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Chong-Yu Ruan

Publications and source records attributed to Chong-Yu Ruan.

14 recordsLinked to original sources
arXiv2024

Precision-controlled ultrafast electron microscope platforms. A case study: Multiple-order coherent phonon dynamics in 1T-TaSe$_2$ probed at 50 femtosecond - 10 femtometer scales

We report on the first detailed beam test attesting the fundamental principle behind the development of high-current-efficiency ultrafast electron microscope systems where a radio-frequency cavity is incorporated as a condenser lens in the beam delivery system. To allow the experiment to be carried out with a sufficient resolution to probe the performance at the emittance floor, a new cascade loop RF controller system is developed to reduce the RF noise floor. Temporal resolution at 50 femtoseconds in full-width-at-half-maximum and detection sensitivity better than 1% are demonstrated on exfoliated 1T-TaSe$_2$ layers where the multi-order edge-mode coherent phonon excitation is employed as the standard candle to benchmark the performance. The high temporal resolution and the significant visibility to very low dynamical contrast in diffraction signals give strong support to the working principle of the high-brightness beam delivery via phase-space manipulation in the electron microscope system.

cond-mat.mtrl-sci↗
arXiv2021

Toward nonthermal control of excited quantum materials: framework and investigations by ultrafast electron scattering and imaging

Quantum material systems upon applying ultrashort laser pulses provide a rich platform to access excited material phases and their transformations that are not entirely like their equilibrium counterparts. The addressability and potential controls of metastable or long-trapped out-of-equilibrium phases have motivated interests both for the purposes of understanding the nonequilibrium physics and advancing the quantum technologies. Thus far, the dynamical spectroscopic probes eminently focus on microscopic electronic and phonon responses. For characterizing the long-range dynamics, such as order parameter fields and fluctuation effects, the ultrafast scattering probes offer direct sensitivity. Bridging the connections between the microscopic dynamics and macroscopic responses is central toward establishing the nonequilibrium physics behind the light-induced phases. Here, we present a path toward such understanding by cross-examining the structure factors associated with different dynamical states obtained from ultrafast electrons scattering, imaging, and modeling. We give the basic theoretical framework on describing the non-equilibrium scattering problems and briefly describe how such framework relates to the out-of-equilibrium phenomena. We give effective models outlining the emergences of nonthermal critical points, hidden phases, and non-equilibrium relaxational responses from vacuum-suspended rare-earth tritellurides, tantalum disulfides thin films, and vanadium dioxide nanocrystalline materials upon light excitations.

cond-mat.str-el↗
arXiv2019

Nonequilibrium dynamics of spontaneous symmetry breaking into a hidden state of charge-density wave

Nonequilibrium phase transition plays a pivotal role in a broad physical context from condensed matter to cosmology. Tracking the formation of non-equilibrium phases in condensed matter is challenging and requires a resolution of the long-range cooperativity on ultra-short timescales. Here, we study the spontaneous symmetry breaking transformation of a charge-density wave system from a stripe phase into a checkerboard state. Such a state is thermodynamically forbidden, but is introduced through a system quench using ultrashort, intense laser pulses. The dynamics is mediated by the soft modes that unfold spontaneously and order the field on a timescale ~1 ps. Using the coherent electron diffraction with ~100 fs resolution, we capture the entire course and demonstrate nonergodic behavior proximal to symmetry breaking that is crucial for stabilizing the hidden states. Remarkably, the thermalization due to carriers cooling arrests the remnants of the transient orders into the topological defects in the eventual state with distinct new properties that last for more than 1 ns. The fundamental dynamics observed here opens an intriguing perspective of controlling phase transitions in quantum materials far from equilibrium.

cond-mat.mes-hall↗
arXiv2017

Dynamical bunching and density peaks in expanding Coulomb clouds

Expansion dynamics of single-species, non-neutral clouds, such as electron bunches used in ultrafast electron microscopy, show novel behavior due to high acceleration of particles in the cloud interior. This often leads to electron bunching and dynamical formation of a density shock in the outer regions of the bunch. We develop analytic fluid models to capture these effects, and the analytic predictions are validated by PIC and N-particle simulations. In the space-charge dominated regime, two and three dimensional systems with Gaussian initial densities show bunching and a strong shock response, while one dimensional systems do not; moreover these effects can be tuned using the initial particle density profile and velocity chirp.

physics.acc-ph↗
arXiv2013

A brief review: Ultrafast electron diffractive voltammetry: General formalism and applications

We present a general formalism of ultrafast diffractive voltammetry approach as a contact-free tool to investigate the ultrafast surface charge dynamics in nanostructured interfaces. As case studies, the photoinduced surface charging processes in oxidized silicon surface and the hot electron dynamics in nanoparticle-decorated interface are examined based on the diffractive voltammetry framework. We identify that the charge redistribution processes appear on the surface, sub-surface, and vacuum levels when driven by intense femtosecond laser pulses. To elucidate the voltammetry contribution from different sources, we perform controlled experiments using shadow imaging techniques and N-particle simulations to aid the investigation of the photovoltage dynamics in the presence of pho- toemission. We show that voltammetry contribution associated with photoemission has a long decay tail and plays a more visible role in the nanosecond timescale, whereas the ultrafast voltammetry are dominated by local charge transfer, such as surface charging and molecular charge transport at nanostructured interfaces. We also discuss the general applicability of the diffractive voltammetry as an integral part of quantitative ultrafast electron diffraction methodology in researching different types of interfaces having distinctive surface diffraction and boundary conditions.

physics.chem-ph↗
arXiv2009

Quantitative nanoparticle structures from ultrafast electron crystallography data

We describe the quantitative refinement of nanoparticle structures from gold nanoparticles probed by ultrafast electron crystallography (UEC). We establish the equivalence between the modified radial distribution function employed in UEC and the atomic pair distribution function (PDF) used in x-ray and neutron powder diffraction analysis. By leveraging PDF refinement techniques, we demonstrate that UEC data are of sufficient quality to differentiate between cuboctahedral, decahedral and icosahedral nanoparticle models. Furthermore, we identify the signatures of systematic errors that may occur during data reduction and show that atomic positions refined from UEC are robust to these errors. This work serves as a foundation for reliable quantitative structural analysis of time-resolved laser-excited nanoparticle states.

cond-mat.mtrl-sci↗
arXiv2009

Ultrafast imaging of photoelectron packets generated from graphite surface

We present an electron projection imaging method to study the ultrafast evolution of photoelectron density distribution and transient fields near the surface. The dynamical profile of the photoelectrons from graphite reveals an origin of a thermionic emission, followed by an adiabatic process leading to electron acceleration and cooling before a freely expanding cloud is established. The hot electron emission is found to couple with a surface charge dipole layer formation, with a sheet density several orders of magnitude higher than that of the vacuum emitted cloud.

cond-mat.mtrl-sci↗
arXiv2009

The development and applications of ultrafast electron nanocrystallography

We review the development of ultrafast electron nanocrystallography as a method for investigating structural dynamics for nanoscale materials and interfaces. Its sensitivity and resolution are demonstrated in the studies of surface melting of gold nanocrystals, nonequilibrium transformation of graphite into reversible diamond-like intermediates, and molecular scale charge dynamics, showing a versatility for not only determining the structures, but also the charge and energy redistribution at interfaces. A quantitative scheme for three-dimensional retrieval of atomic structures is demonstrated with few-particle (< 1000) sensitivity, establishing this nanocrystallographic method as a tool for directly visualizing dynamics within isolated nanomaterials with atomic scale spatio-temporal resolution.

cond-mat.mes-hall↗
arXiv2008

Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of ~< 21mJ/cm^2, lattice vibrations are observed to thermalize on a time scale of ~8ps. At higher fluences approaching the damage threshold, lattice vibration amplitudes saturate. Following a marked initial contraction, graphite is driven nonthermally into a transient state with sp^3-like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photo-excitation.

cond-mat.mtrl-sci↗
arXiv2008

Photovoltage Dynamics of the Hydroxylated Si(111) Surface Investigated by Ultrafast Electron Diffraction

We present a novel method to measure transient photovoltage at nanointerfaces using ultrafast electron diffraction. In particular, we report our results on the photoinduced electronic excitations and their ensuing relaxations in a hydroxyl-terminated silicon surface, a standard substrate for fabricating molecular electronics interfaces. The transient surface voltage is determined by observing Coulomb refraction changes induced by the modified space-charge barrier within a selectively probed volume by femtosecond electron pulses. The results are in agreement with ultrafast photoemission studies of surface state charging, suggesting a charge relaxation mechanism closely coupled to the carrier dynamics near the surface that can be described by a drift-diffusion model. This study demonstrates a newly implemented ultrafast diffraction method for investigating interfacial processes, with both charge and structure resolution.

cond-mat.mtrl-sci↗
arXiv2007

Dynamics of Size-Selected Gold Nanoparticles Studied by Ultrafast Electron Nanocrystallography

We report the studies of ultrafast electron nanocrystallography on size-selected Au nanoparticles (2-20 nm) supported on a molecular interface. Reversible surface melting, melting, and recrystallization were investigated with dynamical full-profile radial distribution functions determined with sub-picosecond and picometer accuracies. In an ultrafast photoinduced melting, the nanoparticles are driven to a non-equilibrium transformation, characterized by the initial lattice deformations, nonequilibrium electron-phonon coupling, and upon melting, the collective bonding and debonding, transforming nanocrystals into shelled nanoliquids. The displasive structural excitation at premelting and the coherent transformation with crystal/liquid coexistence during photomelting differ from the reciprocal behavior of recrystallization, where a hot lattice forms from liquid and then thermally contracts. The degree of structural change and the thermodynamics of melting are found to depend on the size of nanoparticle.

cond-mat.mtrl-sci↗
arXiv1999

Investigation of a 90 Degree Spherical Deflecting Analyzer Operated in an Asymmetrically Charged Configuration

The electron optics of a 90 degree spherical deflecting analyzer (SDA-90) is investigated with an imaging matrix formalism. As a preanalyzer in the UTA-neutrino experiment, high transmission and reasonable energy resolution are the choices of optimization. The magnification of the source through the analyzer plays the key role in determining the energy resolution. The imaging matrix approach provides graphical information to facilitate such an evaluation. We can demonstrate that in case where the analyzer is asymmetrically charged, the rotation of the image helps increase both transmission probability and resolution. A telefocus electron gun is used to check the numerical result, and to investigate the transverse focusing behavior.

physics.optics↗
arXiv1999

Emission Optics of the Steigerwald Type Electron Gun

The emission optics of a Steigerwald type electron gun is re-examined. The virtual and real points of divergence, divergence angles and beam-widths of the electron beams at different telefocusing strength are measured in detail for first time . Two different Wehnelt cylinders are used to establish a contrasting viewpoint. The original `focusing' curves measured by Braucks are reconstructed and will be explained only through a `new' interpretation which is different from the conventional views. While the image of the emitting surface in front of the filament is indeed telefocused beyond the anode, the envelope of the beam does not `focus' as expected. A new model for the emission mechanism is established based on our results.

physics.optics↗