arXiv subjects

Yuri Shvyd'ko

Publications and source records attributed to Yuri Shvyd'ko.

26 records · Page 2Linked to original sources

arXiv2013

Maximizing Spectral Flux from Self-Seeding Hard X-ray FELs

Fully coherent x-rays can be generated by self-seeding x-ray free-electron lasers (XFELs). Self-seeding by a forward Bragg diffraction (FBD) monochromator has been recently proposed [1] and demonstrated [2]. Characteristic time To of FBD determines the power, spectral, and time characteristics of the FBD seed [3]. Here we show that for a given electron bunch with duration sigma_e the spectral flux of the self-seeding XFEL can be maximized, and the spectral bandwidth can be respectively minimized by choosing To ~ sigma_e/pi and by optimizing the electron bunch delay tau_e. The choices of To and tau_e are not unique. In all cases, the maximum value of the spectral flux and the minimum bandwidth are primarily determined by sigma_e. Two-color seeding takes place To >> sigma_e/π. The studies are performed, for a Gaussian electron bunch distribution with the parameters, close to those used in the short-bunch (sigma_e ~ 5 fs) and long-bunch (sigma_e ~ 20 fs) operation modes of the LCLS XFEL.

physics.acc-ph↗

arXiv2012

Hard X-ray Spectrographs with Resolution Beyond 100 micro-eV

Spectrographs take snapshots of photon spectra with array detectors by dispersing photons of different energies into distinct directions and spacial locations. Spectrographs require optics with a large angular dispersion rate as the key component. In visible light optics diffraction gratings are used for this purpose. In the hard x-ray regime, achieving large dispersion rates is a challenge. Here we show that multi-crystal, multi-Bragg-reflection arrangements feature cumulative angular dispersion rates almost two orders of magnitude larger than those attainable with a single Bragg reflection. As a result, the multi-crystal arrangements become potential dispersing elements of hard x-ray spectrographs. The hard x-ray spectrograph principles are demonstrated by imaging a spectrum of photons with a record high resolution of $ΔE \simeq 90 μ$eV in hard x-ray regime, using multi-crystal optics as dispersing element. The spectrographs can boost research using inelastic ultra-high-resolution x-ray spectroscopies with synchrotrons and seeded XFELs.

physics.optics↗

arXiv2012

Spatiotemporal Response of Crystals in X-ray Bragg Diffraction

The spatiotemporal response of crystals in x-ray Bragg diffraction resulting from excitation by an ultra-short, laterally confined x-ray pulse is studied theoretically. The theory presents an extension of the analysis in symmetric reflection geometry [1] to the generic case, which includes Bragg diffraction both in reflection (Bragg) and transmission (Laue) asymmetric scattering geometries. The spatiotemporal response is presented as a product of a crystal-intrinsic plane wave spatiotemporal response function and an envelope function defined by the crystal-independent transverse profile of the incident beam and the scattering geometry. The diffracted wavefields exhibit amplitude modulation perpendicular to the propagation direction due to both angular dispersion and the dispersion due to Bragg's law. The characteristic measure of the spatiotemporal response is expressed in terms of a few parameters: the extinction length, crystal thickness, Bragg angle, asymmetry angle, and the speed of light. Applications to self-seeding of hard x-ray free electron lasers are discussed, with particular emphasis on the relative advantages of using either the Bragg or Laue scattering geometries. Intensity front inclination in asymmetric diffraction can be used to make snapshots of ultra-fast processes with femtosecond resolution.

physics.optics↗

arXiv2012

Disentangling a quantum antiferromagnet with resonant inelastic X-ray scattering

Low-dimensional copper oxide lattices naturally manifest electronic states with strong short range quantum entanglement, which are known to lead to remarkable emergent material properties. However the nanometer scale many-body wavefunction is challenging to measure or manipulate in a simple way. In this study, X-ray induced $dd$ electronic transitions are used to suppress spin entanglement across a single lattice site in the spin-1/2 antiferromagnetic chain compound SrCuO$_2$, revealing a class of cuprate magnetic excitations that result from breaking the spin chain. These measurements are the first to employ two closely spaced X-ray resonances of copper (M$_2$ and M$_3$) as a form of natural 2-slit interferometer to distinguish between different types of electronic transition and resolve how they influence the dynamics of nearby spin-entangled electrons.

cond-mat.str-el↗

arXiv2011

Enhanced X-Ray Angular Dispersion and X-ray Spectrographs with Resolving Power Beyond 10$^{8}$

Spectrograph is an optical device that is used to disperse photons of different energies $E$ into distinct directions and space locations, and to take a snapshot of the whole spectrum of photon energies with a spatially sensitive photon detector. Substantial advantage of a spectrograph over an ordinary spectral analyzer, is its ability to deal with many photon energies simultaneously, thus reducing exposure time per spectrum considerably. To realize a spectrograph, dispersing elements with large angular dispersion rate are required. Here we show, on the example of CDW x-ray optics [1-3], that multi-crystal arrangements may feature cumulative angular dispersion rates more than an order of magnitude larger than those attainable in single Bragg reflections. This makes, first, hard x-ray spectrographs feasible, and, secondly, a resolving power beyond $E/ΔE \gtrsim 10^{8}$ achievable.

physics.optics↗

arXiv2011

Angular Dispersion and Anomalous Transmission Cast Ultramonochromatic X Rays

Optical spectrometers, instruments that work with monochromatic light, are commonly rated by the spectral bandwidth, which defines the ability to resolve closely spaced spectral components. The ability to detect faint objects among these components, spectral contrast, is another desired aspect. Here we demonstrate that a combined effect of angular dispersion (AD) and anomalous transmission (AT) of x rays in Bragg reflection from asymmetrically cut crystals can shape spectral distributions of x rays to profiles with record high contrast and small bandwidths. The AD&AT x-ray optics is implemented as a five-reflection three-crystal arrangement featuring a combination of the above mentioned attributes, so much desirable for x-ray monochromators and analyzers: a spectral contrast of ~ 500, a bandwidth of ~ 0.46 meV and a remarkably large angular acceptance of ~ 107 urad. The new optics can become a foundation for the next generation inelastic x-ray scattering spectrometers for studies of atomic dynamics.

physics.optics↗

arXiv2010

Nanoradian angular stabilization of x-ray optical components

An x-ray free electron laser oscillator (XFELO) has been recently proposed [K. Kim, Y. Shvyd'ko, and S. Reiche, Phys. Rev. Lett. 100, 244802 (2008)]. Angular orientation and position in space of Bragg mirrors of the XFELO optical cavity must be continuously adjusted to compensate instabilities and maximize the output intensity. An angular stability of about 10 nrad (rms) is required [K. Kim and Y. Shvyd'ko Phys. Rev. STAB 12, 030703 (2009)]. To approach this goal, a feedback loop based on a null-detection principle was designed and used for stabilization of a high energy resolution x-ray monochromator ($ΔE/E \simeq 4 \times 10^{-8}$, $E$ = 23.7 keV) and a high heat load monochromator. Angular stability of about 13 nrad (rms) has been demonstrated for x-ray optical elements of the monochromators.

physics.ins-det↗

arXiv2010

Charge-transfer and Mott-Hubbard Excitations in FeBO3: Fe K edge Resonant Inelastic X-ray Scattering Study

Momentum-resolved resonant inelastic x-ray scattering (RIXS) spectroscopy has been carried out at the Fe K edge, successfully for the first time. The RIXS spectra of a FeBO3 single crystal reveal a wealth of information on 1-10 eV electronic excitations. The IXS signal resonates both when the incident photon energy approaches the pre-edge (1s-3d) and the main-edge (1s-4p) of the Fe K edge absorption spectrum. The RIXS spectra measured at the pre-edge and the main-edge show quantitatively different dependences on the incident photon energy, momentum transfer, polarization, and temperature. Electronic excitations observed in the pre-edge and main-edge RIXS spectra are interpreted as Mott-Hubbard (MH) and charge-transfer (CT) excitations, respectively. The charge-transfer gap = 3.81 (0.04) eV and the Mott-Hubbard gap = 3.96 (0.04) eV, are determined, model independently from the experimental data. The CT and MH excitations are assigned using molecular orbitals (MO) in the cluster model and multiplet calculation in the many-electron multiband model, respectively.

cond-mat.str-el↗