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Zhu, D.

Publications and source records attributed to Zhu, D..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Generation of High Peak Power Hard X-Rays at LCLS-II with Double Bunch Self-seeding

We propose to use existing LCLS copper S-band linac double bunch infrastructure to significantly improve LCLS-II hard X-ray performance. In our setup, we use the first bunch to generate a strong seeding X-ray signal, and the second bunch, initially traveling off-axis, to interact with the seed in the amplifier undulator and generate a near TW, 15 fs duration X-ray pulse in the 4 to 8 keV photon energy range. We investigate, via numerical simulations, the required transverse beam dynamics and the four crystals X-ray monochromator to be added to the existing LCLS-II beamline and discuss the final properties of the hard X-ray pulses and their potential application in high intensity, high-field physics experiments, including QED above the Schwinger critical field.

Halavanau, A.↗
DOE OSTI2019

Disentangling transient charge order from structural dynamics contributions during coherent atomic motion studied by ultrafast resonant x-ray diffraction [Disentangling charge and structural contributions during coherent atomic motions studied by ultrafast resonant x-ray diffraction]

We report on the ultrafast dynamics of charge order and structural response during the photoinduced suppression of charge and orbital order in a mixed-valence manganite. Employing femtosecond time-resolved resonant x-ray diffraction below and at the Mn K absorption edge, we present a method to disentangle the transient charge order and structural dynamics in thin films of Pr 0.5 Ca 0.5 MnO 3 . Based on the static resonant scattering spectra, we extract the dispersion correction of charge-ordered Mn 3+ and Mn 4+ ions, allowing us to separate the transient contributions of purely charge order from structural contributions to the scattering amplitude after optical excitation. Furthermore, our finding of a coherent structural mode at around 2.3THz, which primarily modulates the lattice but does not strongly affect the charge order, supports the picture of the charge order being the driving force of the combined charge, orbital, and structural transition.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Coherent order parameter dynamics in SmTe 3

We present ultrafast optical pump-probe and ultrafast x-ray diffraction measurements of the charge density wave dynamics in SmTe3 at 300 K. We performed ultrafast x-ray diffraction measurements at the Linac Coherent Light Source to directly probe the dynamics of the finite-wave-vector order parameter. The dynamics reveal coherent oscillations at ~1.6 THz that become overdamped with increasing fluence. We identify this oscillation with the lattice component of the amplitude mode. Furthermore, our data allow for a clear identification of the amplitude mode frequency in the optical pump-probe data. In both measurements, the system reaches the symmetric phase at high fluence, where the order parameter vanishes and the response (reflectivity and x-ray intensity) is quadratic in the order parameter. This is observed in the x-ray diffraction as a small overdamped modulation near zero intensity. Similar overdamped features are observed in the optical reflectivity at high fluence. In conclusion, a time-dependent Ginzburg-Landau model captures qualitatively the essential features of the experimental observations.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Terahertz-driven phonon upconversion in SrTiO 3

Direct manipulation of the atomic lattice using intense long-wavelength laser pulses has become a viable approach to create new states of matter in complex materials. Conventionally, a high-frequency vibrational mode is driven resonantly by a mid-infrared laser pulse and the lattice structure is modified through indirect coupling of this infrared-active phonon to other, lower-frequency lattice modulations. Here, we drive the lowest-frequency optical phonon in the prototypical transition metal oxide SrTiO 3 well into the anharmonic regime with an intense terahertz field. We show that it is possible to transfer energy to higher-frequency phonon modes through nonlinear coupling. Lastly, our observations are carried out by directly mapping the lattice response to the coherent drive field with femtosecond X-ray pulses, enabling direct visualization of the atomic displacements.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Reply to “Comment on ‘Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO 3 ' ” [Response to Comment by S. Durbin on “Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO 3 ”]

In this reply to S. Durbin's comment on our original paper “Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO 3 ,” we concur that his final equations 8 and 9 more accurately describe the change in diffracted intensity as a function of Ti displacement. Here, we also provide an alternative derivation based on an ensemble average over unit cells. The conclusions of the paper are unaffected by this correction.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Coincident onset of charge-density-wave order at a quantum critical point in underdoped YBa 2 Cu 3 O x

The recently demonstrated x-ray scattering approach using a free electron laser with a high field pulsed magnet has opened new opportunities to explore the charge-density-wave (CDW) order in cuprate high temperature superconductors. Using this approach, in this paper we substantially degrade the superconductivity with magnetic fields up to 33 T to investigate the onset of CDW order in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ at low temperatures near a putative quantum critical point (QCP) at ${p}_{1}{\sim}0.08$ holes per Cu. We find no CDW can be detected in a sample with a doping concentration smaller than ${p}_{1}$. Finally, our results indicate that the onset of the CDW ground state lies inside the zero-field superconducting dome, and broken translational symmetry is associated with the putative QCP at ${p}_{1}$.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Spatial Distortion of Vibration Modes via Magnetic Correlation of Impurities

Long wavelength vibrational modes in the ferromagnetic semiconductor Ga 0.91 M n0.09 As are investigated using time resolved x-ray diffraction. At room temperature, we measure oscillations in the x-ray diffraction intensity corresponding to coherent vibrational modes with well-defined wavelengths. When the correlation of magnetic impurities sets in, we observe the transition of the lattice into a disordered state that does not support coherent modes at large wavelengths. Our measurements point toward a magnetically induced broadening of long wavelength vibrational modes in momentum space and their quasilocalization in the real space. More specifically, long wavelength vibrational modes cannot be assigned to a single wavelength but rather should be represented as a superposition of plane waves with different wavelengths. Lastly, our findings have strong implications for the phonon-related processes, especially carrier-phonon and phonon-phonon scattering, which govern the electrical conductivity and thermal management of semiconductor-based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Diffraction based Hanbury Brown and Twiss interferometry at a hard x-ray free-electron laser

X-ray free-electron lasers (XFELs) provide extremely bright and highly spatially coherent x-ray radiation with femtosecond pulse duration. Currently, they are widely used in biology and material science. Knowledge of the XFEL statistical properties during an experiment may be vitally important for the accurate interpretation of the results. Here, for the first time, we demonstrate Hanbury Brown and Twiss (HBT) interferometry performed in diffraction mode at an XFEL source. It allowed us to determine the XFEL statistical properties directly from the Bragg peaks originating from colloidal crystals. This approach is different from the traditional one when HBT interferometry is performed in the direct beam without a sample. Our analysis has demonstrated nearly full (80%) global spatial coherence of the XFEL pulses and an average pulse duration on the order of ten femtoseconds for the monochromatized beam, which is significantly shorter than expected from the electron bunch measurements.

47 OTHER INSTRUMENTATION↗
DOE OSTI2018

Direct experimental observation of the gas density depression effect using a two-bunch X-ray FEL beam

The experimental observation of the depression effect in gas devices designed for X-ray free-electron lasers (FELs) is reported. The measurements were carried out at the Linac Coherent Light Source using a two-bunch FEL beam at 6.5 keV with 122.5 ns separation passing through an argon gas cell. The relative intensities of the two pulses of the two-bunch beam were measured, after and before the gas cell, from X-ray scattering off thin targets by using fast diodes with sufficient temporal resolution. At a cell pressure of 140 hPa, it was found that the after-to-before ratio of the intensities of the second pulse was about 17% ± 6% higher than that of the first pulse, revealing lower effective attenuation of the gas cell due to heating by the first pulse and subsequent gas density reduction in the beam path. This measurement is important in guiding the design and/or mitigating the adverse effects in gas devices for high-repetition-rate FELs such as the LCLS-II and the European XFEL or other future high-repetition-rate upgrades to existing FEL facilities.

43 PARTICLE ACCELERATORS↗
DOE OSTI2017

Generating Single-Spike Hard X-Ray Pulses with Nonlinear Bunch Compression in Free-Electron Lasers

Here, a simple method for generating single-spike hard x-ray pulses in free-electron lasers (FELs) has been developed at the Linac Coherent Light Source (LCLS). This is realized by nonlinear bunch compression using 20-pC bunch charge, demonstrated in the hard x-ray regime at 5.6 and 9 keV, respectively. Measurements show about half of the FEL shots containing a single-spike spectrum. At 5.6-keV photon energy, the single-spike shots have a mean pulse energy of about 10 μJ with 70% intensity fluctuation and the pulse full width at half maximum is evaluated to be at 200-as level.

47 OTHER INSTRUMENTATION↗