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Solute structure and its evolution in supersaturated aqueous solutions are key clues to understand Ostwald’s step rule. Here, we measure the structural evolution of solute molecules in extremely supersaturated solutions (KH2PO4 (KDP) and NH4H2PO4 (ADP)) using a combination of electrostatic levitation and synchrotron X-ray scattering. The measurement reveals the existence of a solution-solution transition in KDP solution, caused by changing molecular symmetries and structural evolution of the solution with supersaturation. Moreover, we find that the molecular symmetry of H2PO4- impacts on phase selection. These findings manifest that molecular symmetry and its structural evolution can govern the crystallization pathways in aqueous solutions, explaining the microscopic origin of Ostwald’s step rule.

For over two decades, NaCl nucleation from a supersaturated aqueous solution has been predicted to occur via a two-step nucleation (TSN) mechanism, i.e., two sequential events, the formation of locally dense liquid regions followed by structural ordering. However, the formation of dense liquid regions in the very early stage of TSN has never been experimentally observed. By using a state-of-the-art technique, a combination of electrostatic levitation (ESL) and in situ synchrotron X-ray and Raman scatterings, we find experimental evidence that indicates the formation of dense liquid regions in NaCl bulk solution at an unprecedentedly high level of supersaturation (S = 2.31). As supersaturation increases, evolution of ion clusters leads to chemical ordering, but no topological ordering, which is a precursor for forming the dense disordered regions of ion clusters at the early stage of TSN. Moreover, as the ion clusters proceed to evolve under highly supersaturated conditions, we observe the breakage of the water hydration structure indicating the stability limit of the dense liquid regions, and thus leading to nucleation. The evolution of solute clusters and breakage of hydration in highly supersaturated NaCl bulk solution will provide new insights into the detailed mechanism of TSN for many other aqueous solutions., This work provides evidence for two-step nucleation in highly supersaturated bulk NaCl solution, using electrostatic levitation combined with Raman/X-ray scatterings.

Weakly coupled ferroelectric/dielectric superlattice thin-film heterostructures exhibit complex nanoscale polarization configurations that arise from a balance of competing electrostatic, elastic, and domain-wall contributions to the free energy. A key feature of these configurations is that the polarization can locally have a significant component that is along the thin-film surface normal direction with an overall configuration maintaining zero net in-plane polarization. PbTiO_{3}/SrTiO_{3} thin-film superlattice heterostructures on a conducting SrRuO_{3} bottom electrode on SrTiO_{3} have a room-temperature stripe nanodomain pattern with a nanometer-scale lateral period. Ultrafast time-resolved x-ray free electron laser diffraction and scattering experiments reveal that above-bandgap optical pulses induce propagating acoustic pulses and a perturbation of the domain diffuse scattering intensity arising from the nanoscale stripe domain configuration. With 400-nm optical excitation, two separate acoustic pulses are observed: a high-amplitude pulse resulting from strong optical absorption in the bottom electrode and a weaker pulse arising from the depolarization-field-screening effect due to absorption directly within the superlattice. The picosecond scale variation of the nanodomain diffuse scattering intensity is consistent with a larger polarization change than would be expected due to the polarization-tetragonality coupling of uniformly polarized ferroelectrics. The polarization change is consistent, instead, with polarization rotation facilitated by the reorientation of the in-plane component of the polarization at the domain boundaries of the striped polarization structure. The complex steady-state configuration within these ferroelectric heterostructures leads to ultrafast polarization rotation phenomena that have previously been available only through the selection of bulk crystal composition.

The presence of a small concentration of in-plane Fe dopants in ${\mathrm{La}}_{1.87}{\mathrm{Sr}}_{0.13}{\mathrm{Cu}}_{0.99}{\mathrm{Fe}}_{0.01}{\mathrm{O}}_{4}$ is known to enhance stripelike spin and charge density wave (SDW and CDW) order and suppress the superconducting ${T}_{c}$. Here, we show that it also induces highly two-dimensional superconducting correlations that have been argued to be the signatures of a new form of superconducting order, the so-called pair density wave (PDW) order. In addition, using resonant soft x-ray scattering, we find that the two-dimensional superconducting fluctuation is strongly associated with the CDW stripe. In particular, the PDW signature first appears when the correlation length of the CDW stripe grows over eight times the lattice unit ($\ensuremath{\sim}8a$). These results provide critical conditions for the formation of the PDW order.

Photogeneration of excess charge carriers in semiconductors produces electronic strain. Under transient conditions, electron-hole pairs may be separated across a potential barrier. Using time-resolved X-ray diffraction measurements across an intrinsic AlGaAs/n-doped GaAs interface, we find that the electronic strain is only produced by holes, and that electrons are not directly observable by strain measurements. The presence of photoinduced charge carriers in the n-doped GaAs is indirectly confirmed by delayed heat generation via recombination.

We characterize the spatial and temporal coherence properties of hard X-ray pulses from the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL, Pohang, Korea). The measurement of the single-shot speckle contrast, together with the introduction of corrections considering experimental conditions, allows obtaining an intrinsic degree of transverse coherence of 0.85 ± 0.06. In the Self-Amplified Spontaneous Emission regime, the analysis of the intensity distribution of X-ray pulses also provides an estimate for the number of longitudinal modes. For monochromatic and pink (i.e. natural bandwidth provided by the first harmonic of the undulator) beams, we observe that the number of temporal modes is 6.0 ± 0.4 and 90.0 ± 7.2, respectively. Assuming a coherence time of 2.06 fs and 0.14 fs for the monochromatic and pink beam respectively, we estimate an average X-ray pulse duration of 12.6 ± 1.0 fs.

Solution studies have proposed that crystal nucleation can take more complex pathways than previously expected in classical nucleation theory, such as formation of prenucleation clusters or densified amorphous/liquid phases. These findings show that it is possible to separate fluctuations in the different order parameters governing crystal nucleation, that is, density and structure. However, a direct observation of the multipathways from aqueous solutions remains a great challenge because heterogeneous nucleation sites, such as container walls, can prevent these paths. Here, we demonstrate the existence of multiple pathways of nucleation in highly supersaturated aqueous KH2PO4 (KDP) solution using the combination of a containerless device (electrostatic levitation), and in situ micro-Raman and synchrotron X-ray scattering. Specifically, we find that, at an unprecedentedly deep level of supersaturation, a high-concentration KDP solution first transforms into a metastable crystal before reaching stability at room temperature. However, a low-concentration solution, with different local structures, directly transforms into the stable crystal phase. These apparent multiple pathways of crystallization depend on the degree of supersaturation.

The glass forming ability (GFA) of Cu–Zr alloys has been still ambiguous, due to incomplete or lacking thermophysical properties of Cu–Zr liquids in supercooled and stable states, although tremendous effort has been devoted. We provide here the comprehensive thermophysical properties of Cu–Zr liquids, such as undercoolability, density, viscosity, fusion enthalpy, temperature–time-transformation (TTT) diagram, and crystal–liquid interfacial free energy. Three compositions, Cu64Zr36, Cu56Zr44, and Cu50Zr50, show distinctive anomalies in undercoolability, nose time in TTT, and crystal–liquid interfacial free energy, but not in density and viscosity in supercooled and stable liquid states. The anomalies reflect that the GFA is dominantly governed by thermodynamics rather than kinetics in these bulk metallic glasses (BMGs). In addition, we find that positions of nose temperatures in the TTT curves are below 1/2 (Tg + Tl), which implies unequal contribution of thermodynamics and kinetics. We discuss that empirical GFA parameters cannot explain the glass formation of Cu–Zr alloys due to the unequal contribution, and the Turnbull GFA criterion (Trg = Tg/Tl) is valid for the equal contribution of the two effects. The present experimental findings shed light on the ongoing debate about the GFA criterion of Cu–Zr BMGs.

New journal of physics 20(3), 033008 (2018). doi:10.1088/1367-2630/aab548, Arbitrary manipulation of the temporal and spectral properties of x-ray pulses at free-electron lasers would revolutionize many experimental applications. At the Linac Coherent Light Source at Stanford National Accelerator Laboratory, the momentum phase-space of the free-electron laser driving electron bunch can be tuned to emit a pair of x-ray pulses with independently variable photon energy and femtosecond delay. However, while accelerator parameters can easily be adjusted to tune the electron bunch phase-space, the final impact of these actuators on the x-ray pulse cannot be predicted with sufficient precision. Furthermore, shot-to-shot instabilities that distort the pulse shape unpredictably cannot be fully suppressed. Therefore, the ability to directly characterize the x-rays is essential to ensure precise and consistent control. In this work, we have generated x-ray pulse pairs via electron bunch shaping and characterized them on a single-shot basis with femtosecond resolution through time-resolved photoelectron streaking spectroscopy. This achievement completes an important step toward future x-ray pulse shaping techniques., Published by IOP, [London]

We report on high-temperature density measurements of nickel (Ni), zirconium (Zr), niobium (Nb), and hafnium (Hf) in supercooled and stable liquid states by using an electrostatic levitator (ESL) and evaluation of their associated uncertainties. More specifically, this work demonstrates a detailed description of our non-contact measurement method (i.e. schematics of the instrumentation, levitation procedure and density calculation from droplet images). We find that the main contribution of the uncertainties come from measuring sample temperature and mass, aspect ratio of the sample shape, pixel-calibration factors for two-dimensional (2D) detector, and order of the fitting function for calculating the volume. The measurements are typically made with combined uncertainties less than 0.5% and 2.1% for two different types of pyrometers that are used in low temperature (600 K ~ 2800 K) and high temperature (1000 K ~ 3800 K) ranges each operating at a wavelength of 1.6 μm and 0.9 μm, respectively. At melting temperatures, the combined uncertainties of the density measurements of liquid metals are measured less than ± 1.4% for low temperature and ± 2.2% for high temperature cases.

A description of the X-ray Correlation Spectroscopy instrument at the Linac Coherent Light Source is presented. Recent highlights illustrate the coherence properties of the source as well as some recent dynamics measurements and future directions., The X-ray Correlation Spectroscopy instrument is dedicated to the study of dynamics in condensed matter systems using the unique coherence properties of free-electron lasers. It covers a photon energy range of 4–25 keV. The intrinsic temporal characteristics of the Linac Coherent Light Source, in particular the 120 Hz repetition rate, allow for the investigation of slow dynamics (milli­seconds) by means of X-ray photon correlation spectroscopy. Double-pulse schemes could probe dynamics on the picosecond timescale. A description of the instrument capabilities and recent achievements is presented.

Nonlinear optical phenomena in semiconductors present several fundamental problems in modern optics that are of great importance for the development of optoelectronic devices. In particular, the details of photo-induced lattice dynamics at early time-scales prior to carrier recombination remain poorly understood. We demonstrate the first integrated measurements of both optical and structural, material-dependent quantities while also inferring the bulk impulsive strain profile by using high spatial-resolution time-resolved x-ray scattering (TRXS) on bulk crystalline gallium arsenide. Our findings reveal distinctive laser-fluence dependent crystal lattice responses, which are not described by previous TRXS experiments or models. The initial linear expansion of the crystal upon laser excitation stagnates at a laser fluence corresponding to the saturation of the free carrier density before resuming expansion in a third regime at higher fluences where two-photon absorption becomes dominant. Our interpretations of the lattice dynamics as nonlinear optical effects are confirmed by numerical simulations and by additional measurements in an n-type semiconductor that allows higher-order nonlinear optical processes to be directly observed as modulations of x-ray diffraction lineshapes.

In this paper we describe a setup for x-ray scattering experiments on complex fluids using a liquid jet. The setup supports Small and Wide Angle X-ray Scattering (SAXS/WAXS) geometries. The jet is formed by a gas-dynamic virtual nozzle (GDVN) allowing for diameters ranging between 1 μm and 20 μm at a jet length of several hundred μm. To control jet properties such as jet length, diameter, or flow rate, the instrument is equipped with several diagnostic tools. Three microscopes are installed to quantify jet dimensions and stability in situ. The setup has been used at several beamlines performing both SAXS and WAXS experiments. As a typical example we show an experiment on a colloidal dispersion in a liquid jet at the X-ray Correlation Spectroscopy instrument at the Linac Coherent Light Source free-electron laser.

We show that time-resolved x-ray scattering can be applied to polycrystalline materials for the measurement of carrier diffusion. A polycrystalline indium antimonide sample is prepared by high-intensity ultrafast laser surface melting and re-solidification under vacuum to create randomly oriented grains with an average size of 13 nm. Two static diffraction rings are simultaneously observed on a gated pixel array detector. Their centroids move following lower-intensity laser excitation, and utilizing an in-situ angular calibration, the transient lattice spacing is determined with femtometer accuracy, thereby allowing the measurement of charge carrier dynamics. Compared to bulk calculations, we find that carrier diffusion slows by more than one order of magnitude. This result provides evidence for the formation of potential energy barriers at the grain boundaries and demonstrates the capability of time-resolved x-ray scattering to probe nanoscale charge transport in materials other than near-perfect crystals.We show that time-resolved x-ray scattering can be applied to polycrystalline materials for the measurement of carrier diffusion. A polycrystalline indium antimonide sample is prepared by high-intensity ultrafast laser surface melting and re-solidification under vacuum to create randomly oriented grains with an average size of 13 nm. Two static diffraction rings are simultaneously observed on a gated pixel array detector. Their centroids move following lower-intensity laser excitation, and utilizing an in-situ angular calibration, the transient lattice spacing is determined with femtometer accuracy, thereby allowing the measurement of charge carrier dynamics. Compared to bulk calculations, we find that carrier diffusion slows by more than one order of magnitude. This result provides evidence for the formation of potential energy barriers at the grain boundaries and demonstrates the capability of time-resolved x-ray scattering to probe nanoscale charge transport in materials other than near-perfect crystals.

Using a strain-rosette, we demonstrate the existence of transverse strain using time-resolved x-ray diffraction from multiple Bragg reflections in laser-excited bulk gallium arsenide. We find that anisotropic strain is responsible for a considerable fraction of the total lattice motion at early times before thermal equilibrium is achieved. Our measurements are described by a new model where the Poisson ratio drives transverse motion, resulting in the creation of shear waves without the need for an indirect process such as mode conversion at an interface. Using the same excitation geometry with the narrow-gap semiconductor indium antimonide, we detected coherent transverse acoustic oscillations at frequencies of several GHz.

We report on the first integrated apparatus for measuring surface and thermophysical properties and bulk structures of a highly supersaturated solution by combining electrostatic levitation with real-time laser/x-ray scattering. Even today, a proper characterization of supersaturated solutions far above their solubility limits is extremely challenging because heterogeneous nucleation sites such as container walls or impurities readily initiate crystallization before the measurements can be performed. In this work, we demonstrate simultaneous measurements of drying kinetics and surface tension of a potassium dihydrogen phosphate (KH2PO4) aqueous solution droplet and its bulk structural evolution beyond the metastable zone width limit. Our experimental finding shows that the noticeable changes of the surface properties are accompanied by polymerizations of hydrated monomer clusters. The novel electrostatic levitation apparatus presented here provides an effective means for studying a wide range of highly concentrated solutions and liquids in deep metastable states.

We report on the development of a new experimental instrument for time-resolved x-ray scattering (TRXS) at the Pohang Light Source (PLS-II). It operates with a photon energy ranging from 5 to 18 keV. It is equipped with an amplified Ti:sappahire femtosecond laser, optical diagnostics, and laser beam delivery for pump-probe experiments. A high-speed single-element detector and high trigger-rate oscilloscope are used for rapid data acquisition. While this instrument is capable of measuring sub-nanosecond dynamics using standard laser pump/x-ray probe techniques, it also takes advantage of the dense 500 MHz standard fill pattern in the PLS-II storage ring to efficiently record nano-to-micro-second dynamics simultaneously. We demonstrate this capability by measuring both the (fast) impulsive strain and (slower) thermal recovery dynamics of a crystalline InSb sample following intense ultrafast laser excitation. Exploiting the full repetition rate of the storage ring results in a significant improvement in data collection rates compared to conventional bunch-tagging methods.

Physical review letters 109, 185502 (2012). doi:10.1103/PhysRevLett.109.185502, Published by APS, College Park, Md.

We report on the feasibility of high wavevector temporal speckle correlation measurements at the world’s first hard x-ray free electron laser, the Linac Coherent Light Source (LCLS). Due to the chaotic nature of LCLS, the spectral profile of the x-ray radiation fluctuates on a pulse-to-pulse basis. Its impact on the determination of the single shot speckle contrast in a wide angle x-ray scattering geometry is investigated by analyzing FEL power spectra that are simulated based on the nominal operational parameters of LCLS. Ultimately, a potential scheme to deliver a single-mode hard x-ray pulse is proposed by using a narrow bandpass crystal monochromator.

This content may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This material originally appeared in Review of Scientific Instruments 83, 043107 (2012) and may be found at https://doi.org/10.1063/1.3698294., The soft x-ray materials science instrument is the second operational beamline at the linac coherent light source x-ray free electron laser. The instrument operates with a photon energy range of 480���2000 eV and features a grating monochromator as well as bendable refocusing mirrors. A broad range of experimental stations may be installed to study diverse scientific topics such as: ultrafast chemistry, surface science, highly correlated electron systems, matter under extreme conditions, and laboratory astrophysics. Preliminary commissioning results are presented including the first soft x-ray single-shot energy spectrum from a free electron laser.

Physical review letters 108, 267403 (2012). doi:10.1103/PhysRevLett.108.267403, Published by APS, College Park, Md.

Physical review letters 108, 024801 (2012). doi:10.1103/PhysRevLett.108.024801, Published by APS, College Park, Md.

A prototype device capable of splitting an X-ray pulse into two adjustable fractions, delaying one of them with the aim of performing split pulse X-ray photon correlation spectroscopy and pump–probe type studies was designed and manufactured. Time delays up to 2.95 ns have been demonstrated. The achieved contrast values of 56% indicate a feasibility of performing coherence-based experiments with the delay line., A hard X-ray delay line capable of splitting and delaying single X-ray pulses has been developed with the aim of performing X-ray photon correlation spectroscopy (XPCS) and X-ray pump–probe experiments at hard X-ray free-electron laser sources. The performance of the device was tested with 8.39 keV synchrotron radiation. Time delays up to 2.95 ns have been demonstrated. The feasibility of the device for performing XPCS studies was tested by recording static speckle patterns. The achieved speckle contrast of 56% indicates the possibility of performing ultra-fast XPCS studies with the delay line.

The ability to synchronize a femtosecond laser to x-ray pulses is crucial for performing ultrafast time-resolved x-ray scattering experiments at synchrotrons. Conventionally, the task has been achieved by locking a harmonic frequency of the laser oscillator to the storage ring master radio-frequency (RF). However, when the frequency mismatch between the two sources cannot be compensated by small adjustments to the laser cavity length, synchronization to a harmonic frequency requires modifying the optical components of the laser system. We demonstrate a novel synchronization scheme, which is a flexible alternative for synchronizing these two sources operating at arbitrarily different frequencies. First, we find the greatest common divisor (GCD) of the two frequencies that is still within the limited tuning range of the laser cavity length. The GCD is generated by dividing down from the storage ring RF, and is separately multiplied up to provide a feedback signal for synchronizing the laser cavity. Unique to our scheme, the GCD also serves as a harmonic RF source for the laser amplifier such that only laser oscillator pulses at fixed integer multiples of the storage ring RF are selected for amplification and delivery to experiments. Our method is implemented at the Photon Test Facility beamline of Pohang Light Source where timing-jitter less than 4 ps (r.m.s.) is measured using a new shot-to-shot method.

We report an alternate light scattering approach to measure intermediate scattering function and structures of colloidal suspension by using two-pulse speckle contrast correlation analysis. By systematically controlling time-delays between two laser pulses incident on the sample, we are able to monitor transient evolution of coherent diffraction pattern, from which particle dynamics at different length and time scales are obtained simultaneously. Our result demonstrates the feasibility of utilizing a megapixel detector to achieve sufficient data statistics in a short amount of time while enabling microsecond time-resolution. Ultimately, this method provides means to measure high-speed dynamics well beyond the time response limit of a large area two-dimensional (2D) detector. (C) 2014 Optical Society of America

We describe the design and operation of a transmissive single-shot spectrometer for the measurement of hard x-ray free electron laser (FEL) source spectrum at the Linac Coherent Light Source (LCLS). The spectrometer was built around a 10 μm thick near-perfect silicon single crystal that was cylindrically bent. Its energy range covered the full FEL bandwidth while its resolution was sufficient for resolving single spectral spikes characteristics of the FELs. Its application will not only greatly facilitate the understanding and optimization of the x-ray FEL sources, but can also serve as an invaluable inline diagnostic tool for experiments where the spectral content of the source plays an important role in data interpretation.