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Yuelin Li

Publications and source records attributed to Yuelin Li.

13 recordsLinked to original sources
arXiv2026

Fast and bright scintillators for ultrafast materials dynamics using 4th generation synchrotron

We present recent advances in fast and bright scintillators for ultrafast X-ray phase contrast imaging of dynamic materials experiments at the upgraded Advanced Photon Source (APS-U), a fourth generation synchrotron. APS-U enables hard X-ray imaging at frame rates of at least 13 MHz (corresponding to 77 ns or shorter interframe intervals), creating a new need for scintillators with faster response and higher light output than lutetium yttrium oxyorthosilicate (LYSO). For indirect imaging and diffraction with ultrafast cameras, commercial lanthanum bromide (LaBr3) and cerium bromide (CeBr3) are promising candidates. These materials exhibit decay times approximately a factor of two shorter than LYSO (around 40 ns) and lutetium oxyorthosilicate (LSO), while maintaining comparable light yield per incident X-ray photon. However, their implementation at APS-U requires addressing several challenges, including material limitations due to hygroscopicity, efficient optical coupling to imaging systems, and high quantum efficiency for conversion of scintillation light, predominantly at wavelengths below 400 nm, into detectable electronic signals. We report results from material characterization, detector integration and packaging, and beamline experiments of materials with impact. In addition, emerging scintillator classes, including perovskites and high-entropy materials, are discussed as potential alternatives for next-generation ultrafast X-ray diagnostics.

physics.ins-det↗
arXiv2024

Products and powers of principal symmetric ideals

Principal symmetric ideals were recently introduced by Harada, Seceleanu, and Sega, with a focus on their homological properties. They are ideals generated by the orbit of a single polynomial under permutations of variables in a polynomial ring. In this paper we seek to determine when a product of two principal symmetric ideals is principal symmetric and when all the powers of a principal symmetric ideal are again principal symmetric ideals. We characterize the ideals that have the latter property as being generated by polynomials invariant up to a scalar multiple under permutation of variables. Recognizing principal symmetric ideals is an open question for the purpose of which we produce certain obstructions. We also demonstrate that the Hilbert functions of symmetric monomial ideals are not all given by symmetric monomial ideals, in contrast to the non-symmetric case.

math.AC↗
arXiv2020

Detonation-induced transformation of graphite to hexagonal diamond

We explore the structural evolution of highly oriented pyrolytic graphite (HOPG) under detonation-induced shock conditions using in-situ synchrotron X-ray diffraction in the ns time scale. We observe the formation of hexagonal diamond (lonsdaleite) at pressures above 50 GPa, in qualitative agreement with recent gas gun experiments. First-principles density functional calculations reveal that under uniaxial compression the energy barrier for the transition towards hexagonal diamond is lower than cubic diamond. Finally, no indication of cubic diamond formation was observed up to >70 GPa.

cond-mat.mtrl-sci↗
arXiv2018

Giant photoinduced lattice distortion in oxygen-vacancy ordered SrCoO2.5 thin films

Despite of the tremendous efforts spent on the oxygen vacancy migration in determining the property optimization of oxygen-vacancy enrichment transition metal oxides, few has focused on their dynamic behaviors non-equilibrium states. In this work, we performed multi-timescale ultrafast X-ray diffraction measurements by using picosecond synchrotron X-ray pulses and femtosecond table-top X-ray pulses to monitor the structural dynamics in the oxygen-vacancy ordered SrCoO2.5 thin films. A giant photoinduced strain (Δc/c > 1%) was observed, whose distinct correlation with the pump photon energy indicates a non-thermal origin of the photoinduced strain. The sub-picosecond resolution X-ray diffraction reveals the formation and propagation of the coherent acoustic phonons inside the film. We also simulate the effect of photoexcited electron-hole pairs and the resulting lattice changes using the Density Function Theory method to obtain further insight on the microscopic mechanism of the measured photostriction effect. Comparable photostrictive responses and the strong dependence on excitation wavelength are predicted, revealing a bonding to anti-bonding charge transfer or high spin to intermediate spin crossover induced lattice expansion in the oxygen-vacancy films.

cond-mat.mtrl-sci↗
arXiv2017

Photoinduced coherent acoustic phonon dynamics inside Mott insulator Sr2IrO4 films observed by femtosecond X-ray pulses

We investigate the transient photoexcited lattice dynamics in a layered perovskite Mott insulator Sr2IrO4 by femtosecond X-ray diffraction using a laser plasma-based X-ray source. Ultrafast structural dynamics of Sr2IrO4 thin films are determined by observing the shift and broadening of the (0012) Bragg diffraction after excitation by 1.5 eV and 3.0 eV pump photons for films with different thicknesses. The observed transient lattice response can be well interpreted as a distinct three-step dynamics due to the propagation of coherent acoustic phonons generated by the photoinduced quasiparticles (QP). Employing a normalized phonon propagation model, we found that the photoinduced angular shifts of the Bragg peak collapse into a universal curve after introducing a normalized coordinates to account for different thicknesses and pump photon energies, pinpointing the origin of the lattice distortion and its early evolution. In addition, a transient photocurrent measurement indicates that the photoinduced QPs are charge neutral excitons. Mapping the phonon propagation and correlating its dynamics with the QP by ultrafast X-ray diffraction (UXRD) establish a powerful way to study electron-phonon coupling and uncover the exotic physics in strongly correlated systems under nonequilibrium conditions.

physics.atm-clus↗
arXiv2016

Effects of biaxial strain on the improper multiferroicity in h-LuFeO3 films

Elastic strain is potentially an important approach in tuning the properties of the improperly multiferroic hexagonal ferrites, the details of which have however been elusive due to the experimental difficulties. Employing the method of restrained thermal expansion, we have studied the effect of isothermal biaxial strain in the basal plane of h-LuFeO3 (001) films. The results indicate that a compressive biaxial strain significantly enhances the ferrodistortion, and the effect is larger at higher temperatures. The compressive biaxial strain and the enhanced ferrodistortion together, cause an increase in the electric polarization and a reduction in the canting of the weak ferromagnetic moments in h-LuFeO3, according to our first principle calculations. These findings are important for understanding the strain effect as well as the coupling between the lattice and the improper multiferroicity in h-LuFeO3. The experimental elucidation of the strain effect in h-LuFeO3 films also suggests that the restrained thermal expansion can be a viable method to unravel the strain effect in many other epitaxial thin film materials.

cond-mat.mtrl-sci↗
arXiv2015

Giant optical enhancement of strain gradient in ferroelectric thin films and its physics origin

The coupling between strain gradients and polarization, known as flexoelectricity, offers a new mechanism to control the functionality of dielectric materials. However, for the effect to be practically attractive, dynamic control of the strain gradient with magnitudes far exceeding those achievable via mechanical deformation (~10 $m^{-1}$) is needed. Strain-engineered thin films exhibit extraordinary strain gradients of $10^5-10^6 m{-1}$ arising from structural relaxation within a short space range that greatly enhances the steady-state flexoelectric effect. Here we report a giant, optically initiated dynamic enhancement of the strain gradient, also on the order of $10^5 -10^6 m^{-1}$, in ferroelectric BiFeO3 epitaxial thin films via time-dependent coherence analysis of X-ray diffractions. The finding opens the door for dynamic coupling of the flexoelectric effect with light, making optical switching of polarization, and thus application such as direct optical writing of non-volatile ferroelectric memory, possible. A combination of time-resolved X-ray scattering and optical spectroscopy shows that the enhancement of the strain gradient is due to a piezoelectric effect driven by a transient screening electric field, opening the opportunity for new ways of studying flexoelectric effect in strain engineered ferroelectric thin films.

cond-mat.mtrl-sci↗
arXiv2015

Lattice correlation of Hubbard excitons in a Mott insulator Sr2IrO4 and reconstruction of their hopping dynamics via time-dependent coherence analysis of the Bragg diffraction

In correlated oxides the coupling of quasiparticles to other degrees of freedom such as spin and lattice plays critical roles in the emergence of symmetry-breaking quantum ordered states such as high temperature superconductivity. We report a strong lattice coupling of photon induced Hubbard excitonic quasiparticles in spin-orbital coupling Mott insulator Sr2IrO4. Combining time-resolved optical spectroscopy techniques, we further reconstructed spatiotemporal map of the diffusion of quasiparticles via time-dependent coherence analysis of the x-ray Bragg diffraction peak. Due to the unique electronic configuration of the exciton, the strong lattice correlation is unexpected but extends the similarity between Sr2IrO4 and cuprates under highly non-equilibrium conditions. The coherence analysis method we developed may have important implications for characterizing the structure and carrier dynamics in a wider group of oxide heterostructures.

cond-mat.mtrl-sci↗
arXiv2015

Evidence of photo-induced dynamic competition of metallic and insulating phase in a layered manganite

We show evidence that the competition between the antiferromagetic metallic phase and the charge- and orbital-ordered insulating phase at the reentrant phase boundary of a layered manganite, LaSr2Mn2O7, can be manipulated using ultrafast optical excitation. The time-dependent evolution of the Jahn-Teller superlattice reflection, which indicates the formation of the charge and orbital order, was measured at different laser fluences. The laser-induced enhancement and reduction the Jahn-Teller reflection intensity shows a reversal of sign between earlier (~10 ns) and later (~150 ns) time delays during the relaxation after photo excitation. This effect is consistent with a scenario whereby the laser excitation modulates the local competition between the metallic and the insulating phases.

cond-mat.mtrl-sci↗
arXiv2013

Structural and Electronic Recovery Pathways of a Photoexcited Ultrathin VO2 Film

The structural and electronic recovery pathways of a photoexcited ultrathin VO2 film at nanosecond time scales have been studied using time-resolved x-ray diffraction and transient optical absorption techniques. The recovery pathways from the tetragonal metallic phase to the monoclinic insulating phase are highly dependent on the optical pump fluence. At pump fluences higher than the saturation fluence of 14.7 mJ/cm2, we observed a transient structural state with lattice parameter larger than that of the tetragonal phase, which is decoupled from the metal-to-insulator phase transition. Subsequently, the photoexcited VO2 film recovered to the ground state at characteristic times dependent upon the pump fluence, as a result of heat transport from the film to the substrate. We present a procedure to measure the time-resolved film temperature by correlating photoexcited and temperature-dependent x-ray diffraction measurements. A thermal transport model that incorporates changes of the thermal parameters across the phase transition reproduces the observed recovery dynamics. The optical excitation and fast recovery of ultrathin VO2 films provides a practical method to reversibly switch between the monoclinic insulating and tetragonal metallic state at nanosecond time scales.

cond-mat.str-el↗
arXiv2008

"Fractal Expression" in Chinese Calligraphy

We show that from historical record and mathematic analysis, "fractal expressions" may have been a conscious pursuit at least one thousand years ago as an element of beauty in ancient Chinese calligraphy.

physics.gen-ph↗
arXiv2006

Femtosecond electron and x-ray source based on laser wakefield accelerator

A terawatt tabletop laser wakefield acceleration source of relativistic electrons has been developed in our Terawatt Ultrafast High Field Facility (TUHFF). The preliminary results for ultrafast radiolysis of liquid water using this femtosecond electron source are presented. A TUHFF based femtosecond x-ray source is proposed. Thomson scattering of the accelerated electrons off a counterpropagating terawatt laser beam will be used to generate keV x-ray photons. The expected parameters of this x-ray source have been estimated. The short pulse duration, high flux, and good collimation of the resulting x-ray beam would be conducive for ultrafast time-resolved x-ray absorption studies of short-lived transient species in gases, liquids, and solids. It is argued that the solvation dynamics of Br atoms generated in photoinduced electron detachment from aqueous bromide would make a convenient choice for the first pump-probe experiment using this x-ray source.

physics.ins-det↗