice disorder approaching the sub-picosecond scale of the nonthermal regime. The lattice disorder time shows strong nonlinear dependence on the laser fluence with a crossover behavior from thermal-driven to nonthermal-dominant kinetics, which is also verified by ab initio and two-temperature molecular dynamics simulations. This study elucidates the impact of bonding orbitals on lattice stability with a unifying interpretation on photoinduced melting.

4D Visualization of a Nonthermal Coherent Magnon in a Laser Heated Lattice by an X-ray Free Electron Laser.

Ultrafast optical manipulation of magnetic phenomena is an exciting achievement of mankind, expanding one's horizon of knowledge toward the functional nonequilibrium states. The dynamics acting on an extremely short timescale push the detection limits that reveal fascinating light-matter interactions for nonthermal creation of effective magnetic fields. While some cases are benchmarked by emergent transient behaviors, otherwise identifying the nonthermal effects remains challenging. Here, a femtosecond time-resolved resonant magnetic X-ray diffraction experiment is introduced, which uses an X-ray free-electron laser (XFEL) to distinguish between the effective field and the photoinduced thermal effect. It is observed that a multiferroic Y-type hexaferrite exhibits magnetic Bragg peak intensity oscillations manifesting entangled antiferromagnetic (AFM) and ferromagnetic (FM) Fourier components of a coherent AFM magnon. The magnon trajectory constructed in 3D space and time domains is decisive to evince ultrafast field formation preceding the lattice thermalization. A remarkable impact of photoexcitation across the electronic bandgap is directly unraveled, amplifying the photomagnetic coupling that is one of the highest among AFM dielectrics. Leveraging the above-bandgap photoexcitation, this energy-efficient optical process further suggests a novel photomagnetic control of ferroelectricity in multiferroics.

Liquid-liquid phase separation in supercooled water from ultrafast heating of low-density amorphous ice.

Recent experiments continue to find evidence for a liquid-liquid phase transition (LLPT) in supercooled water, which would unify our understanding of the anomalous properties of liquid water and amorphous ice. These experiments are challenging because the proposed LLPT occurs under extreme metastable conditions where the liquid freezes to a crystal on a very short time scale. Here, we analyze models for the LLPT to show that coexistence of distinct high-density and low-density liquid phases may be observed by subjecting low-density amorphous (LDA) ice to ultrafast heating. We then describe experiments in which we heat LDA ice to near the predicted critical point of the LLPT by an ultrafast infrared laser pulse, following which we measure the structure factor using femtosecond x-ray laser pulses. Consistent with our predictions, we observe a LLPT occurring on a time scale < 100 ns and widely separated from ice formation, which begins at times >1 µs.

Subpicosecond Optical Stress Generation in Multiferroic BiFeO3.

Optical excitation leads to ultrafast stress generation in the prototypical multiferroic BiFeO3. The time scales of stress generation are set by the dynamics of the population of excited electronic states and the coupling of the electronic configuration to the structure. X-ray free-electron laser diffraction reveals high-wavevector subpicosecond-time scale stress generation following ultraviolet excitation of a BiFeO3 thin film. Stress generation includes a fast component with a 1/e rise time with an upper limit of 300 fs and longer-rise time components extending to 1.5 ps. The contributions of the fast and delayed components vary as a function of optical fluence, with a reduced a fast-component contribution at high fluence. The results provide insight into stress-generation mechanisms linked to the population of excited electrons and point to new directions in the application of nanoscale multiferroics and related ferroic complex oxides. The fast component of the stress indicates that structural parameters and properties of ferroelectric thin film materials can be optically modulated with 3 dB bandwidths of at least 0.5 THz.

Following the Crystallization of Amorphous Ice after Ultrafast Laser Heating.

Using time-resolved wide-angle X-ray scattering, we investigated the early stages (10 µs-1 ms) of crystallization of supercooled water, obtained by the ultrafast heating of high- and low-density amorphous ice (HDA and LDA) up to a temperature T = 205 K ± 10 K. We have determined that the crystallizing phase is stacking disordered ice (Isd), with a maximum cubicity of χ = 0.6, in agreement with predictions from molecular dynamics simulations at similar temperatures. However, we note that a growing small portion of hexagonal ice (Ih) was also observed, suggesting that within our timeframe, Isd starts annealing into Ih. The onset of crystallization, in both amorphous ice, occurs at a similar temperature, but the observed final crystalline fraction in the LDA sample is considerably lower than that in the HDA sample. We attribute this discrepancy to the thickness difference between the two samples.

Ultrafast Carrier-Lattice Interactions and Interlayer Modulations of Bi2Se3 by X-ray Free-Electron Laser Diffraction.

As a 3D topological insulator, bismuth selenide (Bi2Se3) has potential applications for electrically and optically controllable magnetic and optoelectronic devices. Understanding the coupling with its topological phase requires studying the interactions of carriers with the lattice on time scales down to the subpicosecond regime. Here, we investigate the ultrafast carrier-induced lattice contractions and interlayer modulations in Bi2Se3 thin films by time-resolved diffraction using an X-ray free-electron laser. The lattice contraction depends on the carrier concentration and is followed by an interlayer expansion accompanied by oscillations. Using density functional theory and the Lifshitz model, the initial contraction can be explained by van der Waals force modulation of the confined free carrier layers. Our theoretical calculations suggest that the band inversion, related to a topological phase transition, is modulated by the expansion of the interlayer distance. These results provide insights into the topological phase control by light-induced structural change on ultrafast time scales.

Optical Kerr Effect of Liquid Acetonitrile Probed by Femtosecond Time-Resolved X-ray Liquidography.

Optical Kerr effect (OKE) spectroscopy is a method that measures the time-dependent change of the birefringence induced by an optical laser pulse using another optical laser pulse and has been used often to study the ultrafast dynamics of molecular liquids. Here we demonstrate an alternative method, femtosecond time-resolved X-ray liquidography (fs-TRXL), where the microscopic structural motions related to the OKE response can be monitored using a different type of probe, i.e., X-ray solution scattering. By applying fs-TRXL to acetonitrile and a dye solution in acetonitrile, we demonstrate that different types of molecular motions around photoaligned molecules can be resolved selectively, even without any theoretical modeling, based on the anisotropy of two-dimensional scattering patterns and extra structural information contained in the q-space scattering data. Specifically, the dynamics of reorientational (libration and orientational diffusion) and translational (interaction-induced motion) motions are captured separately by anisotropic and isotropic scattering signals, respectively. Furthermore, the two different types of reorientational motions are distinguished from each other by their own characteristic scattering patterns and time scales. The measured time-resolved scattering signals are in excellent agreement with the simulated scattering signals based on a molecular dynamics simulation for plausible molecular configurations, providing the detailed structural description of the OKE response in liquid acetonitrile.

Experimental observation of the liquid-liquid transition in bulk supercooled water under pressure.

We prepared bulk samples of supercooled liquid water under pressure by isochoric heating of high-density amorphous ice to temperatures of 205 ± 10 kelvin, using an infrared femtosecond laser. Because the sample density is preserved during the ultrafast heating, we could estimate an initial internal pressure of 2.5 to 3.5 kilobar in the high-density liquid phase. After heating, the sample expanded rapidly, and we captured the resulting decompression process with femtosecond x-ray laser pulses at different pump-probe delay times. A discontinuous structural change occurred in which low-density liquid domains appeared and grew on time scales between 20 nanoseconds to 3 microseconds, whereas crystallization occurs on time scales of 3 to 50 microseconds. The dynamics of the two processes being separated by more than one order of magnitude provides support for a liquid-liquid transition in bulk supercooled water.

Mapping the emergence of molecular vibrations mediating bond formation.

Fundamental studies of chemical reactions often consider the molecular dynamics along a reaction coordinate using a calculated or suggested potential energy surface1-5. But fully mapping such dynamics experimentally, by following all nuclear motions in a time-resolved manner-that is, the motions of wavepackets-is challenging and has not yet been realized even for the simple stereotypical bimolecular reaction6-8: A-B + C â†’ A + B-C. Here we track the trajectories of these vibrational wavepackets during photoinduced bond formation of the gold trimer complex [Au(CN)2-]3 in an aqueous monomer solution, using femtosecond X-ray liquidography9-12 with X-ray free-electron lasers13,14. In the complex, which forms when three monomers A, B and C cluster together through non-covalent interactions15,16, the distance between A and B is shorter than that between B and C. Tracking the wavepacket in three-dimensional nuclear coordinates reveals that within the first 60 femtoseconds after photoexcitation, a covalent bond forms between A and B to give A-B + C. The second covalent bond, between B and C, subsequently forms within 360 femtoseconds to give a linear and covalently bonded trimer complex A-B-C. The trimer exhibits harmonic vibrations that we map and unambiguously assign to specific normal modes using only the experimental data. In principle, more intense X-rays could visualize the motion not only of highly scattering atoms such as gold but also of lighter atoms such as carbon and nitrogen, which will open the door to the direct tracking of the atomic motions involved in many chemical reactions.

Intense Reactivity in Sulfur-Hydrogen Mixtures at High Pressure under X-ray Irradiation.

Superconductivity near room temperature in the sulfur-hydrogen system arises from a sequence of reactions at high pressures, with X-ray diffraction experiments playing a central role in understanding these chemical-structural transformations and the corresponding S:H stoichiometry. Here we document X-ray irradiation acting as both a probe and as a driver of chemical reaction in this dense hydride system. We observe a reaction between molecular hydrogen (H2) and elemental sulfur (S8) under high pressure, induced directly by X-ray illumination, at photon energies of 12 keV using a free electron laser. The rapid synthesis of hydrogen sulfide (H2S) at 0.3 GPa was confirmed by optical observations, spectroscopic measurements, and microstructural changes detected by X-ray diffraction. These results document X-ray induced chemical synthesis of superconductor-forming dense hydrides, revealing an alternative production strategy and confirming the disruptive nature of X-ray exposure in studies on high-pressure hydrogen chalcogenides, from water to high-temperature superconductors.

Ultrafast x-ray diffraction study of melt-front dynamics in polycrystalline thin films.

Melting is a fundamental process of matter that is still not fully understood at the microscopic level. Here, we use time-resolved x-ray diffraction to examine the ultrafast melting of polycrystalline gold thin films using an optical laser pump followed by a delayed hard x-ray probe pulse. We observe the formation of an intermediate new diffraction peak, which we attribute to material trapped between the solid and melted states, that forms 50 ps after laser excitation and persists beyond 500 ps. The peak width grows rapidly for 50 ps and then narrows distinctly at longer time scales. We attribute this to a melting band originating from the grain boundaries and propagating into the grains. Our observation of this intermediate state has implications for the use of ultrafast lasers for ablation during pulsed laser deposition.

Hard X-ray self-seeding commissioning at PAL-XFEL.

A wake monochromator based on a large-area diamond single crystal for hard X-ray self-seeding has been successfully installed and commissioned in the hard X-ray free-electron laser (FEL) at the Pohang Accelerator Laboratory with international collaboration. For this commissioning, the self-seeding was demonstrated with a low bunch charge (40 pC) and the nominal bunch charge (180 pC) of self-amplified spontaneous emission (SASE) operation. The FEL pulse lengths were estimated as 7 fs and 29.5 fs, respectively. In both cases, the average spectral brightness increased by more than three times compared with the SASE mode. The self-seeding experiment was demonstrated for the first time using a crystal with a thickness of 30 µm, and a narrow bandwidth of 0.22 eV (full width at half-maximum) was obtained at 8.3 keV, which confirmed the functionality of a crystal with such a small thickness. In the nominal bunch-charge self-seeding experiment, the histogram of the intensity integrated over a 1 eV bandwidth showed a well defined Gaussian profile, which is evidence of the saturated FEL and a minimal electron-energy jitter (∼1.2 × 10-4) effect. The corresponding low photon-energy jitter (∼2.4 × 10-4) of the SASE FEL pulse, which is two times lower than the Pierce parameter, enabled the seeding power to be maximized by maintaining the spectral overlap between SASE FEL gain and the monochromator.

Magnetic Excitations across the Metal-Insulator Transition in the Pyrochlore Iridate Eu_{2}Ir_{2}O_{7}.

We report a resonant inelastic x-ray scattering study of the magnetic excitation spectrum in a highly insulating Eu_{2}Ir_{2}O_{7} single crystal that exhibits a metal-insulator transition at T_{MI}=111(7) K. A propagating magnon mode with a 20 meV bandwidth and a 28 meV magnon gap is found in the excitation spectrum at 7 K, which is expected in the all-in-all-out magnetically ordered state. This magnetic excitation exhibits substantial softening as the temperature is raised towards T_{MI} and turns into a highly damped excitation in the paramagnetic phase. Remarkably, the softening occurs throughout the whole Brillouin zone including the zone boundary. This observation is inconsistent with the magnon renormalization expected in a local moment system and indicates that the strength of the electron correlation in Eu_{2}Ir_{2}O_{7} is only moderate, so that electron itinerancy should be taken into account in describing its magnetism.

Electromagnon with Sensitive Terahertz Magnetochromism in a Room-Temperature Magnetoelectric Hexaferrite.

An electromagnon in the magnetoelectric (ME) hexaferrite Ba_{0.5}Sr_{2.5}Co_{2}Fe_{24}O_{41} (Co_{2}Z-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on the electric field (E^{ω}) of the THz light parallel to the c axis (∥ [001]), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes -8.7% and +5.8% under external magnetic field (H) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from antiphase motion of spins which are coupled with modulating electric dipoles through the exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of the Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different H.

Quantitative Measurements of Size-Dependent Magnetoelectric Coupling in Fe3O4 Nanoparticles.

Bulk magnetite (Fe3O4), the loadstone used in magnetic compasses, has been known to exhibit magnetoelectric (ME) properties below ∼10 K; however, corresponding ME effects in Fe3O4 nanoparticles have been enigmatic. We investigate quantitatively the ME coupling of spherical Fe3O4 nanoparticles with uniform diameters (d) from 3 to 15 nm embedded in an insulating host, using a sensitive ME susceptometer. The intrinsic ME susceptibility (MES) of the Fe3O4 nanoparticles is measured, exhibiting a maximum value of ∼0.6 ps/m at 5 K for d = 15 nm. We found that the MES is reduced with reduced d but remains finite until d = ∼5 nm, which is close to the critical thickness for observing the Verwey transition. Moreover, with reduced diameter the critical temperature below which the MES becomes conspicuous increased systematically from 9.8 K in the bulk to 19.7 K in the nanoparticles with d = 7 nm, reflecting the core-shell effect on the ME properties. These results point to a new pathway for investigating ME effect in various nanomaterials.