Evolution of superconductivity in K2-xFe4+ySe5: Spectroscopic studies of X-ray absorption and emission.

This study investigates the evolution of superconductivity in K2-xFe4+ySe5 using temperature-dependent X-ray absorption and resonant inelastic X-ray scattering techniques. Magnetization measurements show that polycrystalline superconducting (SC) K1.9Fe4.2Se5 has a critical temperature (T c) of ∼31 K with a varying superconducting volume fraction, which strongly depends on its synthesis temperature. An increase in Fe-structural/vacancy disorder in SC samples with more Fe atoms occupying vacant 4d sites is found to be closely related to the decrease in the spin magnetic moment of Fe. Moreover, the nearest-neighbor Fe-Se bond length in SC samples exceeds that in the non-SC (NS) sample, K2Fe4Se5, which indicates a weaker hybridization between the Fe 3d and Se 4p states in SC samples. These results clearly demonstrate the correlations among the local electronic and atomic structures and the magnetic properties of K2-xFe4+ySe5 superconductors, providing deeper insight into the electron pairing mechanisms of superconductivity.

Magnon profile on SrRuO3 films studied by inelastic neutron scattering.

In this study, the inelastic neutron scattering probe of SIKA in ANSTO is employed to investigate the magnon dispersion curve in ferromagnetic SrRuO3 single crystal epitaxial films and to better understand the underlying mechanisms. This report presents the successful measurement of a magnon peak from the SrRuO3 films which contained an amount of material of only 0.9 mg. We reveal one significant magnon dispersion curve along [002] following the quadratic E ∝ Q 2 ) relation, which shows a magnon gap of 0.32 meV. We have discussed several possible mechanisms, such as the higher symmetry structure and the impurity levels, which may contribute to this smaller gap.

Strain effect on orbital and magnetic structures of Mn ions in epitaxial Nd0.35Sr0.65MnO3/SrTiO3 films using X-ray diffraction and absorption.

This study probes the temperature-dependent strain that is strongly correlated with the orbital and magnetic structures of epitaxial films of Nd0.35Sr0.65MnO3 (NSMO) that are fabricated by pulsed laser deposition with two thicknesses, 17 (NS17) and 103 nm (NS103) on SrTiO3 (STO) substrate. This investigation is probed using X-ray diffraction (XRD) and absorption-based techniques, X-ray linear dichroism (XLD) and the X-ray magnetic circular dichroism (XMCD). XRD indicates a significant shift in the (004) peak position that is associated with larger strain in NS17 relative to that of NS103 at both 30 and 300 K. Experimental and atomic multiplet simulated temperature-dependent Mn L3,2-edge XLD results reveal that the stronger strain in a thinner NS17 film causes less splitting of Mn 3d eg state at low temperature, indicating an enhancement of orbital fluctuations in the band above the Fermi level. This greater Mn 3d orbital fluctuation can be the cause of both the enhanced ferromagnetism (FM) as a result of spin moments and the reduced Néel temperature of C-type antiferromagnetism (AFM) in NS17, leading to the FM coupling of the canted-antiferromagnetism (FM-cAFM) state in NSMO/STO epitaxial films at low temperature (T = 30 K). These findings are also confirmed by Mn L3,2-edge XMCD measurements.

The effect of orbital-lattice coupling on the electrical resistivity of YBaCuFeO5 investigated by X-ray absorption.

Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques were used to investigate the valence state, preferred orbital and local atomic structure that significantly affect the electrical and magnetic properties of a single crystal of YBaCuFeO5 (YBCFO). An onset of increase of resistivity at ~180 K, followed by a rapid increase at/below 125 K, is observed. An antiferromagnetic (AFM)-like transition is close to the temperature at which the resistivity starts to increase in the ab-plane and is also observed with strong anisotropy between the ab-plane and the c-axis. The XLD spectra at the Fe L3,2-edge revealed a change in Fe 3d eg holes from the preferential [Formula: see text] orbital at high temperature (300-150 K) to the [Formula: see text] orbital at/below 125 K. The analysis of the Fe K-edge EXAFS data of YBCFO further revealed an unusual increase in the Debye-Waller factor of the nearest-neighbor Fe-O bond length at/below 125 K, suggesting phonon-softening behavior, resulting in the breaking of lattice symmetry, particularly in the ab-plane of Fe-related square pyramids. These findings demonstrate a close correlation between electrical resistivity and coupling of the preferred Fe 3d orbital with lattice distortion of a single crystal of YBCFO.

Correlation between magnetic properties and the electronic structures of soft magnetic ternary Fe(78-x)Y(x)B(22) (x = 4-9) bulk metallic glasses.

Fe and Y K-edge extended x-ray absorption fine structure, Fe(Y) L(3,2)-edge (L(3)-edge) x-ray absorption near-edge structure (XANES) and valence-band photoemission spectroscopy (VB-PES) measurements have been carried out to study soft magnetic ternary Fe(78-x)Y(x)B(22) bulk metallic glasses (BMGs). The combined XANES and VB-PES results do not show broadening of the Fe 3d band to support the previous interpretation of the reduction of the magnetic moment in BMGs by Y-induced decrease of exchange splitting of Fe 3d orbitals. Instead, the density of delocalized/itinerant Fe 3d states in the vicinity of the Fermi level is found to be reduced by Y substitution, which reduces the strength of itinerant-states-mediated ferromagnetic coupling between local spins on the Fe ions and the total magnetic moment of the Fe-based BMGs.

Effect of swift heavy ions in Ni-Al nanocrystalline films studied by X-ray absorption spectroscopy.

X-ray absorption spectroscopic measurements have been used to compare the electronic structures of swift heavy ions (100 MeV Si ions) irradiated and pristine Ni-Al nanocrystalline films. Results from X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES) spectra at Al K-, and Ni L(2,3)-edges and extended X-ray absorption fine structure (EXAFS) at Ni K-edges are discussed. The observed XRD peaks indicate the improvement of crystalline nature and Al(111) clustering after the swift heavy ion interactions. While the XANES spectra at Ni L(2,3)-edges show decrease in the intensity of white line strength, the Al K-edge shows increase in intensity after irradiation. Above results imply that swift heavy ions induce low Z (i.e., Al) ion mass transport, changes in Al sp-Ni-d hybridization, and charge transfer. EXAFS results show that crystalline nature is improved after swift heavy irradiation which is consistent with XRD results.

Anisotropy in the magnetic interaction and lattice-orbital coupling of single crystal Ni3TeO6.

This investigation reports on anisotropy in the magnetic interaction, lattice-orbital coupling and degree of phonon softening in single crystal Ni3TeO6 (NTO) using temperature- and polarization-dependent X-ray absorption spectroscopic techniques. The magnetic field-cooled and zero-field-cooled measurements and temperature-dependent Ni L3,2-edge X-ray magnetic circular dichroism spectra of NTO reveal a weak Ni-Ni ferromagnetic interaction close to ~60 K (TSO: temperature of the onset of spin ordering) with a net alignment of Ni spins (the uncompensated components of the Ni moments) along the crystallographic c-axis, which is absent from the ab-plane. Below the Néel temperature, TN~ 52 K, NTO is stable in the antiferromagnetic state with its spin axis parallel to the c-axis. The Ni L3,2-edge X-ray linear dichroism results indicate that above TSO, the Ni 3d eg electrons preferentially occupy the out-of-plane 3d3z2-r2 orbitals and switch to the in-plane 3dx2-y2 orbitals below TSO. The inherent distortion of the NiO6 octahedra and anisotropic nearest-neighbor Ni-O bond lengths between the c-axis and the ab-plane of NTO, followed by anomalous Debye-Waller factors and orbital-lattice in conjunction with spin-phonon couplings, stabilize the occupied out-of-plane (3d3z2-r2) and in-plane (3dx2-y2) Ni eg orbitals above and below TSO, respectively.

Origin of magnetic properties in carbon implanted ZnO nanowires.

Various synchrotron radiation-based spectroscopic and microscopic techniques are used to elucidate the room-temperature ferromagnetism of carbon-doped ZnO-nanowires (ZnO-C:NW) via a mild C+ ion implantation method. The photoluminescence and magnetic hysteresis loops reveal that the implantation of C reduces the number of intrinsic surface defects and increases the saturated magnetization of ZnO-NW. The interstitial implanted C ions constitute the majority of defects in ZnO-C:NW as confirmed by the X-ray absorption spectroscopic studies. The X-ray magnetic circular dichroism spectra of O and C K-edge respectively indicate there is a reduction in the number of unpaired/dangling O 2p bonds in the surface region of ZnO-C:NW and the C 2p-derived states of the implanted C ions strongly affect the net spin polarization in the surface and bulk regions of ZnO-C:NW. Furthermore, these findings corroborate well with the first-principles calculations of C-implanted ZnO in surface and bulk regions, which highlight the stability of implanted C for the suppression and enhancement of the ferromagnetism of the ZnO-C:NW in the surface region and bulk phase, respectively.

Enhancement of sp(3)-bonding in high-bias-voltage grown diamond-like carbon thin films studied by x-ray absorption and photoemission spectroscopy.

X-ray absorption near-edge structure (XANES) and valence-band photoemission spectroscopy (VB-PES) were used to elucidate the electronic and mechanical properties of diamond-like carbon (DLC) thin films deposited by the plasma-enhanced chemical vapour deposition method at various bias voltages (V(b)) using a C(2)H(2) vapour precursor in an Ar(+) atmosphere. The increase of V(b) is found to increase and decrease the contents of sp(3)- and sp(2)-bonded carbon atoms, respectively, i.e. the films become more diamond-like. The Young's modulus measurements show increases with the increase of the presence of sp(3)-bonded carbon atoms in the structure of the DLC films.

Electronic and atomic structures of the Sr3Ir4Sn13 single crystal: A possible charge density wave material.

X-ray scattering (XRS), x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopic techniques were used to study the electronic and atomic structures of the high-quality Sr3Ir4Sn13 (SIS) single crystal below and above the transition temperature (T* ≈ 147 K). The evolution of a series of modulated satellite peaks below the transition temperature in the XRS experiment indicated the formation of a possible charge density wave (CDW) in the (110) plane. The EXAFS phase derivative analysis supports the CDW-like formation by revealing different bond distances [Sn1(2)-Sn2] below and above T* in the (110) plane. XANES spectra at the Ir L3-edge and Sn K-edge demonstrated an increase (decrease) in the unoccupied (occupied) density of Ir 5d-derived states and a nearly constant density of Sn 5p-derived states at temperatures T < T* in the (110) plane. These observations clearly suggest that the Ir 5d-derived states are closely related to the anomalous resistivity transition. Accordingly, a close relationship exists between local electronic and atomic structures and the CDW-like phase in the SIS single crystal.

Anisotropy in the thermal hysteresis of resistivity and charge density wave nature of single crystal SrFeO3-δ: X-ray absorption and photoemission studies.

The local electronic and atomic structures of the high-quality single crystal of SrFeO3-δ (δ~0.19) were studied using temperature-dependent x-ray absorption and valence-band photoemission spectroscopy (VB-PES) to investigate the origin of anisotropic resistivity in the ab-plane and along the c-axis close to the region of thermal hysteresis (near temperature for susceptibility maximum, Tm~78 K). All experiments herein were conducted during warming and cooling processes. The Fe L 3,2-edge X-ray linear dichroism results show that during cooling from room temperature to below the transition temperature, the unoccupied Fe 3d e g states remain in persistently out-of-plane 3d 3z2-r2 orbitals. In contrast, in the warming process below the transition temperature, they change from 3d 3z2-r2 to in-plane 3d x2-y2 orbitals. The nearest-neighbor (NN) Fe-O bond lengths also exhibit anisotropic behavior in the ab-plane and along the c-axis below Tm. The anisotropic NN Fe-O bond lengths and Debye-Waller factors stabilize the in-plane Fe 3d x2-y2 and out-of-plane 3d 3z2-r2 orbitals during warming and cooling, respectively. Additionally, a VB-PES study further confirms that a relative band gap opens at low temperature in both the ab-plane and along the c-axis, providing the clear evidence of the charge-density-wave nature of SrFeO3-δ (δ~0.19) single crystal.

Understanding of sub-band gap absorption of femtosecond-laser sulfur hyperdoped silicon using synchrotron-based techniques.

The correlation between sub-band gap absorption and the chemical states and electronic and atomic structures of S-hyperdoped Si have been extensively studied, using synchrotron-based x-ray photoelectron spectroscopy (XPS), x-ray absorption near-edge spectroscopy (XANES), extended x-ray absorption fine structure (EXAFS), valence-band photoemission spectroscopy (VB-PES) and first-principles calculation. S 2p XPS spectra reveal that the S-hyperdoped Si with the greatest (~87%) sub-band gap absorption contains the highest concentration of S(2-) (monosulfide) species. Annealing S-hyperdoped Si reduces the sub-band gap absorptance and the concentration of S(2-) species, but significantly increases the concentration of larger S clusters [polysulfides (Sn(2-), n > 2)]. The Si K-edge XANES spectra show that S hyperdoping in Si increases (decreased) the occupied (unoccupied) electronic density of states at/above the conduction-band-minimum. VB-PES spectra evidently reveal that the S-dopants not only form an impurity band deep within the band gap, giving rise to the sub-band gap absorption, but also cause the insulator-to-metal transition in S-hyperdoped Si samples. Based on the experimental results and the calculations by density functional theory, the chemical state of the S species and the formation of the S-dopant states in the band gap of Si are critical in determining the sub-band gap absorptance of hyperdoped Si samples.

Visualizing chemical states and defects induced magnetism of graphene oxide by spatially-resolved-X-ray microscopy and spectroscopy.

This investigation studies the various magnetic behaviors of graphene oxide (GO) and reduced graphene oxides (rGOs) and elucidates the relationship between the chemical states that involve defects therein and their magnetic behaviors in GO sheets. Magnetic hysteresis loop reveals that the GO is ferromagnetic whereas photo-thermal moderately reduced graphene oxide (M-rGO) and heavily reduced graphene oxide (H-rGO) gradually become paramagnetic behavior at room temperature. Scanning transmission X-ray microscopy and corresponding X-ray absorption near-edge structure spectroscopy were utilized to investigate thoroughly the variation of the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups, as well as the C 2p(σ*)-derived states in flat and wrinkle regions to clarify the relationship between the spatially-resolved chemical states and the magnetism of GO, M-rGO and H-rGO. The results of X-ray magnetic circular dichroism further support the finding that C 2p(σ*)-derived states are the main origin of the magnetism of GO. Based on experimental results and first-principles calculations, the variation in magnetic behavior from GO to M-rGO and to H-rGO is interpreted, and the origin of ferromagnetism is identified as the C 2p(σ*)-derived states that involve defects/vacancies rather than the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups on GO sheets.

The effect of thermal reduction on the photoluminescence and electronic structures of graphene oxides.

Electronic structures of graphene oxide (GO) and hydro-thermally reduced graphene oxides (rGOs) processed at low temperatures (120-180°C) were studied using X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). C K-edge XANES spectra of rGOs reveal that thermal reduction restores C = C sp(2) bonds and removes some of the oxygen and hydroxyl groups of GO, which initiates the evolution of carbonaceous species. The combination of C K-edge XANES and Kα XES spectra shows that the overlapping π and π* orbitals in rGOs and GO are similar to that of highly ordered pyrolytic graphite (HOPG), which has no band-gap. C Kα RIXS spectra provide evidence that thermal reduction changes the density of states (DOSs) that is generated in the π-region and/or in the gap between the π and π* levels of the GO and rGOs. Two-dimensional C Kα RIXS mapping of the heavy reduction of rGOs further confirms that the residual oxygen and/or oxygen-containing functional groups modify the π and σ features, which are dispersed by the photon excitation energy. The dispersion behavior near the K point is approximately linear and differs from the parabolic-like dispersion observed in HOPG.

X-ray absorption spectroscopy studies of Ba(1-x)Ca(x)TiO3.

We report x-ray absorption near edge spectroscopy (XANES) of Ca and O K-edges of Ba(1-x)Ca(x)TiO3 (x = 0, 0.01, 0.08, 1) and understand the spectral features related to the electronic structure of these perovskites. The XANES spectra of Ca K-edge possess a pre-edge peak similar to other 3d transition metals like Ti, Ni when present in perovskite structure and provides information about p-type or hole doping. Presence of considerable amount of 3d states justifies the reason to consider it as a light 3d transition metal. The O K-edge spectra display characteristic spectral features assigned as e(g) and t2g and show strong dependence on concentration.

X-ray absorption spectroscopy investigations on oxidized Ni/Au contacts to p-GaN.

X-ray absorption spectroscopy was used to investigate the electronic structure of as-deposited and oxidized Ni/Au contacts to p-GaN and to elucidate the mechanism responsible for low impedance. X-ray absorption near edge spectra of Ni K- and L3,2-edges clearly indicate formation of NiO on the sample surface after annealing. The reason for low impedance may be attributed to increase in hole concentration and existence of p-NiO layer on the surface.