Anomalous X-ray Research Articles

Elastic flow instabilities and macroscopic textures in graphene oxide lyotropic liquid crystals

Graphene oxide (GO) forms a well-aligned lyotropic liquid crystal (LC) phase in aqueous dispersions at relatively low concentrations. Under a remarkably wide range of shear rates, we report hitherto unobserved shear-induced polarized light image patterns, a Maltese cross combined with shear banding, recorded in real time and in situ during rheological measurements. This is shown to be a result of elastic flow instabilities that manifest as a helical flow in alternating bands of left- and right-handed helices, arising from a combination of shear flow and Taylor-type vortex flow. The instability is observed for LCs formed from large aspect ratio GO particles owing to their unique viscoelastic properties, but not for smaller aspect ratio particles. This phenomenon coincides with rheopecty and anomalous small-angle X-ray scattering patterns under shear flow, which confirm the instabilities. The results presented here could lead to advanced control over macroscopic periodic alignment in technologically relevant dispersions of two-dimensional material particles.

Pulsars in supernova remnants

ABSTRACT A comparative analysis of the properties of 60 pulsars located in supernova remnants (SNRs) was carried out. We found that (1) anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs) are characterized by longer periods and higher values of period derivatives, (2) the remainder of the pulsars in this sample have shorter periods compared to the bulk of radio pulsars, (3) rotation of their neutron stars is braking faster, (4) this braking seems unlikely to be solely a result of the loss of angular momentum through magnetic dipole radiation. Using the measured and the estimated braking indices of individual pulsars, we calculated their birth periods. We found that (1) AXPs/SGRs are formed with periods of order of several seconds at the time of birth, (2) the remainder of the pulsars show the wide distribution of initial periods from 13 to 1300 ms with no predominance of millisecond values. Estimates of angles β between the rotation axis and the magnetic moment of the neutron stars are obtained. We found that the distribution of these angles is very close to the uniform distribution. This suggests that pulsars are born with arbitrary values of β. We found that the radio luminosities of pulsars observed inside their SNRs are, on average, one order of magnitude less than the average for neutron stars outside their SNRs. While we note that our data set of pulsars within SNRs of 60 is rather small our findings suggest that these pulsars do not show a decrease in their radio luminosity during their evolution.

Photoluminescence Studies of CaAl2O4: EU2+ Blue Phosphors Co-Doped with ND3+ Ion Synthesized by Solid- State Reaction Method

In this work, the polycrystalline CaAl2O4: Eu2+, Nd3+ based blue phosphor was prepared by solid-state reaction method. The rare earth metal ions doped calcium aluminate phosphors have been studied in depth. The high quantum efficiency make them useful for anomalous long phosphorescence, good stability, luminous paints, radiation dosimetry, X-ray imaging, color display, and light emitting diodes. Phase, crystallinity and luminescent properties of the synthesized phosphors were examined by using powder X-ray diffraction (XRD). In the phosphor material, the photoluminescence (PL) spectrum has a broad emission peak of Eu2+ ion in blue region at (λmax = 440 nm) due to the energy transitions between the ground state 4f7 and the excited state 4f65d1 of the Eu2+ ion. This has importance in making the visible phosphor devices from this material. In this study the single dopant phosphors were synthesized; therefore, it is important to improve the long afterglow of the phosphors by codoping. The phosphors of CaAl2O4: Eu2+, Nd3+ had long persistence luminescence. In these phosphors, Eu2+ ions play the role of activators. Whereas Nd3+ ions generate deep traps that lead to the long persistent phosphorescence and act as the co-activators in the phosphors simultaneously. The concentration of Nd3+ ion co-doped has strong influence on the luminescence of phosphor.

Evaluation of Semiconductors using Anomalous X-ray Scattering

Evaluation of Semiconductors using Anomalous X-ray Scattering

Fine Structure of Zr<sub>80</sub>Pt<sub>20</sub> Amorphous Alloy Determined from Anomalous X-ray Scattering (AXS) Data by Applying Reverse Monte-Carlo (RMC) Simulation Method

The fine structure of a Zr80Pt20 amorphous alloy was evaluated by anomalous X-ray scattering (AXS) coupled with reverse Monte Carlo (RMC) simulation. Featureless short-range ordered structures were preferentially formed by the Zr component, similar to those described by dense random packing (DRP), while the Pt atoms adopted a somewhat different structure. The Pt atoms adopted Zr-rich coordination and an icosahedral local atomic arrangement, in unique chemical short-range ordering (CSRO) as well as topological short-range ordering (TSRO). The present AXS-RMC analysis also suggested that the pre-peak signal at 17 nm−1 in the X-ray diffraction profile was largely attributed to the correlation between the Pt–Pt pair with a separation of approximately 0.4–0.5 nm. The geometrical and chemical features of the common neighbors of the middle-range Pt–Pt pairs indicate the unique CSRO and TSRO, where both chemical and density fluctuations were observed in two distinct regions: a Zr-rich region with high packing density and a Pt-rich region with low packing density.

Semi-Analytical Monte Carlo Method to Simulate the Signal of the VIP-2 Experiment

The VIP-2 collaboration runs an apparatus in the Gran Sasso underground laboratories of the Italian Institute for Nuclear Physics (INFN) designed to search for anomalous X-rays from electron-atom interactions due to violations of the fundamental antisymmetry of multi-electron wavefunctions. The experiment implements the scheme first proposed by Ramberg and Snow, where a current source injects electrons into a metal strip (the experiment’s target). In this paper we describe the structure of a Monte Carlo program to simulate a new upgrade of the experiment, where the anomalous X-ray emission is modulated by an arbitrary time-varying input current. A novel feature of the simulation algorithm is that the Monte Carlo program is based on a mixture of analytical and numerical methods. We report preliminary, exploratory results on the expected detection rate for different modulations of the injected current; these results are a starting point on the way to optimize the modulation scheme and indicate a large potential improvement of the detection sensitivity.

Analysis of Homopolymer Distribution in a Polymer Blend Thin Film by Anomalous Grazing Incidence Small-Angle X-ray Scattering at the Bromine K-Edge

Polystyrene-block-poly(4-hydroxystyrene) (PS–PHS)/poly(4-hydroxystyrene) brominated (Br-PHS) blend thin films were analyzed using anomalous grazing incidence small-angle X-ray scattering (anomalous...

Detailed structural analysis of amorphous Pd40Cu40P20: Comparison with the metallic glass Pd40Ni40P20 from the viewpoint of glass forming ability

To clarify the relationship between the glass-forming ability (GFA) and local atomic structure of Pd-based metallic glasses, the structures of an amorphous Pd40Cu40P20 (PCP) were investigated using the same methods as for Pd40Ni40P20 (PNP) metallic glass, i.e., by a combination of anomalous X-ray scattering, X-ray and neutron diffraction, and reverse Monte Carlo modeling. From the comparison between the results of PNP with an excellent GFA and the PCP with a worse GFA, important features characteristic to the GFA are found in the hyper-ordered structures. Firstly, the compositional inhomogeneity of Cu in PCP is larger than Ni in PNP. Secondly, a Voronoi tessellation reveals that icosahedral arrangements are found with considerable probability around the Ni atoms in PNP, while a less existence around the Cu atoms in PCP. Thirdly, a persistent homology (PH) analysis shows smaller intermediate-size Cu PH rings in PCP than Ni PH rings in PNP.

Cation distribution in Cu2ZnSnSe4, Cu2FeSnS4 and Cu2ZnSiSe4 by multiple-edge anomalous diffraction.

Multiple-Edge Anomalous Diffraction (MEAD) has been applied to various quaternary sulfosalts belonging to the adamantine compound family in order to validate the distribution of copper, zinc and iron cations in the structure. Semiconductors from this group of materials are promising candidates for photovoltaic applications. Their properties strongly depend on point defects, in particular related to cation order-disorder. However, Cu+, Zn2+ and Fe2+ have very similar scattering factors and are all but indistinguishable in usual X-ray diffraction experiments. Anomalous diffraction utilizes the dependency of the atomic scattering factors f' and f'' of the energy of the radiation, especially close to the element-specific absorption edges. In the MEAD technique, individual Bragg peaks are tracked over an absorption edge. The intensity changes depending on the structure factor can be highly characteristic for Miller indices selected for a specific structural problem, but require very exact measurements. Beamline KMC-2 at synchrotron BESSY II, Berlin, has been recently upgraded for this technique. Anomalous X-ray powder diffraction and XAFS compliment the data. Application of this technique confirmed established cation distribution in Cu2ZnSnSe4 (CZTSe) and Cu2FeSnS4 (CFTS). In contrast to the literature, cation distribution in Cu2ZnSiSe4 (CZSiSe) is shown to adopt a highly ordered wurtz-kesterite structure type.

Effects of the atomic order on the half-metallic electronic structure in the Co2Fe(Ga0.5Ge0.5) Heusler alloy thin film

The atomic order and its effect on the electronic structure is a key issue in the application of half-metallic Heusler alloys as spin-polarized electron sources. In this study, we investigated the atomic ordering and electronic structures of $\mathrm{C}{\mathrm{o}}_{2}\mathrm{Fe}(\mathrm{G}{\mathrm{e}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5})$ (CFGG) Heusler alloy thin films at various annealing temperatures using anomalous x-ray diffraction (AXRD) and hard x-ray photoelectron spectroscopy (HAXPES). AXRD measurements clearly showed that the Co-Fe disorder is large in the as-deposited state, which is reduced to almost zero by annealing at ${T}_{\mathrm{an}}=\phantom{\rule{0.16em}{0ex}}500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ for 30 min. The reduction of the Co-Fe disorder explains the observed increase in the spin polarization of currents estimated by ordinary and anisotropic magnetoresistance measurements. The photoelectron spectra of the valence band in CFGG thin films with different ${T}_{\mathrm{an}}$ agree well with the first-principles calculations that considers the atomic disorder. We also found that the characteristic peaks appearing in the valence band HAXPES spectra shifted to higher binding energies, compared to the calculated density of states for stoichiometric $\mathrm{L}{2}_{1}$-ordered CFGG. This indicates that the Fermi level is shifted and located in the vicinity of the conduction band edge in the present CFGG thin film due to electron doping by a mildly Co-rich chemical composition. Our first-principles calculations of partial density of state of $sp$ electron predicted that the high spin-polarization is obtainable in a wider energy region near Fermi level in the $\mathrm{L}{2}_{1}$-ordered structure than the B2 structure, which explains the enhancement of spin-polarization by promoting $\mathrm{L}{2}_{1}$-ordering in the present CFGG film.

An observational argument against accretion in magnetars

The phenomenology of anomalous X-ray pulsars is usually interpreted within the paradigm of very highly magnetized neutron stars, also known as magnetars. According to this paradigm, the persistent emission of anomalous X-ray pulsars (AXPs) is powered by the decay of the magnetic field. However, an alternative scenario in which the persistent emission is explained through accretion is also discussed in literature. In particular, AXP 4U 0142+61 has been suggested to be either an accreting neutron star or a white dwarf. Here, we rule out this scenario based on the observed X-ray variability properties of the source. We directly compare the observed power spectra of 4U 0142+61 and of two other magnetars, 1RXS J170849.0−400910 and 1E 1841−045 with that of the X-ray pulsar 1A 0535+262, and of the intermediate polar GK Persei. In addition, we include a bright young radio pulsar PSR B1509-58 for comparison. We show that, unlike accreting sources, no aperiodic variability within the expected frequency range is observed in the power density spectrum of the magnetars and the radio pulsar. Considering that strong variability is an established feature of all accreting systems from young stellar objects to super-massive black holes and the absence of the variability reports from other magnetars, we conclude that our results also indicate that magnetars, in general, are not powered by accretion.

Identifying crystal structures and chemical reactions at the interface of stanene on Bi2Te3

Synthesizing monolayers and heterostructures is an enabling approach to extract new physical phenomena from bulk materials. Among the structures amenable to this approach is stanene, which is a monolayer of tin, similar to graphene, and has been predicted to host one-dimensional topological states at its edges. Stanene can be tuned by decorating with different adatoms, which makes it a promising platform on which to engineer topological devices. Here, we deposit Sn on Bi2Te3 and characterize the growth using anomalous synchrotron x-ray scattering and x-ray photoelectron spectroscopy (XPS). X-ray diffraction data reveal the formation of epitaxial Sn-based structures, along with penetration of Sn into the Bi2Te3, with Sn intercalating between the upper 10 Bi2Te3 quintuple layers. Additionally, XPS data show deposited Sn reacting to form SnTe and Bi at the Bi2Te3 surface. The calculated heat of reaction for Sn and Bi2Te3 is consistent with an exothermic reaction to SnTe and Bi. Using thermodynamic calculations as a guide, we identify several candidate substrates that can stabilize the stanene phase.

Gravitational waves from SGRs and AXPs as fast-spinning white dwarfs

ABSTRACT In our previous article we have explored the continuous gravitational waves (GWs) emitted from rotating magnetized white dwarfs (WDs) and their detectability by the planned GW detectors such as Laser Interferometer Space Antenna (LISA), Deci-hertz Interferometer Gravitational wave Observatory (DECIGO), and Big Bang Observer (BBO). Here, GWs’ emission due to magnetic deformation mechanism is applied for soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs), described as fast-spinning and magnetized WDs. Such emission is caused by the asymmetry around the rotation axis of the star generated by its own intense magnetic field. Thus, for the first time in the literature, the GW counterparts for SGRs/AXPs are described as WD pulsars. We find that some SGRs/AXPs can be observed by the space detectors BBO and DECIGO. In particular, 1E 1547.0−5408 and SGR 1806−20 could be detected in 1 yr of observation, whereas SGR 1900+14, CXOU J171405.7−381031, Swift J1834.9−0846, SGR 1627−41, PSR J1622−4950, SGR J1745−2900, and SGR 1935+2154 could be observed with a 5-yr observation time. The sources XTE J1810−197, SGR 0501+4516, and 1E 1048.1−5937 could also be seen by BBO and DECIGO if these objects have $M_{\mathrm{ WD}} \lesssim 1.3 \, \mathrm{M}_{\odot }$ and $M_{\mathrm{ WD}} \lesssim 1.2 \, \mathrm{M}_{\odot }$, respectively. We also found that SGRs/AXPs as highly magnetized neutron stars are far below the sensitivity curves of BBO and DECIGO. This result indicates that a possible detection of continuous GWs originated from these objects would corroborate the WD pulsar model.

Parameter Comparison of Several Types of Pulsars

We have carried out a comparative analysis of the parameters of two samples of radio pulsars. The first of them is characterized by periods $$P > 2$$ s, the second one—by the surface magnetic fields $${{B}_{s}} > 4.4 \times $$ 1013 G. The purpose for selecting these values is for the understanding the origin of the difference between normal pulsars and anomalous X-ray pulsars (AXP) and gamma-repeaters (SGR) with similar values of the periods and magnetic fields. It is shown that the derivative dependence of the period on its value $$P$$ in the long-period pulsars does not agree with the model of magneto-dipole braking and requires other mechanisms or the search for specific distributions of their parameters. In the pulsars with large magnetic fields and in AXP/SGR, this dependence does not contradict the magnetic dipole model. For pulsars with $$P > 2$$ s, the transformation efficiency of the rotation energy into the observed radiation turns out to be several times higher than for the objects with shorter periods. The pulsars with long periods have, on average, two orders of magnitude larger age than the sources with $${{B}_{s}} > 4.4 \times $$ 1013 G and AXP/SGR, and are, on average, located higher above the Galactic plane. It is concluded that the main physical parameters of the pulsars with large surface magnetic fields and their spatial distribution are close to the parameters and distribution of AXP/SGR. The comparison shows that it is not the rotation period, but the strong surface magnetic field is the decisive factor for the AXP/SGR peculiarity.

Thickness dependence of degree of B2 order of polycrystalline Co2(Mn0.6Fe0.4)Ge Heusler alloy films measured by anomalous X-ray diffraction and its impacts on current-perpendicular-to-plane giant magnetoresistance properties

We investigated the degree of B2 order (SB2) of 4-10 nm-thick polycrystalline Co2(Mn0.6Fe0.4)Ge (CMFG) Heusler alloy films using anomalous X-ray diffraction (AXRD) and its correlation to the magnitude of current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) effect. We found that SB2 of the CMFG films is degraded for the film thickness below 6 nm, which is an issue for practical applications of CPP-GMR devices such as read head sensors of hard disk drives. Since SB2 has a thickness-dependence, the bulk spin polarization (β) of the CMFG layers is also expected to vary depending on the thickness; thus, the deduction of β using the Valet-Fert model, where β is assumed to be thickness-independent, is not valid for these CPP-GMR devices.

Sodium Induced Morphological Changes of Carbon Coated TiO2 Anatase Nanoparticles - High-Performance Materials for Na-Ion Batteries

The most promising candidate as an everyday alternative to lithium-ion batteries (LIBs) are sodium-ion batteries (NIBs). This is not only due to Na abundance, but also because the main principles and cell structure are very similar to LIBs. Due to these benefits, NIBs are expected to be used in applications related to large-scale energy storage systems and other applications not requiring top-performance in terms of volumetric capacity. One important issue that has hindered the large scale application of NIBs is the anode material. Graphite and silicon, which have been widely applied as anodes in NIBs, do not show great performance. Hard carbons look very promising in terms of their abundance and low cost, but they tend to suffer from instability, in particular over the long term. In this work we explore a carbon-coated TiO2 nanoparticle system that looks very promising in terms of stability, abundance, low-cost, and most importantly that safety of the cell, since it does not suffer from potential sodium plating during cycling. Maintaining a nano-size and consistent morphology of the active material is a crucial parameter for maintaining a well-functioning cell upon cycling. In this work we applied Anomalous Small Angle X-Ray Scattering (ASAXS) for the first time at the Ti K-edge of TiO2 anatase nanoparticles on different cycled composite electrodes in order to have a complete morphological overview of the modifications induced by sodiation and desodiation. This work also demonstrates for the first time that the nanosize of the TiO2 is maintained upon cycling, which is in agreement with the electrochemical stability.

Detection of the magnetar XTE J1810−197 at 150 and 260 GHz with the NIKA2 kinetic inductance detector camera

Context. The investigation of pulsars between millimetre and optical wavelengths is challenging due to the faintness of the pulsar signals and the relative low sensitivity of the available facilities compared to 100 m class telescopes operating in the centimetre band. The kinetic inductance detector (KID) technology offers large instantaneous bandwidths and a high sensitivity that can help to increase the ability of existing observatories at short wavelengths substantially to detect pulsars and transient emission. Aims. To investigate whether pulsars can be detected with KIDs, we observed the anomalous X-ray pulsar XTE J1810−197 with the New IRAM KIDs Array-2 (NIKA2) camera installed at the IRAM 30 m telescope in Spain. Methods. Several short observations of XTE J1810−197 were made on 2019 March 25 under good weather conditions to verify the stability of the KIDs and to try to detect the expected broadband pulsations from the neutron star. Results. We detected the pulsations from the pulsar with NIKA2 at its two operating frequency bands, 150 and 260 GHz (λ = 2.0 and 1.15 mm, respectively). This is the first time that a pulsar is detected with a receiver based on KID technology in the millimetre band. In addition, this is the first report of short millimetre emission from XTE J1810−197 after its reactivation in December 2018, and it is the first time that the source is detected at 260 GHz, which gives us new insights into the radio emission process of the star. Conclusions. We demonstrate that KIDs can fulfil the technical requirements for detecting pulsed emission from neutron stars in the millimetre band. We show that the magnetar XTE J1810−197 is again emitting strong pulsations in the short millimetre band.

Cu/Zn disorder in stoichiometric Cu2ZnSn(S1-xSex)4 semiconductors: A complementary neutron and anomalous X-ray diffraction study

The quaternary compound semiconductor Cu2ZnSn(S1-xSex)4 (CZTSSe) which crystallizes in the kesterite-type structure, is a promising material to be used as p-type absorber layer in thin film solar cell applications based on earth abundant elements. The absorber band tailing caused by an exceptionally high Cu/Zn disorder is believed to be one of the reasons for the limited open-circuit voltage in CZTSSe-based photovoltaic devices. This work is an experimental study of the Cu/Zn disorder in a unique set of single phase, stoichiometric CZTSSe mixed crystals, synthesized by solid state reaction, by means of neutron powder diffraction and anomalous X-ray powder diffraction. The existence of Cu/Zn disorder was revealed as the only intrinsic point defect in these mixed crystals within the detection limits of our measurements. The order parameter Q was calculated on the basis of the occurring CuZn and ZnCu anti site defects causing the Cu/Zn disorder. Variations of the order parameter with anion composition and the effect on the optoelectronic properties of the partial substitution of Se with S in Cu2ZnSn(S1-xSex)4 was elaborated.

Structure of amorphous Cu2GeTe3 and the implications for its phase-change properties

The structure of amorphous ${\mathrm{Cu}}_{2}{\mathrm{GeTe}}_{3}$ is investigated by a combination of anomalous x-ray scattering and extended x-ray absorption fine-structure experiments. The experimental data are analyzed with reverse Monte Carlo modeling, and they are interpreted in terms of short-range-order parameters as well as by using ring statistics and persistent homology to study the intermediate-range order. Based on this information, the structural relationship of the amorphous phase to the corresponding crystal is discussed. It is found that the amorphous network can be rationalized by small atomic displacements of the crystal structure, directed toward the intrinsic void regions. This structural similarity establishes the possibility of a fast phase-change process. On the other hand, the atomic rearrangements also lead to the formation of new chemical bonds and to distortions on the intermediate-range-order level. These are realized by a collapse and contraction of the strict hexagonal ring arrangements of the crystal and by the formation of small, triangular rings as well as Cu cluster configurations. These structural features allow for a new understanding of the phase-change property contrast of this material, especially concerning the density change and the optical contrast.

Application of Anomalous X-ray Scattering Method to Liquid Electrolytes Used in a Battery: Local Structural Analysis around a Dilute Metallic Ion.

In liquid electrolytes used for a battery, various metal complexes are formed as a result of ion-solvent and ion-ion interactions, which strongly influence the properties of the electrolyte and thus the performance of the battery. Therefore, the structural characterization of such complexes is of great importance. In this study, the anomalous X-ray scattering (AXS) technique was applied to the potassium hydroxide solution including ∼0.3 mol % zinc, which is widely used in various batteries such as the alkaline battery. In spite of the small amount of the metallic ions, we have successfully extracted a local structure around zinc after careful data analysis. The obtained pair distribution function exhibited not only the short-range correlation corresponding to the Zn-O bond within the zincate anion but also a medium-range correlation above 3.5 Å. The present study demonstrates the capability of the AXS technique to detect local structures around dilute metallic ions in liquid electrolytes, which will largely extend the applicable range of this technique, especially to the field related to batteries.