arXiv subjects

S. Chernov

Publications and source records attributed to S. Chernov.

9 recordsLinked to original sources

arXiv2024

Multimode Objective Lens for Momentum Microscopy and XPEEM: Experiments

A new type of objective lens has recently been proposed for use in X-ray photoemission electron microscopes (XPEEMs) and momentum microscopes. Adding a ring electrode concentric with the extractor allows the field in the gap between the sample and the extractor to be shaped. Forming a lens field in this gap reduces the field strength at the sample by up to an order of magnitude. This mitigates the risk of field emission, particularly for cleaved samples with sharp edges. A retarding field can redirect all slow electrons, thus eliminating the primary contribution to the space-charge interaction. Here we present the first experimental investigation of the new lens, examining its performance at photon energies ranging from the extreme ultraviolet produced by a high-harmonic generation (HHG)-based source to soft and hard X-rays at two synchrotron facilities. The gap lens in a region without electrodes enables large working distances up to 23 mm. Reduced aberrations allow for larger fields of view in both k-space and real-space imaging, with resolutions comparable to those of conventional cathode lenses. However, field strengths are an order of magnitude smaller. The zero-field mode enables the study of 3D structured objects and is therefore beneficial for small cleaved samples as well as for operando devices involving top electrodes. The repeller mode reduces space-charge effects, but results in a smaller k-field diameter. This reduction ranges from 10% at hard X-ray energies to 50% in the XUV range. The usable energy interval is also reduced by a factor of two. In time-of-flight XPEEM mode the raw data show a resolution of 250 nm, which can be improved to better than 100 nm through data processing.

cond-mat.mtrl-sci↗

arXiv2022

Flares and their echoes can help distinguish photon rings from black holes with space-Earth very long baseline interferometry

Photon rings near the edge of a black hole shadow is supposed to be a unique tool to validate general relativity and provide reliable measurements of principal black hole parameters: spin and mass. Such measurements are possible though only for nearby supermassive black holes (SMBH) with Space-Earth Very Long Baseline Interferometry (S-VLBI) in the submillimeter wavelength range. For subrings to be distinguished S-EVLBI observations with long baselines at the Lagrangian Sun-Earth L2 libration point are needed. However, the average fluxes of nearby SMBH: Sagittarius A$^\ast$ (Sgr A$^\ast$) and M87$^\ast$ -- $F_\nu\sim 1$ Jy, are still insufficient to detect the signal from the photon rings with even such long baselines. We argue that only manifestations of flares in the submillimeter waveband in their accretion disks can reveal observable signals from the photon rings with the S-EVLBI at L2. Such observations will become possible within the planned join program of the {\it Event Horizon Telescope} (EHT) and {\it Millimetron Space Observatory} (MSO), and within the planned {\it next generation EHT} (ngEHT) project. Two different observational tests for photons rings are discussed. The first one involves observations of a time series of responds from subsequent subrings as can be seen in a 1D visibility function within the join EHT-MSO configuration, the second one -- measurements of an increase of the angle between subsequent subrings in the 2D VLBI image which can be obtained within the ngEHT project.

astro-ph.HE↗

arXiv2021

Time-of-Flight Photoelectron Momentum Microscopy at 100-500 MHz Synchrotron Sources: Electron-Optical Chopping or Bandwidth Pre-Selection

The small time gap of synchrotron radiation in conventional multi-bunch mode (100-500MHz) is prohibitive for time-of-flight (ToF) based electron spectroscopy. Even the new generation of delay-line detectors with improved time resolution (<100ps) yields only 20-100 resolved time slices within a 2-10ns gap. Here we present two techniques of implementing efficient ToF recording at sources with high repetition rate. A fast electron-optical beam blanking unit with GHz bandwidth, integrated in a photoelectron momentum microscope, allows chopping the photon-pulse train to any desired repetition period. Aberration-free momentum distributions have been recorded at chopped pulse periods of 5MHz (at MAX II) and 1.25MHz (at BESSY II). The approach is benchmarked against the alternative way of implementing a dispersive element, e.g. a hemispherical analyzer, in the electron optics. Both approaches, chopping in the time domain as well as bandpass pre-selection in the energy domain, can enable efficient ToF spectroscopy, spectroscopic real-space imaging and momentum microscopy with few-meV resolution using 100-500MHz Synchrotron radiation, highly-repetitive lasers or cavity-enhanced high-harmonic sources. For comparison, we show results recorded at BESSY II with a parasitic 4-bunch island-orbit pulse train, coexisting with the 500MHz filling pattern on the main orbit.

physics.ins-det↗

arXiv2020

Hard X-ray Photoelectron Momentum Microscopy and Kikuchi Diffraction on (In,Ga,Mn)As Thin Films

Recent advances in the brilliance of hard-X-ray beamlines and photoelectron momentum microscopy facilitate bulk valence-band mapping and core-level-resolved hard-X-ray photoelectron diffraction (hXPD) for structural analysis in the same setup. High-quality MBE-grown (In,Ga,Mn)As films represent an ideal testing ground, because of the non-centrosymmetric GaAs crystal structure itself and In and Mn doping concentrations of few percent. Here we present results of k-mapping and hXPD for the title compound with 3% In and 2.5 or 5.6% Mn using hard X-ray photons (3 to 5 keV) at beamline P22 at PETRA III (DESY, Hamburg). Numerical processing (difference or ratio images) emphasizes subtle differences of hXPD patterns like the fingerprint-like hXPD-signatures of As and Ga sites. XPD calculations using the Bloch-wave method show a one-to-one correspondence with the measurements. The hXPD results reveal a predominant Ga substitutional site for Mn. Valence band mapping shows that the Fermi energy lies within the valence band and decreases as the Mn concentration increases. The results support the p-d Zener model of ferromagnetism in the title compound. In addition to the shift of the Fermi energy, the band splitting increases with increasing Mn content, which we attribute to an increase of many-body correlations with increasing metallicity of the sample.

physics.ins-det↗

arXiv2019

Band structure tuning of Heusler compounds revisited: Spin- and momentum-resolved electronic structure analysis of compounds with different band filling

Spin-filtered time-of-flight photoelectron momentum microscopy reveals a systematic variation of the band structure within a series of highly spin-polarized ferromagnetic Heusler compounds with increasing number of valence electrons (Co2MnGa, Co2MnSi and Co2Fe0.4Mn0.6Si). The positions of the Fermi energy for minority and majority electrons deviate strongly from a simple band-filling model. Photoexcitation at h$\nu$=6.05 eV (4th harmonic of a Ti:sapphire laser) gives access to the spin-polarization texture P(EB,kx,ky) of the bulk bands in a (kx,ky)-range with diameter 1.4{\AA}$^{-1}$ and energies from the Fermi energy EF to a binding energy of EB=2 eV. The minority bands of Co2MnGa cross the Fermi level, inhibiting half-metallicity; the crossing points allow a precise adjustment of experimental and theoretical majority and minority bands, requiring shifts in opposite directions. The top of the minority band lies only 0.15 eV above EF, i.e. Co2MnGa is much closer to being half-metallic than predicted by calculations. For half-metallic Co2MnSi and Co2Fe0.4Mn0.6Si clear minority band gaps are visible, the topmost occupied minority bands lie 0.5 and 0.35 eV below EF, in reasonable agreement with theory; the exchange splitting is significantly smaller than in theory. The comparison of all three compounds uncovers the surprising fact that with increasing number of valence electrons the frontier majority bands (close to EF) exhibit an increasing deficiency in filling, in comparison with the prediction of a DFT calculation. The same trend is visible in comparison with a DMFT calculation. For s-polarized excitation both half-metallic compounds exhibit nearly complete positive spin polarization close to EF, consistent with previous work in literature.

cond-mat.mtrl-sci↗

arXiv2019

Narrow-Band Pulsed Electron Source Based on Near-Threshold Photoionization of Cs in a Magneto-Optical Trap

The newly developed method of time-of-flight (ToF) momentum microscopy was used to analyse the cold electron emission from a Cs 3D magneto-optical trap (MOT). Three-step resonant photoionization was implemented via two intermediate states (6P3/2 pumped with 852 nm laser and 7S1/2 with 1470 nm) and a tuneable femtosecond Ti:sapphire laser for the final ionization step. The magnetic field of the MOT is switched off during the photoionization step. The natural bandwidth of the fs-laser is reduced to 4 meV using optical spectral filters. Precise tuning of the photon energy makes it possible to observe the transition regime between direct photoemission into the open continuum and field induced ionization of highly-excited Rydberg states. The paths can be identified by their characteristic dependency on the extraction field and on the Ti:sapphire polarization. ToF analysis allowed us to disentangle the ionization paths and the dependence of the spatio-temporal distribution of the cold electrons on the polarization of the ionizing laser.

physics.atom-ph↗

arXiv2018

Breakthrough in HAXPES Performance Combining Full-Field k-Imaging with Time-of-Flight Recording

We established a new approach to hard-X-ray photoelectron spectroscopy (HAXPES). The instrumental key feature is an increase of the dimensionality of the recording scheme from 2D to 3D. A high-energy momentum microscope can detect electrons with initial kinetic energies more than 6 keV with high angular resolution < 0.1{\deg}. The large k-space acceptance of the special objective lens allows for simultaneous full-field imaging of many Brillouin zones. Combined with time-of-flight parallel energy recording, this method yields maximum parallelization of data acquisition. In a pilot experiment at the new beamline P22 at PETRA III, Hamburg, count rates of more than $10^{6}$ counts per second in the d-band complex of transition metals established an unprecedented HAXPES recording speed. It was found that the concept of tomographic k-space mapping previously demonstrated in the soft X-ray regime works equally well in the hard X-ray range. Sharp valence band k-patterns of Re collected at an excitation energy of 6 keV correspond to direct transitions to the 28th repeated Brillouin zone. Given the high X-ray brilliance (1.1x$10^{13}$ hv/s in a spot of less than 20x15 $mu^{2}$), the 3D bulk Brillouin zone can be mapped in a few hours. X-ray photoelectron diffraction (XPD) patterns with < 0.1{\deg} resolution are recorded within minutes. Previously unobserved fine details in the diffractograms reflect the large number of scatterers, several $10^{4}$ to $10^{6}$, depending on energy. The short photoelectron wavelength (an order of magnitude smaller than the interatomic distance) amplifies phase differences and makes hard X-ray XPD with high resolution a very sensitive structural tool. The high count rates pave the way towards spin-resolved HAXPES using an imaging spin filter.

cond-mat.mtrl-sci↗

arXiv2018

Momentum-Transfer Model of Valence-Band Photoelectron Diffraction

Owing to strongly enhanced bulk sensitivity, angle- or momentum-resolved photoemission using X-rays is an emergent powerful tool for electronic structure mapping. A novel full-field k-imaging method with time-of-flight energy detection allowed rapid recording of 4D (EB,k) data arrays (EB binding energy; k final-state electron momentum) in the photon-energy range of 400-1700eV. Arrays for the d-band complex of several transition metals (Mo, W, Re, Ir) reveal numerous spots of strong local intensity enhancement up to a factor of 5. The enhancement is confined to small (EB,k)-regions (dk down to 0.01 A-1; dEB down to 200 meV) and is a fingerprint of valence-band photoelectron diffraction. Regions of constructive interference in the (EB,k)-scheme can be predicted in a manner resembling the Ewald construction. A key factor is the transfer of photon momentum to the electron, which breaks the symmetry and causes a rigid shift of the final-state energy isosphere. Working rigorously in k-space, our model does not need to assume a localization in real space, but works for itinerant band states without any assumptions or restrictions. The role of momentum conservation in Fermi's Golden Rule at X-ray energies is revealed in a graphical, intuitive way. The results are relevant for the emerging field of time-resolved photoelectron diffraction and can be combined with standing-wave excitation to gain element sensitivity.

cond-mat.mtrl-sci↗

arXiv2008

Ultra-hard fluid and scalar field in the Kerr-Newman metric

An analytic solution for the accretion of ultra-hard perfect fluid onto a moving Kerr-Newman black hole is found. This solution is a generalization of the previously known solution by Petrich, Shapiro and Teukolsky for a Kerr black hole. Our solution is not applicable for an extreme black hole due to violation of the test fluid approximation. We also present a stationary solution for a massless scalar field in the metric of a Kerr-Newman naked singularity.

gr-qc↗