Search

Physical review / B 103(5), 054407 (2021). doi:10.1103/PhysRevB.103.054407, Half-metallic ferromagnetic Heusler compounds represent an important class of materials for spintronic applications. We experimentally observe the dispersion of electronic bands with spin resolution in three representative Heusler compounds (Co$_2$MnGa, Co$_2$MnSi, and Co$_2$Fe$_{0.4}$Mn$_{0.6}$Si). Bulk-sensitive measurements at very low photon energy are complemented by spin-integrated hard x-ray angular resolved photoemission spectroscopy. The dispersion of majority and minority electrons allows us to link exchange splitting and band filling to the number of valence electrons Nv. Photoexcitation at $hν=6.05$ $ eV$ gives access to the spin-polarization texture $P(E_B,k_x,k_y)$ of the bulk bands covering a $(k_x,k_y) $ range of 60% of the Brillouin zone. We find that Co$_2$MnSi and Co$_2$Fe$_{0.4}$Mn$_{0.6}$ exhibit minority band gaps of 0.5 and 0.35 eV, i.e., decreasing band gap with increasing NV in contrast to the rigid-band expectation. For Co$_2$MnGa, the minority valence state maximum lies at approx. 0.2 eV above $E_F$., Published by Inst., Woodbury, NY

Seminar, Deutschland; Journal of physics / Condensed matter 00(00), 1-20 (2021). doi:10.1088/1361-648X/abe479, The heavy-fermion behavior in intermetallic compounds manifests itself in a quenching of local magnetic moments by developing Kondo spin-singlet many-body states combined with a drastic increase of the effective mass of conduction electrons, which occurs below the lattice Kondo temperature $T_K$. This behavior is caused by interactions between the strongly localized 4f electrons and itinerant electrons. A controversially discussed question in this context is how the localized electronic states contribute to the Fermi surface upon changing the temperature. One expects that hybridization between the local moments and the itinerant electrons leads to a transition from a small Fermi surface in a non-coherent regime at high temperatures to a large Fermi surface once the coherent Kondo lattice regime is realized below $T_K$. We demonstrate, using hard X-ray angle-resolved photoemission spectroscopy that the electronic structure of the prototypical heavy fermion compound YbRh$_2$Si$_2$ changes with temperature between 100 and 200 K, i.e. far above the Kondo temperature of this system. Our results suggest a transition from a small to a large Fermi surface with decreasing temperature. This result is inconsistent with the prediction of the dynamical mean-field periodic Anderson model and supports the idea of an independent energy scale governing the change of band dispersion., Published by IOP Publ., Bristol

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., 7 pages, 5 figures

New journal of physics 22(10), 103002 (1-13) (2020). doi:10.1088/1367-2630/abb68b, High-resolution full-field imaging of (k$_x$, k$_y$) photoelectron distributions (k-resolution 0.03 Å$^{−1}$, angular resolution 0.03° at 6.7 keV) in a large field of view (up to 16 Å$^{−1}$ dia.) allows to observe fine details in Kikuchi-type diffractograms. Alongside with the element specificity via core-level spectra, this method opens a new avenue to structural analysis using hard x-ray photoelectron diffraction (hXPD). Here we present a theoretical study of the emitter-site specificity by simulating hXPD patterns for arbitrary positions of emitter atoms in the unit cell. Using the Bloch wave approach to photoelectron diffraction from lattice planes, the diffraction patterns from a number of positions in the unit cell can be obtained simultaneously exploiting the reciprocity theorem. Simulations for dopant atoms and dopant multimers (dimers, trimers, clusters) in the Si lattice at various positions in the unit cell reveal a strong site-sensitivity in terms of dramatic changes in the diffraction patterns with emitter-atom position. The results are compared with measurements for Si hyperdoped with Te., Published by IOP, [London]

The coupling of real and momentum space is utilized to tailor electronic properties of the collinear metallic antiferromagnet Mn2Au by aligning the real space Neel vector indicating the direction o...

New journal of physics 21(11), 113031 - (2019). doi:10.1088/1367-2630/ab51fe, Hard x-ray photoelectron diffraction (hXPD) patterns recorded with a momentum microscope withhigh k-resolution (0.025 Å−1 equivalent to an angular resolution of 0.034° at 7 keV) revealunprecedented rich fine structure.Wehave studied hXPD of theC1s core level in the prototypicallow-Z material Graphite at 20 photon energies between 2.8 and 7.3 keV. Sharp bright and dark linesshift with energy; regions of Kikuchi band crossings near zone axis exhibit a filigree structure whichvaries rapidly with energy. Calculations based on the Bloch wave approach to electron diffraction fromlattice planes show excellent agreement with the experimental results throughout the entire energyrange. The main Kikuchi bands in the [001] zone axis appear fixed on the momentum scale with awidth of the corresponding reciprocal lattice vector, allowing to reconstruct the size of the projectedBrillouin zone. The newly developed high-energy k-microscope allows full-field imaging of(kx, ky)-distributions in large k-fields (up to>22 Å−1 dia.) and time-of-flight energy recording., Published by IOP, [London]

New journal of physics 20(3), 033004 - (2018). doi:10.1088/1367-2630/aaa262, Photoelectron spectroscopy, especially at pulsed sources, is ultimately limited by the Coulomb interaction in the electron cloud, changing energy and angular distribution of the photoelectrons. A detailed understanding of this phenomenon is crucial for future pump–probe photoemission studies at (x-ray) free electron lasers and high-harmonic photon sources. Measurements have been performed for Ir(111) at hν =1000 eV with photon flux densities between ∼10$^2$ and 10$^4$ photons per pulse and μm$^2$ (beamline P04/PETRA III, DESY Hamburg), revealing space-charge induced energy shifts of up to 10 eV. In order to correct the essential part of the energy shift and restore the electron distributions close to the Fermi energy, we developed a semi-analytical theory for the space-charge effect in cathode-lens instruments (momentum microscopes, photoemission electron microscopes). The theory predicts a Lorentzian profile of energy isosurfaces and allows us to quantify the charge cloud from measured energy profiles. The correction is essential for the determination of the Fermi surface, as we demonstrate by means of ‘k-space movies’ for the prototypical high-Z material tungsten. In an energy interval of about 1 eV below the Fermi edge, the bandstructure can be restored up to substantial shifts of ∼7 eV. Scattered photoelectrons strongly enhance the inelastic background in the region several eV below $E_F$, proving that the majority of scattering events involves a slow electron. The correction yields a gain of two orders of magnitude in usable intensity compared with the uncorrected case (assuming a tolerable shift of 250 meV). The results are particularly important for future experiments at SASE-type free electron lasers, since the correction also works for strongly fluctuating (but known) pulse intensities., Published by IOP Publ., [Bad Honnef]

The effect of quantization of the electron energy in a magnetic field on the collisionless damping of radio-frequency modes in aluminum has been investigated theoretically. In the geometry where a propagation vector k and a constant magnetic field H are directed along the C4 axis in aluminum there is a magnetic Landau damping caused by electrons whose orbits are inclined to the transverse plane. Despite a relatively low concentration of electrons, this damping can significantly affect the damping of a helicon and a doppleron. It has been shown that the quantization of the electron energy leads to giant oscillations of the damping of these modes.

We investigated the field emission current from a p-type silicon tip with large resistivity of 4 × 103 Ω cm for light illumination with a photon energy of 1.3 eV and tip-anode voltages of 0.7–5.0 kV. Additional AC voltage with amplitude 30–60 V and frequency varying in the range of 10–107 Hz was applied to the tip which resulted in variations of emission current. We investigated the dependence of this phenomenon on the AC signal parameters, light intensity and temperature. The resonant-like frequency dependence of the emission current is because the tip acts as a driven plasmonic resonator. The results represent an important step forward for the development of high-frequency display systems based on electron field emission.

Journal of physics / Condensed matter 29(25), 255001 - (2017). doi:10.1088/1361-648X/aa7173, Spin-momentum locking of surface states has attracted great interest in recent years due to envisioned technological applications in the field of spintronics. Normal metal surfaces like W(1 1 0) and Ir(1 1 1) show surface states with energy dispersions and spin-polarization textures, which are reminiscent of topologically non-trivial surface states. In order to understand this phenomenon the connection of bulk and surface states has to be explored. Using time-of-flight momentum microscopy with soft x-ray excitation, we present a comprehensive analysis of the bulk bands of W and Ir. Surface states are determined by the same method with photon excitation in the vacuum ultraviolet region. The superposition of both spectral densities reveals the hosting of surface states within the gap structure of bulk bands projected on the surface Brillouin zone. Quantitative differences in the extension of experimental and theoretical local band gaps indicate an underestimation of electron correlation effects in theory., Published by IOP Publ., Bristol

Ultramicroscopy 183, 19 - 29 (2017). doi:10.1016/j.ultramic.2017.06.025, The combination of momentum microscopy (high resolution imaging of the Fourier plane) with an imaging spin filter has recently set a benchmark in k-resolution and spin-detection efficiency. Here we show that the degree of parallelization can be further increased by time-of-flight energy recording. On the quest towards maximum information (in earlier work termed “complete” photoemission experiment) we have studied the prototypical high-Z fcc metal iridium. Large partial bandgaps and strong spin-orbit interaction lead to a sequence of spin-polarized surface resonances. Soft X-rays give access to the 4D spectral density function ρ (E$_B$,k$_x$,k$_y$,k$_z$) weighted by the photoemission cross section. The Fermi surface and all other energy isosurfaces, Fermi velocity distribution v$_F$(k$_F$), electron or hole conductivity, effective mass and inner potential can be obtained from the multi-dimensional array ρ by simple algorithms. Polarized light reveals the linear and circular dichroism texture in a simple manner and an imaging spin filter exposes the spin texture. One-step photoemission calculations are in fair agreement with experiment. Comparison of the Bloch spectral function with photoemission calculations uncovers that the observed high spin polarization of photoelectrons from bulk bands originates from the photoemission step and is not present in the initial state., Published by Elsevier Science, Amsterdam

The possibility of the nonlinear wave propagation occurring in semimetals in the geometry where a constant magnetic field H is directed along the trigonal axis of the crystal has been investigated theoretically. In the linear regime in this geometry, there is a strong magnetic Landau damping without the wave propagation. It has been shown that the electron trapping by the magnetic field of a large-amplitude radio-frequency wave decreases the efficiency of this damping. As a result, nonlinear dopplerons can propagate in arsenic and, possibly, in antimony.

We study theoretically the effect of electron energy quantization in a magnetic field on the penetration of radio waves into a semimetal in a geometry in which a constant magnetic field H is directed along the trigonal axis of a crystal. In this geometry, strong magnetic Landau damping in semimetals prevents wave propagation and is responsible for the skin effect. It is shown that quantization of the transverse energy of electrons considerably influences the effectiveness of collisionless absorption of the wave. As a result, the magnetic Landau damping and the skin depth experience giant oscillations upon a change in magnetic field H.

Time-of-flight momentum microscopy is developed. It enables direct three-dimensional mapping of the topology of the Fermi surface, identification of electron and hole pockets, and quantification of Fermi velocity as a function of wavevector.

We find in the case of W(110) previously overlooked anomalous surface states having their spin locked at right angle to their momentum using spin-resolved momentum microscopy. In addition to the well known Dirac-like surface state with Rashba spin texture near the "Equation missing"-point, we observe a tilted Dirac cone with circularly shaped cross section and a Dirac crossing at 0.28 × "Equation missing" "Equation missing" within the projected bulk band gap of tungsten. This state has eye-catching similarities to the spin-locked surface state of a topological insulator. The experiments are fortified by a one-step photoemission calculation in its density-matrix formulation.

The leading edge of intrinsic luminescence pulse in PbWO 4 and ZnWO 4 single crystals is studied. The rise of luminescence intensity is observed after excitation with electron beam pulse. The luminescence rise is shown tobe a result of luminescence center formation via electron-hole recombination within spatially correlated pairs. An electron of a geminate electron-hole pair of PbWO 4 crystal self-traps and stimulates self-trapping of a hole at a close distance. In the ZnWO 4 crystal a hole of a geminate electron-hole pair self-traps and stimulates self-trapping of the electron producing a W 5 + center at a short distance. The rise-time of luminescence depends on spatial separation of recombining components. It is short even at 80 K (100 ns in PbWO 4 and 10 ns in ZnWO 4 ). The reasons for different rise time and different spatial distribution of electron and hole self-trapped centers are discussed.

The electronic surface states on Mo(110) have been investigated using time-of-flight momentum microscopy with synchrotron radiation (hν=35 eV). This novel angle-resolved photoemission approach yields a simultaneous acquisition of the E-vs-k spectral function in the full surface Brillouin zone and several eV energy interval. (kx,ky,EB)-maps with 3.4 A(-1) diameter reveal a rich structure of d-like surface resonances in the spin-orbit induced partial band gap. Calculations using the one-step model in its density matrix formulation predict an anomalous state with Dirac-like signature and Rashba spin texture crossing the bandgap at Γ¯ and EB=1.2 eV. The experiment shows that the linear dispersion persists away from the Γ¯-point in an extended energy- and k∥-range. Analogously to a similar state previously found on W(110) the dispersion is linear along H¯-Γ¯-H¯ and almost zero along N¯-Γ¯-N¯. The similarity is surprising since the spin-orbit interaction is 5 times smaller in Mo. A second point with unusual topology is found midway between Γ¯ and N¯. Band symmetries are probed by linear dichroism.

The Doppler-shifted cyclotron resonance in an aluminum plate in the geometry when constant magnetic field H is directed along the [100] crystallographic axis oriented normally to the surface of the plate is studied theoretically. The analysis is performed for a simple model Fermi surface possessing fourth-order symmetry. Capture of holes by the magnetic field of a radio-frequency wave is shown to considerably decrease the effectiveness of cyclotron absorption at large exciting field amplitudes. This suppresses the collisionless damping of dopplerons (propagating modes related to odd cyclotron resonance harmonics). As a result, the sample becomes more transparent to radio-frequency radiation.

The effect of nonlinearity on the possibility of Group III metals to support propagating radio waves is investigated theoretically. A nonlinear wave is shown to be able to propagate in a metal when the magnetic field of a wave of a large amplitude traps holes and causes collisionless cyclotron absorption to be suppressed.

A theoretical study is presented of the effect of hole energy quantization in a dc magnetic field on microwave penetration through a noble-metal plate. It is shown that quantization results in strong oscillations of cyclotron absorption. Between the absorption peaks, the damping decreases sufficiently to enable propagation of unique quantum waves. Excitation of such waves in a metallic plate gives rise to sharp oscillations of its surface resistance with magnetic field. The shape of these oscillations is also very unusual.

A theoretical analysis is made of Doppler-shifted cyclotron resonance in zinc in linear and nonlinear regimes. It is shown that the absence of a threshold for cyclotron absorption by holes makes the doppleron strongly damped or eliminates it for low-amplitude wave-fields. At high amplitudes capture of holes by the magnetic field of the wave suppresses collisionless absorption and the doppleron can propagate. As a result, the impedance of the plate is an oscillating function of the magnetic field. It is shown that the effect should be observed at frequencies of the order of a few tens of kilohertz, in magnetic fields of the order of a few kilogauss, and for exciting field amplitudes of the order of a few tens of gauss.

It is shown that the decrease of the collisionless absorption in the magnetic-field region below the helicon absorption edge might be so considerable in the nonlinear regime that propagation of a new rf wave becomes possible. The wave frequency is proportional to the fourth power of its wave number, and its damping is inversely proportional to the carrier mean free path. The wave is excited most effectively at magnetic-field values much lower than the helicon threshold. Conditions necessary for observing the wave are presented.

It is shown that quantization of electron energy levels in a strong magnetic field H can give rise to giant oscillations of cyclotron absorption of a radio wave in a metal as a function of H .

The Doppler shift of the frequency of a wave field acting on electrons in a metal can be very large: in the rf range it exceeds the frequency by several orders of magnitude. Therefore, in a dc magnetic field perpendicular to the metal surface the cyclotron resonance is shifted into the frequency range which extends from a few kilohertzs up to a few gigahertz. Unlike the Azbel'-Kaner cyclotron resonance, which is temporal, the Doppler-shifted cyclotron resonance (DSCR) is a spatial resonance. It occurs when the rf wavelength coincides with the extremal displacement of the conduction electrons for one cyclotron period. Owing to Fermi degeneration of the electrons, the DSCR manifests itself as particular components of the electromagnetic field: a doppleron and a Gantmakher-Kaner component. The former is a natural mode of the electromagnetic field in a degenerated plasma of a metal, whose wavelength is close to the extremal displacement of the electrons for a cyclotron period. The latter is also associated with the mentioned electrons, but it is not a natural mode of a plasma. Penetration of the components through a metal slab results in oscillations of the surface impedance of the slab as a function of a dc magnetic field. In this review we present the main results of theoretical and experimental studies of the DSCR in metals carried out during the last three decades. In Part I we describe the properties of dopplerons: their spectra, damping, polarizations, the range of the magnetic field values where they exist, the role of the Fermi surface anisotropy, etc. Part II is devoted to a quantitative description of the excitation of the dopplerons and Gantmakher-Kaner components in metals and to comparison of the theory with experimental data. An important role of the non-specularity of reflection of the conduction electrons from metal surfaces is established. For building the theory, nontrivial methods of solution of the corresponding integrodifferential equations are developed; thus, for example, a method of analytical solution of the two-sided Wiener-Hopf problem is developed. Part III is devoted to nonlinear effects in the DSCR, unexpected for the dense electron plasma in metals. Although the wave field has a little effect on the trajectories of most of the electrons on the Fermi surface, it can significantly alter the motion of the small electron groups responsible for the collisionless wave absorption. When the wave field amplitude is large enough, the electrons are trapped by the field and the collisionless absorption is suppressed. This is valid both for the magnetic Landau damping and for the cyclotron absorption. In the latter case the effect turns out to be so drastic that it results in the appearance of a new doppleron mode which does not exist at small wave field amplitudes. The appearance of the magnetic Landau damping in the case when the wave propagates parallel to a dc magnetic field presents another type of nonlinear effect. It is associated with the fact that the magnetic field of a wave of large amplitude causes the total magnetic field vector to deviate from the wave propagation vector.

(1993). Space ecology and observational astronomy. Astronomical & Astrophysical Transactions: Vol. 4, No. 1, pp. 47-48.

Nonlinear reduction of collisionless cyclotron absorption reduced the permittivity of an electronhole plasma in cadmium so mach that a new Doppler wave was observed. The new doppleron was due to a Doppler-shifted cyclotron resonance of “lens” electrons, but its circular polarization was opposite to that of the familiar doppleron. A linear plasma bleaching effect was observed also in the samples of tungsten.

An investigation is made of the influence of a transverse macroinhomogeneity in a solid state laser on the emission kinetics. It is shown that when a fairly definite minimum is located in the central part of the transverse intensity distribution, the emission is in the form of spontaneous fluctuations whose growth increment increases with increasing depth of this minimum. The results obtained are in agreement with the experimental data on the correlation between the transverse field distribution and the laser emission kinetics.

An investigation was made of the spectral characteristics of the radiation emitted by a ruby laser with a pronounced transverse field structure when a narrow spectral line of a second laser was injected into the resonator of the first laser as a stimulus. The dynamics of the emission spectrum was found to be governed by the gap between the frequency of the stimulating radiation and the maximum of the gain profile.