arXiv subjects

Stephen Weathersby

Publications and source records attributed to Stephen Weathersby.

15 recordsLinked to original sources

arXiv2026

Nanoscale Confinement Enhances Ultrafast Demagnetization

Nanoscale miniaturization has revolutionized the field of spintronics by enabling exponential growth in areal bit density. A similar leap is also expected in device speeds through successfully harnessing femtosecond magnetization dynamics. However, combining this with the miniaturization of realistic devices is challenging. To address this, we studied the effect of dimensional confinement on the femtosecond demagnetization of Fe. By gradually increasing the level of confinement while keeping excitation conditions constant, we found that Fe layers thinner than 10 nm exhibit enlarged demagnetization amplitudes, reaching a $\sim75\%$ increase at 2 nm. By combining ultrafast experiments sensitive to the spins, the charge carriers, and the phonons, we establish that this finite$\text{-}$size effect is magnetic in origin and is not phonon$\text{-}$driven. With the support of ab$\text{-}$initio calculations and atomistic spin dynamics simulations, we identify the enhancement effect as due to local weakening of spin order at the Fe$\text{'}$s interface, which becomes significant upon increased confinement.

cond-mat.mes-hall↗

arXiv2025

Dynamics of a jointly commensurate moir\'e charge density wave

The advent of two-dimensional moir\'e systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moir\'e superstructure formed by two coexisting charge density waves (CDWs) with slightly mismatched wavevectors has been realized. These incommensurate CDWs can collectively exhibit commensurability, resulting in the jointly commensurate CDW (JC-CDW) and establishing a new paradigm for controlling moir\'e potential and periodicity. Achieving such functionality, however, hinges on a key open question: how do the amplitude, phase coherence, and periodicity of this order respond to external perturbations? Here, we address this question using a suite of time- and momentum-resolved diffraction and spectroscopic techniques to probe light-induced CDW dynamics in EuTe$_4$. Our time-resolved diffraction measurements distinguish the instantaneous quenching of the JC-CDW amplitude, as verified by time-resolved photoemission spectroscopy, from the much slower evolution of phase fluctuations. Furthermore, while the JC-CDW wavevector remains locked along the CDW direction upon photoexcitation, indicating a preserved moir\'e periodicity, the correlation length of JC-CDW shows an exclusive reduction perpendicular to its wavevector, unveiling the formation of previously unexplored shear-type defects. Together, this multimodal methodology reconstructs the spatiotemporal evolution of the JC-CDW upon excitation. These findings not only highlight the remarkable robustness of JC-CDWs out of equilibrium, but also provide insight into optical manipulation and engineering of moir\'e quantum materials through defect control.

cond-mat.str-el↗

arXiv2024

Calculation of RF-induced Temporal Jitter in Ultrafast Electron Diffraction

A significant contribution to the temporal resolution of an ultrafast electron diffraction (UED) instrument is arrival time jitter caused by amplitude and phase variation of radio frequency (RF) cavities. In this paper, we present a semi-analytical approach for calculating RF-induced temporal jitter from klystron and RF cavity parameters. Our approach allows fast estimation of temporal jitter for MeV-UED beamlines and can serve as a virtual timing tool when shot-to-shot measurements of RF amplitude and phase jitters are available. A simulation study for the SLAC MeV-UED instrument is presented and the temporal resolution for several beamline configurations are compared.

physics.acc-ph↗

arXiv2024

Element-specific ultrafast lattice dynamics in FePt nanoparticles

Light-matter interaction at the nanoscale in magnetic alloys and heterostructures is a topic of intense research in view of potential applications in high-density magnetic recording. While the element-specific dynamics of electron spins is directly accessible to resonant x-ray pulses with femtosecond time structure, the possible element-specific atomic motion remains largely unexplored. We use ultrafast electron diffraction to probe the temporal evolution of lattice Bragg peaks of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. The diffraction interference between Fe and Pt sublattices enables us to demonstrate that the Fe mean-square vibration amplitudes are significantly larger that those of Pt as expected from their different atomic mass. Both are found to increase as energy is transferred from the laser-excited electrons to the lattice. Contrary to this intuitive behavior, we observe a laser-induced lattice expansion that is larger for Pt than for Fe atoms during the first picosecond after laser excitation. This effect points to the strain-wave driven lattice expansion with the longitudinal acoustic Pt motion dominating that of Fe.

cond-mat.mtrl-sci↗

arXiv2024

Multi-Objective Bayesian Active Learning for MeV-ultrafast electron diffraction

Ultrafast electron diffraction using MeV energy beams(MeV-UED) has enabled unprecedented scientific opportunities in the study of ultrafast structural dynamics in a variety of gas, liquid and solid state systems. Broad scientific applications usually pose different requirements for electron probe properties. Due to the complex, nonlinear and correlated nature of accelerator systems, electron beam property optimization is a time-taking process and often relies on extensive hand-tuning by experienced human operators. Algorithm based efficient online tuning strategies are highly desired. Here, we demonstrate multi-objective Bayesian active learning for speeding up online beam tuning at the SLAC MeV-UED facility. The multi-objective Bayesian optimization algorithm was used for efficiently searching the parameter space and mapping out the Pareto Fronts which give the trade-offs between key beam properties. Such scheme enables an unprecedented overview of the global behavior of the experimental system and takes a significantly smaller number of measurements compared with traditional methods such as a grid scan. This methodology can be applied in other experimental scenarios that require simultaneously optimizing multiple objectives by explorations in high dimensional, nonlinear and correlated systems.

physics.acc-ph↗

arXiv2023

Investigating dissociation pathways of nitrobenzene via mega-electron-volt ultrafast electron diffraction

As the simplest nitroaromatic compound, nitrobenzene is an interesting model system to explore the rich photochemistry of nitroaromatic compounds. Previous measurements of nitrobenzene's photochemical dynamics have probed structural and electronic properties, which, at times, paint a convoluted and sometimes contradictory description of the photochemical landscape. A sub-picosecond structural probe can complement previous electronic measurements and aid in determining the photochemical dynamics with less ambiguity. We investigate the ultrafast dynamics of nitrobenzene triggered by photoexcitation at 267 nm employing megaelectronvolt ultrafast electron diffraction with femtosecond time resolution. We measure the first 5 ps of dynamics and, by comparing our measured results to simulation, we unambiguously distinguish the lowest singlet and triplet electronic states. We observe ground state recovery within 160 +/- 60 fs through internal conversions and without signal corresponding to photofragmentation. Our lack of dissociation signal within the first 5 ps indicates that previously observed photofragmenation reactions take place in the vibrationally "hot" ground state on timescales considerably beyond 5 ps.

physics.chem-ph↗

arXiv2022

Applying Bayesian Inference and deterministic anisotropy to retrieve the molecular structure $|\Psi(\boldsymbol{R})|^2$ distribution from gas-phase diffraction experiments

Currently, our general approach to retrieving molecular structures from ultrafast gas-phase diffraction heavily relies on complex ab initio electronic or vibrational excited state simulations to make conclusive interpretations. Without such simulations, inverting this measurement for the structural probability distribution is typically intractable. This creates a so-called inverse problem. In this work, we develop a broadly applicable method that addresses this inverse problem by approximating the molecular frame structure $|\Psi(\boldsymbol{R}, t)|^2$ distribution independent of these complex simulations. We retrieve the vibronic ground state $|\Psi(\boldsymbol{R})|^2$ for both simulated stretched NO$_2$ and measured N$_2$O. From measured N$_2$O, we observe 40 mAngstroms coordinate-space resolution from 3.75 inverse Angstroms reciprocal space range and poor signal-to-noise, a 50X improvement over traditional Fourier transform methods. In simulated NO$_2$, typical to high signal-to-noise levels predict 100--1000X resolution improvements, down to 0.1 mAngstroms. By directly measuring the width of $|\Psi(\boldsymbol{R})|^2$, we open ultrafast gas-phase diffraction capabilities to measurements beyond current analysis approaches. This method has the potential to effectively turn gas-phase ultrafast diffraction into a discovery-oriented technique to probe systems that are prohibitively difficult to simulate.

physics.atom-ph↗

arXiv2022

Phonon-Mediated Attractive Interactions between Excitons in Lead-Halide-Perovskites

Understanding the origin of electron-phonon coupling in lead-halide perovskites (LHP) is key to interpreting and leveraging their optical and electronic properties. Here we perform femtosecond-resolved, optical-pump, electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in LHP nanocrystals. Photoexcitation is found to drive a reduction in lead-halide octahedra tilts and distortions in the lattice, a result of deformation potential coupling to low energy optical phonons. Our results indicate particularly strong coupling in FAPbBr3, and far weaker coupling in CsPbBr3, highlighting differences in the dominant machanisms governing electron-phonon coupling in LHPs. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multiexcitonic state constructively interfere, giving rise to a coupling strength which scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in LHPs.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast formation of transient 2D diamond-like structure in twisted bilayer graphene

Due to the absence of matching carbon atoms at honeycomb centers with carbon atoms in adjacent graphene sheets, theorists predicted that a sliding process is needed to form AA, AB, or ABC stacking when directly converting graphite into sp3 bonded diamond. Here, using twisted bilayer graphene, which naturally provides AA and AB stacking configurations, we report the ultrafast formation of a transient 2D diamond-like structure (which is not observed in aligned graphene) under femtosecond laser irradiation. This photo-induced phase transition is evidenced by the appearance of new bond lengths of 1.94A and 3.14A in the time-dependent differential pair distribution function using MeV ultrafast electron diffraction. Molecular dynamics and first principles calculation indicate that sp3 bonds nucleate at AA and AB stacked areas in moire pattern. This work sheds light on the direct graphite-to-diamond transformation mechanism, which has not been fully understood for more than 60 years.

cond-mat.mtrl-sci↗

arXiv2021

Visualizing femtosecond dynamics with ultrafast electron probes through terahertz compression and time-stamping

Visualizing ultrafast dynamics at the atomic scale requires time-resolved pump-probe characterization with femtosecond temporal resolution. For single-shot ultrafast electron diffraction (UED) with fully relativistic electron bunch probes, existing techniques are limited by the achievable electron probe bunch length, charge, and timing jitter. We present the first experimental demonstration of pump-probe UED with THz-driven compression and time-stamping that enable UED probes with unprecedented temporal resolution. This technique utilizes two counter-propagating quasi-single-cycle THz pulses generated from two OH-1 organic crystals coupled into an optimized THz compressor structure. Ultrafast dynamics of photoexcited bismuth films show an improved temporal resolution from 178 fs down to 85 fs when the THz-compressed UED probes are used with no time-stamping correction. Furthermore, we use a novel time-stamping technique to reveal transient oscillations in the dynamical response of THz-excited single-crystal gold films previously inaccessible by standard UED, achieving a time-stamped temporal resolution down to 5 fs.

physics.optics↗

arXiv2020

Experimental demonstration of particle acceleration with normal conducting accelerating structure at cryogenic temperature

Reducing the operating temperature of normal conducting particle accelerators substantially increases their efficiency. Low-temperature operation increases the yield strength of the accelerator material and reduces surface resistance, hence a great reduction in cyclic fatigue could be achieved resulting in a large reduction in breakdown rates compared to room-temperature operation. Furthermore, temperature reduction increases the intrinsic quality factor of the accelerating cavities, and consequently, the shunt impedance leading to increased system efficiency and beam loading capabilities. In this paper, we present an experimental demonstration of the high-gradient operation of an X-band, 11.424 GHz, 20-cells linear accelerator (linac) operating at a liquid nitrogen temperature of 77 K. The tested linac was previously processed and tested at room temperature. We verified the enhanced accelerating parameters of the tested accelerator at cryogenic temperature using different measurements including electron beam acceleration up to a gradient of 150 MV/m, corresponding to a peak surface electric field of 375 MV/m. We also measured the breakdown rates in the tested structure showing a reduction of two orders of magnitude, x100, compared to their values at room temperature for the same accelerating gradient.

physics.acc-ph↗

arXiv2019

Light-Induced Charge Density Wave in LaTe$_3$

When electrons in a solid are excited with light, they can alter the free energy landscape and access phases of matter that are beyond reach in thermal equilibrium. This accessibility becomes of vast importance in the presence of phase competition, when one state of matter is preferred over another by only a small energy scale that, in principle, is surmountable by light. Here, we study a layered compound, LaTe$_3$, where a small in-plane (a-c plane) lattice anisotropy results in a unidirectional charge density wave (CDW) along the c-axis. Using ultrafast electron diffraction, we find that after photoexcitation, the CDW along the c-axis is weakened and subsequently, a different competing CDW along the a-axis emerges. The timescales characterizing the relaxation of this new CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. The new density wave represents a transient non-equilibrium phase of matter with no equilibrium counterpart, and this study thus provides a framework for unleashing similar states of matter that are "trapped" under equilibrium conditions.

cond-mat.mtrl-sci↗

arXiv2018

Ultrafast manipulation of mirror domain walls in a charge density wave

Domain walls (DWs) are singularities in an ordered medium that often host exotic phenomena such as charge ordering, insulator-metal transition, or superconductivity. The ability to locally write and erase DWs is highly desirable, as it allows one to design material functionality by patterning DWs in specific configurations. We demonstrate such capability at room temperature in a charge density wave (CDW), a macroscopic condensate of electrons and phonons, in ultrathin 1T-TaS$_2$. A single femtosecond light pulse is shown to locally inject or remove mirror DWs in the CDW condensate, with probabilities tunable by pulse energy and temperature. Using time-resolved electron diffraction, we are able to simultaneously track anti-synchronized CDW amplitude oscillations from both the lattice and the condensate, where photo-injected DWs lead to a red-shifted frequency. Our demonstration of reversible DW manipulation may pave new ways for engineering correlated material systems with light.

cond-mat.mtrl-sci↗

arXiv2016

Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules

Observing the motion of the nuclear wavepackets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wavepacket in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 Å and temporal resolution of 230 fs full-width at half-maximum (FWHM). The method is not only sensitive to the position but also the shape of the nuclear wavepacket.

physics.chem-ph↗

arXiv2015

Diffractive imaging of a molecular rotational wavepacket with femtosecond Megaelectronvolt electron pulses

Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angstrom spatial precision is one of the critical challenges in the chemical sciences, since the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. We performed a gas-phase electron diffraction experiment using Megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved an unprecedented combination of 100 fs root-mean-squared (RMS) temporal resolution and sub-Angstrom (0.76 Å) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.

physics.atom-ph↗