Bulk spin polarization of magnetite from spin-resolved hard x-ray photoelectron spectroscopy

Physical review / B 104(4), 045129 (1-10) (2021). doi:10.1103/PhysRevB.104.045129

Matching emission centers of electrons and photons in current-carrying silver nanoparticle films

Current flow through a nanoparticle film (two-dimensional ensemble of small tunnel-coupled metal particles on a dielectric substrate) is accompanied by electron and photon emission. It has a localized character (originates from emission centers). With an increase in applied voltage, the number of emission centers increases, and with further increase, some of them may burn out. In dark conditions, photon emission centers are visible with a bare eye. To visualize electron emission centers, emission electron microscopy is used. The conducted measurements allow comparison of the number and relative positions of electron and photon emission centers. It is shown that electrons and photons are emi…

The Model for Luminescence Center Excitation in PbWO4

Spinovo rozlisena time-of-flight k-reozlisena fotoemissia Ir-- Kompletny fotoemissny experiment.

Ultramicroscopy 183, 19 - 29 (2017). doi:10.1016/j.ultramic.2017.06.025

Momentum-resolved photoelectron absorption in surface barrier scattering on Ir(111) and graphene/Ir(111)

Physical review / B 96(15), 155108 (2017). doi:10.1103/PhysRevB.96.155108

Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens

Review of scientific instruments 92(5), 053703 (2021). doi:10.1063/5.0046567

Luminescence Properties and Energy Transfer Processes in Nanosized Cerium Doped YAG

Luminescence properties of cerium doped Y3Al5O12 (YAG) nanocrystals in form of nanopowders and nanoceramics have been studied. The comparative analysis of luminescence characteristics for nano- and single-crystals has been done. It was detected that an excitonic mechanism of the energy transfer from the host lattice to cerium ions does not work in nanopowders and nanoceramics. It was also shown that antisite-related and self-trapped exciton-related luminescence bands are suppressed strongly in nanopowders, and it can be excited only under certain circumstances. These bands practically disappeared in the nanoceramic samples. It was suggested that nanoparticle's surface, which is an efficient…

Rashba splitting of the Tamm surface state on Re(0001) observed by spin-resolved photoemission and scanning tunneling spectroscopy

Physical review research 2(1), 013296 (2020). doi:10.1103/PhysRevResearch.2.013296

The Temperature Dependence of Scintillation Parameters in PbWO4 Crystals

The luminescence spectra, decay kinetics and yield of luminescence in undoped PbWO 3 crystals were studied after pulsed electron beam irradiation. The rise time of luminescence pulses shows that two mechanisms - excitonic and recombination - were involved in luminescence center excited state formation. It is proposed that excited states of WO 3 and WO 2- 4 luminescence centers were formed from some metastable state, possibly from Pb related excitation.

Spin texture of time-reversal symmetry invariant surface states on W(110)

AbstractWe 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.

Formation of luminescence centres under excitation with electron beams in PbWO 4 and ZnWO 4 crystals

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 recom…

Soft X-ray emission spectroscopy used for the characterization of a-C and CNx thin films

Abstract We present the results of a soft X-ray emission spectroscopy study of a-C and CNx films on a Si(100) substrate. Also for the characterization of the homogeneity in depth of these films electron energy loss spectroscopy measurements with localization better than 4 nm were carried out. In case of CNx films the highest diamond-like modification occurs in the region close to the Si(100) substrate. The film density decreases with increasing distance from the substrate and becomes almost constant in range of thicknesses more than ~ 2 nm.

Transient absorption spectra and relaxation kinetics in Nb-doped PbWO4 scintillating crystals

Abstract The spectra and relaxation kinetics of absorption induced by a pulsed electron beam were studied for undoped and Nb-doped PbWO 4 single crystals. A significant influence of Nb doping on transient absorption was found. The influence of Nb doping on the relaxation kinetics is proposed to be due to an oxygen vacancy involving electron–hole recombination. A delay was observed (35– 50 ns ) between the growth of F-centers and the irradiation pulse. A large portion of the F-centers decays within ∼16 μs due to recombination with mobile holes. The prolonged recombination processes are responsible for the decrease of the scintillation yield in PbWO 4 : Nb crystals.

The study of time-resolved absorption and luminescence in PbWO4 crystals

Abstract The transient absorption and luminescence under pulsed electron beam excitation were studied for undoped and La3+ doped PbWO4 crystals. The absorption band at ∼1.0 eV is suggested to be due to self-trapped electrons and the absorption band at 3.5 eV may be due to self-trapped holes. The formation of luminescence centers via electron–hole recombination is affected by La3+ in doped crystals. The large fraction of electrons and holes undergoes thermostimulated recombination within geminate pairs and the spatial separation of geminate pairs components is important in recombination process. The intrinsic (blue) luminescence arises in this recombination. The mechanism of La3+ influence o…

Mechanism for energy transfer processes between Ce3+ and Tb3+ in LaPO4:Ce,Tb nanocrystals by time-resolved luminescence spectroscopy

The energy transfer mechanisms between Ce 3+ and Tb 3+ in LaPO 4 :Ce,Tb nanocrystals have been studied by means of time-resolved luminescence spectroscopy in a wide temperature range (10-300K). Special attention was paid to detailed comparative analysis of both rise and decay emission components of both Ce 3+ and Tb 3+ . Surprisingly, a relatively slow rise (several microseconds) of Tb 3+ emission under 266-nm laser excitation was detected, which corresponds to the 4f-5d transition of Ce 3+ in LaPO 4 . It was shown that this rise of Tb 3+ emission could not have arisen due to relaxation of Ce 3+ ions, whose excited state has a lifetime of about 20ns. It was demonstrated that the generally a…

Luminescence spectra and decay kinetics in ZnWO4 and CdWO4 crystals

The luminescence spectra and luminescence decay kinetics in ZnWO4 (nominal pure and doped with Fe and Mo) under pulsed electron beam and pulsed nitrogen laser excitation have been investigated. It is suggested that the two-stage intrinsic luminescence decay observed under ionizing radiation excitation in ZnWO4 and CdWO4 was due to two different self-trapped exciton (STE) configurations in these crystals. The role of Fe and Mo impurities for scintillation efficiency in ZnWO4 and CdWO4 has been discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transient Optical Absorption and Luminescence in Calcium Tungstate Crystal

Iron-related luminescence centers in ZnWO 4 :Fe

A systematic spectroscopic study of single ZnWO4 :Fe crystals with different iron concentrations has been performed under excitation by ultraviolet light, by synchrotron radiation or under photostimulation by near-infrared light. The luminescence of Fe3+-related centres has been studied. It is shown that iron centres of different types efficiently promote the formation of crystal defects at low temperatures. Electrons and holes can be trapped near Fe2+ or Fe3+ ions, which is further revealed in phosphorescence, thermostimulated or photostimulated luminescence. At room temperature the main effect of iron impurity is to reduce the light yield of a ZnWO4 scintillator.

Time-resolved luminescence and absorption in CdWO4

Abstract The luminescence and short-lived absorption are studied in CdWO 4 single crystals under pulsed laser beam as well as pulsed electron beam excitation. The luminescence spectrum and decay kinetics for CdWO 4 powder are studied. The short-lived absorption in single crystals arises from electron transitions from the luminescence center excited state to the conduction band. From luminescence and short-lived absorption spectra the energy depths of luminescence center excited and ground states are estimated.

Relaxation of electron-hole pairs and scintillation mechanism in alkali halide crystals

Abstract The mechanism of the ionizing radiation energy transfer from host lattice to luminescence centers is discussed taking into account the results of the recent experimental investigations of electron-hole pair relaxation in alkali iodide crystals. The high scintillation yield in CsI-Tl, CsI-Na and partially in Nal-Tl crystals is explained by the process of the motion and capture of a one-center self-trapped exciton by impurity ions.

Characterization and magnetic properties of nanoparticles based on FePt solid solution with an oxide shell

Abstract We present the results of magnetic properties investigations of ferromagnetic nanoparticles which ensembled with ferromagnetic shell. The aim of this work is to study the connection of structure–phase composition and thermal treatment with the magnetic properties, partially, with the coercive force and the exchange bias field in a two-dimension spin system of core–shell type. It is found that the exchange bias field increases with the growth of the oxide shell thickness. This is realized by annealing in a dosed oxygen flow. The decrease of the coercive force with the increase of annealing temperature is connected with the concentration effect (the concentration of ferromagnetic par…

Luminescence center excited state absorption in tungstates

The excited state absorption of intrinsic luminescence center (self-trapped exciton) in tungstates (CaWO4, ZnWO4, PbWO4 and CdWO4) was studied. The transient absorption and luminescence spectra, decay kinetics and lifetime dependencies on temperature have been measured. The model of self-trapped exciton and nature of observed absorption bands were discussed.

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

Physical review / B 103(5), 054407 (2021). doi:10.1103/PhysRevB.103.054407

Progress in HAXPES performance combining full-field k-imaging with time-of-flight recording

Journal of synchrotron radiation 26(6), 1996-2012 (2019). doi:10.1107/S1600577519012773

Spectral Measurement of Photon Emission from Individual Gold Nanoparticles Using Scanning Tunneling Microscopy

The light emission spectra of individual Au nanoparticles induced by a scanning tunneling microscope (STM) have been investigated. Two-dimensional ensembles of tunnel-coupled Au particles were prepared by thermal evaporation onto a native oxide silicon wafer in ultrahigh vacuum (10 – 9 mbar). Our STM measurements show a single peak at photon energy 1.6 eV in the tunneling mode and two peaks at 2.2 eV (connected with the Mie plasmon) and 1.45 eV (a new peak which was not discussed in literature before) in the field emission mode.

Multi-MHz time-of-flight electronic bandstructure imaging of graphene on Ir(111)

In the quest for detailed spectroscopic insight into the electronic structure at solid surfaces in a large momentum range, we have developed an advanced experimental approach. It combines the 3D detection scheme of a time-of-flight momentum microscope with an optimized filling pattern of the BESSY II storage ring. Here, comprehensive data sets covering the full surface Brillouin zone have been used to study faint substrate-film hybridization effects in the electronic structure of graphene on Ir(111), revealed by a pronounced linear dichroism in angular distribution. The method paves the way to 3D electronic bandmapping with unprecedented data recording efficiency.

Néel Vector Induced Manipulation of Valence States in the Collinear Antiferromagnet Mn 2 Au

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...

Temperature-Dependent Change of the Electronic Structure in the Kondo Lattice System $YbRh_{2}Si_{2}$

Seminar, Deutschland; Journal of physics / Condensed matter 00(00), 1-20 (2021). doi:10.1088/1361-648X/abe479

Site-specific atomic order and band structure tailoring in the diluted magnetic semiconductor (In,Ga,Mn)As

Physical review / B 103(7), 075107 (1-13) (2021). doi:10.1103/PhysRevB.103.075107

Hosting of surface states in spin–orbit induced projected bulk band gaps of W(1 1 0) and Ir(1 1 1)

Journal of physics / Condensed matter 29(25), 255001 - (2017). doi:10.1088/1361-648X/aa7173

Luminescence and Transient Optical Absorption in CdWO<sub>4</sub>

Luminescence and Transient Absorption of CdWO4Crystals

Time-resolved luminescence and induced absorption in PbWO4

Abstract Luminescence and short-lived induced absorption are studied for two undoped PbWO4 crystals. Luminescence decay at LNT is delayed relative to irradiation pulse. The delay observed is either due to reabsorption of luminescence or due to creation of luminescence center excited states via energy or/and charge transfer after irradiation pulse. Short-lived absorption is observed within 1.1–3.6 eV spectral region at LNT as well as at RT. It is proposed that the short-lived absorption band peaking at ~ 3.5 eV is intrinsic.

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 i…

Growth and defect studies of CdTe particles

The paper reports the epitaxial growth of cadmium telluride (CdTe) particles by thermal deposition on cleaved planes of (001)NaCl and (001)KBr. Using high resolution transmission electron microscopy and electron diffraction it was shown that CdTe particles could have different orientation and phase (cubic or hexagonal) depending on the substrate temperature. Their most common defects are twins and stacking faults.

Multidimensional photoemission spectroscopy—the space-charge limit

New journal of physics 20(3), 033004 - (2018). doi:10.1088/1367-2630/aaa262

The experimental observation of the potential barrier for self-trapped exciton decay into F-H pair in KCl-Na crystals

Abstract The optical absorption induced by the electron pulse irradiation of Na+ doped KCl has been measured. Transient optical absorption band of FA centers was observed at 80 K (LNT). The temperature dependence of FA center formation was studied. It is proposed that the obtained activation energy originates from the potential barrier between the STE perturbed by the cation impurity and the nearest neighbour FA-H pair. The mechanism of the suppression of the defect formation by the monovalent cation impurity in alkali halide crystals is discussed.

Photostimulated processes in the CsI-Tl crystal after UV irradiation

The photostimulated luminescence is studied for CsI-Tl crystal after irradiation with ultraviolet light in the 80 - 300 K temperature range. The PSL creation spectrum coincides with the D absorption band at 80 K. Three bands are observed in the stimulation spectra at 80 K: 1400, 950, and 600 nm. The 1400 and 950 nm stimulation bands are presumably explained as optical transitions in the Tl 0 and V k centers situated in the spatial correlated pairs. The stimulation at 600 nm band is ascribed to the unperturbed Tl 0 centers.

Anomalous d-like surface resonances on Mo(110) analyzed by time-of-flight momentum microscopy.

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 dispersi…

Direct 3D mapping of the Fermi surface and Fermi velocity.

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.

The energy transfer to the luminescence centers in PbWO4

Abstract The luminescence spectra, rise and decay kinetics and the yield of luminescence in undoped PbWO 4 crystals were studied after pulsed electron beam irradiation. The luminescence intensity rise observed after irradiation pulse showed two mechanisms—excitonic relaxation and electron–hole recombination—were involved in the formation of the excited states of the blue luminescence centers. It is proposed that the excited states of WO 3 and WO 2− 4 luminescence centers were formed from some intermediate metastable state.

Extraction dynamics of electrons from magneto-optically trapped atoms

Pulsed photoionization of laser-cooled atoms in a magneto-optical trap (MOT) has the potential to create cold electron beams of few meV bandwidths and few ps pulse lengths. Such a source would be highly attractive for the study of fast low-energy processes like coherent phonon excitation. To study the suitability of MOT-based sources for the production of simultaneously cold and fast electrons, we study the photoionization dynamics of trapped Cs atoms. A momentum-microscope-like setup with a delay-line detector allows for the simultaneous measurement of spatial and temporal electron distributions. The measured patterns are complex, due to the Lorentz force inducing spiral trajectories. Ray-…

Luminescence and transient absorption in ZnWO4 and ZnWO4–Fe crystals

Abstract The transient absorption spectra and relaxation kinetics under pulse electron beam excitation in ZnWO 4 and ZnWO 4 –Fe, as well as luminescence spectra and decay kinetics, were studied. It is shown that a fraction of transient absorption is due to luminescence center excited state. The energy transfer from intrinsic excitations to the defect or impurity states is not efficient. The role of Fe-impurities in the decay process is discussed.

High-resolution hard-x-ray photoelectron diffraction in a momentum microscope—the model case of graphite

New journal of physics 21(11), 113031 - (2019). doi:10.1088/1367-2630/ab51fe

Time-resolved spectroscopy of ZnWO4

The luminescence spectra of ZnWO 4 excited by a pulsed electron beam, pulsed nitrogen laser and X-ray are studied. The luminescence decay kinetics are compared with transient absorption relaxation kinetics. Broad band transient absorption spectra are observed and it is shown that the slow decay component (∼14 μs at RT) of the transient absorption matches well with the luminescence decay therefore this absorption is due to the excited state absorption of the luminescence center.

Investigation of a Ge nanoparticle film by means of electron stimulated photon emission spectroscopy

Abstract Electron stimulated photon emission spectroscopy was used for the study of the electronic structure of Ge nanoparticles. A nanoparticle film was prepared by thermal deposition on a quartz substrate. Photon emission was stimulated by electron bombardment at energies of several hundred electron volts. Electron field emission from a W-tip was used at tip voltage U t  = 100–600 eV. A spectrometer in combination with a liquid nitrogen cooled charge-coupled device (CCD) camera was used for light detection. Light emission spectra were measured in the energy range 1.18–4.2 eV. They are characterized by features at ∼1.6 and ∼3.1 eV. A comparison with light emission spectra obtained with ano…

Emitter-site specificity of hard x-ray photoelectron Kikuchi-diffraction

New journal of physics 22(10), 103002 (1-13) (2020). doi:10.1088/1367-2630/abb68b

Time-of-flight photoelectron momentum microscopy with 80–500 MHz photon sources: electron-optical pulse picker or bandpass pre-filter

Journal of synchrotron radiation 28(6), 1891 - 1908 (2021). doi:10.1107/S1600577521010511